Nicolas Djeghri

Angus Atkinson

Elaine S Fileman

Rachel A Harmer

Claire E Widdicombe

Andrea J Mcevoy

Louise Cornwell

Daniel J Mayor

High prey-predator size ratios and unselective feeding in copepods: A seasonal comparison of five species with contrasting feeding modes

Keywords: Copepod, zooplankton biomass, feeding, selectivity, intraguild predation, predator-prey size ratio, Western Channel Observatory, sloppy feeding

Introduction

Copepods play a central role in pelagic food webs. They form the link between microplankton and fish, and their feeding activities contribute to global biogeochemical cycles. Copepods can feed on a wide variety of food items, including phytoplankton, microzooplankton [START_REF] Stoecker | Predation on Protozoa: its importance to zooplankton[END_REF][START_REF] Calbet | The ciliate-copepod link in marine ecosystems[END_REF], copepod eggs and nauplii [START_REF] Boersma | Predation of calanoid copepods on their own and other copepods' offspring[END_REF] and detritus [START_REF] Roman | Utilisation of detritus by a copepod, Acartia tonsa[END_REF][START_REF] Iversen | Coprorhexy, coprophagy, and coprochaly in the copepods Calanus helgolandicus, Pseudocalanus elongatus, and Oithona similis[END_REF]. Prey selection by copepods has been studied for decades [START_REF] Brooks | Predation, Body Size, and Composition of Plankton[END_REF][START_REF] Steele | The Structure of Plankton Communities[END_REF] and recent studies [START_REF] Kiørboe | How zooplankton feed: mechanisms, traits and trade-offs[END_REF][START_REF] Kiørboe | Foraging mode and prey size spectra of suspension-feeding copepods and other zooplankton[END_REF] have emphasised two broad types of feeding mechanisms: 1) ambush feeding, which is more effective on motile prey that alert predators to their presence via hydrodynamic disturbance; 2) more active feeding modes, such as cruise-and feeding-current feeding, thought to be more effective against non-motile prey that cannot detect and escape from the movement created by the copepod. The distinction between these modes is increasingly emphasised in "trait-based" modelling approaches [START_REF] Kiørboe | How zooplankton feed: mechanisms, traits and trade-offs[END_REF][START_REF] Mariani | Control of plankton seasonal succession by adaptive grazing[END_REF][START_REF] Litchman | Trait-based approaches to zooplankton communities[END_REF][START_REF] Sailley | Impact of zooplankton food selectivity on plankton dynamics and nutrient cycling[END_REF][START_REF] Kenitz | Seasonal succession in zooplankton feeding traits reveals trophic trait coupling[END_REF], with implications for ecosystem function.

Alongside prey motility, prey size is considered to be a "master trait" for understanding food web dynamics, and an increasing number of studies are exploring the inter-relationships between size and feeding modes in predator-prey interactions (e.g. [START_REF] Fuchs | Plankton community properties determined by nutrients and size-selective feeding[END_REF][START_REF] Wirtz | Who is eating whom? Morphology and feeding type determine the size relation between planktonic predators and their ideal prey[END_REF][START_REF] Visser | Optimal foraging in marine ecosystem models: selectivity, profitability and switching[END_REF][START_REF] Kiørboe | Foraging mode and prey size spectra of suspension-feeding copepods and other zooplankton[END_REF]Stamiesszkin et al., 2017). The component of the available prey size spectrum that is accessible to a predator, and the rate at which it can be ingested, can be represented numerically by a kernel function [START_REF] Fuchs | Plankton community properties determined by nutrients and size-selective feeding[END_REF][START_REF] Wirtz | A biomechanical and optimality-based derivation of prey-size dependencies in planktonic prey selection and ingestion rates[END_REF]. These are important for modelling, but quantifying them is problematic, particularly when using data from grazing experiments in which predators are fed artificial prey assemblages that do not reflect the diversity of their normal prey [START_REF] Wirtz | A biomechanical and optimality-based derivation of prey-size dependencies in planktonic prey selection and ingestion rates[END_REF]. Furthermore the size-based view of feeding is confounded by the other factors such as food motility, nutritional quality (e.g. essential fatty acid content), or stoichiometry, all of which influence selection and in turn growth and egg production [START_REF] Koski | Seasonal changes in food quality of the common North Sea copepods Temora longicornis and Pseudocalanus elongatus: a bioassay approach[END_REF].

These interacting factors affecting selectivity are one reason for seemingly contradictory findings on the prey preferences of individual species derived from field experiments. For example, ecologically important copepods such as Oithona similis and Calanus spp. have been reported to select motile prey [START_REF] Castellani | Feeding of Calanus finmarchicus and Oithona similis on the microplankton assemblage in the Irminger Sea, North Atlantic[END_REF][START_REF] Zamora-Terol | Plankton community structure and role of Oithona similis on the western coast of Greenland during the winter-spring transition[END_REF], non-motile prey [START_REF] Hopkins | Midwater food web in McMurdo Sound, Ross Sea, Antarctica[END_REF][START_REF] Atkinson | Subantarctic copepods in an oceanic, low chlorophyll environment: ciliate predation, food selectivity and impact on prey populations[END_REF], or show no clear feeding preference [START_REF] Mayor | Feeding and reproduction of Calanus finmarchicus during non-bloom conditions in the Irminger Sea[END_REF][START_REF] Mayor | Egg production and associated losses of carbon, nitrogen and fatty acids from maternal biomass in Calanus finmarchicus before the spring bloom[END_REF]. Differences in experimental methods further confuse the picture, making it problematic to parameterise zooplankton feeding in models [START_REF] Mitra | Bridging the gap between marine biogeochemical and fisheries sciences; configuring the zooplankton link[END_REF]. This highlights the need for methodologically consistent studies that compare copepods with ambush and active feeding modes across the naturally occurring spectrum of food types.

Here we present data from copepod grazing experiments in which five biomassdominant copepod taxa were fed natural prey assemblages. These species contrast both in size and feeding mode [START_REF] Benedetti | Identifying copepod functional groups from species functional traits[END_REF] and are well studied; Table 1 summarises some of the more recent work. We performed food removal experiments throughout one year from a stratifying shelf site to assess how copepod feeding behaviour differs according to feeding mode and the prey assemblage offered. Recent work near the site [START_REF] Sailley | Impact of zooplankton food selectivity on plankton dynamics and nutrient cycling[END_REF][START_REF] Kenitz | Seasonal succession in zooplankton feeding traits reveals trophic trait coupling[END_REF] provides a central hypothesis: that seasonal changes in prey motility influence the feeding traits displayed by copepods. We examine firstly whether the classic pattern of succession from non-motile towards motile protists exists in our study area, and secondly, whether this leads to a seasonal pattern in copepods with feeding types that reflect their optimum prey motility, and thirdly, whether selectivity differs substantially between species. We focused attention on the large food items which also allowed us to examine maximum prey:predator ratios and the incidence of feeding on copepod nauplii.

