Emilie Houliez
email: emilie.houliez@outlook.fr

François G Schmitt

Elsa Breton

Dimitra-Ioli Skouroliakou

Urania Christaki

On the conditions promoting Pseudo-nitzschia spp. blooms in the eastern English Channel and southern North Sea

This study investigated the drivers of the blooms of Pseudo-nitzschia seriata and Pseudonitzschia delicatissima complexes in the eastern English Channel and southern North Sea.

Phytoplankton data series acquired from 1992 to 2020 were analyzed with a multivariate statistical approach based on Hutchinson's niche concept. P. seriata and P. delicatissima complexes were found to be typically present year round, but they bloomed at different periods because they occupied different realized ecological niches. P. delicatissima complex occupied a more marginal niche and was less tolerant than P. seriata complex. P. delicatissima complex typically bloomed in April-May at the same time as Phaeocystis globosa while P. seriata complex blooms were more frequently observed in June during the decline of low intensity P. globosa blooms. P. delicatissima and P. seriata complexes were both favored by low-silicate environments and relatively low turbulence but they responded differently to water temperature, light, ammonium, phosphate and nitrite + nitrate conditions.

Niche shifts and biotic interactions played important roles in the control of the blooms of P. delicatissima and P. seriata complexes. The two complexes occupied different sub-niches during their respective low abundance and bloom periods. The phytoplankton community structure and the number of other taxa presenting a niche overlapping the niches of P. delicatissima and P. seriata complexes also differed between these periods. P. globosa was the taxa contributing the most to the dissimilarity in community structure. P. globosa interacted positively with P. delicatissima complex and negatively with P. seriata complex.

Introduction:

Blooms of the diatom Pseudo-nitzschia are recognized worldwide as a public health threat because several species in this genus are known to produce the neurotoxin domoic acid (DA) causing amnesic shellfish poisoning (ASP) [START_REF] Bates | Pseudonitzschia, Nitzschia, and domoic acid: New research since 2011[END_REF][START_REF] Grattan | Harmful algal blooms and public health[END_REF][START_REF] Lelong | Pseudo-nitzschia (Bacillariophyceae) species, domoic acid and amnesic shellfish poisoning: revisiting previous paradigms[END_REF][START_REF] Petroff | Public health risks associated with chronic, low-level domoic acid exposure: a review of the evidence[END_REF][START_REF] Trainer | Toxic diatoms[END_REF]. Since the report of the first ASP event in 1987 [START_REF] Perl | An outbreak of toxic encephalopathy caused by eating mussels contaminated with domoic acid[END_REF], Pseudo-nitzschia has become the focus of considerable international scientific attention. Temperature, salinity, nutrients, irradiance, photoperiod and upwelling events have been identified as factors influencing its growth and toxicity [START_REF] Bates | Bloom dynamics and physiology of domoicacid-producing Pseudo-nitzschia species[END_REF][START_REF] Bates | Pseudonitzschia, Nitzschia, and domoic acid: New research since 2011[END_REF][START_REF] Clark | Pseudo-nitzschia bloom dynamics in the Gulf of Maine: 2012-2016[END_REF][START_REF] Downes-Tettmar | Seasonal variation in Pseudo-nitzschia spp. and domoic acid in the Western English Channel[END_REF][START_REF] Fehling | Seasonality of Pseudo-nitzschia spp.(Bacillariophyceae) in western Scottish waters[END_REF][START_REF] Klein | Dynamics of Pseudonitzschia spp. and domoic acid production in a macrotidal ecosystem of the eastern English Channel (Normandy, France)[END_REF][START_REF] Lelong | Pseudo-nitzschia (Bacillariophyceae) species, domoic acid and amnesic shellfish poisoning: revisiting previous paradigms[END_REF][START_REF] Marchetti | Environmental conditions and phytoplankton dynamics associated with Pseudo-nitzschia abundance and domoic acid in the Juan de Fuca eddy[END_REF][START_REF] Mccabe | An unprecedented coastwide toxic algal bloom linked to anomalous ocean conditions[END_REF][START_REF] Thorel | Nutrient ratios influence variability in Pseudo-nitzschia species diversity and particulate domoic acid production in the Bay of Seine (France)[END_REF][START_REF] Trainer | Pseudo-nitzschia physiological ecology, phylogeny, toxicity, monitoring and impacts on ecosystem health[END_REF][START_REF] Trainer | Toxic diatoms[END_REF]. However, in spite of intense research, the current understanding of the conditions controlling Pseudo-nitzschia spp. blooms remains elusive since the aforementioned abiotic factors are generally insufficient to explain the occurrence, intensity and demise (often abrupt) of Pseudo-nitzschia spp. blooms [START_REF] Azanza | The role of life cycle characteristics in harmful algal bloom dynamics[END_REF]. A part of our lack of understanding is because environmental cues triggering Pseudo-nitzschia spp. blooms are complex and can be unique to the bay, coastal or open ocean region where they occur making any generalization of the findings almost impossible [START_REF] Trainer | Pseudo-nitzschia physiological ecology, phylogeny, toxicity, monitoring and impacts on ecosystem health[END_REF]. Another reason might be the methodology employed.

Harmful algae (including Pseudo-nitzschia spp.) are usually studied in monospecific cultures in the laboratory and field studies often neglect to characterize surrounding phytoplankton community [START_REF] Wells | Harmful algal blooms in a changing ocean[END_REF]. As such, competitive interactions are not taken into account.

Biotic interactions between Pseudo-nitzschia spp. and the other members of the phytoplankton community are of interest because competition for resources is well known to impact the dominance of certain phytoplankton species [START_REF] Litchman | CHAPTER 16 -Resource Competition and the Ecological Success of Phytoplankton[END_REF][START_REF] Sommer | The Role of Competition for Resources in Phytoplankton Succession[END_REF][START_REF] Tilman | Resource Competition between Plankton Algae: An Experimental and Theoretical Approach[END_REF]. Besides, positive interactions such as facilitations (Bruno et al., 2003) have also recently been shown to play a role in some microalgae species successions [START_REF] Krichen | Demonstration of facilitation between microalgae to face environmental stress[END_REF]. Therefore, as it has already been demonstrated in terrestrial plants community successions [START_REF] Brooker | Facilitation in plant communities: the past, the present, and the future[END_REF], some pioneer microalgae species could modify their surrounding environment, making it more favorable for the subsequent colonization by other microalgae species. Some examples of Pseudo-nitzschia species attached on colonies of other phytoplankton species have also been observed in the field, such as Pseudo-nitzschia pseudodelicatissima on Chaetoceros socialis, Pseudo-nitzschia americana and Pseudonitzschia linea on Chaetoceros spp. and Odontella spp., Pseudo-nitzschia delicatissima on Phaeocystis globosa or Pseudo-nitzschia cf. granii var. curvata on Phaeocystis pouchettii, suggesting that some kind of relationships may exist between these species (Lelong et al., 2012 and references therein). However, to our knowledge, the potential role of biotic interactions influencing Pseudo-nitzschia spp. blooms has never been explored. Turbulence is another factor that has not been considered and would deserve more attention since it affects plankton dynamics [START_REF] Estrada | Phytoplankton in a turbulent world[END_REF][START_REF] Gibson | Laboratory and ocean studies of phytoplankton response to fossil turbulence[END_REF][START_REF] Mann | Dynamics of marine ecosystems: biological-physical interactions in the oceans[END_REF][START_REF] Schmitt | Turbulence et écologie marine[END_REF]. There is evidence that turbulence affects most of the parameters influencing diatoms survival. It influences settling velocity and cell re-suspension [START_REF] Estrada | Phytoplankton in a turbulent world[END_REF][START_REF] Ruiz | Turbulence increases the average settling velocity of phytoplankton cells[END_REF], nutrients acquisition and availability [START_REF] Dell'aquila | Nutrient consumption and chain tuning in diatoms exposed to storm-like turbulence[END_REF][START_REF] Estrada | Phytoplankton in a turbulent world[END_REF][START_REF] Pahlow | Impact of cell shape and chain formation on nutrient acquisition by marine diatoms[END_REF], genes expression [START_REF] Amato | Marine diatoms change their gene expression profile when exposed to microscale turbulence under nutrient replete conditions[END_REF], chain morphology [START_REF] Amato | Marine diatoms change their gene expression profile when exposed to microscale turbulence under nutrient replete conditions[END_REF][START_REF] Clarson | Some observations of diatoms under turbulence[END_REF][START_REF] Dell'aquila | Nutrient consumption and chain tuning in diatoms exposed to storm-like turbulence[END_REF] and encounterbased processes such as interactions with grazers and diatom-diatom encounter rate for reproduction and chain formation [START_REF] Rothschild | Small-scale turbulence and plankton contact rates[END_REF].

Pseudo-nitzschia spp. are regularly present along all the French coasts [START_REF] Belin | Three decades of data on phytoplankton and phycotoxins on the French coast: Lessons from REPHY and REPHYTOX[END_REF] but in some regions, the blooms of Pseudo-nitzschia spp. have received few attention. This is the case of the eastern English Channel and southern North Sea. Phytoplankton research in this area has mainly focused on the understanding of the massive P. globosa blooms that occur every spring [START_REF] Breton | Annual variations of phytoplankton biomass in the eastern English Channel: comparison by pigment signatures and microscopic counts[END_REF][START_REF] Breton | Multiple phytoplankton community responses to environmental change in a temperate coastal system: A trait-based approach[END_REF][START_REF] Genitsaris | Seasonal variations of marine protist community structure based on taxon-specific traits using the eastern English Channel as a model coastal system[END_REF][START_REF] Grattepanche | Succession of primary producers and micrograzers in a coastal ecosystem dominated by Phaeocystis globosa blooms[END_REF][START_REF] Hernández-Fariñas | Temporal changes in the phytoplankton community along the French coast of the eastern English Channel and the southern Bight of the North Sea[END_REF][START_REF] Peperzak | The wax and wane of Phaeocystis globosa blooms[END_REF].

