Mélanie Ourgaud
email: melanie.ourgaud@mio.osupytheas.fr

François Carlotti
email: francois.carlotti@mio.osupytheas.fr

Keywords:

Food quality is recognized as a key parameter of food web functioning in which zooplankton plays a crucial role not only in linking lower to upper trophic levels but also in transforming the quality of the organic matter available to predators. The influence of size and taxonomic group composition of zooplankton in these processes was assessed in eastern Kerguelen waters (Southern Ocean) at the onset of the spring bloom in 2011. Biochemical (lipids, proteins and carbohydrates) and elemental (carbon and nitrogen) composition were measured in five size-fractions of bulk zooplankton ranging from 80 µm to > 2000 µm and in large copepods, euphausiids, annelids and salps, and energy content was derived from biochemical contents. Proteins were the dominant component of zooplankton dry weight (21.5% dw), followed by lipids (8.9% dw), soluble carbohydrates (2.2% dw) and insoluble carbohydrates (1.0% dw). A concentration increase with zooplankton size for all biochemical components was observed, particularly stronger for proteins and lipids. Copepods and salps provided, respectively, the highest and the lowest amount of lipids and energy. A four-fold increase in energy content was observed from the smallest to the largest fraction inducing a significant increase (> 1 1 2 Page 2 sur 42 e.Proofing 21/01/2019 http://eproofing.springer.com/journals_v2/printpage.php?token=yVZa0lxp-10rN8EC...

10 kJ g dw) in the quality of zooplankton matter. This may explain why large zooplankton represent a major food resource for numerous fish, seabirds and marine mammals in the Southern Ocean. Such unique results are required to better quantify energy dynamics in polar food webs.

Introduction

Zooplankton plays a central role in ecosystem functioning and food webs in transforming and transferring the organic matter from lower (phytoplankton sensu lato) to higher trophic levels (zooplanktivorous carnivores) up to large predators [START_REF] Banse | Zooplankton: pivotal role in the control of ocean production[END_REF][START_REF] Frederiksen | From plankton to top predators: bottom-up control of a marine food web across four trophic levels[END_REF]). In the Southern Ocean, zooplankton constitutes a huge biomass of prey consumed not only by small planktivorous fishes such as myctophids, melamphaids and gempylids [START_REF] Pakhomov | Prey composition and daily rations of myctophid fishes in the Southern Ocean[END_REF][START_REF] Cherel | Isotopic niches and trophic levels of myctophid fishes and their predators in the Southern Ocean[END_REF] but also by a number of seabirds and marine mammals (Cherel et al. 2002[START_REF] Cherel | Feeding habits of seabirds and marine mammals of the Kerguelen Archipelago[END_REF]. The Kerguelen Islands support large colonies of landbased seabirds, seals and elephant seals that forage directly on zooplankton or zooplanktivorous consumers [START_REF] Guinet | Consumption of marine resources by seabirds and seals in Crozet and Kerguelen waters: Changes in relation to consumer biomass. 1962-1985[END_REF]Bocher et al. 2002;[START_REF] Hindell | Foraging habitats of top predators, and areas of ecological significance, on the Kerguelen plateau[END_REF]. Meso-and macrozooplankton copepods, euphausiids and amphipods may each represent up to 50 -80% of the food ingested by different species of diving seabirds, such as petrels and penguins [START_REF] Ridoux | The diets and dietary segregation of seabirds at the subantarctic Crozet Islands[END_REF]Bocher et al. 2001;[START_REF] Cherel | Feeding habits of seabirds and marine mammals of the Kerguelen Archipelago[END_REF].

Many studies have underlined the importance of considering predator size-prey size relationships to describe properly the length, trophic structure and functioning of food webs, and to predict their dynamics and evolution under the climate change [START_REF] Barnes | Global patterns in predator-prey size relationships reveal size dependency of trophic transfer efficiency[END_REF]Blanchard -1 et al. 2017). In pelagic ecosystems, predator-prey size relationships are also of major importance within planktonic communities among species and developmental stages of organisms, particularly for enhancing biomass trophic transfer to consumers [START_REF] Hansen | The size ratio between planktonic predators and their prey[END_REF]Garcia-Comas et al. 2016). If zooplankton were previously considered as a single functioning entity in early food web models, size-based groups of zooplanktonic organisms are now entered in ecosystem modeling (Carlotti et al. 2000;[START_REF] Travers-Trolet | Towards endto-end models for investigating the effects of climate and fishing in marine ecosystems[END_REF]. Three groups of zooplankton based on taxonomy, trophic level and maximum size were included in an ecosystem model of the Kerguelen Islands waters [START_REF] Pruvost | An ecosystem model of the Kerguelen Islands'EEZ[END_REF]). But size is not the only important parameter. To ensure a true understanding of planktonic food webs, it is necessary to consider the abundance, biomass and composition of the different size fractions of zooplankton, along with the environmental drivers of their population dynamics (Carlotti et al. 2015).

In addition, the knowledge of the biochemical composition of zooplankton size fractions may enable us to approximate the energy density of an environment, the efficiency of organic matter transfer along food webs and the energy amount that the high-level predators may obtain when feeding on one size fraction or another. Many studies have been published on the elemental or biochemical composition of zooplankton (see Donnelli et al. 1994;[START_REF] Postel | Biomass and abundance[END_REF][START_REF] Hagen | Seasonal adaptations and the role of lipids in oceanic zooplankton[END_REF]among others). However, most of them were performed either on bulk zooplankton without information on species and size composition [START_REF] Bhat | Biochemical composition of zooplankton from the northern Arabian sea[END_REF][START_REF] Rao | Biochemical composition of zooplankton from Visakhapatnam harbour waters, east coast of India[END_REF][START_REF] Kumar | Biochemical composition and calorific value of zooplankton from the coastal waters of South Andaman[END_REF], or on the dominant species [START_REF] Percy | The biochemical composition and energy content of Arctic marine macrozooplankton[END_REF]Reinardt and van Vleet 1986;[START_REF] Torres | Proximate composition and overwintering strategies of Antarctic micronektonic Crustacea[END_REF]. Much information is available on lipid content, particularly for polar species [START_REF] Ward | Rhincalanus gigas and Calanus simillimus: lipid storage patterns of two species of copepod in the seasonally ice-free zone of the Southern Ocean[END_REF][START_REF] Falk-Petersen | Functional biodiversity of lipids in Antarctic zooplankton: Calanoides acutus, Calanus propinquus, Thysanoessa macrura and Euphausia crystallorophias[END_REF][START_REF] Lee | Lipid storage in marine zooplankton[END_REF][START_REF] Mayzaud | Seasonal and growth stage changes in lipid and fatty acid composition in the multigeneration copepod Drepanus pectinatus from Iles Kerguelen[END_REF]), but less for proteins, carbohydrates or energy content [START_REF] Donnelly | Proximate and elemental composition and energy content of mesopelagic crustaceans from the Eastern Gulf of Mexico[END_REF][START_REF] Färber-Lorda | Elemental composition, biochemical composition and caloric value of Antarctic krill. Implications in energetics and carbon balances[END_REF][START_REF] Kumar | Biochemical composition and calorific value of zooplankton from the coastal waters of South Andaman[END_REF].

AQ1

The aim of our study was thus to provide comprehensive information on the biochemical (lipids, proteins and carbohydrates) and elemental (C and N) compositions, and energy content (joules), of zooplankton size-based fractions at different stations east of Kerguelen Islands during early phytoplankton spring bloom. Our more specific objectives were:

(1) to determine the variations of zooplankton biochemical and elemental compositions according to fraction size and stations (2) to relate these variations to the broad composition by taxonomic groups of fractions and to environmental parameters (temperature, salinity and chlorophyll a concentration), and (3) to quantify the energy content of zooplankton in eastern Kerguelen waters.