Material and Methods

Western Channel Observatory study location

This study was made at a pair of neighbouring sites in the English Channel forming the "Western Channel Observatory" http://www.westernchannelobservatory.org.uk/ (Smyth et al., 2015, Fig. 1). These seasonally stratifying sites have been sampled intermittently since 1903 [START_REF] Southward | Long-term oceanographic and ecological research in the western English Channel[END_REF] and have comparable spring and autumn bloom dynamics.

The most intensively sampled is L4, which is closest inshore (13 km from Plymouth) in ~54 m water depth. It has been visited weekly (weather permitting) since 1988 and is subject to a comprehensive suite of planktonic measurements.

The offshore site, E1, is 27 km further out in ~75 m water depth and sampled less regularly. This was selected as the main location for the present experimental study because, being further offshore and less subject to coastal influences, it forms a clearer comparison to the dynamics of other open shelf sites described within this Special Issue (Giering et al., in review). However, to test our hypothesis on seasonal succession we have used the much longer time series from nearby L4, which comprises sufficient sampling time points to provide robust generalisations.

Micro-and mesozooplankton seasonality at E1 and L4

The seasonal plankton cycle at site E1 has been estimated with near-monthly sampling during 2014 and 2015 (Fig. 2), while longer-term context has been provided by weekly sampling at site L4 from 1988-2015 (Fig. 3). Microplankton biomass data from both sites is based on CTD Niskin bottle water samples from 10m depth. This is within the upper mixed layer when a thermocline is present; typically from May to September.

At each sampling time-point from E1 and L4, an unfiltered 200 mL water sample was immediately preserved in acid Lugol's iodine (2 % final concentration) for the subsequent enumeration of phyto-and microzooplankton species. A second 200 mL subsample was also preserved in neutral formaldehyde (1 % final concentration) for the enumeration of coccolithophore species. Cell counts were conducted following the European Standard protocol (EN 15204) "Water quality -Guidance standard on the enumeration of phytoplankton using inverted microscopy" (Utermöhl, 1958) technique. Specifically, between 50 mL and 100 mL sub-samples were settled and on average 897 cells were identified and enumerated in each weekly sample from L4; similar numbers of cells were counted from E1.

This exceeds the abovementioned (EN 15204) protocol's recommended limit of 400 cells to be counted for mixed natural samples to obtain a 95% confidence limit. Because of the high diversity in natural populations it is not possible (or recommended) to count 400 individuals for each species. Median abundance values for diatoms, dinoflagellates, ciliates, coccolithophores, flagellates from L4 were 17 mL -1 , 17 mL -1 , 5 mL -1 , 10 mL -1 , 1993 mL -1 respectively. Further details of the light microscopy are provided in [START_REF] Widdicombe | Long-term phytoplankton community dynamics in the Western English Channel[END_REF]. Cell biovolumes for each taxon were calculated assuming appropriate geometric shapes according to [START_REF] Kovala | Computation of phytoplankton cell numbers, cell volume, cell surface area and plasma volume per litre, from microscopical counts[END_REF] using average cell length, width and depth measurements of 10-50 individual cells. Carbon conversions were made using the conversions of [START_REF] Menden-Deuer | Carbon to volume relationships for dinoflagellates, diatoms, and other protist plankton[END_REF]. Median biomass values for diatoms, dinoflagellates, ciliates, coccolithophores, flagellates were 2.33 mgC m -3 , 5.95 mgC m -3 , 3.18 mgC m -3 , 0.18 mgC m -3 , 11.48 mgC m -3 respectively. Nauplii were under-sampled by the 200 µm net and too rare in the Lugols volumes settled. Their biomass was derived from larger volume (2 L) water samples from 4 depths (surface, 10m, 25m and 50m) with the CTD.

These were first pre-screened through 300 µm mesh, concentrated by reverse filtration and analysed on flowCAM to determine naupliar abundances and lengths. These lengths were then converted to biomass via the length-mass relationships in Supplementary Table 1.

Mesozooplankton were collected using a series of vertical net hauls with a UNESCO (1968) standard WP2 nets (57 cm diameter, 200 µm mesh) from either 70 m (E1) or from 50 m depth (L4) to the surface at 0.2 m sec -1 . Two net hauls from each site were preserved in 4 % formalin for microscopic analysis, and one additional haul from E1 was filtered onto a 10 cm square of 200 µm gauze and frozen on board for bulk zooplankton biomass estimates.

Mesozooplankton from the formalin-preserved vertical net hauls were enumerated and identified by microscopy as detailed in [START_REF] Atkinson | Questioning the role of phenology shifts and trophic mismatching in a planktonic food web[END_REF]. Two sub-samples of different size were analysed per sample. The smaller one was extracted with a Stempel pipette for the numerous taxa (including the 5 copepod species examined in this study). Typical subsamples ranged from 1-10ml from the 300 ml original sample. A second, larger aliquot was analysed for rarer and large taxa, typically either 12.5%, 25% or 50%. The number of copepods (excluding nauplii) counted in each weekly sub-sample ranged from 70 to 300 individuals, with a median of 194. Abundances across the two hauls were averaged and numbers expressed as individuals per m 3 allowing for a 95% net efficiency (UNESCO, 1968).

To estimate mesozooplankton carbon biomass from the abundance data we measured 3780 individuals of the more common taxa collected from L4 throughout the 2015 season. Taxon-specific length-mass conversion factors obtained from the literature were applied to the seasonal lengths derived separately for the periods spring (March to May), summer (June to August), autumn (September to November) and winter (December to February). Supplementary Table 1 lists the source references for these conversion factors.

We also obtained dry mass and carbon masses directly from bulk net catches from E1 during 2014 and 2015. This provided an independent check on the method based on length-mass conversions described above. Frozen samples were defrosted, dried for 5 days at 60˚C until reaching constant weight, removed from the oven and placed in a desiccator to weigh for dry mass, prior to CHN analysis of subsamples. For each sample the plankton was removed from the gauze, homogenised and four replicates weighed out for analysis using a

Thermoquest FlashEA 1112 elemental analyser.

Feeding experiments

In total 11 experiments were run at E1, spanning March 2014 to March 2015. The complete experimental setup is summarised in Table 2 and the environmental conditions in Table 3. Sampling was conducted between mid-morning and midday on each visit, and comprised of first a CTD profile and collection of incubation water. This was collected from 10 m depth and was gently drained from the 10m Niskin bottles into a large acid washed and rinsed carboy via silicon tubing through a submerged 200 µm mesh bag, to exclude larger grazers. This water was kept cool and in darkness until return to the Plymouth laboratory within 3 hours of collection. It was then left overnight in the dark at ambient E1 surface temperature.