Consequently, the number of studies dealing with Pseudo-nitzschia spp. ecology in this region is very limited. [START_REF] Delegrange | Pseudo-nitzschia sp. diversity and seasonality in the southern North Sea, domoic acid levels and associated phytoplankton communities[END_REF] described the diversity and toxicity of Pseudonitzschia spp. during one year (2012) at a single station in the southern Bight of the North Sea. [START_REF] Lefebvre | Trajectories of Changes in Phytoplankton Biomass, Phaeocystis globosa and Diatom (incl. Pseudo-nitzschia sp.) Abundances Related to Nutrient Pressures in the Eastern English Channel, Southern North Sea[END_REF] studied the possibility to use P. globosa and diatoms (including Pseudo-nitzschia spp.) as indicators of eutrophication. [START_REF] Karasiewicz | Environmental Impact on Harmful Species Pseudonitzschia spp. and Phaeocystis globosa Phenology and Niche[END_REF] found that Pseudo-nitzschia spp. could bloom several times during the same year or bloom over a large period of time; however, these blooms patterns could be driven by different species of Pseudo-nitzschia. Three Pseudo-nitzschia species (P. delicatissima, P. fraudulenta and P. pungens) belonging to two different complexes (Pseudo-nitzschia delicatissima complex and Pseudo-nitzschia seriata complex) have been detected in this region by [START_REF] Delegrange | Pseudo-nitzschia sp. diversity and seasonality in the southern North Sea, domoic acid levels and associated phytoplankton communities[END_REF]. In the present study, it was thus hypothesized that the two Pseudo-nitzschia complexes might occupy different ecological niches and present contrasted bloom periods controlled by different environmental factors. Understanding these potential differences in the ecology of the two Pseudo-nitzschia complexes is important because the species belonging to these complexes can present different levels of toxicity (e.g. [START_REF] Trainer | Pseudo-nitzschia physiological ecology, phylogeny, toxicity, monitoring and impacts on ecosystem health[END_REF]. It was also thought that biotic interactions with the other members of the phytoplankton community and turbulence might influence Pseudo-nitzschia spp. blooms.

By using a methodology based on the concept of realized ecological niche (i.e. the set of favorable conditions under which a species survives, grows, reproduces and can be observed [START_REF] Hutchinson | Concluding remarks[END_REF]), the present study aimed: 1) to determine the bloom periods of Pseudo-nitzschia delicatissima complex and Pseudo-nitzschia seriata complex, 2) to investigate the main factors favoring these blooms and, 3) to define the role of the surrounding phytoplankton community by identifying potential biotic interactions between Pseudo-nitzschia spp. and the other phytoplankton species.

2 Materials and Methods:

Study area

The eastern English Channel and southern North Sea are temperate eutrophic shallow shelf seas with a macrotidal range (3-9 m). The water column is constantly well mixed. Tides are characterized by a residual water circulation parallel to the coast with coastal waters drifting from the English Channel to the North Sea. This so-called "coastal flow" separates the coastal waters from offshore waters by a transient tidally controlled frontal area [START_REF] Brylinski | Le "fleuve côtier": un phénomène hydrologique important en Manche orientale. Exemple du Pas-de-Calais[END_REF]. Coastal waters are influenced by freshwater runoffs from the Seine, Somme, Authie and Canche estuaries and from the Liane river. The eastern English Channel and southern North Sea present an unusual phytoplankton succession occurring every spring. Each year, the Haptophyte P. globosa forms massive blooms between March and May and these blooms are preceded and followed by two diatom blooms with distinct species composition [START_REF] Breton | Annual variations of phytoplankton biomass in the eastern English Channel: comparison by pigment signatures and microscopic counts[END_REF][START_REF] Breton | Multiple phytoplankton community responses to environmental change in a temperate coastal system: A trait-based approach[END_REF][START_REF] Genitsaris | Seasonal variations of marine protist community structure based on taxon-specific traits using the eastern English Channel as a model coastal system[END_REF][START_REF] Grattepanche | Succession of primary producers and micrograzers in a coastal ecosystem dominated by Phaeocystis globosa blooms[END_REF][START_REF] Hernández-Fariñas | Temporal changes in the phytoplankton community along the French coast of the eastern English Channel and the southern Bight of the North Sea[END_REF].

Data sets and sampling strategy

Data come from three monitoring programs with similar methodology: 1) SNO-SOMLIT (National coastal observation service, www.somlit.fr), 2) IFREMER French Observation and Monitoring program for Phytoplankton and Hydrology in coastal waters (REPHY, 2021) and 3) Regional Observation and Monitoring Program for Phytoplankton and Hydrology in the eastern English Channel (SRN, 2017). The complete dataset comprised a total of 1213 samples collected over 28 years of phytoplankton observations (SNO-SOMLIT: 1996[START_REF] Schmitt | Turbulence et écologie marine[END_REF]and REPHY/SRN: 1992[START_REF] Mammola | Assessing similarity of n-dimensional hypervolumes: Which metric to use?[END_REF].

Samples were collected at 13 stations distributed along four coastal-offshore transects located in three main areas: the coasts of Boulogne-sur-Mer and Dunkerque and the Bay of Somme (Fig. 1). Water samples for laboratory analyses of nutrient concentrations and phytoplankton were collected at high tide in subsurface with a five-liter Niskin bottle. These sampling were done fortnightly during the SNO-SOMLIT monitoring while during the REPHY/SRN monitoring, samples were collected fortnightly from March to June and monthly the rest of the year. Phytoplankton samples (1 L) were fixed with neutral or acid Lugol's Iodine solution (final concentration 2%).

Abiotic parameters

Seawater temperature and salinity profiles were measured in situ with a CTD probe (conductivity, temperature, depth). Silicate (SiOH4), nitrate + nitrite (NO3 -+ NO2 -), ammonium (NH4 + ) and phosphate (PO4 3-) concentrations were analyzed following the [START_REF] Aminot | Hydrologie des écosystèmes marins. Paramètres et analyses[END_REF][START_REF] Litchman | CHAPTER 16 -Resource Competition and the Ecological Success of Phytoplankton[END_REF]'s methodology. Turbulence was characterized by a proxy of the mean dissipation rate (ε in m²/s 3 ) using MacKenzie and Leggett (1993)'s equation: ε = (5.82 10 -6 ) W 3 / z with z = sampling depth (in m) and W = wind speed (in m s -1 ). Wind speed was measured at the meteorological stations of Boulogne-sur-Mer, Dunkerque and Cayeux-sur-Mer and was provided by Meteo France. Daily photosynthetically active radiation (PAR in einstein m -2 day - was estimated from satellite products using [START_REF] Frouin | The SeaWIFS PAR product[END_REF]'s algorithm. Five-day averages of turbulence and PAR values were used in the realized ecological niche analyses (four previous days and day of phytoplankton sampling) [START_REF] Hernández Fariñas | Assessing phytoplankton realized niches using a French national phytoplankton monitoring network[END_REF][START_REF] Houliez | Spatio-temporal drivers of microphytoplankton community in the Bay of Biscay: Do species ecological niches matter?[END_REF].

2.4 Taxonomic determination of phytoplankton species 10-100 mL of the fixed phytoplankton samples were settled for at least 24h following the Utermöhl method [START_REF] Utermöhl | Zur Ver vollkommung der quantitativen phytoplankton-methodik[END_REF]. Phytoplankton species were identified to the lowest possible taxonomic level and enumerated using an inverted optical microscope (x 200 to x 400 magnification). To reduce bias in the datasets due to difficulties in differentiating some species or genera using an optical microscope and to guarantee homogeneity over time and among sampling sites, some species and genera were grouped into taxonomic units. A taxonomic unit was thus composed either by a single species (easily identifiable) or a group of several species or genus difficult to distinguish. Pseudo-nitzschia spp. were separated into three groups: Pseudo-nitzschia americana and two complexes with different cell width and shape: Pseudo-nitzschia delicatissima complex (cell width <3µm, linear shape, delicate forms with very fine structure) and Pseudo-nitzschia seriata complex (cell width >3µm, lanceolate shape) (Hasle and Syvertsen, 1996). This classification based on the size and shape of the cells has already demonstrated its usefulness in the understanding of Pseudo-nitzschia spp. ecology [START_REF] Ajani | Fifteen years of Pseudo-nitzschia in an Australian estuary, including the first potentially toxic P. delicatissima bloom in the southern hemisphere[END_REF][START_REF] Bowers | Diversity and toxicity of Pseudo-nitzschia species in Monterey Bay: Perspectives from targeted and adaptive sampling[END_REF][START_REF] Delegrange | Pseudo-nitzschia sp. diversity and seasonality in the southern North Sea, domoic acid levels and associated phytoplankton communities[END_REF][START_REF] Downes-Tettmar | Seasonal variation in Pseudo-nitzschia spp. and domoic acid in the Western English Channel[END_REF][START_REF] Stern | Spatial and temporal patterns of Pseudo-nitzschia genetic diversity in the North Pacific Ocean from the Continuous Plankton Recorder survey[END_REF][START_REF] Thorel | Nutrient ratios influence variability in Pseudo-nitzschia species diversity and particulate domoic acid production in the Bay of Seine (France)[END_REF].

Statistical analyses

Realized ecological niches and sub-niches

The Outlying Mean Index (OMI, [START_REF] Dolédec | Niche separation in community analysis: a new method[END_REF] and Within Outlying Mean Indexes (WitOMI, Karasiewicz et al., 2017) were used to characterize respectively the realized ecological niche and realized ecological sub-niches of the two Pseudo-nitzschia complexes and of the other species compositing surrounding phytoplankton community. OMI and WitOMI are both based on [START_REF] Hutchinson | Concluding remarks[END_REF] realized ecological niche concept (for more details read the supplementary materials). The OMI analysis was carried out using the "niche" and "rtest" functions available in the R ade4 package [START_REF] Dray | The ade4 package: implementing the duality diagram for ecologists[END_REF] while the WitOMI analysis was performed with the R subniche package (Karasiewicz et al., 2017).

The OMI analysis is a multivariate statistical analysis defining the realized ecological niche of each taxa composing a community along environmental gradients [START_REF] Dolédec | Niche separation in community analysis: a new method[END_REF]Karasiewicz et al., 2020). It depends on the response of each taxa to their environment and describes their habitat. Three parameters characterize the realized niche of each taxa: tolerance, marginality, and residual tolerance. Tolerance (also known as niche breadth) corresponds to the niche extent. It describes the range of conditions tolerated and used by a given taxa. Taxa with high tolerance can persist under a wide range of environmental conditions. They are generalists. Inversely, a low tolerance corresponds to specialists i.e. taxa encountered under specific environmental conditions. Marginality (also known as niche position) is the distance between the average conditions (or resources) used by a given taxa and the average conditions (or resources) available in the study area. Taxa with high marginality occupy a marginal realized ecological niche and occur in atypical habitats within the study area. Inversely, taxa with low marginality occupy a non-marginal realized ecological niche and occur in habitats commonly found in the study area. Residual tolerance represents the part of variance that is not explained by marginality. It helps to determine whether the dataset of environmental conditions used to perform the OMI analysis is relevant to define the realized ecological niche of each taxa. The OMI analysis provides graphical outputs which are described in the supplementary materials.

