S De La Peña-Lastra

L Affre

X L Otero

Jan Willem Van Groenigen

Soil nutrient dynamics in colonies of the yellow-legged seagull (Larus michahellis) in different biogeographical zones

Keywords:

Seabirds drastically alter the ecosystems where they establish their colonies. However, previous studies have not considered how colonies of the same species affect their surroundings in different environmental contexts. The main objective of this study was to determine the degree to which environmental factors (particularly climate and lithology) modulate the impact of seabird colonies on soil nutrients. For this purpose, two breeding colonies of the yellow-legged gull were selected: one located in the Atlantic Islands National Park (AINP, Atlantic region) and the other in Calanques National Park (CNP, Mediterranean region). In both parks, samples of soil and excrement were obtained from colonies with different densities of birds and in control zones, without birds, in two different seasons (winter and summer). The samples were analysed to determine the concentrations of N-NO 3 -, N-NH 4 + , total P and different geochemical P forms, including bioavailable P. The colony soils were enriched in N and P relative to soils from the control zones. However, the annual nutrient dynamics were modulated by the environmental conditions in each park. In winter in CNP, NH 4 + concentrations were low and similar to those in the control zones, while the concentrations of NO 3 -were highest at this time of year. By contrast, in AINP, the annual N cycling appeared to be less variable, although the NH 4 + concentrations were lower than in the control zone in winter, while those of NO 3 -remained high, despite the high rainfall in this season. The concentrations of P (total and bioavailable) remained high in soils in both parks throughout the year. However, fractionation of the P forms revealed different geochemical behaviour at the two sites. In CNP, calcium phosphate and residual P were the dominant fractions. In AINP, the P was distributed more homogeneously between the different fractions, with a slight predominance of the P associated with Al hydroxides and clays. The findings clearly show alteration of the cycling of both nutrients in both parks, although the impact is modulated by the environmental conditions at each location.

Introduction

The yellow-legged gull Larus michahellis occurs throughout Europe, the Middle East and North Africa. In Europe, the overall population of the species increased slowly until 1980, after which it underwent exponential growth until becoming one of the most abundant seabird species in Mediterranean and Atlantic coastal regions [START_REF] Barros | Censo da poboación reprodutora de corvo mariño cristado (Phalacrocorax aristotelis), gaivota patiamarela (Larus michahellis), gaivota escura (Larus fuscus) e gaivotón (Larus marinus) no Parque Nacional Marítimo-Terrestre das Illas Atlánticas de Galicia: resultados de 2015[END_REF][START_REF] Rizet | Dynamique de la population de goélands leucophée (Larus michahellis N., 1840) sur les îles de Marseille et test de la méthode de Distance Sampling[END_REF]Vidal et al., 1998). However, the numbers have decreased drastically in recent years [START_REF] Barros | Censo da poboación reprodutora de corvo mariño cristado (Phalacrocorax aristotelis), gaivota patiamarela (Larus michahellis), gaivota escura (Larus fuscus) e gaivotón (Larus marinus) no Parque Nacional Marítimo-Terrestre das Illas Atlánticas de Galicia: resultados de 2015[END_REF][START_REF] Galarza | Is the population of Cantabrian yellow-legged gull Larus michahellis lusitanius Naumann, 1840 decreasing? 2013/2014 Census in Biscay (Basque Country)[END_REF][START_REF] Rizet | Dynamique de la population de goélands leucophée (Larus michahellis N., 1840) sur les îles de Marseille et test de la méthode de Distance Sampling[END_REF], probably due to closure of urban waste dumps and reductions in fish discards [START_REF] Oro | Effects of a trawling moratorium on the breeding success of the Yellow-legged gull Larus cachinnans[END_REF][START_REF] Ramos | Diet of Yellow-legged Gull[END_REF]. Previously, the enormous increase in the population had led to some controversy regarding the need to take control measures as the huge numbers of gulls threatened the conservation of habitats and species in several island systems (e.g. Berlengas, Chafarinas and Cíes islands).

Colonies of the yellow-legged gull have previously been studied with the aim of evaluating the effect of the species in shaping ecosystem biodiversity. The findings of different studies have revealed changes in plant composition, vigour of native species and also the appearance of ruderal species on the island cliffs where the colonies are established [START_REF] Guitián | La influencia de las colonias de aves marinas en la vegetación de los acantilados del noroeste ibérico[END_REF]Vidal et al., 1998a,b;[START_REF] Mouriño | Caracterización de la vegetación de los acantilados del Parque Natural de las Islas Cíes y su relación con la colonia de gaviota patiamarilla (Larus cachinnans)[END_REF][START_REF] García | Seagull influence on soil properties, chenopod shrub distribution, and leaf nutrient status in semi-arid Mediterranean islands[END_REF]. More recently, different studies have revealed significant changes in various properties of soils and waters and in the concentrations of nutrients (N, P, K) and trace elements (Zn, Pb, Hg). The presence of yellow-legged gulls on Chafarinas Islands (North Africa) induced eutrophication, salinization, acidification and nutrient T imbalances [START_REF] García | Seagull influence on soil properties, chenopod shrub distribution, and leaf nutrient status in semi-arid Mediterranean islands[END_REF][START_REF] Signa | Gull-derived trace elements trigger small-scale contamination in a remote Mediterranean nature reserve[END_REF].. [START_REF] De | Enrichment of trace elements in colonies of the yellow-legged gull (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF] observed that the presence of yellow legged-gull colonies in the Atlantic Islands National Park (NW Spain) led to the incorporation of oligoelements (e.g. Cu, Zn, Co) and P in the colony soils, via inputs of biological materials, mainly excrement and pellets. These researchers also observed that the total concentrations of oligoelements and P in the soil were related to the density of gulls in each subcolony.

However, the aforementioned and other previous studies (see e.g. [START_REF] Sobey | The relationship between herring gulls and the vegetation of their breeding colonies[END_REF][START_REF] Hogg | The effect of nesting gulls on the vegetation and soil of islands in the great lakes[END_REF][START_REF] Lindeboom | The nitrogen pathway in a penguin rookery[END_REF] mainly focused on comparisons between soils and water in the colonies and in control zones (without seabirds). To our knowledge, no previous studies have considered or compared the effects of colonies of the same gull species in different environments. The mobility and bioavailability of macro and micronutrients in soil are strongly dependent on climatic conditions and lithology [START_REF] Sposito | The Chemistry of Soils[END_REF]). In the first instance, climate and lithology exert a strong influence on soil composition and properties (i.e. pH, texture, mineralogy, etc). Furthermore, climate parameters (including temperature, hydric and edaphic availability) and pH regulate essential processes involved in soil nutrient dynamics, such as nitrification, the composition of interstitial water, solubility and partitioning of soil nutrients, especially P (for more details, see [START_REF] Bonilla | Soil nitrogen dynamics in a Holm oak forest[END_REF]Rodà, 1992, Rutigliano et al., 2009).

Taking all of the above into account, the present study aimed to evaluate how the impact of seabirds on soil nutrient (N and P) dynamics is modulated by climate and lithology in areas where yellow-legged gull colonies are established. For this purpose, breeding colonies were selected for study in two areas with different climatological and geological characteristics: the Atlantic Islands National Park (AINP) located in the NW Iberian Peninsula, characterised by a wet Atlantic climate and a geological substrate formed by granite, with acid soils; and Calanques National Park (CNP) situated in SW France, characterised by a dry Mediterranean climate and a substrate formed by limestone. Sampling zones with different densities of gull were established in the breeding colonies and in an area without gulls (control zone) in each park. Sampling was performed seasonally to evaluate the effects of seasonal changes on soil nutrient dynamics.