After the water collection, 0-70 m WP2 net hauls were used to collect zooplankton.

The cod end contents were placed in lidded 5 L containers, topped up with surface seawater and maintained in a flowing water bath while in transit to the laboratory. Immediately on return to the laboratory, actively swimming representatives of the most common copepods were picked out. They were transferred to 0.2 µm filtered seawater and then left overnight to acclimate. In total, adult females of six species were incubated; Oithona similis, Acartia clausi, Pseudocalanus elongatus, Centropages typicus, Calanus helgolandicus, and C.

finmarchicus, as well as Calanus spp. CV. Most of these Calanus incubations were with the dominant species C. helgolandicus (Table 2). However, all results for this genus are presented together simply as "Calanus spp". because the two species and stages are of similar size and feeding types,

On the morning after sampling the incubation water was gently mixed and used to fill 3 glass control bottles of 1.2 L and between 1 and 3 bottles of 0.6 L or 1.2 L, according to copepod size and availability (Table 2). The experimental animals were then checked and those that were intact and actively swimming were added to the bottles. Each bottle was spiked with ammonium chloride (15 µmol L -1 NH 4 Cl) and disodium hydrogen phosphate (1 µmol L -1 Na 2 HPO 4 ) , in order to counter potential artefacts arising from grazer excretion enhancing specific rates of prey growth in the grazed bottles [START_REF] Båmstedt | Chapter 8: Feeding[END_REF]. All bottles were then filled to the top with mixed incubation water and sealed with Parafilm to exclude air bubbles. At T zero , between 1 and 3 (according to the remaining E1 water volume) 500 mL sub-samples were taken from the remaining incubation water and fixed in acid Lugol's iodine solution (2% final concentration). All experimental bottles were then incubated for 24 h on a plankton wheel (0.5 revolutions min -1 ) in the laboratory maintained at ambient E1 temperature and light conditions. Lighting was at estimated average ambient E1 10 m intensity and switched off at dusk and on at dawn.

After 24 h the copepods were first checked for mortality and then 450-500 mL from each bottle was fixed in acid Lugol's solution (2% final concentration) for microplankton community analysis. As an ongoing check on particle removal during the experiments (in order to adjust stocking densities if necessary between experiments) an additional 150 ml subsample from each bottle was filtered onto a GF/F filter and extracted in 90% aqueous acetone. This allowed fluorometric analysis to determine chlorophyll a (chl a) concentrations and the reduction in chl a due to grazing (Table 2). Removal of the copepods from the incubations for species verification was either by pipette from the abovementioned water sub-samples or by sieving them out of the remaining incubation water.

Analysis of feeding experiments

Copepod feeding-induced changes within the microplankton community were estimated by comparing the abundance of large phytoplankton and microzooplankton among the treatments with and without added copepods. Because direct microscope counting of prey taxa is time consuming, we have channelled our resources into the larger end of their food size spectrum. This is because firstly, the rarity of these cells means that they are seldom enumerated in feeding studies, raising questions on the upper size limit of ingestable food [START_REF] Kiørboe | Foraging mode and prey size spectra of suspension-feeding copepods and other zooplankton[END_REF]. Second, large prey are less prone to bottle incubation-induced "food chain effects" than the smaller cells [START_REF] Båmstedt | Chapter 8: Feeding[END_REF]. These large cells are rarer so the lugols-preserved 450-500 ml sub-samples were concentrated by first passing the sample through a 63 µm mesh. The particles collected on the mesh were then washed into a counting chamber and examined and counted at x200 magnification using an Olympus IMT-2 inverted microscope.

All ciliates, dinoflagellates, diatoms and nauplii > 32 µm in length were enumerated.

Four taxonomic groups of prey were identified: diatoms, ciliates, dinoflagellates and nauplii.

Five size classes were used: 32 -95 µm, 95 -220 µm, 220 -346 µm, 346 -472 µm and 472 -598 µm. Nauplii sizes ranged from 50 -390 µm but were not sufficiently numerous to be divided into different size classes. We acknowledge that the counted abundances of the larger and smaller end of this range are likely underestimates of their natural abundances because we screened the pre-and post-experimental seawater through 200 and 63 µm meshes, respectively. Our calculated total ingestion rates within the above size range are thus minimum estimates. However, we contend that since identical 63 µm screening methods were used for all bottles, our clearance rate values on the 32-95 µm spectrum are robust, with particles smaller than the screen size being retained due to their elongation.

Calculation of feeding rates

For consistency with previous studies of grazing in the Western English Chanel [START_REF] Fileman | Grazing by the copepods Calanus helgolandicus and Acartia clausi on the protozooplankton community at station L4 in the western English Channel[END_REF][START_REF] Fileman | Feeding rates and prey selectivity of planktonic decapod larvae in the Western English Channel[END_REF], clearance rates (ml copepod -1 d -1 ) for diatoms, ciliates and dinoflagellates were calculated according to [START_REF] Frost | Effects of size and concentration of food particles on the feeding of the marine planktonic copepod Calanus pacificus[END_REF] equations incorporating the prey growth term:

F = [ln(C cont / C 0 ) -ln(C exp / C 0 )] x (V / (n x t)) (1)
where C exp, C cont and C 0 , are respectively, the concentration (prey. mL -1 ) in the final experimental bottles, in the final control bottles and in the initial bottles. V is the volume of experimental bottles (ml), n the number of copepods in the incubation and t is the incubation duration in days. Following [START_REF] Fileman | Feeding rates and prey selectivity of planktonic decapod larvae in the Western English Channel[END_REF], we chose a threshold mean of 25 food items for each counted category (i.e. based on taxa and size-class), based on mean numbers counted in each of the final control bottles. If values for a food item were below this, then the results are not presented for the experiment. Rare instances where no cells were counted in the grazed bottles were likewise excluded, as these would provide infinite clearance rates.

Concentrations of nauplii tended to be low in the initial samples and higher in the final controls, likely due to hatching of eggs and the absence of predators in the control bottles.

Because low count numbers in the initial samples would introduce imprecision into the clearance rate calculations, we used the clearance rate equation in [START_REF] Båmstedt | Chapter 8: Feeding[END_REF].:

F = ln(C cont / C exp ) x (V / (n x t)) (2)
To calculate ingestion rates, the carbon contents for protistan preys were first calculated from the conversion factors given by Menden-Deuer and [START_REF] Menden-Deuer | Carbon to volume relationships for dinoflagellates, diatoms, and other protist plankton[END_REF] assuming appropriate geometric shapes [START_REF] Kovala | Computation of phytoplankton cell numbers, cell volume, cell surface area and plasma volume per litre, from microscopical counts[END_REF]. For nauplii the lengths were measured and carbon contents were estimated from morphometric relationships.