The WitOMI analysis uses the environmental space delimited by the OMI axes as a reference. It decomposes the realized niche of each taxa into realized ecological sub-niches (Karasiewicz et al., 2017). This analysis provides an estimation of niche shifts or conservatism in the realized ecological niche of each taxa, under specific sub-environmental conditions (called subsets) corresponding to a given time period or a sub-location within the study area. For each subset studied, two additional marginalities are provided: WitOMIG and WitOMIGK. WitOMIG is the distance between the average conditions (or resources) used by the taxa within the studied subset and the average conditions (or resources) available within the whole sampling period or study area. WitOMIGK is the distance between the average conditions (or resources) used by the taxa within the studied subset and the average conditions (or resources) used by the community. Each of these marginalities is accompanied by an index of tolerance (TolWitOMIG and TolWitOMIGK) and residual tolerances. For each subset considered, the WitOMI analysis provides a graphical representation of: 1) the environmental conditions (or resources) available (sub-environmental space), 2) the potential sub-niche of the taxa, 3) the realized ecological subniche of the taxa and 4) the biological constraints. The potential sub-niche of the taxa is the intersection between the sub-environmental space within the subset considered and the realized ecological niche defined by the OMI analysis. It corresponds to the niche which could be occupied by the taxa in the absence of biotic interactions. The realized ecological sub-niche is the niche currently occupied by the taxa.

The biological constraints correspond to the difference in size between the potential sub-niche and realized ecological sub-niche, and represent the part of the niche that the taxa can't occupy due to biotic interactions [START_REF] Houliez | Spatio-temporal drivers of microphytoplankton community in the Bay of Biscay: Do species ecological niches matter?[END_REF]. In the present study, the environmental variables included in the OMI and WitOMI analyses were water temperature, salinity, nutrient concentrations, turbulence and PAR. These factors are known to influence harmful algal blooms and Pseudo-nitzschia spp. growth (e.g. [START_REF] Bates | Bloom dynamics and physiology of domoicacid-producing Pseudo-nitzschia species[END_REF][START_REF] Bates | Pseudonitzschia, Nitzschia, and domoic acid: New research since 2011[END_REF][START_REF] Lelong | Pseudo-nitzschia (Bacillariophyceae) species, domoic acid and amnesic shellfish poisoning: revisiting previous paradigms[END_REF][START_REF] Wells | Harmful algal blooms in a changing ocean[END_REF]. The WitOMI analysis was used to decompose the realized ecological niche of P. delicatissima and P. seriata complexes by considering two subsets corresponding to two contrasted periods based on their abundance.

For each Pseudo-nitzschia complex, the first subset corresponded to periods of low abundance < 10 3 cells L -1 . The second subset corresponded to bloom periods. Bloom periods were defined by using the French sanitary regulatory thresholds above which Pseudo-nitzschia species are considered as producing blooms potentially harmful, and samples are collected for domoic acid analysis in shellfish by the IFREMER REPHYTOX monitoring network [START_REF] Belin | Three decades of data on phytoplankton and phycotoxins on the French coast: Lessons from REPHY and REPHYTOX[END_REF]. This threshold was ≥ 10 5 cells L -1 for P. seriata complex and ≥ 3x10 5 cells L -1 for P. delicatissima complex. A Mann-Whitney U test [START_REF] Hollander | Nonparametric statistical methods[END_REF] was used to test the significance of differences in abiotic parameters between the blooms and periods of low abundance of P. delicatissima and P. seriata complexes.

Phytoplankton community structure and biotic interactions

For both P. delicatissima and P. seriata complexes, the taxa contributing the most to the dissimilarity in phytoplankton community structure between years with blooms and years without bloom were studied with a SIMPER (SIMilarity PERcentage) analysis performed on a data frame containing fourth root transformed phytoplankton species abundances using PRIMER 6 (PRIMER-E ltd, Plymouth, UK) (Clarke and Warwick, 2001). When taxa contribution was related to abundance changes, the significance of these changes was tested with a Mann-Whitney U test [START_REF] Hollander | Nonparametric statistical methods[END_REF].

To highlight potential biotic interactions between the Pseudo-nitzschia complexes (P. delicatissima and P. seriata) and the other members of the phytoplankton community, an interaction network was built using the [START_REF] Chiquet | The Poisson-Lognormal Model as a Versatile Framework for the Joint Analysis of Species Abundances[END_REF]'s multivariate Poisson-lognormal model (PLN model). This analysis was performed using the R PLNmodels package. PLN models offer the possibility to distinguish pairs of species that are truly interacting from indirect covariations due to similar response to environmental variables. From the matrix of partial correlation coefficients obtained with the PLN model, a simplified version of the network representing only the taxa interacting with P. delicatissima and P. seriata complexes was represented by using Cytoscape v.3.8.2.

Niche overlaps

The realized ecological niche, potential sub-niche and realized ecological sub-niche of P. delicatissima and P. seriata complexes were compared to the niches of the other taxa by using the R hypervolume [START_REF] Blonder | The n-dimensional hypervolume[END_REF] and BAT (Biodiversity Assessment Tools) packages [START_REF] Cardoso | BAT: Biodiversity Assessment Tools. R package version 2.6.1[END_REF][START_REF] Mammola | Assessing similarity of n-dimensional hypervolumes: Which metric to use?[END_REF]. Niche hypervolumes were built by using the function "hypervolume_svm" on a data frame containing the coordinates of the realized ecological niche, potential sub-niche and realized ecological sub-niche along the first two axes of the OMI and WitOMI analyses. The total volume of each niche and sub-niche was calculated with the function "kernel.alpha" [START_REF] Mammola | Functional diversity metrics using kernel density ndimensional hypervolumes[END_REF]. The function "kernel.similarity" was used to compare the different niches [START_REF] Mammola | Functional diversity metrics using kernel density ndimensional hypervolumes[END_REF]. This function computes pairwise comparisons between kernel density niche hypervolumes of multiple species and characterizes the similarity between pairs of species niches with pairwise distance metrics (centroid and minimum distance) and similarity indices (intersection = niche overlap, Jaccard similarity index and Soerensen-Dice similarity index) (see [START_REF] Mammola | Assessing similarity of n-dimensional hypervolumes: Which metric to use?[END_REF] for details). In the present study, only the niche overlaps were reported. They were expressed as percentage of the volumes of the realized ecological niche, potential sub-niche and realized ecological sub-niches of P. seriata or P. delicatissima complexes.

3 Results:

Bloom periods

Pseudo-nitzschia delicatissima and Pseudo-nitzschia seriata complexes are typically present year round, but their bloom periods are different (Fig. 2 A&B). P. delicatissima complex blooms mainly at the same time as P. globosa blooms from April to May. From 2009, P. delicatissima complex blooms were observed every year (data not shown). P. seriata complex typically blooms from March to September but the blooms were more frequently observed in June during the decline periods of low intensity P. globosa blooms. P. seriata complex started to bloom either at the same time as P. globosa or with a small delay after the start of P. globosa blooms. However in all cases, the peak of the blooms of P. seriata complex was always reached during the decline of P. globosa blooms or shortly after. P. globosa bloomed every year but it was not the case for P. seriata complex (data not shown).

During some years, low intensity P. globosa blooms were not accompanied by blooms of P. seriata complex.

Realized ecological niches of Pseudo-nitzschia spp.

The OMI analysis revealed that P. delicatissima and P. seriata complexes occupied different realized ecological niches and presented different affinities for abiotic environmental conditions.

The first two axes of the OMI analysis explained 88.0% of total inertia (OMI1 = 70.3% and OMI2 = 17.6%, Fig. 3). OMI1 was mainly explained by NO3 -+ NO2 -and PAR while OMI2 was mainly explained by water temperature (T). The OMI plane reflected the seasonal variations of environmental conditions and delimited three main periods. The right side of the OMI plane corresponded to autumn-winter conditions characterized by high nutrient concentrations, well-mixed waters (high turbulence) and low PAR. The bottom left side corresponded to spring conditions with low nutrient concentrations, high PAR and high salinity. The top left side corresponded to summer conditions with also low nutrient concentrations, high PAR and high salinity but with higher temperature. The realized ecological niche center of gravity (marginality) of each taxa is positioned within this plane according to their affinities for environmental conditions. The position of the marginality of P. delicatissima and P. seriata complexes showed that P. delicatissima complex had affinity for spring conditions while P. seriata complex had affinity for spring-summer conditions.

Along OMI2, the realized ecological niche of P. seriata complex was centered at a higher temperature than the realized ecological niche of P. delicatissima complex. High abundances of P. delicatissima complex were more frequently observed when temperature ranged from 11 to 14°C while high abundances of P. seriata complex were more frequently observed when temperature ranged from 14 to 16°C (supplementary materials Fig. S2). Along OMI1, the realized ecological niche of P. seriata complex was also centered at higher PAR than the realized ecological niche of P. delicatissima complex. High abundances of P. delicatissima complex were more frequently observed when PAR ranged from 25 to 45 einstein m -2 day -1 while high abundances of P. seriata complex were more frequently observed when PAR ranged from 40 to 55 einstein m -2 day -1 . P. delicatissima and P. seriata complexes also differed in their affinity for NO3 -+ NO2 -conditions. For both P. delicatissima and P. seriata complexes, high abundances were more frequently observed when NO3 -+ NO2 - concentrations were ≤ 1 µM. However, when NO3 -+ NO2 -concentrations ranged from 5 to 12 µM, high abundances of P. delicatissima complex were more frequently observed than high abundances of P. seriata complex.

P. delicatissima complex had a higher marginality and lower tolerance than P. seriata complex (Table 1). In comparison to the other taxa composing the phytoplankton community, P. delicatissima and P. seriata complexes were among the ten taxa with the lowest marginality. P. delicatissima complex was the 17 th taxa with the lowest tolerance while P. seriata complex presented an intermediate tolerance (38 th over the 74 diatoms with significant realized niches).

3.3 Realized ecological sub-niches of Pseudo-nitzschia spp.

Environmental conditions

All abiotic environmental conditions differed between low abundance and bloom periods of P. delicatissima complex (Fig. 4). During the blooms of P. delicatissima complex, median water temperature, salinity, turbulence and nutrient concentrations (NH4 -, NO2 -+ NO3 -, Si(OH)4 and PO4 3-) were significantly lower and median PAR was significantly higher than during its low abundance periods. By contrast, only median turbulence, NO2 -+ NO3 - concentrations, Si(OH)4 concentrations and PAR significantly differed between the blooms and low abundance periods of P. seriata complex. Median turbulence, NO2 -+ NO3 - concentrations and Si(OH)4 concentrations were significantly lower while median PAR was significantly higher during the blooms of P. seriata complex than during its low abundance periods. During the years with blooms of P. seriata complex, Si(OH)4 and NO2 -+ NO3 -winter stocks were lower than during the years without blooms of P. seriata complex (supplementary materials, Fig. S3).