Material and methods

Environmental context

The study area comprised two zones: the Atlantic Islands National Park (AINP), located on Atlantic side of Iberian Peninsula (NW Iberian Peninsula); and Calanques National Park (CNP) situated on Mediterranean coast of France (SW France): (Fig. 1; Table 1). In AINP, yellow-legged gull colonies on the cliffs in Cíes and Ons islands were selected for study, and a control plot (without gulls) was established on the mainland, on the Cabo Home cliffs (Fig. 1B). In the Mediterranean zone, the gull colonies and the control plot selected for study were located in the Frioul archipelago (Fig. 1C).

Atlantic Islands National Park (AINP)

The geological substrate of AINP is constituted by a two-mica granite. The elevation ranges between 10 and 80 m a.s.l., and the mean precipitation recorded during the study period (from winter 2011 to winter 2013) in the weather station installed at Figueiras cliffs (Cíes Islands) was 1419 mm. More than 80% of the precipitation was concentrated between September and March (~1100 mm) (Supplementary Fig. 1). The mean annual temperature is 15.3 °C (ranging from 4 °C in winter to 34 °C in summer) [START_REF] Meteogalicia | Consellería de Medio Ambiente[END_REF].

According to the classification of [START_REF] Allué | Subrregiones fitoclimáticas de España[END_REF], the area is characterised by a subhumid Mediterranean climate with Atlantic tendency [START_REF] Vilas | Guía de visita Parque Nacional Marítimo-Terrestre de las Islas Atlánticas de Galicia[END_REF]. The soils in the gull colonies in AINP and the control zone (Cabo Home) have been analysed in previous studies and are characterized as follows: poorly developed; depth to underlying rock, 15-25 cm; sandy texture (sand fraction ~70%); slightly acidic (pH 5.1-5.8); high organic C content (11-16%); and low electrical conductivity (despite the proximity to the sea), ranging between 522 and 771 µS cm -1 in the colony soils and 1308 ± 1319 µS cm -1 in the control zone (De La Peña, 2012;[START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF][START_REF] Otero | Variabilidad espaciotemporal de formas de nitrógeno y fósforo en suelos de las colonias de gaviota patiamarilla (Larus michaellis, Naumann 1840) en el Parque Nacional Marítimo-Terrestre de las Islas Atlánticas de Galicia[END_REF]. The soils are classified as Mollic Leptosols (Arenic, Humic) (De La Peña, 2012;[START_REF] Wrb | World Reference Base for Soil Resources. International soil classification system for naming soils and creating legends for soil maps[END_REF] (Supplementary Table 1).

The cliffs in AINP are partly covered by different plant communities, except in zones of granite outcrops (Fig. 1). In the absence of gulls (i.e. Cabo Home cliffs), Chrithmo-Armerietum pubigerae association occurs on the lowest parts of the cliffs, which are most exposed to wave splash. A community formed by Dauco-Festucetum pruinosae occurs on higher slopes, where the soil is deeper. In Galicia, this community is dominated by the grass Festuca rubra subsp. pruinosa. Finally, the upper part of the slope is occupied by aerohalophilic shrubs, dominated by the endemic gorse Ulex europaeus subsp. latebracteatus [START_REF] Otero | Parque Nacional de Marítimo Terrestre de las Islas Atlánticas de Galicia[END_REF]. However, on the cliffs where the yellow-legged gulls are established, the distribution of the plant communities does not follow any clear pattern, and new communities of halo-ornithcoprophilic character, such as Cochelario danicae-Matricarietum maritimae and Calendulo algarbiensis-Parietaritarietum judaicae, have been described (Guitián and Guitián, 1990).

Calanques National Park (CNP)

In CNP, the study was conducted in the Frioul archipelago. The islands in this archipelago are covered by a mosaic of Mediterranean shrubs (e.g. Pistacia lentiscus, Rosmarinus officinalis, Coronilla juncea and Lavatera arborea) and xeric grassland, with a high proportion of bare soil and rocky patches [START_REF] Vidal | Seabirds drive plant species turnover on small Mediterranean islands at the expense of native taxa[END_REF]. The geological substrate is constituted by limestone (CaCO 3 ). The elevation ranges between 3 and 36 m. a.s.l. The climate is arid Mediterranean (semi-arid thermo Mediterranean zone, [START_REF] Médail | Biodiversity hotspots in the Mediterranean basin: setting global conservation priorities[END_REF]: the mean annual temperature is 15.2 °C [START_REF] Bonnet | Analyse diachronique des changements floristiques sur un archipel méditerranéen périurbain (îles du Frioul, Marseille)[END_REF] with large annual variations (from -16 °C in winter to more than 40 °C in summer), the mean precipitation is around 350 mm y -1 , with more than 80% occurring between November and April (280 mm) (Fig. S1), mainly as cold rain; there is also period of intense drought in summer and strong winds occur on more than 200 days per year [START_REF] Douguedroit | La sécheresse à Marseille: un siècle de longues séquences sèches (études statistiques)[END_REF].

The soils are less well developed than in AINP and are characterised by being of depth < 10-15 cm (Rendzic and Lithic Leptosols, WRB, 2014), neutral-alkaline (pH 7.5-9), with a high total organic C content (8-21%), moderate electrical conductivity, with environments ranging from those with a low ionic charge to slightly saline environments (between 866 ± 429 µS cm -1 in the colony soils and 748 ± 517 µS cm -1 in the control plot). The texture is dominated by fine fractions (silt and clay) (greater than68%) relative to the sand fraction (Supplementary Table 1).

In both study zones, the yellow-legged gull populations are clearly in decline. In AINP, the population decreased from 16,837 pairs in 2011 to 10,795 pairs in 2015 [START_REF] Barros | Censo da poboación reprodutora de corvo mariño cristado (Phalacrocorax aristotelis), gaivota patiamarela (Larus michahellis), gaivota escura (Larus fuscus) e gaivotón (Larus marinus) no Parque Nacional Marítimo-Terrestre das Illas Atlánticas de Galicia: resultados de 2015[END_REF]. The same tendency has been observed in CNP, with 12,099 pairs in 2010 and 10,339 pairs in 2015 [START_REF] Rizet | Dynamique de la population de goélands leucophée (Larus michahellis N., 1840) sur les îles de Marseille et test de la méthode de Distance Sampling[END_REF]. However, the worldwide population of the species is increasing (Birdlife International, 2017).

The characteristics of the plots selected in both parks are detailed in Table 1. The cliffs under study were selected on the basis of the density of gulls, as follows: no gulls (control), low-intermediate density and high density, established in previous surveys of breeding gulls in each park.

Sampling

Sampling of seabird excrement

During the breeding period, samples of yellow-legged gull excrement were collected from the colonies, taking particular care not to include substrate material. The samples were placed in a cool box and transported to the laboratory, where they were frozen at -80 °C until analysis. Each sample comprised 5-10 fresh excrements, which were (caption on next page) homogenized carefully. The concentrations of total N (TN), inorganic N (N-NO 3 -, N-NH 4 + ), total P (TP) and bioavailable P (P-bio) were determined.

Soil sampling

Soil sampling was carried out seasonally, in summer (August-September), which corresponds to the end of the breeding period when the density of the gulls in the colony is maximal, and in winter (February-March), when there are no gulls present in the colony.

