Lola Gilbert
email: lola.gilbert@univ-lr.fr

Jérôme Spitz

Tiphaine Jeanniard-Du-Dot

Pack-ice seals contribute to biological transfers of iron in the Southern Ocean

Keywords: biological iron vector, Antarctica, sea ice, primary productivity, horizontal nutrient transfers, vertical nutrient transfers

The contribution of animals to biological transfers of essential nutrients in ecosystems is increasingly recognised as a significant component of ecosystem functioning. In the Southern Ocean (SO), primary productivity is primarily limited by the availability of iron in the euphotic zone, which makes animals locally releasing iron-rich faeces potential fertilizers of the SO food web. We quantified the amounts of iron released by four species of Antarctic pack-ice seals using a bioenergetic model set up with best available data on species abundance, energetics, diets and prey composition. We estimated that leopard, crabeater, Weddell and Ross seals together release 208 tonnes of iron per year (95 % confidence interval [104 -378]). This is equivalent to the current contribution of SO humpback whales and four times that of SO sperm whales. At the population level, crabeater seals are the major contributors (73%), followed by Weddell (21%), leopard (4%) and Ross seals (1%). Locally, each species shows different daily individual iron release rates, suggesting the patchy and transient impact of these iron releases on primary producers might differ according to species. Beyond quantitative aspects, pack-ice seals' contribution to horizontal, vertical and trophic transfers of iron, depends on their habitat preferences, on their ecology and behaviours at sea and on the ice. Although their role as iron vectors has been mostly overlooked so far, our results place pack-ice seals alongside whales

Introduction

Fluxes and transfers of essential nutrients largely determine the productivity, diversity and structure of trophic webs [START_REF] Atkinson | Consumer-driven nutrient dynamics in freshwater ecosystems: from individuals to ecosystems[END_REF][START_REF] Marzetz | Linking primary producer diversity and food quality effects on herbivores: A biochemical perspective[END_REF]. This applies both to major nutrients (e.g. nitrogen N and phosphorus P), which are present in large quantities throughout the trophic web, and to trace elements (e.g. iron Fe or manganese Mn), which are present in much smaller quantities but are no less essential to ecosystem functioning. Within marine trophic webs, primary producers such as phytoplankton are tied to the euphotic zone, i.e. the surface layer where sunlight penetrates and enables photosynthesis. Nutrients naturally tend to be adsorbed on particles sinking to deeper waters outside of this euphotic zone [START_REF] Boyd | The biogeochemical cycle of iron in the ocean[END_REF], so that phytoplankton face limiting conditions for their growth in many parts of the oceans [START_REF] Moore | Processes and patterns of oceanic nutrient limitation[END_REF]).

In the Southern Ocean (SO), one of the largest High Nutrient Low Chlorophyll (HNLC) regions in the world, the euphotic zone largely contains major nutrients (N and P), but primary productivity is limited by low concentrations of micronutrients, particularly iron (Hassler et al. 2012a;[START_REF] Tagliabue | The integral role of iron in ocean biogeochemistry[END_REF]. Physical iron transfer processes (e.g. atmospheric dust) are limited in large areas of this ocean due to their remoteness [START_REF] Martin | Iron in Antarctic waters[END_REF]). This low iron supply combined with a rapid biological uptake result in a rapid depletion of any form of bioavailable iron (i.e. chemical form that can be taken up and used by primary producers) in surface waters [START_REF] Smith | Circumpolar Deep Water and Shelf Sediments Support Late Summer Microbial Iron Remineralization[END_REF]. Thus, any process providing the SO euphotic zone with iron can shape phytoplankton communities and influence the structure and productivity of trophic webs (Hassler et al. 2012a). Moreover, phytoplankton is the first component of the biological carbon pump: inorganic carbon is transferred to organic matter via photosynthesis, and part of it is sequestered in sediments through sinking particles, passive advection, vertical migration of animals and sinking carcasses [START_REF] Falkowski | Biogeochemical controls and feedbacks on ocean primary production[END_REF][START_REF] Stukel | Stirring Up the Biological Pump: Vertical Mixing and Carbon Export in the Southern Ocean[END_REF].

This has motivated a substantial body of research to underpin iron transfers to the SO euphotic zone to further our understanding of the whole SO system, from processes regulating its productivity to the global cycling of nutrients (including carbon) and climate regulation (Hassler et al. 2012a).