Average food concentrations in each bottle [C] were calculated according to the equation of [START_REF] Conover | Transformation of organic matter[END_REF], as presented in [START_REF] Båmstedt | Chapter 8: Feeding[END_REF]:

[C] = C 0 x (1 -e g ) x (-g) (3)
where g = ln(C exp / C 0 ) for experimental bottles and C exp, C cont and C 0 , are respectively, the concentration in carbon mass (µg C mL -1 ) in the final experimental bottles, in the final control bottles and in the initial bottles. In some instances there were more cells counted in the final experimental bottles than in the final controls. Those data were set to zero for calculations of average concentration and thence ingestion rate but are shown as negative in all calculations of clearance rates.

From these data ingestion rates (I) were estimated using:

I = F x [C] (4)
Selective feeding was evaluated using the electivity index E i * [START_REF] Vanderploeg | Two electivity indices for feeding with special reference to zooplankton grazing[END_REF] as follows:

E i * = [(W i -1) / k] / [(W i + 1) / k] ( 5
)
where k is the total number of prey types in a given experiment and W i is defined as:

W i = F i / Ʃ F i ( 6
)
F i is the clearance rate on the i th type of food and Ʃ F i is the sum of clearance rates of all food types. The value of this index ranges from 1 to -1, positive values indicating selection and negative values indicating avoidance.

Results

Seasonality at E1 during 2015 and 2016

Our experimental period at E1 spanned March 2014 to March 2015, while the nanomicroplankton seasonality was also sampled throughout the rest of 2015 (Fig. 2). The nanomicroplankton biomass at 10 m depth ranged from ~10 mg C m -3 in winter to ~250 mg C m -3 in spring, and was typically dominated by small (2-6 µm) nanoflagellates. Both 2014 and 2015 (and particularly the latter) diverged from the long-term average (Fig. 3), which supports the hypothesised succession of diatoms in the spring to dinoflagellates after the onset of seasonal thermal stratification [START_REF] Widdicombe | Long-term phytoplankton community dynamics in the Western English Channel[END_REF]. Three diatoms blooms occurred in 2014, one in spring (April-May), the second two in autumn (September and October). By contrast 2015 had no clear spring bloom but instead showed major autumn blooms dominated by autotrophic dinoflagellates.

Mesozooplankton abundance varied from 276 ind. m -3 to 3744 ind. m -3 , with an average of 1490 ind. m -3 . Our estimates of their seasonal median biomass based on lengthmass relationships was 4.5 mg.C m -3 ; similar to the value of 5.4 mg.C m -3 based on CHN analysis of the bulk samples (Table 3). However, maximum values from bulk CHN analysis are unrealistically high, due to clogging of the 200 µm nets with diatoms. Notwithstanding these uncertainties of determining biomass, either with length-mass relationships or with CHN, both methods agree that the values at E1 were noticeably less than at other English Channel/Celtic Sea sites sampled over the same time period (Table 4, see also Giering et al., in review, this volume).

Mesozooplankton biomass was dominated by copepods (mean of 75 % of the total; Fig. 2c), and was not correlated with microplankton biomass (Linear regression, p = 0.34).

The 5 copepod taxa studied here represented, on average, 46 % of the total copepod biomass (Fig. 2d, Table 3).

Average plankton seasonality at L4

The long term average picture (Fig. 3) reveals the highest proportion of diatoms from March to June, with an increased proportion of motile cells thereafter. However, there is substantial inter-annual variation in plankton composition and phenology at this site [START_REF] Atkinson | Questioning the role of phenology shifts and trophic mismatching in a planktonic food web[END_REF]. For example diatoms can bloom in autumn and ciliates also increase sharply in spring, and nanoflagellates persist at substantial levels throughout the season [START_REF] Widdicombe | Long-term phytoplankton community dynamics in the Western English Channel[END_REF][START_REF] Atkinson | Questioning the role of phenology shifts and trophic mismatching in a planktonic food web[END_REF].

The L4 and E1 time series do not provide strong support for the hypothesis (Sailley et al., 2013;[START_REF] Kenitz | Seasonal succession in zooplankton feeding traits reveals trophic trait coupling[END_REF] that the seasonality of copepods is congruent with their feeding traits (i.e. that suspension feeders appear during the time of diatom blooms and ambush feeders appear when motile cells are most abundant). In accordance with this hypothesis, Pseudocalanus elongatus peaks during the spring diatom bloom whereas the more carnivorous Centropages typicus peaks in September, when its preferred motile prey is most abundant. By contrast, both the elevated abundance of ambush Oithona similis in spring and the summer abundance maximum of Calanus spp. are counter to the hypothesised relationship between prey motility and the predominant copepod feeding mode.

Copepod feeding experiments: prey composition and potential for artefacts

The larger prey studied in our experiments (nauplii excluded) ranged from 0.32-46% (median 2.8%) of the total microplankton biomass (Table 3), so given these low values we have not expressed our results in terms of total daily ration estimates. Ciliates and dinoflagellates comprised the smaller end of this large food spectrum, with the majority of ciliates belonging to the genera Strombidium spp., Mesodinium sp, and Askenasia sp. and dinoflagellates consisting mainly of Gyrodinium spp., Dinophysis spp. and Protoperidinium spp. Some larger ciliates and dinoflagellates were observed (e.g. Ceratium spp.) but they were never above our counting threshold for inclusion in calculations. Diatoms were the only prey category occurring across all our size-classes. These were mainly composed of large, elongated chain-forming diatoms such as Rhizosolenia spp. Their size categories refer to the chains lengths, not the individual cells. The concentrations of prey items sufficiently numerous to provide clearance rates are presented in Supplementary Fig. 1.

The percentage removal of total chl a in our incubations was relatively low overall (Table 2), possibly reflecting the finding that some of the chl a resides within cells that are too small to be eaten. However in two of the incubations (of Calanus and Oithona in experiment 4) the removal exceeded 50%. These two incubations are not presented here, due to their potential for unrealistic indications of grazing dynamics [START_REF] Båmstedt | Chapter 8: Feeding[END_REF]. The median reduction of other cells was substantially greater, ranging from 25-41% (Table 2) and reflecting the high rates of removal of the largest cells.

Copepod feeding selectivity

For the large particle fraction examined, Fig. 4 summarises the contribution of prey types to the available food and to the diets of the 5 copepod species. The available food varied greatly throughout the experiments with variable biomass dominance of long diatoms, ciliates or nauplii. The main feature of Fig. 4 is that the diet of the species broadly reflected the composition of the available food, in other words the seasonal variation in prey field had a much clearer effect on diet than the identity of the grazer.

The estimated rates at which the five copepod species cleared each of the identified prey items are presented in detail in Supplementary Fig. 2. Maximal significant clearance rates varied from 31 mL cop -1 d -1 for Oithona similis to 523 mL cop -1 d -1 for Calanus spp.