Material and methods

Study site and sampling

The naturally iron-fertilized region of the Kerguelen Islands is characterized by a complex mesoscale structure of water masses with contrasted biogeochemical characteristics, generating highphytoplankton and zooplankton biomass [START_REF] Blain | The natural iron fertilization experiment KEOPS (Kerguelen Ocean Plateau Compared Study): an overview[END_REF]Carlotti et al. 2015). The polar front (PF), defined as the northern most boundary of the winter water colder than 2 °C, passes above the plateau south of the Kerguelen Islands and then follows northwards the eastern shelf break before turning eastwards and southwards in a meandering course (Fig. 1), separating the cold Antarctic Surface Water (AASW) in the south from the warm Subantarctic Surface Water (SASW) in the north [START_REF] Park | Polar Front around the Kerguelen Islands: An up-to-date determination and associated circulation of surface/subsurface waters[END_REF][START_REF] Pauthenet | Seasonal meandering of the Polar Page 39 sur 42 e[END_REF], and inducing the mixing of Feenriched shelf waters in the oceanic waters east of the archipelago [START_REF] Trull | Chemometric perspectives on plankton community responses to natural iron fertilization over and downstream of the Kerguelen Plateau in the Southern Ocean[END_REF]. Located between the PF in the south and the Subantarctic front (SAF) in the north, the Kerguelen Islands are bathed by the polar frontal surface water (PFSW) [START_REF] Park | Polar Front around the Kerguelen Islands: An up-to-date determination and associated circulation of surface/subsurface waters[END_REF]). Sampling of zooplankton was performed in early austral spring (15 October-20 November 2011) during the Kerguelen Ocean and Plateau Compared Study cruise II (KEOPS2 survey) at 13 stations east of the Kerguelen Islands (Fig. 1). Most stations were located in AASW south of the meandering polar front. One station (R2) was located in the deep water south-west of the Kerguelen Islands in high-nutrients low-chlorophyll (HNLC) waters, two stations (A3 and E-4W) were located over the Kerguelen plateau, and seven stations (TNS-7, E-1, E-2, E-3, E-4E, E-5 and TEW-4) in the northernmost AASW in a quasi-stationary meander of the PF. Two other stations (F-L and TEW-8) were located in deep PFSW at the extreme north-east of the study region [START_REF] Park | Polar Front around the Kerguelen Islands: An up-to-date determination and associated circulation of surface/subsurface waters[END_REF]. Station A3 was sampled twice, in pre-spring bloom (A3-1) and spring bloom conditions (A3-2). The six stations E (E-1 to E-5) corresponded to a quasi-Lagrangian survey of a stationary meander of the PF over a 20-day period during which the same water mass was sampled repeatedly. Detailed information on sampling strategy, hydrodynamic conditions and plankton communities during the KEOPS2 survey were given in Carlotti et al. (2015) and [START_REF] Trull | Chemometric perspectives on plankton community responses to natural iron fertilization over and downstream of the Kerguelen Plateau in the Southern Ocean[END_REF]. Zooplankton was collected with a double Bongo (60 cm mouth diameter) with one 330 µm and one 120 µm mesh nets mounted with filtering cod ends. For biochemical analyzes, one haul was performed at each station from 250 m depth to the surface at a speed of 0.5 m s during the day (Carlotti et al. 2015). Samples were filtered through five sieves arranged in a column (2000,1000,500,200 and 80 µm mesh size) to obtain five size-based fractions (80-200 µm, 200-500 µm, 500 -1000 µm, 1000-2000 µm and > 2000 µm). When abundant, some large organisms from the fraction > 2000 µm were sorted by broad taxonomic groups (copepods, euphausiids, annelids and salps) and kept separately. All samples were then frozen at -20 °C until analysis.

Fig. 1

Localization of stations analyzed for biochemical and elemental composition of plankton east of the Kerguelen Islands during the KEOPS2 survey, with position of the polar front and water masses (modified from [START_REF] Park | Polar Front around the Kerguelen Islands: An up-to-date determination and associated circulation of surface/subsurface waters[END_REF]. AASW Antarctic surface water, PFSW polar frontal surface water, SASW subantarctic surface water -1

Differences in elemental and biochemical composition of zooplankton among stations were related to some environmental parameters of water masses: temperature (T, °C) and salinity (S) of surface water [START_REF] Park | Polar Front around the Kerguelen Islands: An up-to-date determination and associated circulation of surface/subsurface waters[END_REF][START_REF] Trull | Chemometric perspectives on plankton community responses to natural iron fertilization over and downstream of the Kerguelen Plateau in the Southern Ocean[END_REF], and vertically integrated chlorophyll a concentration (Chl a, mg m ) from the surface to 200 m [START_REF] Lasbleiz | Pigments, elemental composition (C, N, P, and Si), and stoichiometry of particulate matter in the naturally iron fertilized region of Kerguelen in the Southern Ocean[END_REF], along with the chemometric characteristics of phytoplankton accounting for level of iron fertilization, phytoplankton biomass and growth rate as defined by [START_REF] Trull | Chemometric perspectives on plankton community responses to natural iron fertilization over and downstream of the Kerguelen Plateau in the Southern Ocean[END_REF], and further referred to as T-groups. T-group 1 (stations R2, TNS-7 and TEW-4) and T-group 2 (stations E-1, E-2, E-3, E4-E and E-5) were characterized by low phytoplankton biomass, and low to moderate iron supply and growth rate. T-group 3 (A3-1, A3-2 and E-4W), located over the plateau, were characterized by moderate to high phytoplankton biomass and iron supply, and high growth rate, while T-group 5 (TEW-8 and F-L), located downstream the PF in Subantarctic mode waters, presented high phytoplankton biomass, moderate to high iron supply and moderate -2 growth (see [START_REF] Trull | Chemometric perspectives on plankton community responses to natural iron fertilization over and downstream of the Kerguelen Plateau in the Southern Ocean[END_REF] for more details). T-group 4 corresponded to coastal stations not sampled for biochemical analyzes.

Biochemical and elemental analyzes

Before processing, identification of the main taxonomic groups in each fraction was performed under a binocular microscope and the relative importance of the different groups was visually estimated. Then, samples were freeze-dried and ground into a homogeneous fine powder with an agate mortar and pestle. The biochemical analyzes of lipids (LIP), proteins (PRO), and soluble (SC) and insoluble (IC) carbohydrates were determined by spectrophotometric methods. Lipids were measured following the method of [START_REF] Bligh | A rapid method of total lipid extraction and purification[END_REF], modified by [START_REF] Mayzaud | Some aspects of the biochemical and mineral composition of marine plankton[END_REF], and expressed as tripalmitic acid equivalent. Protein concentrations were determined according to the method of [START_REF] Lowry | Protein measurements with the Folin phenol sulfuric acid method[END_REF] and expressed as bovine seroalbumin equivalent. Soluble carbohydrates were extracted with distilled hot water (100 °C, 20 mn) and insoluble carbohydrates in the residue. Both soluble and insoluble carbohydrates were determined by a modified method of [START_REF] Dubois | Colorimetric method for determination of sugar and related substances[END_REF], and expressed as glucose equivalents. Elemental analyzes of carbon (%C) and nitrogen (%N) were obtained using the elemental analyzer (Flash EA1112 Thermo Scientific, Milan, Italy) during stable isotope measurements (see Carlotti et al. 2015). Three replicates were run on well-homogenized material for each type of analysis. Thus, at each station, 12 to 15 analyzes were done for each biochemical or elemental component (4 or 5 size classes × 3 replicates). All concentrations (mg g ) and percentages (%) were expressed on a dry weight basis (dw).