WitOMI analysis

The WitOMI analysis showed that the sub-environmental space defined by the available environmental conditions was narrower during the blooms of P. delicatissima and P. seriata complexes than during their low abundance periods (Fig. 5). Consequently, the realized ecological sub-niches of P. delicatissima and P. seriata complexes shifted during their blooms. P. delicatissima and P. seriata complexes were both restricted by biological constraints during their respective low abundance periods and they could not take up the totality of their potential sub-niche. P. delicatissima and P. seriata complexes took up 63% and 90% of their potential sub-niche respectively (Table 1). By contrast, during their blooms, there was no biological constraint and they both took up the totality of their potential subniche. However, the realized ecological sub-niche occupied was more marginal than during their low abundance periods and their tolerance was lower.

Phytoplankton community structure and potential biotic interactions

To understand what were the biological constraints influencing the realized ecological sub-niches of P. delicatissima and P. seriata complexes, the surrounding phytoplankton community structure was compared between the years with blooms and years without bloom to determine the taxa influencing the most the bloom periods of P. delicatissima and P. seriata complexes. For both P. delicatissima and P. seriata complexes, the surrounding phytoplankton community structure was significantly different between their blooming years and the years without bloom (PERMANOVA, p<0.001). For P. delicatissima complex, the dissimilarity between the phytoplankton community present during the blooming years and that present during the years without bloom equaled 68% (SIMPER analysis, supplementary materials Table S1). Twenty-four taxa were identified as the most influential taxa explaining this dissimilarity. Among them, P. globosa was the major contributor. For P. seriata complex, the dissimilarity in phytoplankton community between the blooming years and years without bloom equaled 71%. Twenty-three taxa were identified as the most influential and P. globosa was also the major contributor to this dissimilarity. However, contrasting interactions between P. globosa and each of the Pseudonitzschia complexes (P. delicatissima and P. seriata) were observed. The interaction network highlighted a positive interaction between P. delicatissima complex and P. globosa while interaction between P. seriata complex and P. globosa was negative (Fig. 6). Positive interactions between P. seriata complex and 5 other diatoms: Guinardia delicatula, Leptocylindrus danicus, Chaetoceros curvisetus + Chaetoceros debilis and Chaetoceros danicus were also highlighted. P. globosa abundance was thus significantly higher during the blooms of P. delicatissima complex than during its low abundance periods (Fig. 7A) while the opposite trend was observed during the blooms and low abundance periods of P. seriata complex (Fig. 7B). G. delicatula (Fig. 7C), C. curvisetus + C. debilis (Fig. 7D) and C. danicus (Fig. 7E) abundances were significantly higher during the blooming years of P. seriata complex while for L. danicus (Fig. 7F), no significant difference was observed between the blooming years of P. seriata complex and years without bloom. G. delicatula, C. curvisetus + C. debilis, C. danicus and L. danicus were not present in high abundance at the same period. G. delicatula was present in high abundance (≥ 10 5 cells L -1 ) from March to June during the blooms of P. globosa and P. seriata complex (Fig. 7G). C. curvisetus + C. debilis high abundances (≥ 10 5 cells L -1 ) were typically observed from March to October but they were more frequently observed in March-April during the blooms of P. globosa and in June during the blooms of P. seriata complex (Fig. 7H). C. danicus high abundances (≥ 10 4 cells L -1 ) were more frequently observed from February to March during the first diatom bloom and at the beginning of the P. globosa bloom period (Fig. 7I). L. danicus high abundances (≥ 10 5 cells L -1 ) were more frequently observed during the blooms of P. seriata complex in June and July (Fig. 7J).

3.5 Realized ecological niches and sub-niches of the taxa interacting with Pseudo-nitzschia spp. and niche overlaps Overlaps between the realized ecological sub-niches of the taxa composing the surrounding phytoplankton community and the realized ecological sub-niches of P. delicatissima and P. seriata complexes were calculated to better understand the biological constraints influencing the realized ecological sub-niches of P. delicatissima and P. seriata complexes.

The number of other taxa present and showing a niche overlapping the realized ecological sub-ecological niche of P. delicatissima complex was lower during the blooms of P. delicatissima complex than during its low abundance periods. 113 other taxa were present during low abundance periods of P. delicatissima complex vs. 74 during its blooms. 86% of them had a realized ecological sub-niche overlapping the realized ecological sub-niche of P. delicatissima complex during its low abundance periods vs. 77% during its blooms. The same pattern was observed during low abundance periods and blooms of P. seriata complex. There were 108 other taxa during the low abundance periods of P. seriata complex vs. 64 during its blooms. 92% of them had a realized ecological sub-niche overlapping the realized ecological sub-niche of P. seriata complex during its low abundance periods vs. 77% during its blooms.

Among these taxa, P. globosa, G. delicatula, L. danicus, C. curvisetus + C. debilis and C. danicus were the taxa interacting the most with P. delicatissima complex and/or P. seriata complex. The realized ecological niche and sub-niches of P. globosa, G. delicatula, L. danicus, C. curvisetus + C. debilis and C. danicus differed from the realized ecological niche and sub-niches of P. delicatissima and P. seriata complexes (Fig. 8 & Fig. 5). C. curvisetus + C. debilis presented a lower marginality than P. delicatissima and P. seriata complexes while P. globosa, G. delicatula, L. danicus and C. danicus had a higher marginality (Table 1). Among these five taxa, P. globosa was the least tolerant while C. danicus was the more tolerant. C. danicus had affinity for winter-spring conditions, C. curvisetus + C. debilis for spring-summer-autumn conditions, P. globosa for spring conditions and L. danicus and G. delicatula for spring-summer conditions (Fig. 3).

The realized ecological niche of P. globosa was more similar to the realized ecological niche of P. delicatissima complex than the realized ecological niche of P. seriata complex.

The realized ecological niche of P. globosa overlapped 56 % of the realized ecological niche of P. delicatissima complex and 52 % of the realized ecological niche of P. seriata complex (Table 2). The realized ecological niche of P. globosa differed from realized ecological niches of P. delicatissima and P. seriata complexes mainly in the temperature, PAR and NO2 -+ NO3 -conditions favoring its development (supplementary materials, Fig. S2). High P. globosa abundances were more frequently observed when PAR ranged from 30 to 45 einstein m -2 day - 1 i.e. at values close to the PAR conditions favoring the development of P. delicatissima complex but at lower PAR than P. seriata complex. High P. globosa abundances were also more frequently observed at lower temperature (9-12°C) and higher NO3 -+ NO2 - concentrations than P. delicatissima and P. seriata complexes. Indeed, when NO3 -+ NO2 - concentrations were > 10 µM high P. globosa abundances were more frequently observed than high abundances of P. delicatissima and P. seriata complexes.

During low abundance periods of P. delicatissima complex, P. globosa occupied 49% of its potential sub-niche (Table 1) and its realized ecological sub-niches were smaller than the realized ecological sub-niches of P. delicatissima complex (Fig. 8A) P. globosa potential and realized ecological sub-niches overlapped respectively 52% and 38% of the potential and realized ecological sub-niches of P. delicatissima complex (Table 2). During the blooms of P. delicatissima complex, the environmental conditions were different from its low abundance periods. The realized ecological sub-niches of P. delicatissima complex and P. globosa shifted (Fig. 5B &8B). Consequently, the realized ecological sub-niches of P. globosa overlapped a higher proportion of the potential and realized ecological sub-niches of P. delicatissima complex (Table 2). Contrary to P. delicatissima complex that occupied the totality of its potential sub-niche, P. globosa only occupied 87% of its potential sub-niche.

During low abundance periods of P. seriata complex, P. globosa occupied 74% of its potential sub-niche and the sub-niches P. globosa were smaller than the sub-niches of P. seriata complex (Fig. 8C). The potential and realized ecological sub-niches of P. globosa overlapped respectively 57% and 43% of the potential and realized ecological sub-niches of P. seriata complex. During low abundance periods of P. seriata complex, P. globosa occupied a smaller proportion of its potential sub-niche due to stronger environmental and biological constraints (Table 1, Fig. 8D). Its potential sub-niche became identical to the potential sub-niche of P. seriata complex and the overlap between the realized ecological subniches of P. globosa and P. seriata complex increased (Table 2).

Among the diatoms interacting with P. seriata complex, the realized ecological niche of G. delicatula was the most similar to the realized ecological niche of P. seriata complex and completely overlapped it (Fig. 8E & F, Table 2). The realized ecological niche of P. seriata complex was however smaller than the realized ecological niche of G. delicatula (Fig. 8 & Fig. 5) since high abundances of G. delicatula occurred over a larger range of temperatures than high abundances of P. seriata complex (supplementary materials, Fig. S2).

The other diatoms were ranked as follows: C. danicus, C. curvisetus + C. debilis and L. danicus with their realized ecological niches overlapping respectively 73%, 62% and 55% of the realized ecological niche of P. seriata complex (Table 2). High abundances of C. danicus were frequently observed over a larger range of NO3 -+ NO2 -concentrations than high abundances of P. seriata complex and at lower temperature and PAR (supplementary materials, Fig. S2). High abundances of C. curvisetus + C. debilis and L. danicus were frequently observed at higher temperatures than high abundances of P. seriata complex.

However, high abundances of C. curvisetus + C. debilis were favored by lower PAR than P. seriata complex while L. danicus and P. seriata complex were favored by relatively similar PAR. High abundances of C. curvisetus + C. debilis were also observed over a larger range of NO3 -+ NO2 -concentrations than P. seriata complex and L. danicus.

During low abundance periods of P. seriata complex, C. curvisetus + C. debilis

occupied the totality of its potential sub-niche while G. delicatula, C. danicus and L. danicus occupied respectively 82%, 90% and 79% of their potential sub-niche (Fig. 8E,G,I,K & Table 1). The sub-niches of G. delicatula, C. curvisetus + C. debilis and C. danicus were larger than the sub-niches of P. seriata complex while the sub-niches of L. danicus were smaller. The potential sub-niches of L. danicus, C. curvisetus + C. debilis and C. danicus overlapped respectively 60%, 68%, and 96% of the potential sub-niche of P. seriata complex while the potential sub-niche of G. delicatula overlapped the totality of the potential subniche of P. seriata complex (Table 2). The realized ecological sub-niches of L. danicus, C. curvisetus + C. debilis, C. danicus and G. delicatula overlapped respectively 37%, 77%, 99% and 53% of the realized ecological sub-niche of P. seriata complex.

During the blooms of P. seriata complex, the environmental conditions constrained the realized ecological sub-niches of all the taxa interacting with P. seriata complex (Fig. 8F, H, J & L). Consequently, all of them had the same potential sub-niche as P. seriata complex.