-In AINP, the sampling was carried out in August and September in 2011 and 2012 and in February and March in 2011 and 2013, in gull colonies on Cíes and Ons islands and in the control zone (without gulls, Cabo Home) (Fig. 1B). -In CNP, the sampling was carried out in September 2017 and in January 2018 (3 days after strong rainfall) in different parts of the Frioul archipelago with different densities of gulls, and in the control zone (without gulls) (Fig. 1C).

Two plots of 7 × 5 m were established in the cliff sites -one in the lower and one in the upper part of the cliffs. In two sites (Monte das Herbas and Ruzo cliffs, AINP) only one plot was able to be established, in the middle part of the cliff. The plots were established in homogeneous areas, in terms of slope, stoniness and plant cover (see [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF], De la Peña et al., 2019).

A 50x50cm sampling frame was thrown to fall at random 6-8 times in each plot, and a sample of soil was taken from within the frame each time. Soil samples (0-15 cm depth) were collected within the frame with a trowel (length 20 cm) and stored in plastic bags at 3-5 °C until analysis.

Sample analysis

The nitrogen forms (N-NH 4 + , N-NO 3 -) were determined in samples of excrement and soils. The samples (5 g) were extracted with 50 ml of a 2 M KCl solution. The samples were maintained with continuous shaking for 1 h and were then filtered. Ammonia was determined by a steam distillation method. For this purpose, 5 ml of indicator solution (boric acid, water and ethanol) was added to 20 ml of the extract, along with MgO, which enables conversion of the NH 4 + to NH 3 in alkaline medium. Once the NH 4 + was extracted, 0.2 g of ball-milled Devarda's alloy was added to the same solution, to enable the reduction of NO 3 -to NH 4 + , which then forms NH 3 and is steam distilled. Finally, the concentration of NH 4 + is determined by titration with a 0.005 N H 2 SO 4 solution and a mixed indicator of bromocresol green and methyl red (for more details on the method, see [START_REF] Mulvaney | Nitrogen-Inorganic forms[END_REF].

In air-dried soil samples (0.5 g), the total P (TP) was extracted by acid digestion (9 ml of 14.4 M HNO 3 : 3 ml of 12 M HCl for 25 min) in a microwave oven (Ethos Easy). Bioavailable P (P-bio) was extracted by the Mehlich 3 method [START_REF] Mehlich | Mehlich 3 soil test extractant: a modification of the Mehlich 2 extractant[END_REF], and crystalline Fe oxyhydroxides (FeD) were extracted with a sodium dithionite-citrate solution (0.5 M) [START_REF] Holmgren | A rapid citrate-dithionite extractable iron procedure[END_REF]. In addition, a few samples (3-4 per zone) were subjected to sequential P extraction. Two grams of dried, ground (< 2mm) soil were subjected to sequential extraction to yield the following P fractions (for further details, see [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF]:

• F1-P: (P-ads), i.e. weakly adsorbed and soluble phosphorus. The sample was extracted with 20 ml of 1 M MgCl 2 solution, with continuous stirring for 30 m. Samples were centrifuged at 10,000 rpm and filtered through Albet filter paper, and the extract thus obtained was stored at 3 °C until analysis. The residue was washed twice with Milli-Q water (18 Ω) before the next extraction was started. The centrifugation, filtration and washing procedures were repeated at the end of each extraction step.

• F2-P (P-Fe/Mn), i.e. P adsorbed to Fe and Mn oxides and oxyhydroxides. Twenty ml of a sodium bicarbonate-dithionite solution (Na 2 S 2 O 4 0.11 M + 0.11 M NaHCO 3 ; pH 7) was added to the residue from the prior extraction, and the mixture was stirred continuously for 1 h.

• F3-P (P-clay/Al), i.e. P bound to Al hydroxides and clay. The samples were shaken for 18 h with 20 ml of sodium hydroxide (0.1 M NaOH).

• F3b-P (P-HA), i.e. P associated with soil humic substances. The extract obtained in step F3 was dark brown (almost black) due to the dissolution of humic acids under alkaline conditions (Jiménez-Cárceles and Alvarez-Rogel, 2008). To avoid interference in the colorimetric quantification and for simultaneous quantification of the concentration of P associated with these soil components, the extract was acidified with 2.5 ml of concentrated sulphuric acid (H 2 SO 4 ) and allowed to stand overnight. The organic flocs were allowed to settle at the bottom of the container before being separated by filtration. The filter with the retained material was dried (45 °C) and subsequently calcined at 520 °C for 2 h in a muffle furnace. The ash was dissolved in 5 ml of 1 M HCl by boiling the mixture for 30 min, and the extract was made up to 50 ml with Milli-Q water.

• F4-P (P-Ca), i.e. P bound to Ca phosphates. The samples were shaken for 1 h with 20 ml of 0.5 MHCl.

• F5-P (P-res), i.e. P mainly associated with refractory organic matter [START_REF] Schlichting | Sequentially extracted phosphorus fractions in peat-derived soils[END_REF][START_REF] Largeau | Refractory macromolecular fraction of soil organic matter: chemical composition, sources and relationship with the stable fraction[END_REF]. The material remaining after the above extractions was calcined and processed as described in step F3b-P (P-HA).

Reference material (NIST2782 and SO2) was used to determine the accuracy of the P extraction processes. The mean percent recovery of total P was 91 ± 2% (n = 6, and the sum of all fractions yielded by sequential extraction was 88.6 ± 4% (n = 3) [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF].

The concentrations of total P (TP) and bioavailable P (P-bio) were determined colorimetrically, by the molybdenum blue method [START_REF] Bowman | A rapid method to determine total phosphorus in soils[END_REF]. The P fractions and FeD were determined in an ICP-OES spectrometer (Perkin Elmer Optima 4300 DV). All equipment used during extraction of the different P forms was washed thoroughly with HCl (5%), for at least 48 h, and then with ultrapure water (Milli Q).

Statistical analysis

One-way ANOVA and a post hoc Holm-Sidack method test were used to detect significant differences between the mean concentrations of nutrients in excrement samples from each park, as well as differences in the mean nutrient concentrations between seasons (winter, summer), between sampling zones (control zone, low-intermediate density, high density) and overall between both national parks (CNP, AINP). Prior to ANOVA, all variables were checked for normality and homoscedasticity. Non-normally distributed data were transformed using a natural logarithm transformation. All statistical analyses were carried out using Sigma Plot 12,0. In all tests, a significance level of p < 0.05 was applied.

Results

Nutrients in excrement

The gull excrements contained high quantities of N and P, and a high level of inter-sample variability was observed for both parks. The samples from AINP contained significantly higher amounts of TP and P- = 2,258 ± 1,552 mg kg -1 , N-NO 3 -= 12.8 ± 9.8 mg kg -1 ; Table 2).

N And P in the gull colony soils

In both parks, the colony soils contained significantly higher amounts of N forms and of P (especially bioavailable P) than in the corresponding control zones (Table 3). In addition, the concentrations of N forms and P-bio were higher in AINP than in CNP (Table 3).