Energy content estimation

The energy value (Ei) of each size fraction in kilo Joules per gram dw of plankton was estimated using the equation (Eq. 1):

where C = concentration of lipids in the fraction i in mg g dw, C = concentration of proteins in the fraction i in mg g dw, C = concentration of soluble carbohydrates in the fraction i in mg g dw, [START_REF] Postel | Biomass and abundance[END_REF] for zooplankton, following [START_REF] Beukema | Caloric values of marine invertebrates with an emphasis on the soft parts of marine bivalves[END_REF].

The total amount of energy provided by zooplankton per square meter per station (E kJ m ) in the 0-250 m water column was calculated as the sum of the biomass (Bi in g m dw) of each size fraction at this station multiplied by its energy value Ei (kJ g ) (Eq. 2), for the 4 fractions > 200 µm:

Biomasses (g m dw) of the four larger zooplankton size fractions at each station were extracted from data given by Carlotti et al. (2015).

The lowest plankton size fractions (80-200 µm) were not taken into account in energy calculation, as they were mainly composed of phytoplankton.

Data analyzes

After testing for normality and homogeneity of variances, the effect of size was tested following one-way ANOVA or non-parametric Kruskal 2012). As variables were expressed in different units (mg g , kJ g , %), the analysis was done on transformed reduced centered data. F tests based on 999 permutations were used to test the significance of RDA axes, the global effect of the environment (all variables taken into

InsCarb -1 T -2 -2 -1 = ∑ (Bi × Ei) E T -2 -1 -1
account) and the effect of each environmental variable. The forward selection of RDA was used to determine the minimum number of explanatory factors that could explain significant proportions of variation in biochemical data. This RDA analysis was done using the software R (package 'vegan').

Results

Biochemical composition and energy value of size fractions and groups 1). They were also the most variable as indicated by higher coefficients of variation (CV, not shown) (CV = 45 as a mean for fraction 80-200 µm and only 20 for fraction > 2000 µm). Among the biochemical components, LIP and IC presented the most variable concentrations in all size fractions (CV = 39 for both), followed by PRO (CV =2 8), while SC were the least variable within and between fractions (CV = 21). For all components, except SC, mean concentrations did not differ between the three larger fractions (500-1000 µm, 1000-2000 µm and > 2000 µm), as indicated by the results of post hoc comparisons (Table 1). PRO and LIP concentrations increased more with size (× 2.9

and × 2.7, respectively, between the lowest and the largest fractions), than SC and IC (× 1.7 and × 1.4, respectively). The energy content of zooplankton fractions was strongly correlated to LIP concentration (Pearson linear regression, r = 0.92, p < 0.001, n = 186), as lipids were the most energy-rich components, and secondarily with PRO (r = 0.89, p < 0.001, n = 186), which were the dominant components in zooplankton. Accordingly, the energy content of zooplankton fractions increased with increasing fraction size (Table 1). The energy content of zooplankton during KEOPS2 cruise increased from a mean of 3.9 kJ g dw in the fraction 80-200 µm (from 2.1 to 7.9 kJ g dw at stations E-1 and R2, respectively) to a mean of 10.4 kJ g dw in the fraction > 2000 µm (from 7.8 to 13.6 kJ g dw at stations E1 and E5, respectively). Percentages of organic C and nitrogen, and C/N ratio also significantly increased with fraction size (Table 1). The C/N ratio was not significantly different between the two smallest fractions (mean ± SD = 4.9 ± 0.6, n = 36, and 5.5 ± 0.7, n = 39, in the 80-200 µm and 200 -500 µm fractions, respectively). In the largest fractions, C/N increased from 6.1 ± 0.9, n = 39, in the 500-1000 µm fraction to 6.6 ± 1.6, n = 36, in the > 2000 µm fraction, but the difference was not significant. Significant linear correlations were observed between %C, %N and C/N ratio, and all biochemical components and energy values in zooplankton fractions (Table 2). Both %C and %N were best correlated with proteins, while C/N was best correlated with lipids, and energy content with %C. Size fractions differed in their taxonomic composition (Table 3), which in turn influenced their elemental and biochemical compositions. The lowest size fraction (80-200 µm) was generally composed of a mixture of phytoplankton, detritus and small zooplankton, but was sometimes dominated by diatoms (A3-2, E-4W), foraminifera (A3-1) or small copepods (R2) at some stations. g dw), followed by annelids (10.4 kJ g dw), while salps were the least energetic (6.3 kJ g dw) (Table 4).

Spatial variation in biochemical composition and energy content

While station position were linked with temperature and salinity of water masses (cold and more salty at R2, A3-1 and A3-2 in AASW, and warm and less salty at F-L and TEW-8 in PFSW), chlorophyll a concentration displayed a patchy distribution (Table 5). High Chl a values (> 300 mg m ) were recorded at F-L and A3-2, low-Chl a values (<120 mg m ) at R2, A3-1, E-2, E-3 and E-4E, and intermediate values (< 120 < Chl a < 200 mg m ) at the other stations. The mean biochemical composition of zooplankton, all fractions combined, differed significantly with station for LIP, SC and IC, but not for PRO (Table 5). The highest difference in component concentrations with space was observed for LIP (× 3 from stations with the lowest to the highest values) and IC (× 2.9), while the difference was 1.7 for PRO and 1.6 for SC. Mean concentrations of LIP peaked at station F-L and

 

were the lowest at E-1 and E-2. PRO and SC concentrations were slightly higher at stations A3-1 and TEW-8, and IC concentration at F-L. During the Lagrangian survey, the mean LIP concentration of zooplankton was multiplied by 3.8 from E-1 to E-5, IC concentration by 1.9 and PRO concentration by 1.1, while SC concentration did not vary. However, while LIP concentration increased in time from E-1 to E-5, PRO concentration was much higher at station E-3 in the middle of the survey than at station E-5 at the end (Fig. 2). The total amount of energy (E ) provided by zooplankton biomass in the 0-250 m water column at each station during the KEOPS2 cruise was related to two main parameters, the biochemical composition and the biomass of zooplankton in each size fraction at station level. E ranged from 26.35 kJ m dw at station E-4E to 116.73 kJ m dw at station F-L (mean 51.78 ± 24.67 J m dw, n = 186). As illustrated in Fig. 3,E was mainly related to the two largest size fractions (1000-2000 µm and > 2000 µm), which represented, respectively, an average of 23% and 59% of the total amount of energy provided by zooplankton in the 250m-deep surface-water layer. Fraction 500-1000 µm constituted 17% of E and fraction 200-500 µm only 1%.

Relative importance of factors

The RDA explained 77.5% of the inertia of the biochemical composition of zooplankton (permutations F test, p < 0.001). The forward selection procedure of the RDA retained 1ten variables (three size classes, four stations, one T-group, sea surface temperature and chlorophyll a concentration) explaining significantly the variation in the biochemical characteristics of plankton (Fig. 4). Date and salinity were not significant. The overall ordination was significant (p < 0.001). The RDA indicated that size was the most important factor in explaining the biochemical composition of zooplankton in Kerguelen waters (axis 1), followed by station, surface temperature and chlorophyll a concentration (axis 2). The first axis explained 58.9% of the total variance and revealed the importance of size in accounting the biochemical and energy concentrations of zooplankton. E, SC, PRO, LIP, %C and %N were all positively correlated with the largest plankton fraction (> 2000 µm) and negatively to the smallest fractions (80-200 µm and 200-500 µm). The second axis summarized 11.8% of the total variance and detected the importance of some stations, surface temperature and chlorophyll a concentration in explaining differences associated with IC and LIP concentrations in zooplankton fractions as opposed to ISC concentration. Higher IC and LIP concentrations were positively associated with stations F-L and E5, higher sea surface temperature, and higher Chl a concentration. Higher IC concentration were also observed in T-group 2, but this factor was less important than T (°C) and Chl a. On the negative part of axis 2, higher SC concentrations were associated with station TNS-7 and the 200-500 µm size class.