These species tended to clear diatoms at the highest rates, with the exception of Centropages typicus, whose maximal clearance rates were on ciliates. For Acartia clausi, Oithona similis and Centropages typicus, clearance rates on diatoms tended to be higher when the prey was large.

The feeding preferences of the 5 copepods are presented for each predator-prey combination in terms of electivity indices (Supplementary Fig. 3), and summarised by comparing the proportional contribution of available and ingested prey in each experiment (Fig. 5). The overall pattern was for predominantly unselective feeding, with copepod dietary composition tending to be proportional to prey availability. However, instances of significant, positive selection (t-test p<0.05) for both motile (dinoflagellates, ciliates or nauplii) and nonmotile (diatoms) prey were identified (Supplementary Fig. 3). Acartia clausi, Oithona similis

and Centropages typicus all showed a preference for ciliates and their selectivity towards diatoms typically increased as a function of cell size (Supplementary Fig. 3). These tendencies were less apparent in Calanus spp. and Pseudocalanus elongatus, both of which displayed instances both of positive and negative selection towards ciliates and diatoms.

Fig. 6 provides a summary overview of the differences in selectivity among the species.

Overall, the contribution of non-motile cells (i.e. diatoms) to diets decreased from Calanus spp. (94%), Pseudocalanus elongatus (91 %), Acartia clausi (62 %), Oithona similis (57%),

Centropages typicus (39 %) (Fig. 6a). However an ANOVA failed to identify significant (p < 0.05) inter-species differences, supporting the view depicted in Fig. 4 of a degree of unselective feeding, such that the diet strongly reflected the food composition offered. Fig. 6b shows that, for the prey cells enumerated, the maximum prey/predator length ratio ranged from 0.14 to 0.51. The order of species along this spectrum was similar to the contribution of non-motile diatoms to the diet (Fig. 6a); for example the suspension feeders Pseudocalanus elongatus and Calanus spp. had the lower maximum prey/predator ratio and the ambushing

Oithona similis the highest ratio.

Discussion

In combination, our findings provide little support for a clear, predictable seasonal succession of traits displayed by copepods in relation to their microplankton prey. First, we found highly variable patterns of plankton succession between years, with no clear link between the seasonality of ambush or feeding current copepods and the respective motile or non-motile preys. Second, we found reduced selectivity but considerable dietary diversity, including very high maximum prey sizes and ingestion of copepod nauplii. In combination, these factors blur the hypothesised coupling between microplankton motility and copepod feeding selectivity [START_REF] Mariani | Control of plankton seasonal succession by adaptive grazing[END_REF][START_REF] Sailley | Impact of zooplankton food selectivity on plankton dynamics and nutrient cycling[END_REF][START_REF] Kenitz | Seasonal succession in zooplankton feeding traits reveals trophic trait coupling[END_REF]. Below we discuss our main feeding results, namely reduced feeding selectivity of copepods, their ability to eat or fragment even very large diatoms and the role of intra-guild predation.

Selection for motile and non-motile cells

Ambush-feeding copepods such as Oithona similis locate their prey by detecting hydro mechanical signals, suggesting that they are best suited to catching motile prey [START_REF] Paffenhöfer | On the ecology of marine cyclopoid copepods (Crustacea, Copepoda)[END_REF][START_REF] Kiørboe | How zooplankton feed: mechanisms, traits and trade-offs[END_REF]. Conversely, those that can create feeding currents, such as Calanus helgolandicus, are considered to be more suited to catching the non-motile prey that cannot detect and escape from these currents. Our data broadly support these assertions, since the diet of Oithona similis contained the highest proportion of motile prey, and that of Calanus spp. contained the lowest (Fig. 6 a). However, both species also displayed statistically significant preference towards ciliates at times and conversely, all five species of copepods showed at least one significant instance of preference for diatoms in the experiments. Overall the diets were diverse, reflecting the composition of the available food (Figs. 4,5). Our results thus provide only partial support for the generalisation that ambush feeders select for motile cells while more active feeding modes select for non-motile cells, so we investigate this further using the example of Oithona similis.

Previous studies have also reported the ingestion of diatoms by Oithona similis [START_REF] Hopkins | Midwater food web in McMurdo Sound, Ross Sea, Antarctica[END_REF][START_REF] Atkinson | Subantarctic copepods in an oceanic, low chlorophyll environment: ciliate predation, food selectivity and impact on prey populations[END_REF][START_REF] Castellani | Feeding of Calanus finmarchicus and Oithona similis on the microplankton assemblage in the Irminger Sea, North Atlantic[END_REF], confirming that these ambush-feeders are indeed capable of detecting and ingesting non-motile cells. Their positive selection for diatoms >300 μm in length is consistent with the physics of fluid disturbance;

Oithona similis is expected to be able to detect sinking, immobile phytoplankton cells ≥ 80 μm in diameter [START_REF] Kiørboe | Predator and prey perception in copepods due to hydromechanical signals[END_REF]. We therefore speculate that small, ambushfeeding copepods such as Oithona spp. have a bimodal distribution of clearance rates in relation to prey size, with the smaller peak reflecting the ingestion of motile prey that is not large enough to escape from the ambusher, and the larger peak occurring where larger, nonmotile cells become detectable (e.g. Experiment 5 in Supplementary Fig. 2). Notwithstanding the mechanisms involved, the example of Oithona removing large diatoms will contribute to a blurring of the selectivity and enhance the generalist feeding abilities of copepods.

How does our suggestion of limited food selectivity fit with a multitude of studies showing clear selective abilities? Perhaps the method used is critical here, because the large majority of studies that find, like us, a degree of unselective feeding with respect to motility have all used the prey removal method in incubations with natural seston (e.g. [START_REF] Poulet | Comparison between five coexisting species of marine copepods feeding on naturally occurring particulate matter[END_REF][START_REF] Huntley | Nonselective, nonsaturated feeding by three calanoid copepod species in the Labrador Sea[END_REF][START_REF] Atkinson | Omnivory and feeding selectivity in five copepod species during spring in Bellingshausen Sea, Antarctica[END_REF][START_REF] Mayor | Feeding and reproduction of Calanus finmarchicus during non-bloom conditions in the Irminger Sea[END_REF][START_REF] Castellani | Feeding of Calanus finmarchicus and Oithona similis on the microplankton assemblage in the Irminger Sea, North Atlantic[END_REF][START_REF] Isari | Copepod foraging on the basis of food nutritional quality: can copepods really choose?[END_REF].