Spatio-temporal variation in the N forms

The concentrations of N-NH 4 + showed a marked seasonal variability in AINP but not in PNC. Thus, in CNP the mean concentrations of N-NH 4 + in the colony soils were similar in summer (CNP:

14.2-25.3 mg kg -1 ) and in winter (CNP: 10.8-16.9 mg kg -1 ; Fig. 2). In CNP, there were no significant differences in the mean concentrations of N-NH 4 + in winter (CNP winter: N-NH 4 high density = 16.9 ± 11 mg kg -1 ; low-intermediate density: 10.8 ± 4.2 mg kg -1 ; control zone: 8.96 ± 3.5 mg kg -1 ; Fig. 2). However, in summer although there were no significant differences between the soil in low-intermediate density zone and the control soil, there were differences between the control plot and the high density colony (CNP summer: N-NH 4 + high density = 25.3 ± 13 mg kg -1 ; low-intermediate density:

14.2 ± 8.8 mg kg -1 ; control zone: 11.0 ± 5.2 mg kg -1 ; Fig. 2). A similar situation was observed in AINP in winter (CNP winter: N-NH 4 + high density = 36.7 ± 26 mg kg -1 ; low-intermediate density:

28.6 ± 23 mg kg -1 ; control zone: 27.2 ± 27 mg kg -1 ; Fig. 2). In summer, the mean concentration of N-NH 4 + in the zone with a high density of gulls was statistically significantly higher than in the zone with a low-intermediate density of gulls, and the mean concentration in this zone was, in turn, significantly higher than in the control zone (Fig. 2). In summer, the mean concentration of N-NH 4 + in the colony soils was significantly higher than in the control zone (Fig. 2). The seasonal pattern in the concentrations of N-NO 3 -was not the same as observed for N-NH 4 + in both parks. Thus, in CNP the highest concentrations occurred in winter, considering the three zones together, and in AINP, the highest concentrations occurred in summer, as observed for N-NH 4 + (Fig. 2).

Spatio-temporal variation in the total (TP) and bioavailable P (Pbio) concentrations

The mean TP concentrations in the gull colony soils were similar in both parks (AINP: 1,211 ± 957; CNP: 1,119 ± 620 mg kg -1 , Table 3), while the concentration of P-bio was significantly higher in AINP than in CNP (AINP: 169 ± 130, CNP: 126 ± 88 mg kg -1 , Table 3). However, the increase in P-bio in the soils influenced by the gull colonies relative to the control plot did not differ significantly between the parks (AINP: 127 ± 130 mg kg -1 , CNP: 111 ± 88 mg kg -1 ).

In CNP, significant seasonal differences in the TP concentrations were observed in both the control and colony soils, with the highest values occurring in summer (Fig. 3). In winter, there was a substantial decrease in the TP in all plots (e.g. high density of gulls: TP summer: 1,553 ± 550, TP winter 783 ± 237 mg kg -1 ; Fig. 3). The TP content showed significant differences between control zones and colony soils but not between colonies.

In AINP, the TP concentrations were clearly affected by the presence of gulls, increasing with the density of gulls, as follows: control zone < low-intermediate < high density. By contrast, there were no significant seasonal differences in either the control zone or in the colony soils (Fig. 3). The soil in the control zone contained an unusually high amount of TP, presumably due to the presence of pegmatite intrusions with high contents of apatite (calcium phosphate) [START_REF] Igme | Mapa geológico de España[END_REF][START_REF] González | Guia Geológica del Parque Nacional de las Islas Atlánticas de Galicia[END_REF].

In both summer and winter in CNP, the concentration of P-bio in the zones with high densities of gulls was significantly higher (mean values winter/summer 131-153 mg kg -1 ) than in the zones with low densities of gulls (mean values winter/summer 133-87.5 mg kg -1 ) and than in the control zone (winter: 15.4 ± 8.8, summer. 14.4 ± 5.0 mg kg -1 ) (Fig. 3). Likewise, the concentrations were significantly higher in the zones with intermediate densities of gulls than in the control zone.

In AINP the concentration of P-bio was significantly higher in the colony soils than in the corresponding control soil (Fig. 3). The highest values corresponded to the plots with the highest density of gulls (summer: 185 ± 52, winter: 199 ± 44 mg kg -1 ), followed by zones with low-intermediate density of gulls (winter: 159 ± 65, summer

Table 2

Concentrations of N forms, total P and bioavailable P (mg kg -1 ) in fresh excrements of the yellow-legged gull. In each column, different letters indicate significant differences (P < 0.05) between national parks. 523 ± 280Aa(4 3 1)36 1,119 ± 620Ab(1,070)72 826 ± 106Ba(8 0 8)10 1,211 ± 957Ab(9 5 9)100 154 ± 79 mg kg -1 ) and by the control zone (winter 54.5 ± 24, summer 32.5 ± 8.10 mg kg -1 ). The differences in P-bio between zones with high and low-intermediate densities of gulls did not show a clear pattern. Thus, in summer the concentrations were similar in both zones, while in winter the concentration of P-bio was significantly higher in the zone with a high density of gulls (Fig. 3).

Phosphorus partitioning

The distribution of P in the different geochemical fractions showed significant differences in the comparison between the colony and the corresponding control zone and the between-park comparison (Table 4).

In CNP, most of the P (96%) was distributed between residual P (Pres: 872 ± 185 mg kg -1 ) and Ca phosphate fractions (P-Ca: 708 ± 397 mg kg -1 , Table 4). These fractions were followed in abundance by the P associated with Fe oxides (P-Fe/Mn) ~P-HA > Pclay/Al ~P-ads. By contrast, in AINP, the P was more homogeneously distributed. The dominant form was P-clay/Al (139 ± 49 mg kg -1 ), which represents 41% of the sum of the six fractions. The second most abundant P form was P-res, followed by P-Ca > P-HA ~P-ads > P-Fe/Mn (Table 4).

Finally, the concentrations of the most labile and mobile P fractions (P-ads, P-HA) in soil were much higher in AINP than in CNP (AINP = Pads: 78.4 ± 73; P-HA: 87.7 ± 34 mg kg -1 ; CNP = P-ads: 27.5 ± 14 mg kg -1 ; P-HA: 35.2 ± 12 mg kg -1 ).