Fig. 4

Redundancy analysis (RDA): ordination diagram on the effects of environmental (station, date, TT, SS, Chl a and T-group) and morphological (size) factors on the biochemical and elemental characteristics of zooplankton (concentrations in macronutrients, energy, %C and %N) in Kerguelen waters

Discussion

The biochemical composition of zooplankton is an essential factor in the understanding of food web functioning and predator population dynamics, as zooplankton acts as 'a complex functional hub' in the transfer of organic matter between the lowest and the highest trophic levels in aquatic ecosystems [START_REF] Banse | Zooplankton: pivotal role in the control of ocean production[END_REF][START_REF] Travers-Trolet | Towards endto-end models for investigating the effects of climate and fishing in marine ecosystems[END_REF]). The main results of the present study indicated that (1) proteins formed the major biochemical component of zooplankton in Kerguelen waters (22% dw), followed by lipids (9% dw), while carbohydrates (3% dw) represented only a weak percentage of organic constituents, (2) concentrations of all biochemical components increased with the size of zooplankton fractions, (3) the main part (> 80%) of the energy provided by zooplankton per station was due to large-sized organisms (> 1000 µm), particularly copepods, which were the most energy-rich group analyzed, and (4) a high patchiness among stations was observed in the total energy provided by zooplankton, but the richest station was located in the polar front zone.

The dominance of proteins and the low percentage of carbohydrates in zooplankton are recorded in all oceanographic regions from tropical [START_REF] Goswami | Biochemical composition of zooplankton from the Andaman Sea[END_REF][START_REF] Kumar | Biochemical composition and calorific value of zooplankton from the coastal waters of South Andaman[END_REF]) and temperate waters [START_REF] Raymont | The biochemical composition of Euphausia superba[END_REF][START_REF] Mayzaud | Some aspects of the biochemical and mineral composition of marine plankton[END_REF] to polar zones [START_REF] Percy | The biochemical composition and energy content of Arctic marine macrozooplankton[END_REF][START_REF] Torres | Proximate composition and overwintering strategies of Antarctic micronektonic Crustacea[END_REF][START_REF] Yun | Comparison of phytoplankton macromolecular compositions and zooplankton proximate compositions in the northern Chukchi Sea[END_REF]. Lipids present the most variable concentrations representing generally high percentages in zooplankton from Arctic and Antarctic waters (e.g. [START_REF] Donnelly | Chemical composition of Antarctic zooplankton during austral fall and winter[END_REF][START_REF] Falk-Petersen | Functional biodiversity of lipids in Antarctic zooplankton: Calanoides acutus, Calanus propinquus, Thysanoessa macrura and Euphausia crystallorophias[END_REF], and lower percentages in tropical zones (e.g. Arun [START_REF] Kumar | Biochemical composition and calorific value of zooplankton from the coastal waters of South Andaman[END_REF]but see Cass et al. 2014 and[START_REF] Lee | Lipid storage in marine zooplankton[END_REF] for a review). However, the biochemical composition of zooplankton varies largely with species, size, ontogenic stage, site and season.

Issues concerning zooplankton biochemical values

The comparison between the numerous studies published on the biochemical composition of planktonic organisms and their energetic content is made difficult by differences in sampling methods (net mesh size, depth range), analytical methods, units in which results are expressed (most often in dry weight but also wet weight, ash-free dry weight, carbon weight), and the conversion factors used (see [START_REF] Postel | Biomass and abundance[END_REF][START_REF] Yebra | Advances in biochemical indices of zooplankton production[END_REF]). In the present study, the residue (ashes and not recovered organic components) represents a high percentage of dry weight (44-84% dw), depending on fraction size or group of organisms. This high residue percentage, particularly in the two smallest size fractions, could derive in part from large particulate aggregates and detritus (fecal pellets, diatom frustules, exuviae), abundant in the water column during the KEOPS2 survey [START_REF] Jouandet | Rapid formation of large aggregates during the spring bloom of Kerguelen Island: observations and model comparisons[END_REF]. In bulk zooplankton samples collected with a Bongo net of 330 µm mesh size, [START_REF] Bhat | Biochemical composition of zooplankton from the northern Arabian sea[END_REF] recorded mean percentages of residue (54-60% dw) close to those observed in our three largest size fractions (58-60% dw). In the Mediterranean Sea, the residue percentage in similar zooplankton size fractions ranges from 64-84% dw (C.T. Chen pers. comm.). When really quantified by combustion, ash percentages are highly variable among species or groups of organisms (6-57% dw in [START_REF] Ikeda | Metabolism and elemental composition of zooplankton from the Barrents Sea during early Arctic summer[END_REF], 7-31% dw in [START_REF] Donnelly | Proximate and elemental composition and energy content of mesopelagic crustaceans from the Eastern Gulf of Mexico[END_REF], 7-71% dw in Donnelly et al. (1994), 6-70% dw in Percy and Fife (199481), and 2-77% in [START_REF] Postel | Biomass and abundance[END_REF]), being generally the lowest in copepods and the highest in tunicates and ctenophores. [START_REF] Clarke | Proximate and elemental composition of gelatinous zooplankton from the Southern Ocean[END_REF] recorded high-ash contents (60-73% dw) in gelatinous zooplankton from the Southern Ocean, but only 12% dw in the annelid Tomopteris. A mean ash content of 25.7% dw (12.4_ 35.5% dw) was recorded by [START_REF] Postel | Biomass and abundance[END_REF] in different size classes of plankton from 55 to > 1000 µm. However, in addition to ash, there is still a proportion of non-explained organic matter, the sum of the biochemical components recovered plus ashes representing rarely 100% of the matter analyzed whatever the study and the environment. In effect, the analytical methods used may generate significant differences in component percentages, and do not quantify all the biochemical components present. For example, chitin, which may represent from 1 to 14% dw of zooplankton [START_REF] Mayzaud | Some aspects of the biochemical and mineral composition of marine plankton[END_REF][START_REF] Donnelly | Proximate and elemental composition and energy content of mesopelagic crustaceans from the Eastern Gulf of Mexico[END_REF][START_REF] Torres | Proximate composition and overwintering strategies of Antarctic micronektonic Crustacea[END_REF], was not quantified in our study. Total lipids are generally measured following the [START_REF] Bligh | A rapid method of total lipid extraction and purification[END_REF] method, as we did. But [START_REF] Iverson | Comparison of the Bligh and Dyer and Folch methods for total lipid determination in a broad range of marine tissue[END_REF] demonstrate that this method may underestimate lipid content by up to 50% when the percentage of lipids is > 2%. When using the correction factor indicated by [START_REF] Iverson | Comparison of the Bligh and Dyer and Folch methods for total lipid determination in a broad range of marine tissue[END_REF], the mean percentage of lipids in the zooplankton of Kerguelen Islands increases from 9 to 11% dw. The difference between the two methods increases with the size and concentration of zooplankton fraction, from 1.0% in the 80-200 µm fraction to 2.7% in the fraction > 2000 µm. Proteins are most often quantified by the [START_REF] Lowry | Protein measurements with the Folin phenol sulfuric acid method[END_REF] method, but proteins do not represent all the nitrogen compounds of the organic matter (amino acids, chitin, etc.). From 12 to 42% dw of total nitrogen can be represented by non-protein nitrogen components in zooplankton [START_REF] Mayzaud | Some aspects of the biochemical and mineral composition of marine plankton[END_REF][START_REF] Donnelly | Proximate and elemental composition and energy content of mesopelagic crustaceans from the Eastern Gulf of Mexico[END_REF]. Thus, when the protein content is calculated in multiplying nitrogen percentages (%N) by the classical 'Kjeldahl' conversion factor of 6.25, much higher percentages of proteins are obtained. If we had used that kind of conversion factor, the mean percentage of proteins in Kerguelen zooplankton would have increased from 22 to 44% dw, with an increase difference of 19% for the 80-200 µm fraction up to 27% for the fraction > 2000 µm. Similarly, the conversion factors used to transform the biochemical component concentrations into energy (calories and/or joules) differ among studies. These calculations may also differ from direct quantification of energy in a microbomb calorimeter. Conversion factors in caloric equivalents of organic constituents range from 9 -9.5 kcal g for lipids, from 4 to 5.7 kcal g for proteins and from 4 to 4.2 kcal g for carbohydrates [START_REF] Clarke | Proximate and elemental composition of gelatinous zooplankton from the Southern Ocean[END_REF][START_REF] Postel | Biomass and abundance[END_REF][START_REF] Rao | Biochemical composition of zooplankton from Visakhapatnam harbour waters, east coast of India[END_REF]. The conversion factors in joules used in the present paper (35.6, 21.4 and 17.2 kJ g for lipids, proteins and carbohydrates, respectively) are those recommended by [START_REF] Postel | Biomass and abundance[END_REF] for zooplankton. Despite these problems in data comparison, our results agree with the general trends observed for biochemical composition variations with size and specific composition recorded elsewhere for zooplankton, and corresponded to minimum values.