This contrasts strongly with reductionist-type studies which, by offering formulated diets under controlled conditions, have indicated the presence of particle selection on the basis of motility, taste etc. (e.g. [START_REF] Marshall | Respiration and feeding in copepods[END_REF][START_REF] Conover | Transformation of organic matter[END_REF][START_REF] Strickler | Calanoid Copepods, Feeding Currents, and the Role of Gravity[END_REF]. These conflicting findings may arise partly from differences in prey abundances; copepods feeding on highly

diverse, but much diluted prey in the natural world cannot afford or do not need to be too selective.

This discrepancy raises some important issues for modelling zooplankton feeding.

Trait-based models, for example, provide a mechanistic basis for understanding predatorprey interactions. They can address mechanisms behind the succession of zooplankton feeding mode and prey motility [START_REF] Mariani | Control of plankton seasonal succession by adaptive grazing[END_REF][START_REF] Sailley | Impact of zooplankton food selectivity on plankton dynamics and nutrient cycling[END_REF][START_REF] Kenitz | Seasonal succession in zooplankton feeding traits reveals trophic trait coupling[END_REF],

and have provided the central hypothesis for this present study. However zooplankton feeding can also be interpreted and modelled in other ways, for instance according to prey size or stoichiometry (e.g. [START_REF] Fuchs | Plankton community properties determined by nutrients and size-selective feeding[END_REF][START_REF] Wirtz | Who is eating whom? Morphology and feeding type determine the size relation between planktonic predators and their ideal prey[END_REF][START_REF] Wirtz | A biomechanical and optimality-based derivation of prey-size dependencies in planktonic prey selection and ingestion rates[END_REF][START_REF] Mitra | Bridging the gap between marine biogeochemical and fisheries sciences; configuring the zooplankton link[END_REF][START_REF] Stamieszkin | Zooplankton grazing and egestion shifts particle size distribution in natural communities[END_REF]. This makes it important to determine the degree of food selectivity that zooplankton show within natural food assemblages.

Selection according to cell size

In a meta-analysis of predator-prey ratios in plankton, [START_REF] Hansen | The size ratio between planktonic predators and their prey[END_REF] found an

optimal ratio for copepods of 18:1. However, our study suggests that even small copepods can display near maximal clearance rates on prey that are much longer relative to their body size. Our equivalent upper predator:prey length ratios range from 6:1 to 2:1 (depending on species). The ability of copepods to tackle large prey has also been found in previous studies [START_REF] Lampitt | Carnivorous feeding by a small marine copepod[END_REF][START_REF] Atkinson | Diets and feeding selectivity among the epipelagic copepod community near South Georgia in summer[END_REF][START_REF] Calbet | The feeding ecology of the copepod Centropages typicus (Kröyer)[END_REF]. The absolute maximum lengths of food items that the copepods can remove are likely to be even higher than those we present, because we only calculated values for prey items that decreased significantly in abundance over the experiments. Other, larger prey were present but were not sufficiently abundant to discern robustly the changes in their numbers. Our data add to the evidence [START_REF] Kiørboe | Foraging mode and prey size spectra of suspension-feeding copepods and other zooplankton[END_REF][START_REF] Atkinson | Sardine cycles, krill declines and locust plagues: revisiting "wasp-waist" food webs[END_REF]) that questions the use of single fixed predator:prey ratios.

While our experiments demonstrate that small copepods are capable of removing diatoms colonies 600 μm long, it is unclear whether these are eaten intact or fragmented. For example, a brittle prey item, such as a diatom colony half of a copepod's body length, could easily break up during handling. Previous observations of copepod feeding have revealed that, whilst they can successfully feed on large, elongated diatom colonies, most is ultimately lost because of handling difficulties [START_REF] Vanderploeg | Diaptomus vs. net phytoplankton: effects of algal size and morphology on selectivity of a behaviourally flexible, omnivorous copepod[END_REF]. [START_REF] Svensen | Production of dissolved organic carbon by Oithona nana (Copepoda: Cyclopoida) grazing on two species of dinoflagellates[END_REF] recently showed that sloppy feeding of Oithona nana on a dinoflagellate released 6-15% of carbon egested as DOC, so DOC release could also be substantial when copepods tackle large diatoms.

Overall, the capture and partial fragmentation of very large particles by copepods is increasingly becoming recognised for its consequences on nutrient fluxes (Noji et al., 1991, Iversen and[START_REF] Iversen | Coprorhexy, coprophagy, and coprochaly in the copepods Calanus helgolandicus, Pseudocalanus elongatus, and Oithona similis[END_REF][START_REF] Mayor | Microbial gardening in the ocean's twilight zone: Detritivorous metazoans benefit from fragmenting, rather than ingesting, sinking detritus[END_REF][START_REF] Anderson | The role of microbes in the nutrition of detritivorous invertebrates: A stoichiometric analysis[END_REF]. It seems likely that copepod feeding activities could also influence the size of phytoplankton colonies through fragmentation. [START_REF] Bergkvist | Grazer-induced chain length plasticity reduces grazing risk in a marine diatom[END_REF] showed that diatom colonies tend to reduce their length when they are exposed to chemical cues derived from copepods, suggesting an evolutionary response to grazing pressure. Irrespective of the mechanisms involved, it is clear that large but rare food items can be important for copepods and represent an understudied aspect of their trophic ecology [START_REF] Gifford | Protozoa in the diets of Neocalanus spp. in the oceanic subarctic Pacific[END_REF].

Importance of intraguild predation.

The role of nauplii as prey items for copepods remains poorly understood, partly because few studies have quantified feeding rates on them and even fewer have offered them alongside the natural seston in feeding experiments [START_REF] Sells | Predation by omnivorous copepods on early developmental stages of Calanus finmarchicus and Pseudocalanus spp[END_REF][START_REF] Bonnet | Calanus the cannibal[END_REF][START_REF] Boersma | Predation of calanoid copepods on their own and other copepods' offspring[END_REF]. In our experiments significant positive selection for nauplii was only observed for Centropages typicus and only in experiment 6. Given the size and suggested weak escape responses, this apparent lack of clear selection for nauplii was surprising.

All the copepod species investigated here were found to be capable of feeding on copepod nauplii, and in several experiments these formed important contributions to the large particle component of the diet. Given the biomass dominance of copepods (Fig 2c)

these results support the view that copepods could be an important mortality agent for their own naupliar stages [START_REF] Lampitt | Carnivorous feeding by a small marine copepod[END_REF][START_REF] Bonnet | Calanus the cannibal[END_REF][START_REF] Boersma | Predation of calanoid copepods on their own and other copepods' offspring[END_REF]. The fact that even the small copepods such as Oithona similis were feeding on nauplii emphasises that multiple trophic levels exist within relatively small increments of size.