Discussion

Variability in nutrient concentrations in the excrements

Seabird excrements are characterised by high concentrations of nutrients, particularly of N and P [START_REF] García | Seagull influence on soil properties, chenopod shrub distribution, and leaf nutrient status in semi-arid Mediterranean islands[END_REF][START_REF] Zwolicki | Guano deposition and nutrient enrichment in the vicinity of planktivorous and piscivorous seabird colonies in Spitsbergen[END_REF][START_REF] Otero | Variabilidad espaciotemporal de formas de nitrógeno y fósforo en suelos de las colonias de gaviota patiamarilla (Larus michaellis, Naumann 1840) en el Parque Nacional Marítimo-Terrestre de las Islas Atlánticas de Galicia[END_REF][START_REF] Otero | Trace elements in biomaterials and soils from a yellow-legged gull (Larus michahellis) colony in the Atlantic Islands of Galicia National Park (Galicia-NW Spain)[END_REF]. The high, variable concentrations of N and P in the excrements of the yellow-legged gull, as well as in other seabirds (see also [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF], appear to be derived from the diet. The yellow-legged gull is considered an opportunistic omnivore, incorporating substances of marine and terrestrial origin in its diet, along with a wide variety of waste products of anthropogenic and terrestrial origin [START_REF] Pierotti | Diet choice in the Herring Gull: constraints imposed by (Larus michahellis) chicks along the Spanish Western Mediterranean coast: the relevance of refuse dumps[END_REF][START_REF] Munilla | Henslow's swimming crab (Polybius henslowii) as an important food for yellow-legged gulls (Larus cachinnans) in NW Spain[END_REF]. A previous study in AINP has shown that various components of the gull diet occur in proportions higher than 5%: waste remains (40.1%), Polybius henslowii (36.4%), Mytilus galloprovincialis (13.5%), Micromesistius poutassou (9.8%), Sardina pilchardus (8.4%) and Trachurus spp. (7.6%) [START_REF] Munilla | Henslow's swimming crab (Polybius henslowii) as an important food for yellow-legged gulls (Larus cachinnans) in NW Spain[END_REF]. Although the N and P contents of each of the elements that the gulls consume are not known, mussels (Mytilus galloprovincialis) cultivated in Galician rias have been found to contain much more P (314 mg/100 g) than populations from the Mediterranean Sea (149 mg/ 100 g Delta del Ebro; 239 mg/100 g Valencia) [START_REF] Fuentes | Comparison of physicochemical parameters and composition of mussels (Mytilus galloprovincialis Lmk.) from different Spanish origins[END_REF]. This type of difference may partly explain the higher concentrations of nutrients present in the excrements of the yellow-legged gull in AINP than in gulls from CNP (8 times more total P and 4 times more P-bio). Although there are no comparative data on populations of the same species, it has been concluded that differences in seabird diets represent the main cause of differences observed in tundra soils [START_REF] Zwolicki | Guano deposition and nutrient enrichment in the vicinity of planktivorous and piscivorous seabird colonies in Spitsbergen[END_REF]; see also [START_REF] Hutchison | Survey of contemporary knowledge of biochemistry. 3. The biochemistry of vertebrate excretion[END_REF][START_REF] Bédard | Assessment of the importance of nutrient recycling by seabirds in the St. Lawrence Estuary[END_REF][START_REF] Stempniewicz | Keystone species and ecosystem functioning. Seabirds in polar ecosystems[END_REF].

Effect of environmental context on soil nutrient dynamics

The results of the present study show that yellow-legged gull colonies exert a strong impact on N and P cycling in breeding colonies [START_REF] Otero | Seasonal variation in inorganic nitrogen content of soils from breeding sites of yellow-legged gulls (Larus Cachinnans) in the Cies Islands Natural Park (NW Iberian Peninsula)[END_REF]2015[START_REF] García | Seagull influence on soil properties, chenopod shrub distribution, and leaf nutrient status in semi-arid Mediterranean islands[END_REF]. However, other environmental factors in addition to nutrient supply can influence the mobility and bioavailability of these nutrients. Previous studies have highlighted the role of climate conditions on N dynamics [START_REF] Riddick | The Global Ammonia Emission from Seabirds[END_REF][START_REF] Riddick | Measurement of ammonia emissions from temperate and polar seabird colonies[END_REF], but as far as we are aware, no previous studies have considered the role of lithology on nutrient dynamics in seabird colony soils.

N Dynamics

The concentrations of N forms in soil were generally higher in AINP

Table 4

Geochemical forms of P in the soil (mg kg -1 ). Capital letters indicate significant differences between equivalent zones in each national park (e.g. control zone in the AINP and control zone in the CNP). Lowercase letters indicate significant differences in each P form between the control zone and the colony zone in the same national park (e.g. control zone and colony in the AINP). than in CNP, and in both parks they were highest at the end of summer (Tables 3,4), when the presence of gulls and chicks in the colony is greatest. However, a more detailed analysis shows that the seasonal patterns are more complex due to the high level of variability in both the between-park comparison and between-plot comparison in each park. For example, in CNP, the concentration of N-NO 3 -was higher in winter, whereas in AINP the corresponding values were highest in summer. The seasonal differences between parks suggest that environmental factors, especially climate factors (i.e. precipitation) modulate the impact of seabird colonies on nutrient dynamics (see e.g. [START_REF] Otero | Seasonal variation in inorganic nitrogen content of soils from breeding sites of yellow-legged gulls (Larus Cachinnans) in the Cies Islands Natural Park (NW Iberian Peninsula)[END_REF][START_REF] Otero | Variabilidad espaciotemporal de formas de nitrógeno y fósforo en suelos de las colonias de gaviota patiamarilla (Larus michaellis, Naumann 1840) en el Parque Nacional Marítimo-Terrestre de las Islas Atlánticas de Galicia[END_REF][START_REF] García | Seagull influence on soil properties, chenopod shrub distribution, and leaf nutrient status in semi-arid Mediterranean islands[END_REF].

The main N form present in seabird excrement is uric acid [START_REF] Riddick | The Global Ammonia Emission from Seabirds[END_REF]. Uric acid undergoes diverse transformations in excrement that affect its geochemical mobility and loss from soil. Mineralization of organic N involves transformation into the ammonium ion (NH 4 + , ammonification), which can be adsorbed onto soil colloids thereby reducing leaching from soil. However, transformation of N to nitrate (NO 3 -, nitrification) favours N loss from soil during heavy rainfall [START_REF] Bernal | Variability of DOC and nitrate responses to storms in a small Mediterranean forested catchment[END_REF][START_REF] Fenn | Temporal and spatial trends in streamwater nitrate concentrations in the San Bernardino Mountains, southern California[END_REF]. The elevated precipitation that usually occurs in AINP between October and March, together with the sandy soils, enhances this process in autumn and winter [START_REF] Otero | Seasonal variation in inorganic nitrogen content of soils from breeding sites of yellow-legged gulls (Larus Cachinnans) in the Cies Islands Natural Park (NW Iberian Peninsula)[END_REF]. Despite the significant loss of N in the colony soils in AINP, the higher concentrations of inorganic N in the soil from AINP than in soil from CNP appear to indicate that the behaviour of N is less variable in the former than in the latter. In both parks the gull colonies are of similar size and developmental stage; however, the greater edaphic development, both at depth and at the surface (Fig. 1D), appears to contribute to greater retention of this element in the colony soils in AINP. The higher concentration of N in gull excrement in AINP may also contribute to the differences between the two parks.

In addition to N leaching, other geochemical processes that contribute to accelerating N loss may be particularly important in CNP. A high proportion of N (sometimes greater than 60%) may be lost due to volatization as ammonia (NH 3 ) during the early stages of mineralization of the uric acid present in the excrement [START_REF] Woodmansee | Additions and losses of nitrogen in grasslands ecosystems[END_REF]. In CNP, this process is favoured by high summer temperatures and the alkaline reaction between soils and limestone (CaCO 3 ) [START_REF] Riddick | The global distribution of ammonia emissions from seabird colonies[END_REF]. Moreover, N loss from excrement is greater when the excrement is deposited directly on the rock, as is usual in CNP (see Fig. 1E), than when it is deposited on soil or vegetation (see e.g. [START_REF] Lindeboom | The nitrogen pathway in a penguin rookery[END_REF][START_REF] Blackall | Temporal variation in atmospheric ammonia concentrations above seabird colonies[END_REF]. In fact, NH 3 volatization from excrement is considered to be very low or nil when the excrement is deposited directly on vegetation [START_REF] Riddick | The Global Ammonia Emission from Seabirds[END_REF]. These factors contribute to understanding why the concentrations of N-NH 4 + and NO 3 -were lower in the soils in CNP. Furthermore, in Mediterranean climate zones, N cycling is strongly influenced by water availability in soil [START_REF] Austin | Water pulses and biogeochemical cycles in arid and semiarid ecosystems[END_REF][START_REF] Ochoa-Hueso | Nitrogen deposition effects on Mediterranean-type ecosystems: An ecological assessment[END_REF] and shows marked seasonal patterns [START_REF] Bonilla | Soil nitrogen dynamics in a Holm oak forest[END_REF]Rodà, 1992, Rutigliano et al., 2009). The concentration of inorganic N in soil is related to wetting-drying cycles that lead to lysis of microbial cells [START_REF] Austin | Water pulses and biogeochemical cycles in arid and semiarid ecosystems[END_REF] and net mineralization of N [START_REF] Bytnerowicz | Nitrogen deposition in California forests: a review[END_REF][START_REF] Fenn | Methods for measuring atmospheric nitrogen deposition inputs in arid and montane ecosystems of western North America[END_REF][START_REF] Padgett | Changes in soil inorganic nitrogen as related to atmospheric nitrogenous pollutants in southern California[END_REF]. Regarding the results of the previously cited studies, the increase in N-NO 3 observed in winter in CNP may be may be due to the fact that sampling was conducted immediately after a period of rainfall following a prolonged drought period. The findings suggest that the winter rains may reactivate N mineralization processes (especially nitrification) after long dry periods that occur in arid climates.