Effect of size and group composition on zooplankton biochemical composition

Fraction size was the main factor explaining the biochemical composition of zooplankton in Kerguelen waters, with generally an increase in all component concentrations with size (Table 1, Fig. 4). Similar results are obtained at a specific level by different authors for all biochemical components [START_REF] Guisande | Biochemical fingerprints in zooplankton[END_REF] or lipids [START_REF] Hagen | Seasonal adaptations and the role of lipids in oceanic zooplankton[END_REF][START_REF] Lee | Lipid storage in marine zooplankton[END_REF]. The larger the organisms, the higher their concentrations in biochemical components. However, in the Mediterranean Sea, the highest concentrations are observed in the medium-size fractions of zooplankton (200-500 and 500-1000 µm), with a decrease in concentrations in the largest fractions (1000-2000 and > 2000 µm) (C.T. Chen pers. comm.). Such a decrease is attributed to the dominance of gelatinous organisms (salps, siphonophores) in the largest size fractions.

-

1 -1 -1 -1
Concentrations in biochemical components, particularly lipids and proteins, differ considerably among species and groups of organisms [START_REF] Percy | The biochemical composition and energy content of Arctic marine macrozooplankton[END_REF]Reinhardt and Van Vleet 1986;[START_REF] Torres | Proximate composition and overwintering strategies of Antarctic micronektonic Crustacea[END_REF]; among many others). Therefore, the species or group composition of plankton size fractions greatly influences their biochemical composition. In this way, the higher PRO concentrations in all size fractions observed at station E-3, compared with the other stations E (see Fig. 2), could be related to a higher proportion of copepods (> 90%), amphipods and chaetognaths (all groups rich in proteins).

Similarly, the 80-200 µm fraction contained higher concentrations of proteins and lipids (22.0% and 7.3%, respectively) at station R2, where this fraction was composed of small copepods, than at station A3-2 (5.3% and 2.7% respectively) where this fraction was dominated by diatoms.

Among the few groups of organisms analyzed in Kerguelen waters, copepods showed the highest lipid, protein and energy content, followed by euphausiids, while the lowest values were recorded in salps. However, the %N was higher in euphausiids (11% dw) than in copepods (6% dw) (Table 4). Similar results are observed in other studies, whatever the geographical region. Higher concentrations of proteins and lipids in copepods than in euphausiids are indicated by [START_REF] Goswami | Biochemical composition of zooplankton from the Andaman Sea[END_REF] in the Andaman Sea, who recorded similarly a higher %N (9% dw) in euphausiids than in copepods (7% dw), perhaps due to a higher chitin content in euphausiids. Reinhardt and Van Vleet (1986) also found more lipids in copepods than in euphausiids of similar size in the Antarctic. In contrast, lower concentrations of biochemical components are generally recorded in gelatinous organisms, such as ctenophores, hydromedusae, tunicates and siphonophores (e.g. [START_REF] Goswami | Biochemical composition of zooplankton from the Andaman Sea[END_REF][START_REF] Clarke | Proximate and elemental composition of gelatinous zooplankton from the Southern Ocean[END_REF][START_REF] Donnelly | Chemical composition of Antarctic zooplankton during austral fall and winter[END_REF], while protein concentration could be high in chaetognaths and pteropods [START_REF] Mayzaud | Some aspects of the biochemical and mineral composition of marine plankton[END_REF][START_REF] Percy | The biochemical composition and energy content of Arctic marine macrozooplankton[END_REF][START_REF] Donnelly | Chemical composition of Antarctic zooplankton during austral fall and winter[END_REF], as observed in Kerguelen waters.

Both %C and %N were more strongly correlated with proteins than with lipids, while energy was more strongly correlated with %C than with % N (r = 0.914 vs 0.712, n = 186, respectively) (see Table 2). The high dependence of energy on organic carbon content is well known for aquatic organisms [START_REF] Salonen | The relation of energy to organic carbon in aquatic invertebrates[END_REF]). Similar results were also observed in Antarctic krill by [START_REF] Färber-Lorda | Elemental composition, biochemical composition and caloric value of Antarctic krill. Implications in energetics and carbon balances[END_REF], with the best correlation found between joules and carbon. As expected, the C/N ratio was better correlated with lipids (r = 0.662) than with other components, a well-documented relation in zooplankton (e.g. [START_REF] Postel | Biomass and abundance[END_REF][START_REF] Färber-Lorda | Elemental composition, biochemical composition and caloric value of Antarctic krill. Implications in energetics and carbon balances[END_REF].

Variations of biochemical composition among stations

Wide spatial and temporal variations in the biochemical composition of zooplanktonic organisms are reported in the literature [START_REF] Raymont | The biochemical composition of Euphausia superba[END_REF][START_REF] Donnelly | Proximate and elemental composition and energy content of mesopelagic crustaceans from the Eastern Gulf of Mexico[END_REF][START_REF] Rao | Biochemical composition of zooplankton from Visakhapatnam harbour waters, east coast of India[END_REF].