Concluding remarks

This diversity of food types and sizes ingested by copepods reflects their wide range of ecological and biogeochemical roles. Irrespective of the subsequent fate (fragmentation or ingestion), the removal of particles larger than their own faecal pellets adds to the evidence that copepods do not simply repackage small particles into larger, faster sinking pellets [START_REF] Iversen | Coprorhexy, coprophagy, and coprochaly in the copepods Calanus helgolandicus, Pseudocalanus elongatus, and Oithona similis[END_REF]. Reduced selectivity has other, fundamental, ramifications. For the grazers, it would increase resilience to highly unpredictable seasonality of prey resources (Fig. 2, [START_REF] Mazzocchi | Stability and resilience in coastal copepod assemblages: the case of the Mediterranean longterm ecological research at Station MC (LTER-MC)[END_REF]. Coincidental ingestion of non-food items also has a series of implications. For example copepods also consume lithogenic sediment particles [START_REF] Paffenhöfer | The feeding response of a marine planktonic copepod to quantity and quality of particles[END_REF][START_REF] Arendt | Effects of suspended sediments on copepods feeding in a glacial influenced sub-Arctic fjord[END_REF] and acidic digestion in the gut can mobilise the attached iron to enhance productivity [START_REF] Schmidt | Zooplankton gut passage mobilises lithogenic iron for ocean productivity[END_REF]. Some studies have found that inert particles are selected against compared to nutritious ones (e.g. [START_REF] Paffenhöfer | The feeding response of a marine planktonic copepod to quantity and quality of particles[END_REF] whereas interestingly, recent microplastic studies emphasise the fact that copepods can and do ingest these inert particles [START_REF] Cole | Microplastic ingestion by zooplankton[END_REF]. Overall, the natural diversity of possible prey needs to be taken into account when interpreting copepod feeding.

The variety of conclusions arising from zooplankton feeding studies over the last century raises some fundamental questions over how to incorporate this process into foodweb models. On one hand, evidence of copepods feeding unselectively seems contrary to models stressing selection based on optimal size, motility or nutritional quality [START_REF] Litchman | Trait-based approaches to zooplankton communities[END_REF][START_REF] Mitra | Bridging the gap between marine biogeochemical and fisheries sciences; configuring the zooplankton link[END_REF][START_REF] Kenitz | Seasonal succession in zooplankton feeding traits reveals trophic trait coupling[END_REF]). On the other hand, size-based models can have difficulties with the parametrization of their kernel function [START_REF] Wirtz | A biomechanical and optimality-based derivation of prey-size dependencies in planktonic prey selection and ingestion rates[END_REF], often emphasizing an "optimal" size of prey by characterising a unimodal kernel function. Our study shows that copepods are able to process very large prey with high clearance rates, questioning the extent of the unimodal feeding kernels. We speculate that, when large enough prey cells are available, this function could be bimodal, particularly for ambush feeders. Overall this study adds to the evidence that we should encapsulate the natural diversity of particle types, sizes and trophic levels into what we count as copepod food.

Acknowledgements:

Table1: Size, feeding mode, and prey according to the literature for the five copepod genera studied here. The third and fourth column are not an exhaustive reviews, but gives consensual findings for our particular study species and then, when available, some different findings.

Genus

Female length (mm)*

Feeding mode Prey

Calanus 2.4 -3.9 Mainly feeding-current feeding?

Diatoms selected or omnivorous [START_REF] Irigoien | Feeding selectivity and egg production of Calanus helgolandicus in the English Channel[END_REF][START_REF] Fileman | Grazing by Calanus helgolandicus and Para-Pseudocalanus spp. on phytoplankton and protozooplankton during the spring bloom in the Celtic Sea[END_REF][START_REF] Leiknes | Feeding selectivity of Calanus finmarchicus in the Trondheimsfjord[END_REF], but some records of selection for large motile prey [START_REF] Fileman | Grazing by the copepods Calanus helgolandicus and Acartia clausi on the protozooplankton community at station L4 in the western English Channel[END_REF].

Pseudocalanus 0.93 -1.77 Feeding-current feeding [START_REF] Tiselius | Sensory capabilities and food capture of two small copepods, Paracalanus parvus and Pseudocalanus sp[END_REF] Generally opportunistic eating on the most abundant food [START_REF] Cotonnec | Nutritive value and selection of food particles by copepods during a spring bloom of Phaeocystis sp. In the English Channel, as determined by pigment and fatty acid analyses[END_REF][START_REF] Fileman | Grazing by Calanus helgolandicus and Para-Pseudocalanus spp. on phytoplankton and protozooplankton during the spring bloom in the Celtic Sea[END_REF][START_REF] Cleary | Feeding by Pseudocalanus copepods in the Bering Sea: Trophic linkages and a potential mechanism of niche partitioning[END_REF] Acartia 0.81 -1.47 Feeding-current and ambush feeding [START_REF] Kiørboe | Prey switching behaviour in the planktonic copepod Acartia tonsa[END_REF] Protozooplankton mainly [START_REF] Wiadnyana | Selective feeding of Acartia clausi and Centropages typicus on microzooplankton[END_REF][START_REF] Fileman | Grazing by the copepods Calanus helgolandicus and Acartia clausi on the protozooplankton community at station L4 in the western English Channel[END_REF] but records of feeding on phytoplankton [START_REF] Cotonnec | Nutritive value and selection of food particles by copepods during a spring bloom of Phaeocystis sp. In the English Channel, as determined by pigment and fatty acid analyses[END_REF] Oithona 0.68 -0.96 Strictly ambush feeding [START_REF] Paffenhöfer | On the ecology of marine cyclopoid copepods (Crustacea, Copepoda)[END_REF] Ciliates mainly [START_REF] Atkinson | Omnivory and feeding selectivity in five copepod species during spring in Bellingshausen Sea, Antarctica[END_REF][START_REF] Nakamura | Predation and respiration by the small cyclopoid copepod Oithona similis: How important is feeding on ciliates and heterotrophic flagellates[END_REF][START_REF] Castellani | Feeding and egg production of Oithona similis in the North Atlantic[END_REF]Saiz et al., 2007;[START_REF] Zamora-Terol | Plankton community structure and role of Oithona similis on the western coast of Greenland during the winter-spring transition[END_REF] But records of feeding on diatoms [START_REF] Hopkins | Midwater food web in McMurdo Sound, Ross Sea, Antarctica[END_REF][START_REF] Atkinson | Subantarctic copepods in an oceanic, low chlorophyll environment: ciliate predation, food selectivity and impact on prey populations[END_REF] Centropages 1.6 -2.0 Feeding-current and ambush feeding [START_REF] Cowles | Characterization of feeding activity patterns in the planktonic copepod Centropages typicus Kröyer under various food conditions[END_REF] Omnivorous with a preference for large and motile prey [START_REF] Wiadnyana | Selective feeding of Acartia clausi and Centropages typicus on microzooplankton[END_REF][START_REF] Calbet | The feeding ecology of the copepod Centropages typicus (Kröyer)[END_REF] *Maximum total body length based on the species used for the present study: data from Conway (2012) * Biomasses have been obtained from lugols cell counts and conversion factors. Cell dimensions were converted to volumes based on [START_REF] Kovala | Computation of phytoplankton cell numbers, cell volume, cell surface area and plasma volume per litre, from microscopical counts[END_REF] and thence to carbon using [START_REF] Menden-Deuer | Carbon to volume relationships for dinoflagellates, diatoms, and other protist plankton[END_REF].