G. delicatula and L. danicus occupied almost the totality of their potential sub-niche while, due to stronger biological constraints, C. curvisetus + C. debilis and C. danicus occupied a smaller proportion of their potential sub-niche than during low abundance periods of P. seriata complex (Table 1). Overlaps between the realized ecological sub-niche of P. seriata complex and the realized ecological sub-niches of C. curvisetus + C. debilis and C. danicus were thus reduced. By contrast, overlaps between the realized ecological sub-niche of P. seriata complex and the realized ecological sub-niches of G. delicatula and L. danicus increased (Table 2). Overlaps between the realized ecological sub-niche of P. globosa and the realized ecological sub-niche of the other diatoms interacting with P. seriata complex were also modified. Overlap between the realized ecological sub-niche of P. globosa and the realized ecological sub-niche of C. curvisetus + C. debilis was reduced from 57% to 2%. By contrast, overlaps between the realized ecological sub-niche of P. globosa and the realized ecological sub-niches of C. danicus and L. danicus increased (C. danicus: from 56% to 71%, L. danicus: from 43% to 81%).

Discussion

The complexes Pseudo-nitzschia delicatissima and Pseudo-nitzschia seriata are typically present year round but they bloom at different periods and occupy different realized ecological niches. P. delicatissima complex showed affinity for spring conditions and blooms more frequently from April to May at the same time as P. globosa blooms. P. seriata complex has affinity for spring-summer conditions. P. seriata complex is able to bloom within a larger seasonal window but its blooms are more frequently observed in June during the decline period of low intensity P. globosa blooms. A similar succession in the bloom seasonality of these two complexes has been observed by [START_REF] Fehling | Seasonality of Pseudo-nitzschia spp.(Bacillariophyceae) in western Scottish waters[END_REF] in western Scottish coastal waters and [START_REF] Delegrange | Pseudo-nitzschia sp. diversity and seasonality in the southern North Sea, domoic acid levels and associated phytoplankton communities[END_REF] in the southern North Sea. High abundances of P. delicatissima complex in spring have also been previously reported in the Skagerrak and Norwegian coastal waters (Hasle et al., 1996) and in the southern North Sea [START_REF] Delegrange | Pseudo-nitzschia sp. diversity and seasonality in the southern North Sea, domoic acid levels and associated phytoplankton communities[END_REF]. This difference in seasonality can be explained by the two complexes occupying different realized ecological niches. P. delicatissima complex occupies a more marginal niche and is less tolerant than P. seriata complex. The realized ecological niches of P. delicatissima and P. seriata complexes differ mainly in the temperature, light and NO2 -+ NO3 -conditions allowing their growth and survival. High abundances of P. seriata complex are observed at higher temperature and PAR than P. delicatissima complex but at lower NO2 -+ NO3 - concentrations. The abiotic conditions favoring the blooms of P. delicatissima and P. seriata complexes also differ. The blooms of P. delicatissima complex are influenced by water temperature, salinity, turbulence, PAR and nutrients (NH4 -, NO2 -+ NO3 -, Si(OH)4 and PO4 3- concentrations). By contrast, the blooms of P. seriata complex are not influenced by water temperature, salinity and NH4 -and PO4 3-concentrations.

In addition to environmental conditions, biotic interactions and niche shifts seem to play an important role in the control of the blooms of P. delicatissima and P. seriata complexes. During their respective low abundance periods, P. delicatissima and P. seriata complexes were limited by strong biological constraints and they could not occupy the totality of their potential sub-niche. For both complexes, the surrounding community structure was different between their respective blooms and low abundance periods. The number of other taxa present and proportions of taxa presenting a niche overlapping the niches of P. delicatissima and P. seriata complexes were higher during their respective low abundance periods, suggesting that P. delicatissima and P. seriata complexes had potentially to compete with a higher number of other taxa during their respective low abundance periods than during their blooms. P. globosa is the taxa that contributed the most to the dissimilarity in community structure between these periods. P. globosa interacted positively with P. delicatissima complex, but negatively with P. seriata complex. P. globosa also occupied different sub-niches during the respective blooms and low abundance periods of P. delicatissima and P. seriata complexes.

The positive interaction between P. delicatissima complex and P. globosa is in line with observations frequently made in the eastern English Channel and southern North Sea. In these areas, during P. globosa blooms, it is very frequent to observe P. globosa colonies heavily colonized by P. delicatissima complex (e.g. [START_REF] Pannard | Short-term variability of the phytoplankton community in coastal ecosystem in response to physical and chemical conditions' changes[END_REF][START_REF] Sazhin | The colonization of two Phaeocystis species (Prymnesiophyceae) by pennate diatoms and other protists: a significant contribution to colony biomass[END_REF]. [START_REF] Sazhin | The colonization of two Phaeocystis species (Prymnesiophyceae) by pennate diatoms and other protists: a significant contribution to colony biomass[END_REF] observed up to 130 cells of P. delicatissima complex per P. globosa colony. Cells of P. delicatissima complex associated with P. globosa colonies have also been commonly observed in the Gulf of Maine (northwestern Atlantic Ocean) [START_REF] Fernandes | Diversity and toxicity of the diatom Pseudonitzschia Peragallo in the Gulf of Maine, Northwestern Atlantic Ocean[END_REF]. In the northern Adriatic Sea, [START_REF] Totti | Phytoplankton size-distribution and community structure in relation to mucilage occurrence in the northern Adriatic Sea[END_REF] observed higher proportions of P. delicatissima complex in large mucilage aggregates than in the water column. The reasons for the presence of P. delicatissima complex in mucilage aggregates and its association with P. globosa are currently unknown. However, it is thought that P. delicatissima complex would be able to use the organic substances contained in the mucus of P. globosa colonies and mucilage aggregates [START_REF] Sazhin | The colonization of two Phaeocystis species (Prymnesiophyceae) by pennate diatoms and other protists: a significant contribution to colony biomass[END_REF]. P. globosa colonies would thus provide a favorable habitat for the growth of P. delicatissima complex [START_REF] Sazhin | The colonization of two Phaeocystis species (Prymnesiophyceae) by pennate diatoms and other protists: a significant contribution to colony biomass[END_REF]. In accordance with this hypothesis, [START_REF] Loureiro | The significance of organic nutrients in the nutrition of Pseudo-nitzschia delicatissima (Bacillariophyceae)[END_REF] observed that in laboratory cultures, P. delicatissima is able to use organic nutrients to maintain its growth when inorganic nutrients are limiting. In the present study, the blooms of P. globosa and P. delicatissima complex occurred after a first diatom bloom that started to consume the nutrient stocks, and silicates were potentially limiting for diatoms during this period. Like any other diatoms, P. delicatissima complex requires silicate to grow. However, since Pseudo-nitzschia spp. have lightly silicified frustules, they seem better adapted to low silicate conditions than many other diatoms [START_REF] Marchetti | Environmental conditions and phytoplankton dynamics associated with Pseudo-nitzschia abundance and domoic acid in the Juan de Fuca eddy[END_REF]. Indeed, several laboratory studies showed that Pseudo-nitzschia species were able to grow under low silicate concentrations (e.g. [START_REF] Fehling | Growth and domoic acid production by Pseudo-nitzschia seriata (Bacillariophyceae) under phosphate and silicate limitation[END_REF]Pan et al., 1996a;Pan et al., 1996b). Natural blooms of Pseudo-nitzschia spp. have also been repeatedly observed in association with depleted silicate concentrations and this genus has often been found to outcompete other phytoplankton species during the late stage of a spring diatom blooms [START_REF] Anderson | Circulation and environmental conditions during a toxigenic Pseudo-nitzschia australis bloom in the Santa Barbara Channel, California[END_REF][START_REF] Anderson | Predicting potentially toxigenic Pseudo-nitzschia blooms in the Chesapeake Bay[END_REF][START_REF] Bates | Bloom dynamics and physiology of domoicacid-producing Pseudo-nitzschia species[END_REF][START_REF] Davidson | Harmful algal blooms: how strong is the evidence that nutrient ratios and forms influence their occurrence?[END_REF][START_REF] Dortch | Abundance and vertical flux of Pseudo-nitzschia in the northern Gulf of Mexico[END_REF][START_REF] Fehling | Growth and domoic acid production by Pseudo-nitzschia seriata (Bacillariophyceae) under phosphate and silicate limitation[END_REF][START_REF] Marchetti | Environmental conditions and phytoplankton dynamics associated with Pseudo-nitzschia abundance and domoic acid in the Juan de Fuca eddy[END_REF]Pan et al., 1996a;Pan et al., 1996b). It is thus frequent to find significant negative correlations between Pseudonitzschia spp. blooms and silicate concentrations, Si:N or Si:P ratios (e.g. [START_REF] Anderson | Circulation and environmental conditions during a toxigenic Pseudo-nitzschia australis bloom in the Santa Barbara Channel, California[END_REF][START_REF] Anderson | Predicting potentially toxigenic Pseudo-nitzschia blooms in the Chesapeake Bay[END_REF]. In the present study, high abundances of P. delicatissima complex were observed as associated with low Si:N ratios. Similar observations were made in the southern North Sea in 2012 [START_REF] Delegrange | Pseudo-nitzschia sp. diversity and seasonality in the southern North Sea, domoic acid levels and associated phytoplankton communities[END_REF] and in the Bay of Seine (central English Channel) in 2013 [START_REF] Thorel | Nutrient ratios influence variability in Pseudo-nitzschia species diversity and particulate domoic acid production in the Bay of Seine (France)[END_REF]. The capacity of P. delicatissima complex to grow at low silicate concentrations associated with its potential ability to use organic substances provided by P. globosa colonies mucus might be a competitive advantage over other diatoms and might explain the positive interaction between the blooms of P. globosa and P. delicatissima complex.

The negative interaction between P. seriata complex and P. globosa is more complex since other diatoms: C. curvisetus + C. debilis, C. danicus, L. danicus and G. delicatula also interacted with P. globosa and/or P. seriata complex. Interactions between P. seriata complex, L. danicus and G. delicatula seem to have been direct, while interactions between C. curvisetus + C. debilis, C. danicus and P. seriata seem to have been indirect because these species were not present with high abundance at the same period of the year and were not favored by the same environmental conditions. P. seriata complex was able to bloom after that a sequence of events created conditions favorable to its growth. Its bloom periods were accompanied by shifts in its realized ecological sub-niche but also in the realized ecological sub-niches of its interacting taxa. P. seriata complex always bloomed during years of low intensity P. globosa blooms. During the blooming years of P. seriata complex, the winter stocks of NO2 -+ NO3 -and Si(OH)4 were lower while PAR and abundances of C. danicus and C. curvisetus + C. debilis were higher than during years without bloom of P. seriata complex.