Major variations are particularly observed in polar regions, where lipid storage is an important seasonal phenomenon [START_REF] Percy | The biochemical composition and energy content of Arctic marine macrozooplankton[END_REF][START_REF] Ward | Rhincalanus gigas and Calanus simillimus: lipid storage patterns of two species of copepod in the seasonally ice-free zone of the Southern Ocean[END_REF][START_REF] Lee | Lipid storage in marine zooplankton[END_REF][START_REF] Mayzaud | Seasonal and growth stage changes in lipid and fatty acid composition in the multigeneration copepod Drepanus pectinatus from Iles Kerguelen[END_REF]. Wide variations in the biochemical composition of size-fractionated zooplankton among stations were also recorded in Kerguelen waters during the onset of the spring bloom (Table 5). Two types of component varied particularly with ontogeny and time, lipids as involved in energy storage, and insoluble carbohydrates as involved in cellular membrane formation. While LIP were significantly positively related to fraction size (Table 1), IC concentrations were significantly related to the abundance of organisms (r = 0.55, p = 0.034, n = 62) for fractions between 200 µm and 2000 µm, mainly composed of copepods (Table 3). The similar mean concentration of proteins among stations (Table 5) could reflect the fact that proteins are the main structural component in zooplankton growth (Carlotti et al. 1993), which fluctuates less than non-structural components, such as some lipid classes linked to energy storage [START_REF] Falk-Petersen | Functional biodiversity of lipids in Antarctic zooplankton: Calanoides acutus, Calanus propinquus, Thysanoessa macrura and Euphausia crystallorophias[END_REF][START_REF] Hagen | Seasonal adaptations and the role of lipids in oceanic zooplankton[END_REF]Cass et al. 2014). Variation in biochemical component concentrations among stations reflected the patchiness in composition and population response of zooplankton communities to the spring bloom, as stations were sampled at different dates and bloom conditions. The patchiness of rapidly changing biomass at the onset of the spring bloom in this region, evidenced for both phytoplankton [START_REF] Lasbleiz | Pigments, elemental composition (C, N, P, and Si), and stoichiometry of particulate matter in the naturally iron fertilized region of Kerguelen in the Southern Ocean[END_REF][START_REF] Trull | Chemometric perspectives on plankton community responses to natural iron fertilization over and downstream of the Kerguelen Plateau in the Southern Ocean[END_REF] and zooplankton communities (Carlotti et al. 2015), was thus also observed in their biochemical composition.

The data obtained during the Lagrangian survey between station E-1 and E-5 (eddy in oceanic AASW) gave some information on the biochemical changes in zooplankton as the bloom developped (Fig. 2). Carlotti et al. (2015) observed a strong increase in zooplankton abundance during this survey, with the cumulative contribution of various larval stages of the dominant copepods and euphausiids. Protein concentrations (× 1.1 between E-1 and E-5) responded less than lipids (× 3.8 in the same time) to the increase in zooplankton abundance. As proteins were mostly involved in structural weight (Carlotti et al. 1993), while lipids were the most efficient component for non-structural energy storing, this could suggest an increase in feeding activity of zooplankton during the development of the bloom.

Strong seasonality in total lipid content has been well studied in Arctic and Antarctic planktonic organisms, particularly in crustaceans, such as copepods, euphausiids and amphipods, that have to respond to food shortage and diapause for winter survival, then to reproduction and ontogeny during phytoplankton bloom [START_REF] Percy | The biochemical composition and energy content of Arctic marine macrozooplankton[END_REF]Reinhard and van Vleet 1986;[START_REF] Mayzaud | Seasonal and growth stage changes in lipid and fatty acid composition in the multigeneration copepod Drepanus pectinatus from Iles Kerguelen[END_REF]. The energy content of planktonic organisms varies widely with season according to trophic resource variations and zooplankton growth (e.g. [START_REF] Percy | The biochemical composition and energy content of Arctic marine macrozooplankton[END_REF][START_REF] Donnelly | Chemical composition of Antarctic zooplankton during austral fall and winter[END_REF][START_REF] Mayzaud | Seasonal and growth stage changes in lipid and fatty acid composition in the multigeneration copepod Drepanus pectinatus from Iles Kerguelen[END_REF]). An increase in both lipid and energy content of zooplankton as the season progresses, reflecting an increase in feeding at all trophic levels, is observed in all organisms, but modulated by species physiology [START_REF] Lee | Lipid storage in marine zooplankton[END_REF][START_REF] Kattner | Perspectives on marine zooplankton lipids[END_REF]Cass et al. 2014). Lipid storage is particularly important in herbivorous copepods experiencing winter diapause in deep waters such as Rhincalanus gigas [START_REF] Ward | Rhincalanus gigas and Calanus simillimus: lipid storage patterns of two species of copepod in the seasonally ice-free zone of the Southern Ocean[END_REF][START_REF] Hagen | Seasonal adaptations and the role of lipids in oceanic zooplankton[END_REF][START_REF] Lee | Lipid storage in marine zooplankton[END_REF], which was the most abundant copepod species in the largest plankton size fraction in Kerguelen waters (> 75% in number) (Carlotti et al. 2015). However, as emphasized by many authors, as proteins are the major organic components, they may also serve as metabolic reserve in zooplankton organisms, in addition to lipids, but such a role may be more important in temperate and tropical waters (Arun [START_REF] Kumar | Biochemical composition and calorific value of zooplankton from the coastal waters of South Andaman[END_REF] than in polar regions.

   

Role of zooplankton energy in food web functioning

The energy content of zooplankton in Kerguelen waters increased with the size of the organisms, the three largest and biomass-dominant fractions analyzed (500-1000 µm, 1000-2000 µm and > 2000 µm) having a mean energy value of ≥10 kJ g . A mean energy value of 12.5 kJ g for total zooplankton (> 300 µm) has been recorded by [START_REF] Goswami | Biochemical composition of zooplankton from the Andaman Sea[END_REF] in the Andaman Sea, but only 7.5 kJ g by Arun [START_REF] Kumar | Biochemical composition and calorific value of zooplankton from the coastal waters of South Andaman[END_REF] in the same region, and 10.6 kJ g by [START_REF] Bhat | Biochemical composition of zooplankton from the northern Arabian sea[END_REF] in the Arabian Sea. The energy content of Kerguelen organisms were within the range of those recorded in other studies for similar groups, with high values recorded in copepods and annelids, and low values in salps (Table 6). The energy content (E) of zooplankton was positively and significantly related both to lipid and protein concentrations of the organisms, the highest correlation being naturally observed with lipids due to their high energy content (nearly twice that of proteins). However, being the dominant biochemical components, proteins constituted an average of 55% of the energy provided by zooplankton in Kerguelen, while lipids represented 38% and carbohydrates only 7%. Zooplankton grazing on phytoplankton corresponded not only to the OM transfer up into the food web but also to a crucial qualitative leap of increasing nutritive and energetic density of this matter. Differences in biochemical composition and energy content between the 80-200 µm size class at stations where it was dominated by diatoms (representative of 'phytoplankton') and those of the > 500 µm size classes (representative of 'zooplankton') illustrated this phenomenon (Fig. 5). The shift from phyto to zooplankton enhanced by four (× 4.2) the energy density of each gram of organic matter (2.42 kJ g dw and 10.14 kJ g dw, respectively), providing a more nutritive and energetic food source to fulfill the metabolic requirements of upper level predators. Such a qualitative leap was linked to an increase in protein and lipid concentrations in zooplankton, compared to those of phytoplankton (Fig. 5). By comparison, an increase in OM energy density of only ×1.3 was observed from euphausiids to myctophids using the mean energy value for these fish species (13.6 kJ g ) recorded by [START_REF] Lea | Fatty acid composition of lipid-rich myctophids and mackerel icefish (Chamsocephalus gunnari) -Southern Ocean food-web implications[END_REF] in Kerguelen waters (Fig. 5). Similarly, [START_REF] Waluda | Diet variability and reproductive performance of macaroni penguins Eudyptes chrysolophus at Bird Island, South Georgia[END_REF] observe an increase of ×1.5 in OM energy value from krill to planktivorous fish such as myctophids in Subantarctic waters (4.65. kJ g ww and 6.99 kJ g ww, respectively).