** Biomasses obtained mainly from seasonal length measurements and conversions using literature lengthmass relationships. Values in brackets refer to values derived from dry mass and carbon mass determination on a separate net haul, often including phytoplankton as well as zooplankton.

Fig 1

 1 Figure captions :

Fig. 2 .

 2 Fig. 2. Environmental conditions at E1 during the two year study including the times of the 11

Fig 3 .

 3 Fig 3. L4 station medians, quartiles and ranges of variation in nano-microplankton biomass

Fig 4 .

 4 Fig 4. Proportion in terms of percentage biomass of available large food (top panel) and the

Fig 5

 5 Fig 5 Numerical contribution of food to the diet plotted against its numerical proportion of

Table 3 :

 3 Environmental conditions at the site E1 at the time of our experiments.

	Experiment number 1	2	3	4	5	6	7	8	9	10	11
	Date	12/03/2014 24/04/2014 14/05/2014 17/06/2014 02/07/2014 22/07/2014 16/09/2014 14/10/2014 18/11/2014 10/02/2015 10/03/2015
	Temperature (°C) 9.73	10.78	11.34	16.34	17.69	18.72	16.09	15.31	13.66	9.97	9.65
	Biomass of	14.1	45.76	246.49	28.91	44.09	18.61	105.28	14.45	5.58	10.14	21.2
	microplankton
	*(mgC.m -3 )
	% of this biomass	N/A	0.32	0.87	3.36	14.32	2.76	0.18	0.19	N/A	46.22	29.25
	enumerated in the
	large food category
	Biomass of	1.03	1.47	4.74	4.93	3.24	4.5	8.01	6.12	0.75	4.01	7.26
	mesozooplankton**
	(mgC.m -3 )	(0.89)	(5.13)	(9.12)	(5.18)	(1.79)		(14.75)	13.99)	(5.62)	(3.89)	(31.16)
	% of this biomass	17	19.43	52.9	27.81	29.18	32.71	25.47	12.16	9.19	13.8	12.66
	represented in the
	experiments
	Biomass copepods	0.92	0.66	4.29	2.61	1.07	2.71	7.29	5.23	0.57	3.19	6.24
	(mgC.m -3 )
	% of this biomass	19.12	43.05	58.5	52.49	88.06	54.27	27.98	14.23	11.96	17.33	14.74
	represented in the
	experiments

Table 4 .

 4 Mesozooplankton biomasses during this study, as compared to 2014-2015 values reported during parallel, near full-depth sampling in the Celtic sea reported byGiering et al. (this issue)

	Site	Median	Seasonal range of	Sampling	Sampling period	Method Notes
		mesozooplankton	mesozooplankton	depth
		biomass (mg C m -3 )	biomass (mg C m -3 )	(m)
	E1	4.5	0.75-8.0	0-70	March2014-March 2015: see	Use of length mass
					Table 3	conversions derived for L4
						site*
	E1	5.4	0.89-32*	0-70	March2014-March 2015: see	Weighing and C analysis of
					Table 3	separate catches
	L4	9.9	2.2-53**	0-50	45 sampling time-points spanning	Use of length mass
					March 2014-March 2015	conversions derived for L4
						site*
	Central Celtic
	Sea: CCS
	site

We first thank the crews of the Plymouth Quest for the collection of samples at the E1 site during our field seasons, and Joanna Nunes and Sarah Dashfield for their help with sample collection in 2015. We are also indebted to the crews and scientists for supporting the ongoing L4 time series that provides the context for our seasonal observations. We are grateful to Denise Cummings who analysed the C and N contents of the zooplankton samples. This work was funded jointly through the NERC and Department for Environment, Food and Rural Affairs, Shelf Seas Biogeochemistry Programme and the Marine Ecosystems Research Programme (Grant nos. NE/K001833/1, NE/K001833/2, NE/L003279/1).

Fig. 6 Summary of copepod feeding selectivity across all available experiments. a: the "diatom index", defined as the mean proportion of non-motile prey (diatoms) in the ingestion rate (number of prey cop -1 (mean ± CI 95%). b: Maximum Prey/predator ratio (length of the largest prey on which we found a significant positive clearance rate (t-test, p<0.05) divided by the length of the predator). Copepod lengths are from Conway (2012).

Supplementary Fig 1

Concentration of food items that were above the threshold for inclusion in feeding rate calculations. Points are means, with bars representing 95% confidence intervals.

Supplementary Fig. 2 Clearance rates (mL.cop -1 .d -1 , median ± range across the replicate experimental bottles)

versus prey length (µm; horizontal bars show the range of the size-classes). Black stars indicate clearance rates significantly different from 0 (t-test p<0.05). In experiments 1 and 9 none of our large food item categories reached the threshold so these experiments are omitted. Note that rarity of these large cells precludes precise estimates of the concentration in the incubation bottles, contributing to the sometimes large range in clearance rates between replicate bottles.

Supplementary Fig 3

Median and range values for Electivity index [START_REF] Vanderploeg | Two electivity indices for feeding with special reference to zooplankton grazing[END_REF] McLaren [START_REF] Mclaren | Population and production ecology of zooplankton in Ogac Lake, a landlocked fiord on Baffin Island[END_REF][START_REF] Larson | Water content, organic content, and carbon and nitrogen composition of medusae from the northeast Pacific[END_REF][START_REF] Mizdalski | Weight and length data of zooplankton in the Weddell Sea in austral spring 1986[END_REF], Daan1989, Clarke et al. 1992[START_REF] Mutlu | Distribution and abundance of ctenophores, and their zooplankton food in the Black Sea. I. Pleurobrachia pileus[END_REF][START_REF] Gibson | Feeding and growth rates of the doliolid, Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea)[END_REF][START_REF] Båmstedt | Significance of food type for growth of ephyrae Aurelia aurita (Scyphozoa)[END_REF][START_REF] Persad | Abundance, biomass and production of ctenophores and medusae off Kingston, Jamaica[END_REF][START_REF] Bastian | Ecosystem relevance of variable jellyfish biomass in the Irish Sea between years, regions and water types[END_REF]Other rarer, non-copepod taxa Little and Copley 2003[START_REF] Mcconville | Disentangling the counteracting effects of water content and carbon mass on zooplankton growth[END_REF] Supplementary Table 1 references