Overall, the findings suggest that in CNP, mineralization of the organic N present in excrement occurs later in the year than in AINP, where the greater availability of water throughout almost of the year will favour nitrification processes in summer. In CNP this will not occur until after the first rainfall in autumn-winter.

P dynamics

The colony soils in both parks were enriched in P, especially the most bioavailable forms (P-bio, P-ads), although also in the more recalcitrant forms (i.e. P-res, P-Ca). Thus, the concentration of P-bio in the colony soils was around 5-7 times higher than in the control zones. However, there were substantial differences between the zones in the annual dynamics. The behaviour of P in the soil also differed, which affected both the total P content and the geochemical forms of P.

The TP content of the colony soils in AINP was almost identical in summer and winter, whereas in CNP the TP content was much higher in summer than in winter (Fig. 3). The lower TP in CNP is consistent with the intense precipitation that occurs in winter in Mediterranean climate zone, which causes high rates of run-off and strong erosion of the soil surface (Plan Blue, 2003) and also loss of the P accumulated on the soil surface during the breeding period.

In addition to the P enrichment observed in the yellow-legged gull colony soils, the P fractioning indicated differences in geochemical forms that were consistent with marked edapho-geochemical differences (depth, texture, pH, organic matter content, etc. Table S1) between the national parks.

The order of abundance of the different P forms in the yellow-legged gull excrement was as follows: P-Ca≫ P-res > P-Fe/Mn ~P-HA > P-ads≫ P-clay/Al [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF]. In both parks, P-Ca was one of the main P forms present in the soil (Table 4), which is consistent with the concentration in the excrements, although also with its presence in the parent material (granites and pegmatites with apatite) in AINP. However, in CNP the concentration of P-Ca reached extremely high values, around 7 times higher than in AINP (CNP: 708 ± 397, AINP: 94.9 ± 78 mg kg -1 ), while the other geochemical forms were present at very low concentrations (in general median values < 40 mg kg -1 ), except for the residual fraction, which was the most abundant. The P incorporated in calcareous soils undergoes a series of fixation reactions that lead to rapid (weeks) immobilization in the soil. Phosphorus fixation is a combination of surface adsorption on clay and lime, and precipitation of various calcium phosphate minerals (mainly dicalcium phosphate and octacalcium phosphate), which gradually decrease the solubility and availability of P to plants, especially in young calcareous soils with high concentrations of P [START_REF] Carreira | Factors affecting phosphate sorption along a Mediterranean dolomite soil and vegetation chronosequence[END_REF]Tunesi et al., 1999, Wandruszaka, 2006). This process explains the enrichment of the P-Ca fraction to the detriment of the other geochemical fractions in the soils in CNP.

In addition, the low importance of Fe oxides and hydroxides in the colony soils in CNP is consistent with the alkaline nature of the soil, which leads to reduction in the positive charge of these colloids (see below), and also with the low concentration of Fe oxyhydroxides in the soil (CNP colonies FeD: 0.07 ± 0.03%, Table S1). The same reasoning can be applied to the small part that the clays and Al oxyhydroxides play in controlling the mobility of P in colony soils in CNP.

By contrast, in acid soils the P solubility is mainly determined by Fe and Al oxyhydroxides [START_REF] Wild | Soils and the Environment[END_REF]. This is particularly relevant in AINP, as together these P-associated fractions represent almost 30% of the total P, which is consistent with the higher content of Fe oxyhydroxides than in CNP (AINP colonies FeD: 0.11-0.14%). In addition, the positive charge associated with the surfaces of the Fe and Al oxyhydroxides, as well as the edges of the phyllosilicates in the clay fraction confer the soil the capacity to adsorb oxyanions such as orthophosphate (Equation 1, 2 However, this process and the posterior migration of P into the soil particles (mainly Al and Fe oxides and hydroxides) or conversion of dicalcium-P (DCP) into octacalcium-P or apatite usually leads to a huge reduction in the bioavailable P and particularly the weakly adsorbed P (P-ads), an important fraction in the gull colony soils in AINP (representing 11% of the TP). The high concentration of P-ads is compatible with the fact that the colony soils in AINP are saturated in P due to the input from the yellow-legged gull (for further details, see [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF].

Unlike in CNP, the P fraction associated with humic acids was also an important fraction in AINP. The P-HA represented 13% of the TP in AINP, which is consistent with the high content of organic C in the colony soils and the high primary productivity observed in the yellowlegged gull colonies relative to the control zone (Fig. 1E). The dominant forms of organic P in the colony soils in AINP were orthophosphate monoesters (39.5%; [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF], which are the major functional and most stable class of organic P in most soils [START_REF] Turner | Rapid estimation of microbial biomass in grassland soils by ultra-violet absorbance[END_REF]. Furthermore, output of orthophosphates is known to occur via the small temporary streams that run through the colony, as well as loss of high amounts of dissolved organic carbon (DOC) in the run-off from the colonies (DOC colonies: 12.2 ± 8.1 mg l -1 ; [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF] relative to the amount present in the rivers that run through forest areas in Cíes Islands (6.0 ± 4.5 mg l -1 ). Orthophosphate and dissolved organic P (DOP) are P forms that are bioavailable to marine plankton [START_REF] Dyhrman | Presence and regulation of alkaline phosphatase activity in eukaryotic phytoplankton from the coastal ocean: implications for dissolved organic phosphorus remineralization[END_REF] and therefore seabird colonies may represent an important source of input of P (and N) to coastal waters [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF][START_REF] Otero | Trace elements in biomaterials and soils from a yellow-legged gull (Larus michahellis) colony in the Atlantic Islands of Galicia National Park (Galicia-NW Spain)[END_REF].

The lower concentration of P-bio in summer in the soils in CNP is also related to the strong seasonal variations that characterize the Mediterranean climate as the mineralization of organic phosphates is a slow process and subject to wetting-drying cycles (Lebedjantzev, 1924, Read and[START_REF] Read | Decomposition and mineralization processes in Mediterranean-type ecosystems and in heathlands of similar structure[END_REF], which lead to the release of important amounts of phosphorus.

Conclusions

In both national parks, yellow-legged gull colonies exert a strong impact on N and P cycling, in an effect recently denominated ornithoeutrophication [START_REF] Otero | Trace elements in biomaterials and soils from a yellow-legged gull (Larus michahellis) colony in the Atlantic Islands of Galicia National Park (Galicia-NW Spain)[END_REF]). However, annual N and P cycles are strongly influenced by environmental conditions.