Fig. 5

Increase in energy density (E, in kJ g ) of the organic matter from phytoplankton to zooplankton, and from euphausiids to myctophids, in Kerguelen waters. The proportions of energy provided by proteins, lipids and carbohydrates are indicated for each group of organisms. Data for phytoplankton, zooplankton and euphausiids are issued from this study, and data for myctophids are issued from [START_REF] Lea | Fatty acid composition of lipid-rich myctophids and mackerel icefish (Chamsocephalus gunnari) -Southern Ocean food-web implications[END_REF] in Kerguelen waters

-1 -1 -1 -1 -1 -1
The two largest zooplankton fractions (1000-2000 µm and > 2000 µm) analyzed, rich in lipids and proteins, represented 80% on average of the biomass (Carlotti et al. 2015) and 82% of the energy provided by zooplankton in the 0-250 m water layer east of the Kerguelen Islands during the KEOPS2 survey (Fig. 3). The fraction 500-1000 µm completed the biomass and energy contribution to the whole zooplankton, particularly at the most productive stations (F-L, E-3, A3-2), while the fraction 200-500 µm contributed only for a few % both in biomass and energy of the whole zooplankton. Among zooplankton organisms, large copepods presented the highest concentrations in lipids and the highest energy content (Table 4). Many studies on food web structure in the Southern Ocean evidence the importance of meso-and macrozooplankton, either as a direct food source for a wide array of zooplankton consumers (e.g. amphipods, euphausiids, fish and seabirds), or indirectly, amphipods, euphausiids and fish being themselves forage species consumed by high-level predators [START_REF] Pruvost | An ecosystem model of the Kerguelen Islands'EEZ[END_REF][START_REF] Hindell | Foraging habitats of top predators, and areas of ecological significance, on the Kerguelen plateau[END_REF]. Euphausiids, while primarily grazing on phytoplankton, are also able to feed on a large size range of zooplankton, mainly copepods (from 100 µm to several mm),

and the varying importance of the copepod contribution in their diets is nowadays recognized [START_REF] Schmidt | Feeding and overwintering of Antarctic krill across its major habitats: The role of sea ice cover, water depth, and phytoplankton abundance[END_REF][START_REF] Sogawa | Feeding habits of six species of euphausiids (Decapoda: Euphausiacea) in the northwestern Pacific Ocean determined by carbon and nitrogen stable isotope ratios[END_REF]. The high energy values of the mesozooplankton size fractions above 200 µm found in our study, mainly boosted by copepod energy value, could represent an essential role for the trophic requirement of some euphausiid species, as shown in other ecosystems [START_REF] Huenerlage | Krill of the northern Benguela Current and the Angola-Benguela frontal zone compared: Page 37 sur 42 e.Proofing 21/01[END_REF][START_REF] Schmidt | Feeding and food processing in Antarctic krill (Euphausia superba Dana)[END_REF][START_REF] Sogawa | Feeding habits of six species of euphausiids (Decapoda: Euphausiacea) in the northwestern Pacific Ocean determined by carbon and nitrogen stable isotope ratios[END_REF]. Myctophid fishes, which are an important trophic link between zooplankton and high-level predators, mainly feed on large (> 1 mm) copepods (> 50% by number), then on euphausiids, hyperiid amphipods and pteropods [START_REF] Pakhomov | Prey composition and daily rations of myctophid fishes in the Southern Ocean[END_REF][START_REF] Cherel | Isotopic niches and trophic levels of myctophid fishes and their predators in the Southern Ocean[END_REF]. Planktonic crustaceans, particularly large copepods, euphausiids and amphipods, also constitute the main prey of many species of seabirds, such as petrels and penguins that breed in the area of the Kerguelen and Crozet Islands [START_REF] Ridoux | The diets and dietary segregation of seabirds at the subantarctic Crozet Islands[END_REF][START_REF] Guinet | Consumption of marine resources by seabirds and seals in Crozet and Kerguelen waters: Changes in relation to consumer biomass. 1962-1985[END_REF]Bocher et al. 2002;[START_REF] Cherel | Feeding habits of seabirds and marine mammals of the Kerguelen Archipelago[END_REF].

The total energy content provided by zooplankton east of Kerguelen Islands presented a patchy distribution (Fig. 3), as also emphasized for chlorophyll a concentration, and phytoplankton and zooplankton abundances [START_REF] Lasbleiz | Pigments, elemental composition (C, N, P, and Si), and stoichiometry of particulate matter in the naturally iron fertilized region of Kerguelen in the Southern Ocean[END_REF]Carlotti et al. 2015). The highest lipid and total energy contents were recorded at F-L, a station located in the PFSW polar front zone [START_REF] Park | Polar Front around the Kerguelen Islands: An up-to-date determination and associated circulation of surface/subsurface waters[END_REF]. This could suggest a role of oceanic front in providing more nutritive food resources for upper-level consumers. However, more sampling in this zone would have been necessary to support this hypothesis. The influence of oceanic front in increasing zooplankton abundance, biomass and grazing activity has been well documented in the Southern Atlantic [START_REF] Mayzaud | Zooplankton nutrition, storage and fecal lipid composition in different water masses associated with the Agulhas and Subtropical Fronts[END_REF][START_REF] Richoux | Trophic ecology of zooplankton at a frontal transition zone: fatty acid signatures at the subtropical convergence, Southern Ocean[END_REF][START_REF] Lopes | Influence of oceanic fronts on mesozooplankton abundance and grazing during spring in the south-western Atlantic[END_REF]. Recently, [START_REF] Woodson | Ocean fronts drive marine fishery production and biogeochemical cycling[END_REF], incorporating hydrological patchiness and front into current fishery models, show the crucial role of oceanic fronts in sustaining global marine fishery production. In the southern oceans, long-term tracking of different marine birds and mammals indicates that front zones constitute preferential foraging zones for many of them (Bost et al. 2009).

Thus, the high abundance, large size and high-energy content of zooplankton of Kerguelen waters could explain why it constitutes a   major food resource for the numerous fish, seabirds and marine mammals of this region. Further studies on the temporal and spatial variation in the biochemical composition of zooplankton in this region would be needed to better understand how the physical parameters and the development of the phytoplankton bloom influence the energy transfer along the food web. However, information provided here could be still used to better estimate the energetic budget of trophic compartments in this region. The functional role of zooplankton involves not only accelerating OM transfer but also increasing its energy performance, a pivotal role that should be better consider in modeling studies. Modeling of food web functioning would be more realistic if based on energy units rather than matter content (C, N, P). However, few data giving the energy value of planktonic community by size classes are still available.