High abundances of C. danicus and C. curvisetus + C. debilis were observed during the diatom bloom preceding the blooms of P. globosa. During blooming years of P. seriata complex, C. danicus, C. curvisetus + C. debilis and the other diatoms consumed prematurely the winter stocks of NO2 -+ NO3 -and Si(OH)4, so that the NO2 -+ NO3 -stock left over after the decline of this first diatom bloom was too low to support high intensity P. globosa blooms. In spite of this, since P. globosa is an Haptophyte, it doesn't require Si(OH)4 to grow, and it managed to dominate the phytoplankton community when diatoms became limited by Si(OH)4. It was the dominant species until temperature and PAR increased and became less favorable to its growth. P. globosa and P. seriata complex did not occupy the same realized ecological niches. Since P. seriata complex tolerates higher temperature and PAR than P. globosa, it took advantage of this situation and started to bloom. In spite of their low intensity, P. globosa blooms continued to consume the NO2 -+ NO3 -stock. Consequently, NO2 -+ NO3 -concentrations left over during the decline of P. globosa blooms were lower than during years without blooms of P. seriata complex. Different forms of nitrogen as well as changes in the concentration of nitrogen and Si(OH)4 can modify the competitive relationships between diatoms species (e.g. [START_REF] Van Ruth | Size Dependent Competition in Centric Diatoms as a Function of Nitrogen and Silicon Availability[END_REF]. In this study, nitrogen and Si(OH)4 appear to influence the ability of P. seriata complex to outcompete the other diatoms present. When P. seriata complex bloomed, the Si(OH)4 concentrations were still relatively low, but since P. seriata complex is able to maintain viable cells under silicate limited conditions and is a good competitor under low Si:N ratios [START_REF] Fehling | Growth and domoic acid production by Pseudo-nitzschia seriata (Bacillariophyceae) under phosphate and silicate limitation[END_REF], it still managed to grow. These blooms of P. seriata complex were accompanied by the presence of G. delicatula and C. danicus that interacted positively with P. seriata complex. The development of P. globosa blooms after Chaetoceros spp. blooms is common in the eastern English Channel and southern North Sea [START_REF] Grattepanche | Succession of primary producers and micrograzers in a coastal ecosystem dominated by Phaeocystis globosa blooms[END_REF][START_REF] Rousseau | Diatom succession, silicification and silicic acid availability in Belgian coastal waters (Southern North Sea)[END_REF]. It was suggested that the winter nutrient stocks determines the composition of the first diatom bloom, and this composition in turn influences the intensity of the blooms of P. globosa through competitive relationships for nutrients [START_REF] Karasiewicz | Realized niche analysis of phytoplankton communities involving HAB: Phaeocystis spp. as a case study[END_REF]. The association of low intensity P. globosa blooms with low NO2 -+ NO3 -concentrations is in accordance with observations made in the southern North Sea suggesting that P. globosa bloom intensity is controlled by nitrate left over after the decline of the first diatom bloom [START_REF] Rousseau | Diatom succession, silicification and silicic acid availability in Belgian coastal waters (Southern North Sea)[END_REF] and references therein). The presence of P. seriata complex during years of low intensity P. globosa blooms has been previously reported in a study examining the conditions favoring P. globosa blooms in the eastern English Channel [START_REF] Karasiewicz | Realized niche analysis of phytoplankton communities involving HAB: Phaeocystis spp. as a case study[END_REF] and in a study describing Pseudo-nitzschia spp. diversity and toxicity in 2012 in the southern North Sea [START_REF] Delegrange | Pseudo-nitzschia sp. diversity and seasonality in the southern North Sea, domoic acid levels and associated phytoplankton communities[END_REF]. As in the present study, the bloom of P. seriata complex in 2012 in the southern North Sea was associated with low NO2 -+ NO3 -and Si(OH)4 concentrations [START_REF] Delegrange | Pseudo-nitzschia sp. diversity and seasonality in the southern North Sea, domoic acid levels and associated phytoplankton communities[END_REF]. Negative relationships between the occurrence of P. seriata complex and nitrate concentrations have also been previously reported in the western English Channel [START_REF] Downes-Tettmar | Seasonal variation in Pseudo-nitzschia spp. and domoic acid in the Western English Channel[END_REF] and western Scottish waters [START_REF] Fehling | Seasonality of Pseudo-nitzschia spp.(Bacillariophyceae) in western Scottish waters[END_REF] suggesting that P. seriata complex responds mainly to nitrate concentrations in surface waters and is able to rapidly use nitrate when it becomes available. The capacity of P. seriata complex to outcompete other diatoms when Si(OH)4 concentrations are low is also in line with competition experiments showing that in comparison with other diatoms, Pseudonitzschia pungens (that belongs to the Pseudo-nitzschia seriata complex) is able to dominate when Si:N ratios are relatively low [START_REF] Sommer | Are marine diatoms favoured by high Si: N ratios?[END_REF]. The presence of Leptocylindrus species and/or Guinardia (Rhizosolenia) delicatula at the same time as Pseudo-nitzschia spp. seems to be recurrent. These taxa have been observed as associated with Pseudo-nitzschia spp. in the Bay of Seine (eastern English Channel) [START_REF] Thorel | Nutrient ratios influence variability in Pseudo-nitzschia species diversity and particulate domoic acid production in the Bay of Seine (France)[END_REF], in the western Scottish waters [START_REF] Fehling | Seasonality of Pseudo-nitzschia spp.(Bacillariophyceae) in western Scottish waters[END_REF], on the Galician coasts [START_REF] Casas | Seasonal succession of phytoplankton species on the coast of A Coruña (Galicia, northwest Spain)[END_REF], in the Adriatic Sea [START_REF] Burić | Pseudonitzschia blooms in the Zrmanja river estuary (eastern Adriactic Sea)[END_REF][START_REF] Totti | Phytoplankton size-distribution and community structure in relation to mucilage occurrence in the northern Adriatic Sea[END_REF] and in the Mediterranean Sea [START_REF] Quiroga | Pseudo-nitzschia blooms in the Bay of Banyuls-sur-Mer, Northwestern Mediterranean Sea[END_REF]. This suggests similarities in their environmental requirements. G. delicatula and L. danicus are often observed together [START_REF] Tomas | Identifying marine phytoplankton[END_REF]. As Pseudo-nitzschia spp., G. delicatula and L. danicus are lightly silicified chain forming diatoms [START_REF] Conley | Differences in silica content between marine and freshwater diatoms[END_REF][START_REF] Davidson | Harmful algal blooms: how strong is the evidence that nutrient ratios and forms influence their occurrence?[END_REF][START_REF] Rousseau | Diatom succession, silicification and silicic acid availability in Belgian coastal waters (Southern North Sea)[END_REF][START_REF] Schlüter | Long-term changes in the seasonality of selected diatoms related to grazers and environmental conditions[END_REF]. The presence of G. delicatula is also frequently observed during periods of low Si(OH)4 and nitrate concentrations since it generally appears after a succession of blooms [START_REF] Del Amo | Impacts of highnitrate freshwater inputs on macrotidal ecosystems. II. Specific role of the silicic acid pump in the year-round dominance of diatoms in the Bay of Brest (France)[END_REF]Rousseau et al., 2002 and references therein). In the present study, the OMI and WitOMI analyses showed that some similarities exist between the realized ecological niches and sub-niches of P. seriata complex, G. delicatula and L.

danicus. They all have affinity for spring-summer conditions and tolerate higher temperature and PAR than P. globosa. However, in spite of these similarities, P. seriata complex, G. delicatula and L. danicus occupy different realized ecological niches and sub-niches. G. delicatula and L. danicus occupy a more marginal niche and are less tolerant than P. seriata complex.

Figure captions:

Fig. 1: Map of the eastern English Channel and southern North Sea showing the location of sampling stations (black points) Fig. 2: Observation frequencies of A) Pseudo-nitzschia delicatissima complex (Pdel), B) Pseudo-nitzschia seriata (Pser) complex, C) all the other diatoms and D) Phaeocystis globosa (Phaeo) during the months of the year. Gray bars correspond to observation frequencies of the considered taxa whatever its abundance. Black bars correspond to observation frequencies of the bloom periods i.e. abundances ≥ 3x10 5 cells L -1 for P. delicatissima complex and ≥ 10 5 cells L -1 for P. seriata complex, all the other diatoms and P. globosa Pseudo-nitzschia seriata (Pser) complex, C) all the other diatoms and D) Phaeocystis globosa (Phaeo) during the months of the year. Gray bars correspond to observation frequencies of the considered taxa whatever its abundance. Black bars correspond to observation frequencies of the bloom periods i.e. abundances ≥ 3x10 5 cells L -1 for P. delicatissima complex and ≥ 10 5 cells L -1 for P. seriata complex, all the other diatoms and P. globosa Realized ecological sub-niches of species 1 (E, F, G) and species 2 (H, I, J) in the first subset (first column), second subset (second column) and third subset (third column). Grey polygon = environmental space. Orange polygon = realized ecological niche. Blue polygon = subenvironmental space. Yellow polygon = potential sub-niche. Green polygon = realized ecological sub-niche. In G, H, I, the realized ecological sub-niche of species 1 and 2 is smaller than their respective potential sub-niche due to biotic interactions. The blue area inside the yellow polygon and around the green polygon represents thus the biological constraints. In E, F, J, the size of the realized ecological sub-niche of species 1 and species 2 is not reduced by biotic interactions and they occupy the totality of their potential sub-niche. Their realized ecological sub-niche has thus the same size as their potential sub-niche. Figure adapted from Karasiewicz et al. (2017) and Karasiewicz et al., 2020).