In CNP, mineralization of the organic N in the excrement occurs more slowly in the summer due to the very dry conditions at this time of year, but is reactivated by the first rainfall in autumn-winter. This leads to higher concentrations of N-NO 3 -in winter than in summer, in contrast to what occurs in AINP.

The annual P dynamics are more strongly conditioned by edaphic and lithological conditions than by climatic conditions. In CNP, calcium phosphate acts as a sink for the P released on mineralization of the gull excrement due to the high concentration of calcium carbonate in the soil. By contrast, in AINP, where the soils are characteristically acidic or strongly acidic, the P is distributed more homogeneously between the different geochemical fractions.

Fig. 1 .

 1 Fig. 1. A) Location of the study areas; B) Location of the sampling zones in Calanques National Park (Marseille, France); C) Location of sampling zones in the Atlantic Islands National Park (AINP; Galicia, Spain); D) General view of the colony in AINP. Note the dense plant cover and appearance of granite outcrops in the upper parts of the slopes. E) General view of the CNP colony. Note the rocky substrate (limestone) and scarce plant cover.

Fig. 2 .

 2 Fig. 2. Spatio-temporal variations in the N forms in the national parks. Different capital letters indicate significant differences between the three sites considered in each park in the same season (i.e. summer control, low-intermediate and high density). Different lowercase letters indicate significant seasonal differences (i.e. summer and winter) for each site.

Fig. 3 .

 3 Fig. 3. Spatio-temporal variations of total and bioavailable P in the national parks. Different capital letters indicate significant differences between the three sites considered in each park in the same season (i.e. summer control, low-intermediate and high density). Different lower-case letters indicate significant seasonal differences (i.e. summer and winter) for each site.

Table 1

 1 General characteristics of the study areas. A) Atlantic Islands National Park (Galicia, Spain) and Calanques National Park (Marseille, France).

						)			)
		Influence of seagulls		Control plot		Low-intermediate (25-50 pairs ha -1			Low-intermediate (25-50 pairs ha -1	High (≥50 pairs ha -1 )			High (≥50 pairs ha -1 )
		Coordinates UTM Exposition. Plant association Representative species		511270.408 4680186.325 W Pastizales de Armeria pubigera A. pubigera		507595.772 4674540.514 N-NE Dauco gummnifer-Festucetum pruinosae Festuca pruinosa; Koeleria glauca	507,662,000 4675667.000 N-NO Cochleario-Matricarietum maritimae Cochlearia danica; Matricaria maritima	507663.946 4675668.334 SW Praderas de Dactylis maritima y Silene uniflora Silene uniflora; Dactylis glomerata	507173.000 4673353.600 S Pastizales de Armeria pubigera A. pubigera	507086.817 4673685.139 S-SW Praderas de Holcus lanatus Holcus lanatus; A. pachycarpa	507468.880 4676710.230 N-NW Praderas de Dactylis maritima and Silene uniflora Silene uniflora; Dactylis glomerata		506388.038 4694062.134 N-NE Ulicetum latebracteato-minoris Ulex latebracteatus	506376.120 4694105.095 N-NE Comunidades A. pachycarpa A. pachycarpa		Coordinates UTM Exposition Plant association Representative species		686482.600 4793464.491 ENE Astragalo tragacanthae-Plantaginetum subulatae Astragalus tragacantha	686499.011 4793532.248 N Crithmo maritimi-Limonietum pseudominuti Limonium pseudominutum	686801.552 4793753.299 E Helianthemo syriaci-Ericetum multiflorae Rosmarinus			686598.283 4794531.904 NW Astragalo tragacanthae-Plantaginetum subulatae Astragalus tragacantha	686566.525 4794481.013 S Crithmo maritimi-Limonietum pseudominuti Limonium pseudominutum	686609.645 4794586.246 W Helianthemo syriaci-Ericetum multiflorae Rosmarinus officinalis		686487.525 4794679.521 NE Astragalo tragacanthae-Plantaginetum subulatae Astragalus tragacantha	686630.880 4794716.194 NE Crithmo maritimi-Limonietum pseudominuti Limonium pseudominutum	686629.526 4794683.430 N Helianthemo syriaci-Ericetum multiflorae Rosmarinus officinalis
	A) Atlantic Islands National Park -AINP-(Galicia, Spain, Zone 29)	Plot	Home cape	Home cape (CH)	Cíes islands	Monte Herbas (MH)	Figueiras (FG1)	Figueiras (FG2)	Faro Porta (FP)	Ruzo (RZ)	Percha	Ons island	Punta Centulo (PC1)	Punta Centulo (PC2)	B) Calanques National Park -CNP-(Marseille, France, Zone 31)	Plot	Pomègues island: Control plot	PCA	PCL	PCR	Ratonneau island: Low-intermediate influence (25-50 pairs	ha -1 )	DBA	DBL	DBR	) Ratonneau island: High influence (90-150 pairs ha -1	DFA	DFL	DFR

Table 3

 3 Concentrations of N and P in colony soils and control soils in each national park, considering the summer and winter values together for the gull colony soils (lowintermediate and high density together) and for the control zone. In the rows, different capital letters indicate significant differences (at p < 0.05) between corresponding zones in each park (e.g. control zone in the AINP and control zone in the CNP). Within each park, different lower-case letters indicate significant differences between different zones (control zone, colonies).

			CNP		AINP
			Control site	Colonies	Control site	Colonies
			mg kg -1
	+ N-NH 4 N-NO 3 -	mean ± SDmediann mean ± SDmediann	9.99 ± 4.5Aa(10.1)36 8.29 ± 7.7Aa(5.04)36	16.8 ± 11Ab(13.7)72 39.0 ± 32Ab(26.9)72	31.5 ± 21.1Ba(38.2)22 25.6 ± 17Ba(20.7)22	43.4 ± 39.9Bb(34.8)300 56.8 ± 48Bb(45.7)300
	Pbio	mean ± SDmediann	14.9 ± 7.1Aa(14.5)36	126 ± 88Ab(1 0 9)72	42.9 ± 20Ba(35.9)20	169 ± 130Bb(1 4 0)260
	Total P	mean ± SDmediann

  ).

	Me-OH + H + →Me-OH 2	+ (Me: AlorFe)	(1)
	Me-OH 2	+ +H 2 PO 4 -→Me-O-PO 3 H 2 + H 2 O	(2)

Acknowledgements

The authors are grateful to the managers and technicians at AINP for the support and facilities provided during the fieldwork. Thanks are also due to María José Santiso, for assistance with laboratory work, and to Esther Sierra Abraín, Pablo Montero and Adrián González, for help with sampling. This work is part of a research project entitled "Monitorización de los procesos biogeoquímicos en suelos de acantilados y aguas costeras en relación con la influencia de las colonias de aves marinas del Parque Nacional Marítimo Terrestre de las Islas Atlánticas de Galicia", funded by Organismo Autónomo Parques Nacionales (Ref. 041/2010, Ministerio de Medio Ambiente, Medio Rural y Marino), Fundación BBVA and Cross-Research in Environmental Technologies (CRETUS) (AGRUP 2015(AGRUP /02, ref. 2018-P G100) -P G100) and Consellería de Educación, Universidade e Formación P rofesional-Xunta de Galicia (Axudas á consolidación e estruturación de unidades de investigación competitivas do SUG del Plan Galego IDT, Ambiosol Group ref. 2018-PG036). Saúl de La Peña was in receipt of a predoctoral fellowship from the FPU P rogram of Spanish Ministry of Education and Innovation. We also thank Christine Francis for improving the English translation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper