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Fig. 2

 2 Fig. 2Variation of lipid (top) and protein (bottom) percentages with size in zooplankton collected during the Lagrangian survey from stations E-1 to station E-5. The best-fitted curves follow the equation y = ln(x)a, where y = compound percentage (%), x = fraction size, a = constant depending on station

Fig. 3

 3 Fig. 3 Total energy content (kJ m ) of zooplankton > 200 µm in the 0-250 m water layer at the different stations sampled during KEOPS2 survey, with the relative importance of each size fraction in total energy -2

  Cherel Y, Labat JP, Mayzaud P, Razouls S, Jouventin P (2001) Amphipod-based food web: Themisto gaudichaudii caught in nets and by seabirds inKerguelen waters, Bocher P, Cherel Y, Alonzo F, Razouls S, Labat JP,Mayzaud P, Jouventin P (2002) Importance of the large copepod Paraeuchaeta antarctica(Giesbrecht, 1902) in coastal waters and the diet of seabirds at Kerguelen, Southern Ocean. J Plankton Res 24:1317 -1333 Bost CA, Cotté C, Bailleul F, Cherel Y, Charrassin JB, Guinet C, Ainley DG, Weimerskirch H (2009) The importance of oceanic fronts to marine birds and mammals of the southern oceans. J Mar Syst 78:363-376 Carlotti F, Krause M, Radach G (1993) Growth and development of Calanus finmarchicus related to the influence of temperature: experimental results and conceptual model. Limnol Oceanogr 38:1125-1134 Carlotti F, Giske J, Werner F (2000) Modelling zooplankton dynamics. In: Harris R, Wiebe P, Lenz J, Skjoldal HR, Huntley M (eds) ICES zooplankton methodology manual. Academic Press, San Diego, pp 571-667 Carlotti F, Jouandet MP, Nowaczyk A, Harmelin-Vivien M, Lefèvre D, Richard P, Zhu Y, Zhou M (2015) Mesozooplankton structure and functioning during the onset of the Kerguelen bloom during Keops2 survey. Biogeosciences 12:4543-4563 Cass CJ, Daly KL, Wakeham SG (2014) Assessment of storage lipid accumulation patterns in eucalanoid copepods from the eastern tropical Pacific Ocean. Deep-Sea Res Pt I 93:117-130 Cherel Y, Bocher P, Trouvé C, Weimerskirch H (2002) Diet and feeding ecology of blue petrels Halobaena caerulea at Iles Kerguelen, Southern Indian Ocean. Mar Ecol Prog Ser 228:283-299

Table 1 Mean

 1

	Size fraction (µm)	n	LIP (mg g dw)	PRO (mg g dw)	SC (mg g dw)	IC (mg g dw)	%C dw
	80-200	36 46.43 ±	89.08 ±	14.78 ±	8.44 ± 3.81
			22.09	54.13	4.97

(± standard deviation) concentrations of lipids (LIP), proteins (PRO), soluble carbohydrates and insoluble carbohydrates (IC), percentages of organic carbon (%C) and nitrogen (%N), and e content (E) in size fractionated zooplankton (0-250 m) of Kerguelen during the KEOPS2 survey Results of post-hoc tests performed on statistical analyses are indicated by letters, different lette indicating statistically significant differences. n = number of analyses One-way ANOVA (F), Tuckey post hoc test Kruskal-Wallis ANOVA (H), post hoc paired comparison test

Table 3

 3 Mean composition of zooplankton size fractions sampled during the KEOPS2 survey in Kerguelen waters

	Size fraction (µm)	Groups of organisms
	80-200	Diatoms (40-90%), Foraminifera, Detritus, Nauplii, Copepods, Eggs, Radiolarians, Pteropods
	200-500	Copepods (50-95%), Diatoms, Foraminifera, Pteropods, Polychaetes, eggs
	500-1000	Copepods (70-95%), Amphipods, Pteropods, Euphausiids, Chaetognaths
	1000-2000	Copepods (70-95%), Amphipods, Chaetognaths, Pteropods, Euphausiids, Salps
	> 2000

Copepods (60-95%), Chaetognaths, Euphausiids, Amphipods, Salps, Fish larvae, Annelids, Appendicularians, Pteropods

The dominant groups are indicated in bold characters in decreasing order of importance. The range of visually estimated volume percentages of the main groups is indicated into brackets Table

4

Mean (± SD) concentrations of lipids (LIP), proteins (PRO), soluble carbohydrates (SC), insoluble carbohydrates (IC), percentages of organic carbon (%C) and nitrogen (%N), and energy content (E) of some groups of large organisms

(> 2000

µm) n = number of samples analyzed. Results of post hoc tests performed on statistical analyzes are indicated by letters, different letters indicating statistically significant differences One-way ANOVA (F), Tuckey post hoc test Kruskal-Wallis ANOVA (H), post hoc paired comparison test http://eproofing.springer.com/journals_v2/printpage.php?token=yVZa0lxp-10rN8EC...   n = number of samples analyzed. Results of post hoc tests performed on statistical analyzes are indicated by letters, different letters indicating statistically significant differences One-way ANOVA (F), Tuckey post hoc test Kruskal-Wallis ANOVA (H), post hoc paired comparison test

Table 5

 5 Mean (± standard deviation) concentrations of lipids (LIP), proteins (PRO), soluble carbohydrates and insoluble carbohydrates (IC) in zooplankton (0-250 m) of Kerguelen during the KEOPS2 su and environmental parameters (TT, SS, Chl a, T-group)

	Station	n	Date	TT (°C )	SS	Chl-a (mg m )	T-group	LIP (mg.g dw)	PRO (mg.g dw)
								74.08 ±	202.19
	R2	12 26/10/2011 1.8 33.9 39	T1	24.19	± 81.92
								107.53	307.49
	A3-1	12 20/10/2011 1.8 34.0 106 T3	± 10.91	± 26.43
								67.66 ±	183.62
	A3-2	15 16/11/2011 1.8 34.0 372 T3	29.54	± 82.28
	TNS-7	15 22/10/2011 2.2 33.9 150 T1	65.58 ±	177.04
								31.19	± 82.67

http://eproofing.springer.com/journals_v2/printpage.php?token=yVZa0lxp-10rN8EC...   NOT in italics but in lower case -group = group of stations defined by Trull et al. (2015) on the chemometric characteristics of phytoplankton Park et al. (2014) and Trull et al. (2015) Lasbleiz et al. (2014) http://eproofing.springer.com/journals_v2/printpage.php?token=yVZa0lxp-10rN8EC...

Table 6

 6 Mean energy content of some groups of zooplankton organisms (kJ g dw)

	Reference Site	Copepods Euphausiids Annelids Salps
	This study	Kerguelen Islands	15.3	10.3	11.4	6.3
	Goswami
	et al.	Andaman Sea 17.2	13.9	-	6.9
	(1981)
	Percy and Fife (1981) Baffin Island	-	24.3	18.8	-
	Clarke et al. (1992)	South Georgia -	-	16.3	5.4
	Donnelly et al.(1994) Weddell Sea	14.4		9.2	-
	Torres et al. (1994)	Weddell Sea	-	17.5	-	-

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Acknowledgements

The authors thank S. Blain, project coordinator, B. Quéguiner chief scientist on board, and Captain B. Lassiette and the crew of the R/V Marion Dufresnes. Special thanks are due to M.P. Jouandet for her help in sample collection, M.F. Fontaine for biochemical analyzes, to L. Guillou and P. Richard (LIENSs Laboratory, La Rochelle University) for C and N analyzes, and to M. Paul, a native English speaker, for English correction. This work was supported by the French Research program of INSU-CNRS LEFE -CYBER ("Les enveloppes fluides et l'environnement-Cycles biogéochimiques, environnement et resources"), the French ANR (Agence Nationale de la Recherche, ANR-10-BLAN-0614 of SIMI-6 program, and ANR-09-CEXC-006-01 to M. Zhou and F. Carlotti), LABEX OT-MED (No. ANR-11-LABX-0061), the French CNES (Centre National d'Etudes Spatiales) and the French Polar Institute IPEV (Insitut Polaire Paul-Emile Victor). The project leading to this publication has received funding from European FEDER Fund under Project 1166-39417. Many thanks are addressed to the three reviewers for their constructive and helpful comments on an earlier version of the manuscript.

Compliance with ethical standards

Conflict of interest The authors declare that they have no conflict of interest.