Fig. 3 :

 3 Fig. 3: Outlying Mean Index (OMI) analysis showing the projection of the environmental parameters (in black) and species niche marginality (in blue) along the two OMI axes (OMI1 and OMI2) delimiting the OMI plane. Dots are sampling units colored according to the seasons of the year. T = seawater temperature, par = photosynthetically active radiation, S = salinity, NH4 = ammonium concentration, SIOH4 = silicate concentration, PO4 = phosphate concentration, Eps = turbulent dissipation rate, Pdel = Pseudo-nitzschia delicatissima complex, Pser = Pseudo-nitzschia seriata complex, Phaeo = Phaeocystis globosa, Gdel = Guinardia delicatula, Ldan = Leptocylindrus danicus, Ccur = Chaetoceros curvisetus + Chaetoceros debilis and Cdan = Chaetoceros danicus Fig. 4: Boxplots representing A & B) seawater temperature (in °C) , C & D) salinity, D & E) photosynthetically active radiation (PAR in einstein m -2 day -1 ), F & G) turbulent dissipation rate (epsilon in 10 -3 W m -3 ), I & J) ammonium concentration (NH4 -in µM), K & L) nitrite + nitrate concentration (NO2 -+ NO3 -in µM), M & N) silicate concentration (Si(OH)4 in µM) and phosphate concentration (PO4 3-in µM) during the low abundance (LA) and bloom periods of Pseudo-nitzschia delicatissima complex (Pdel, left panel) and Pseudo-nitzschia seriata complex (Pser, right panel). P values are Mann-Whitney U test results. ns = non significant Fig. 5: Within Outlying Mean Index (WitOMI) analysis. Sub-niches of A & B) Pseudonitzschia delicatissima complex and C & D) Pseudo-nitzschia seriata complex during their respective low abundance (left panel) and high abundance (right panel) periods. Grey polygon = environmental space, orange polygon = realized ecological niche (RN), blue polygon = subenvironmental space, yellow polygon = potential sub-niche (PS), green polygon = realized ecological sub-niche (RS). Red dot = WitOMIG Fig. 6: Interaction network based on a Poisson lognormal model (PLN) showing the taxa interacting with Pseudo-nitzschia delicatissima complex (Pdel) and Pseudo-nitzschia seriata complex (Pser). Phaeo = Phaeocystis globosa, Gdel = Guinardia delicatula, Ldan = Leptocylindrus danicus, Ccur = Chaetoceros curvisetus + Chaetoceros debilis and Cdan = Chaetoceros danicus. Solid lines represent positive interactions while dashed lines represent negative interactions. Lines width is proportional to interaction strength Fig. 7: Abundance of Phaeocystis globosa (Phaeo) during low abundance (LA) and bloom periods of A) Pseudo-nitzschia delicatissima complex and B) Pseudo-nitzschia seriata complex. Abundance of C) Guinardia delicatula (Gdel), D) Chaetoceros curvisetus + Chaetoceros debilis (Ccur), E) Chaetoceros danicus (Cdan) and F) Leptocylindrus danicus (Ldan) during years of low intensity Phaeocystis globosa blooms without bloom of Pseudo-

Fig. 3 :

 3 Fig. 3: Outlying Mean Index (OMI) analysis showing the projection of the environmental parameters (in black) and species niche marginality (in blue) along the two OMI axes (OMI1 and OMI2) delimiting the OMI plane. Dots are sampling units colored according to the seasons of the year. T = seawater temperature, par = photosynthetically active radiation, S = salinity, NH4 = ammonium concentration, SIOH4 = silicate concentration, PO4 = phosphate concentration, Eps = turbulent dissipation rate, Pdel = Pseudo-nitzschia delicatissima complex, Pser = Pseudo-nitzschia seriata complex, Phaeo = Phaeocystis globosa, Gdel = Guinardia delicatula, Ldan = Leptocylindrus danicus, Ccur = Chaetoceros curvisetus + Chaetoceros debilis and Cdan = Chaetoceros danicus

Fig. 8 :

 8 Fig. 8: Sub-niches of Phaeocystis globosa during the low abundance A) and B) bloom periods of Pseudo-nitzschia delicatissima complex. Sub-niches of C & D) Phaeocystis globosa, E & F) Guinardia delicatula (Gdel), G & H) Chaetoceros curvisetus + Chaetoceros debilis (Ccur), J & J) Chaetoceros danicus (Cdan) and K & L) Leptocylindrus danicus (Ldan) during the low abundance (left panel) and bloom (right panel) periods of Pseudo-nitzschia seriata complex. Grey polygon = environmental space, orange polygon = realized ecological niche (RN), blue polygon = sub-environmental space, yellow polygon = potential sub-niche (PS), green polygon = realized ecological sub-niche (RS). Red dot = WitOMIG

Fig. S1 :

 S1 Fig. S1: Schematic representation of the OMI and WitOMI analyses results. (A) Projection of the environmental variables on the first two axes of the OMI (OMI 1 and OMI2). Env. = environmental variable. (B) Projection of the niche position of each taxa on the first two axes of the OMI. Sp. = species. Realized ecological niches of species 1 (C) and species 2 (D).Realized ecological sub-niches of species 1 (E, F, G) and species 2 (H, I, J) in the first subset (first column), second subset (second column) and third subset (third column). Grey polygon = environmental space. Orange polygon = realized ecological niche. Blue polygon = subenvironmental space. Yellow polygon = potential sub-niche. Green polygon = realized ecological sub-niche. In G, H, I, the realized ecological sub-niche of species 1 and 2 is smaller than their respective potential sub-niche due to biotic interactions. The blue area inside the yellow polygon and around the green polygon represents thus the biological constraints. In E, F, J, the size of the realized ecological sub-niche of species 1 and species 2 is not reduced by biotic interactions and they occupy the totality of their potential sub-niche. Their realized ecological sub-niche has thus the same size as their potential sub-niche. Figure adapted fromKarasiewicz et al. (2017) andKarasiewicz et al., 2020).

Figure S2 :

 S2 Figure S2: Observation frequencies of high abundances of Pseudo-nitzschia delicatissima complex (Pdel), Pseudo-nitzschia seriata complex (Pser), Phaeocytis globosa (Phaeo), Chaetoceros curvisetus (Ccur), Chaetoceros danicus (Cdan), Guinardia delicatula (Gdel) and Leptocylindrus danicus (Ldan) against water temperature (A), PAR (B), NO2 -+ NO3 - concentrations (C) and turbulence (D)

Table 1 :

 1 Realized ecological niche and sub-niches parameters of Pseudo-nitzschia delicatissima complex (Pdel), Pseudo-nitzschia seriata complex (Pser), Phaeocystis globosa (Phaeo), Guinardia delicatula (Gdel), Leptocylindrus danicus (Ldan), Chaetoceros curvisetus + Chaetoceros debilis (Ccur) and Chaetoceros danicus (Cdan)

		Niche parameters (OMI		Sub-niche parameters (WitOMI analysis)
			analysis)
	Taxa	Marginality Tolerance Periods	Marginality	Tolerance	Biological	%	PS
					(WitOMIG)	(TolWitOMIG)	constraints	occupied
	Pdel	0.23	0.95	Pdel LA	0.35	4.19		63
				Pdel blooms 0.84	0.25		100
	Pser	0.15	1.81	Pser LA	0.33	4.30		90
				Pser blooms 0.95	0.77		100
	Phaeo	0.77	0.37	Pdel LA	0.58	0.84		49
				Pdel blooms 0.92	0.21		87
				Pser LA	0.76	0.40		74
				Pser blooms 1.21	0.11		56
	Gdel	0.25	1.78	Pser LA	0.27	1.12		82
				Pser blooms 0.88	0.69		93
	Ldan	1.08	0.99	Pser LA	0.85	0.84		79
				Pser blooms 1.72	0.87		95
	Ccur	0.12	0.79	Pser LA	0.41	2.27		100
				Pser blooms 1.95	0.13		19
	Cdan	0.68	3.24	Pser LA	1.77	3.83		90
				Pser blooms 1.16	0.67		73

OMI = Outlying Mean Index, WitOMI = Within Outlying Mean Indexes, Biological constraints = part of the potential sub-niche volume that could not be occupied due to biological constraints, % PS occupied = percentage of the potential sub-niche volume occupied by the realized ecological sub-niche, Pdel LA = low abundance periods of Pseudonitzschia delicatissima complex, Pdel blooms = blooms of Pseudo-nitzschia delicatissima complex, Pser LA = low abundance periods of Pseudo-nitzschia seriata complex, Pser blooms = blooms of Pseudo-nitzschia seriata complex

Table 2 :

 2 Overlap percentages between the realized ecological niche, potential sub-niche and realized ecological sub-niches of Pseudo-nitzschia delicatissima complex, Pseudo-nitzschia seriata complex and their interacting taxa

		Pdel	Pser		Pdel PS		Pser PS			Pdel RS		Pser RS
		RN	RN		LA	Blooms LA	Blooms		LA	Blooms LA	Blooms
	Phaeo	56	52	Phaeo	52	96	57	100	Phaeo	38	80	43	87
	RN			PS					RS
	Gdel		100	Gdel			100	100	Gdel			53	87
	RN			PS					RS
	Ldan		55	Ldan			60	100	Ldan			37	86
	RN			PS					RS
	Ccur		62	Ccur			68	100	Ccur			77	17
	RN			PS					RS
	Cdan		73	Cdan			96	100	Cdan			99	60
	RN			PS					RS
	RN = realized ecological niche, PS = potential sub-niche, RS = realized ecological sub-niche,

Pdel= Pseudo-nitzschia delicatissima complex, Pser = Pseudo-nitzschia seriata complex, Phaeo = Phaeocystis globosa, Gdel = Guinardia delicatula, Ldan = Leptocylindrus danicus, Ccur = Chaetoceros curvisetus + Chaetoceros debilis, Cdan = Chaetoceros danicus, LA = low abundance period

Table S1 :

 S1 SIMPER analysis identifying the main taxa contributing to the dissimilarity between the phytoplankton community present during blooming years and years without bloom of Pseudo-nitzschia delicatissima complex (Pdel) and Pseudo-nitzschia seriata complex (Pser)