Jérôme Rambaud

PhD Stéphane Dauger

PhD Luc Morin

MD MSc Jean Bergounioux

PhD Pierre-Louis Leger

PhD Ricardo Carbajal
email: ricardo.carbajal@aphp.fr

PhD Sylvain Renolleau

PhD Romain Guedj

M D Phd

Jerome Rambaud

Bronchiolitis Admissions to Intensive Care during COVID

INTRODUCTION

Acute bronchiolitis, a viral seasonal infection of the lower respiratory tract, affects nearly 20% of all children and leads to the hospitalization of 2% to 3% of all children younger than 12 months every year [START_REF] Hasegawa | Trends in bronchiolitis hospitalizations in the United States, 2000-2009[END_REF][START_REF] Meissner | Viral Bronchiolitis in Children[END_REF] . Approximately 2 to 6% of hospitalized children require admission to the pediatric intensive care unit (PICU) [START_REF] Hasegawa | Trends in bronchiolitis hospitalizations in the United States, 2000-2009[END_REF] . Acute bronchiolitis may account for as much as 13% of PICU admissions [START_REF] Green | Admission to hospital for bronchiolitis in England: trends over five decades, geographical variation and association with perinatal characteristics and subsequent asthma[END_REF] and it is responsible for the death of around 3/100 000 children under 12 months each year [START_REF] Fleming | Mortality in children from influenza and respiratory syncytial virus[END_REF] ; e.g., nearly 20 and 100 infants annually in England [START_REF] Fleming | Mortality in children from influenza and respiratory syncytial virus[END_REF] and the United States [START_REF] Byington | Respiratory syncytial virus-associated mortality in hospitalized infants and young children[END_REF] , respectively. Bronchiolitis epidemic has a predictable seasonal pattern, and in France, it starts in September, peaks in November-December, and ends in February. Although children have been relatively spared by the COVID-19 pandemic, the impact that the overlap of the second wave of this pandemic and of the associated public measures have on the 2020-2021 bronchiolitis season in the Northern hemisphere is unknown.

The aim of this multicenter regional study is to determine the effect of the COVID-19 second wave in France and the public measures it elicited, i.e., social distancing, mask-wearing, hand hygiene, curfews, and lockdowns, on the number of admissions for bronchiolitis to all PICUs of the largest region of France, and its related deaths.

METHODS

From the electronic health record systems, we identified all children (age <2 years) admitted to all 5 PICUs of the region of Paris (12.6 million inhabitants representing around 20% of France population) for bronchiolitis from September 2015 through December 2020.

Bronchiolitis was defined using the ICD-10 codes. Based on the 5 bronchiolitis epidemics before COVID-19, (2015-2016 through 2019-2020), we calculated by Time Series Analysis (SPSS, v18; SPSS Inc, Chicago, USA) the predicted number of PICU admissions for the months of September through December 2020 (COVID season) and compared them to actual admissions. Informed consent was waived within the provisions of the French Public Health Code.

RESULTS

During the 5 pre-COVID-19 seasons there were 3099 admissions for bronchiolitis to all 5 PICUs; 2190/3099 (70.7%) occurred between September and December. Figure 1 shows the monthly actual PICU admissions for each of the 5 pre-COVID-19 bronchiolitis seasons, a model fit created by time series analysis (R 2 = 0.93) and the predicted admissions for the first 4 months (September-December) of the COVID-19 bronchiolitis season. While the predicted number of admissions for bronchiolitis between September and December 2020 to all PICUs was 444, the actual number of admissions was 65; i.e., an 85.3% reduction. Figure 2

DISCUSSION

This regional study, including all PICUs of the Paris region, shows a dramatic reduction in the number of admissions of young infants to PICUs for bronchiolitis during the COVID era.

Because of the exhaustive recruitment in a region representing around 20% of France population, these results could be extrapolated to the whole country since the same public measures were taken. During the first 4 months of the current bronchiolitis season, there were nearly 370 fewer PICU admissions for bronchiolitis in infants than expected in the Region of Paris. This represents an important reduction of the severe bronchiolitis burden on infants themselves, infants' families, and the management of PICU beds. Although we are unable to quantify it, it is also possible that the deaths of some infants due to bronchiolitis were avoided. Our findings, in a country of the Northern hemisphere, are consistent with a marked decrease in detections of Respiratory Syncytial Virus (RSV), the main causative virus of bronchiolitis, reported during the 2020 winter in the Southern hemisphere [START_REF] Yeoh | The impact of COVID-19 public health measures on detections of influenza and respiratory syncytial virus in children during the 2020 Australian winter[END_REF] .

The observed results could most probably be the consequence of the measures imposed to curb the COVID-19 pandemic such as social distancing, curfews, lockdowns, as well as hygiene measure, which may have had a major impact on the spreading of bronchiolitis viruses, especially the RSV. This draws attention to the role of adults in bronchiolitis virus spreading and suggests that in the future some of the hygiene measures used could be adapted to decrease the bronchiolitis burden in infants. Interestingly, this reduction was observed while schools and nurseries remained open. Nonetheless, it is also plausible that school attendance may have decreased or that infection control measures instituted in schools and nurseries may have prevented viral transmission. Finally, the decrease in PICU admissions for bronchiolitis cannot be explained by a reduction in available pediatric beds during COVID-19 because children have been spared by the pandemic and also because a marked reduction of total visits to pediatric emergencies departments was observed during COVID-19 [START_REF] Angoulvant | COVID-19 pandemic: Impact caused by school closure and national lockdown on pediatric visits and admissions for viral and non-viral infections, a time series analysis[END_REF] . Although the studied period does not encompass the whole bronchiolitis epidemic, this period usually accounts for more than 70% of the entire bronchiolitis season admissions. End of partial lockdown* on December 15th (week 51), followed by a curfew

  shows the comparison of the predicted and actual admissions for September-December 2020. During the entire 5 pre-COVID-19 seasons there were 19 deaths for bronchiolitis and no death during the 4 first months of the COVID-19 season.

Figure 1 .

 1 Figure 1. Monthly actual admissions of infants for bronchiolitis to all PICUs of the Region of Paris during the 5 pre-COVID-19 epidemics of bronchiolitis and predicted admissions for the season 2020-2021. PICU admissions during the 2020-2021 bronchiolitis season were predicted by time series analysis; the R 2 of the model was 0.93. The curves show the actual Pre-COVID-19 admissions, the fit of the admissions model and the predicted admissions for September through December 2020. The exact number of PICU admissions for bronchiolitis and the related deaths during the 5 pre-COVID-19 epidemic seasons are also shown. PICU: Pediatric Intensive Care Unit. In France, each bronchiolitis epidemic season starts in September.

Figure 2 .

 2 Figure 2. Actual and predicted admissions of infants for bronchiolitis to all PICUs of the Region of Paris (around 20% of all France population) during the first 4 months of the 2020-2021 bronchiolitis epidemic. Dates of start of COVID-19 second wave in France, curfew and lockdown are shown. There was an 85.3% reduction between predicted and actual PICU admissions for bronchiolitis. *schools and nurseries for children remained open. PICU: Pediatric Intensive Care Unit.

  second wave started in October (week 40) Two weeks curfew started on October 17th (week 42) Partial lockdown* started on October 30th (week 44)