	spp. Chaetoceros spp. Leptocylindrus	Bacillariophyceae Ccur 75 Bacillariophyceae Ldan 151	64 177	1.42x10 -2 2.14x10 -2	2.08 3.02
	Pdel: blooming years vs. years without bloom curvisetus + Chaetoceros debilis Ditylum Bacillariophyceae Dbri 1 danicus Paralia sulcata Bacillariophyceae Psul 1x10 3 Synedra Bacillariophyceae Tnit 282 nitzschioides f. Average dissimilarity = 68.05 Taxa Class Code Years without bloom brightwellii Rhizosolenia Bacillariophyceae Rset 131 nitzschioides Thalassiosira Bacillariophyceae Thsp 149 Average abundance (cells L -1 ) setigera Rhizosolenia Bacillariophyceae Dfra 8 spp. Skeletonema Bacillariophyceae Ske 141 Phaeocystis globosa Haptophyceae Phaeo 3x10 5 Asterionellopsis glacialis Bacillariophyceae Agla 2x10 3 Guinardia delicatula Bacillariophyceae Gdel 1x10 4 fragilissima f. fragilissima Guinardia spp. Bacillariophyceae Gui 63 spp. Nitzschia Bacillariophyceae Nlon 301 Cerataulina Bacillariophyceae Cer 90 longissima Raphoneis spp. Bacillariophyceae Rha 154 spp. Navicula spp. + Fallacia spp. + Bacillariophyceae Navi 35 + Delphineis spp. Chaetoceros Bacillariophyceae Ccur 71 Rhizosolenia hebetata Bacillariophyceae Rheb 4x10 3 Haslea spp. Leptocylindrus Bacillariophyceae Ldan 1 curvisetus + Chaetoceros Pseudo-Bacillariophyceae Pser 418 danicus debilis Lauderia spp. + Bacillariophyceae Lann 56 Pser: blooming years vs. years without bloom Schroederella nitzschia seriata complex Chaetoceros spp. Bacillariophyceae Cspp 556 Average dissimilarity = 71.00 Taxa Class Code Years spp. Thalassiosira Bacillariophyceae Tgra 66 Paralia sulcata Bacillariophyceae Psul 734 without bloom gravida Leptocylindrus Bacillariophyceae Lep 4 Chaetoceros socialis + Chaetoceros Bacillariophyceae Csoc 208 Average abundance (cells L -1 ) spp. Cerataulina Bacillariophyceae Cer 21 Phaeocystis Haptophyceae Phaeo 3x10 4 spp. Guinardia spp. Bacillariophyceae Gui 25 socialis f. radians Eucampia striata Bacillariophyceae Gstri 567 Synedra Lauderia spp. + spp. Skeletonema spp. Bacillariophyceae Ske 53 Thalassiosira Thalassiosira Nitzschia Raphoneis spp. + Delphineis Bacillariophyceae Rha 253 Chaetoceros Bacillariophyceae Cspp 654 striata longissima Bacillariophyceae Nlon 237 Eucampia Bacillariophyceae Gstri 1x10 3 radians gravida Bacillariophyceae Tgra 59 socialis f. Chaetoceros spp. Bacillariophyceae Thsp 153 socialis + Chaetoceros Bacillariophyceae Csoc 348 hebetata Rhizosolenia Bacillariophyceae Rheb 4x10 3 delicatula Schroederella Bacillariophyceae Lann 263 Guinardia Bacillariophyceae Gdel 4x10 3 complex nitzschioides delicatissima nitzschioides f. Bacillariophyceae Tnit 502 nitzschia setigera globosa Asterionellopsis Bacillariophyceae Agla 3x10 3 Navicula spp. + Bacillariophyceae Navi 107 Fallacia spp. + glacialis Pseudo-Bacillariophyceae Pdel 2x10 3 Haslea spp. Rhizosolenia Bacillariophyceae Rset 37	214 2x10 3 230 Blooming years 10 267 46 63 Average abundance (cells L -1 ) 9x10 5 8x10 3 1x10 3 1 622 0 381 53 99 8x10 3 36 2x10 3 151 155 Blooming 113 2x10 3 years 101 204 Average abundance 52 (cells L -1 ) 8x10 4 35 91 222 374 885 534 755 231 265 603 4x10 3 119 6x10 3 191 5x10 3 103 2x10 3 59	1.92x10 -2 1.80x10 -2 1.26x10 -2 Average dissimilarity 1.18x10 -2 1.79x10 -2 1.16x10 -2 1.71x10 -2 9.22x10 -2 2.98x10 -2 2.85x10 -2 1.67x10 -2 1.11x10 -2 1.08x10 -2 1.64x10 -2 1.08x10 -2 1.61x10 -2 2.63x10 -2 1.08x10 -2 2.29x10 -2 1.57x10 -2 1.91x10 -2 Average 1.53x10 -2 1.90x10 -2 dissimilarity 1.43x10 -2 1.88x10 -2 1.32x10 -2 7.22x10 -2 1.31x10 -2 1.85x10 -2 1.69x10 -2 1.56x10 -2 2.33x10 -2 1.58x10 -2 2.42x10 -2 1.58x10 -2 1.68x10 -2 2.46x10 -2 2.87x10 -2 1.70x10 -2 2.89x10 -2 1.78x10 -2 3.25x10 -2 1.26x10 -2 3.07x10 -2 1.26x10 -2	2.70 2.54 1.86 Contribution 1.72 2.52 % 1.71 2.40 13.55 4.37 2.36 1.64 1.59 2.31 1.58 2.26 4.18 1.58 3.87 2.21 3.35 Contribution 2.15 2.80 % 2.02 2.79 2.76 1.86 10.16 1.85 2.72 2.30 3.29 2.32 3.41 2.32 2.47 3.46 2.48 4.05 2.50 4.07 2.62 1.78 4.58 4.32 1.77

) from 1998 to

was downloaded on GlobColour website (https://hermes.acri.fr). PAR

Acknowledgements

We thank the reviewers for their constructive comments. We thank the Sepia II crew for operating the research vessel that was used to collect the water samples during the monitoring programs. We thank Eric Lecuyer for the SNO-SOMLIT sampling, operation of the CTD and analyses of ammonium concentrations. We thank Nicole Degros and Muriel Crouvoisier for measuring the SNO-SOMLIT nitrite, nitrate, silicate and phosphate concentrations. We thank Alain Lefebvre, Camille Blondel, Vincent Duquesne, Pascale Herbert, Remy Cordier, Catherine Belin, Antoine Huguet, Gaetane Durand, Dominique Soudan and David Devreker for collecting and analyzing the samples during the SRN monitoring program and for providing us the data set via SEANOE. We thank all the members of the REPHY monitoring program for collecting water samples, identifying and counting the phytoplankton species, operating the CTD, analyzing the nutrient concentrations and for providing us the data set via SEANOE. We thank Amélie Chédru for the work done during her master internship. This research was funded by: 1) the French government and the regional council Hauts-de-France in the framework of the CPER project MARCO, 2) the LEFE AO INSU project Turbu-diatox: "Effets de la turbulence sur la prolifération et la toxicité des diatomées" and 3) an interlaboratories project funded by the GdR PHYCOTOX.

Conclusion

This study showed that in the eastern English Channel and Southern North Sea, the complexes P. delicatissima and P. seriata are typically present year round but their bloom periods are different. This difference in seasonality can be explained by the two complexes occupying different realized ecological niches. The necessity to investigate the surrounding phytoplankton community in addition to the environmental conditions was also highlighted. Indeed, niche shifts and biotic interactions played an important role in the control of the blooms of P. delicatissima and P. seriata complexes. P. globosa interacted with the two complexes but this interaction was positive with P. delicatissima complex and negative with P. seriata complex. Other diatoms also influenced the blooms of P. seriata complex. The positive interaction found between P. delicatissima complex and P. globosa is in accordance with previous observations reporting cells of the P. delicatissima complex associated with P. globosa colonies. However, the underlying processes explaining this association are currently unresolved. Laboratory experiments with co-cultures of P. globosa and Pseudo-nitzschia may help to better understand their relationships. The production of domoic acid associated with the presence of the two Pseudo-nitzschia complexes and its potential impacts for the food web have also not received adequate attention in this area.

Authors' contribution

EH: conceptualization, data analyses and wrote completely the first draft. All other authors read, commented and approved the final version of the manuscript. EB and DS: identification and count of phytoplankton species from the SNO-SOMLIT monitoring program. UK and FS: funding acquisition and supervision of EH's post-doc.

Supplementary materials

On the conditions promoting Pseudo-nitzschia spp. blooms in the eastern English Channel and southern North Sea Emilie Houliez, François G. Schmitt, Elsa Breton, Dimitra-Ioli Skouroliakou, Urania Christaki

Supplementary Materials and Methods

Realized ecological niche and sub-niches

Hutchinson's niche concept [START_REF] Hutchinson | Concluding remarks[END_REF] describes the species ecological niche as a n-dimensional hypervolume delimited by a set of favorable environmental conditions (biotic and abiotic) under which a species can maintain nonnegative growth rates, survive and reproduce. Each of these conditions is referred to as a "niche axis". Distinction is made between the fundamental and realized components of the species ecological niche. The fundamental ecological niche corresponds to the hypervolume of conditions under which a species can sustain nonnegative growth rates and persist in the absence of immigration and competition. This definition has been updated to also assume the absence of different other biotic interactions including the absence of predation, pathogens, facultative mutualists or facilitations (Bruno et al., 2003;[START_REF] Carscadden | Niche Breadth: Causes and Consequences for Ecology, Evolution, and Conservation[END_REF][START_REF] Holt | Bringing the Hutchinsonian niche into the 21st century: Ecological and evolutionary perspectives[END_REF]. A species may fail to occupy a portion of its fundamental ecological niche because these conditions (or resources) do not exist at a specific time in the study area or because interspecific biotic interactions have excluded the species from locations where those conditions occur [START_REF] Carscadden | Niche Breadth: Causes and Consequences for Ecology, Evolution, and Conservation[END_REF]. The hypervolume of conditions actually occupied by the species is thus called the realized ecological niche. It is a subset of its fundamental ecological niche that corresponds to the conditions under which the species can persist and be observed given the presence of other species competing for the same resources and/or in the presence of other kinds of biotic interactions (predation, presence of pathogens or mutualistic and facilitation relationships) [START_REF] Brun | Ecological niches of the open ocean phytoplankton taxa[END_REF].

Outlying Mean Index (OMI) and Within Outying mean Index (WitOMI)

The OMI and WitOMI analyses were both based on a principal component analysis (PCA). They were performed with two data frames. The first data frame contained the environmental variables and included water temperature, salinity, nutrient concentrations, turbulence and PAR data. The second data frame contained fourth root transformed phytoplankton species abundances. The fourth root transformation down-weight the importance of the highly abundant species for a proper characterization of the realized ecological niches of not only the less common ("mid-range") species but also of the rarer species (Clarke and Warwick, 2001).

The OMI analysis is a multivariate analysis defining the realized ecological niche (RN) of each taxa within a given environmental space. It characterizes the RN of each taxa with three parameters: marginality, tolerance and residual tolerance. The OMI analysis is accompanied by a Monte Carlo permutation test defining the statistical significance of the marginality. It compares the observed marginality to the distribution of 1000 random marginality values obtained by permutations under the null hypothesis that the taxonomic unit is indifferent to its environment. If the observed marginality is greater or lower than marginality values obtained by permutations, the null hypothesis can be rejected. The OMI analysis provides several graphics. The first graphic represents the projection of the environmental variables on the OMI axes. The second graphic represents the projection of the marginality of each taxa on the OMI axes. The third graphic represents with polygons the RN of each taxa within the environmental space (previously called "realized environmental space" in Karasiewicz et al., 2017) on the plane defined by the OMI axes (Fig. S1).

The WitOMI analysis uses the environmental space delimited by the OMI axes as a reference. It decomposes the RN of each taxa into realized ecological sub-niches (RS) corresponding to a given time period of interest or a sub-location within the study area. For each subset studied, two additional marginalities, two tolerance indexes and two residual tolerances are provided. Associated Monte Carlo permutation tests define the significance of WitOMIG and WitOMIGK (see Karasiewicz et al., 2017 for more details).