The contribution of animals to biological nutrient cycling is increasingly recognised [START_REF] Wing | Seabirds and marine mammals redistribute bioavailable iron in the Southern Ocean[END_REF][START_REF] Wing | Marine micronutrient vectors: Seabirds, marine mammals and fishes egest high concentrations of bioactive metals in the subantarctic island ecosystem[END_REF][START_REF] Doughty | Global nutrient transport in a world of giants[END_REF][START_REF] Allgeier | Animal pee in the sea: consumer-mediated nutrient dynamics in the world's changing oceans[END_REF][START_REF] Ratnarajah | Pelagic Iron Recycling in the Southern Ocean: Exploring the Contribution of Marine Animals[END_REF]. Grazing and predation by fauna, and the subsequent release of waste (faeces and urine) in the euphotic zone, retain nutrients in the surface ocean for extended periods of time, rather than them being lost to depths with sinking materials. Among the SO fauna, air-breathing megafauna such as cetaceans, seals and penguins may contribute significantly to SO productivity by releasing iron-rich waste products in surface waters [START_REF] Lavery | Iron defecation by sperm whales stimulates carbon export in the Southern Ocean[END_REF][START_REF] Lavery | Whales sustain fisheries: blue whales stimulate primary production in the Southern Ocean[END_REF][START_REF] Nicol | Southern Ocean iron fertilization by baleen whales and Antarctic krill[END_REF][START_REF] Ratnarajah | The biogeochemical role of baleen whales and krill in Southern Ocean nutrient cycling[END_REF][START_REF] Wing | Seabirds and marine mammals redistribute bioavailable iron in the Southern Ocean[END_REF][START_REF] Otero | Seabird colonies as important global drivers in the nitrogen and phosphorus cycles[END_REF][START_REF] Sparaventi | Trace elements in Antarctic penguins and the potential role of guano as source of recycled metals in the Southern Ocean[END_REF]). These diving top predators differ from most marine life in that they are tied to surface waters for breathing.

As they shut down non-vital body functions during their dives to limit oxygen consumption [START_REF] Kooyman | Physiology of diving in marine mammals[END_REF], they release their waste products during their surface intervals while breathing and resting near the surface [START_REF] Roman | The whale pump: Marine mammals enhance primary productivity in a coastal basin[END_REF].

At the scale of the SO, the total amount of iron recycled by top predators is thought to be minor compared to that recycled by the microfauna (microbial community, microzooplankton and krill) [START_REF] Maldonado | Southern Ocean biological iron cycling in the pre-whaling and present ecosystems[END_REF]. The transient and localized enrichments of surface waters from waste release are also unlikely to be detected across large regions, as they operate on a smaller scale than physical iron supply processes [START_REF] Smith | Circumpolar Deep Water and Shelf Sediments Support Late Summer Microbial Iron Remineralization[END_REF]. Nevertheless, the release of iron-rich faecal matter could be locally significant in stimulating phytoplankton productivity.

Predator waste products are specific in that they are heavily concentrated in iron (from 63.4 to 3928 mg.Fe.kg -1 dry weight) [START_REF] Ratnarajah | The biogeochemical role of baleen whales and krill in Southern Ocean nutrient cycling[END_REF][START_REF] Wing | Seabirds and marine mammals redistribute bioavailable iron in the Southern Ocean[END_REF][START_REF] Wing | Marine micronutrient vectors: Seabirds, marine mammals and fishes egest high concentrations of bioactive metals in the subantarctic island ecosystem[END_REF][START_REF] Wing | Penguins and seals transport limiting nutrients between offshore pelagic and coastal regions of Antarctica under changing sea ice[END_REF][START_REF] Sparaventi | Trace elements in Antarctic penguins and the potential role of guano as source of recycled metals in the Southern Ocean[END_REF]. These total iron concentrations (i.e. both dissolved and particulate fraction) are six to eight orders of magnitude higher than nanomolar concentrations of euphotic waters [START_REF] Ratnarajah | The biogeochemical role of baleen whales and krill in Southern Ocean nutrient cycling[END_REF][START_REF] Wing | Seabirds and marine mammals redistribute bioavailable iron in the Southern Ocean[END_REF]. In baleen whale faeces, the dissolved fraction alone can be three orders of magnitude higher than SO surface waters (186 -754 nmol.Fe.L -1 ) [START_REF] Ratnarajah | Physical speciation and solubility of iron from baleen whale faecal material[END_REF]. Uncertainties remain regarding the bioavailability of iron from waste products to primary producers [START_REF] Ratnarajah | Physical speciation and solubility of iron from baleen whale faecal material[END_REF]). Yet, primary producers can use iron in different chemical forms [START_REF] Nodwell | Direct use of inorganic colloidal iron by marine mixotrophic phytoplankton[END_REF][START_REF] Lis | Iron bioavailability to phytoplankton: An empirical approach[END_REF][START_REF] Tagliabue | The integral role of iron in ocean biogeochemistry[END_REF], either dissolved in seawater (< 0.2 μm, labile or colloid forms) or particulate (> 0.2 μm, e.g. bound to organic ligands), and SO phytoplankton strains have shown increased growth and productivity when fertilized by seabird guano and baleen whale faeces [START_REF] Smith | Preliminary investigation into the stimulation of phytoplankton photophysiology and growth by whale faeces[END_REF][START_REF] Shatova | Phytoplankton community structure is influenced by seabird guano enrichment in the Southern Ocean[END_REF]. Thus, at least a fraction of iron released in this waste is likely to be recycled in the trophic web, either immediately upon release, after remineralisation by the microbial community or after complexation with free bacterial ligands in surface waters [START_REF] Ratnarajah | Physical speciation and solubility of iron from baleen whale faecal material[END_REF]. SO megafauna species are also highly mobile and are the largest iron vectors that can operate against physical processes. For example, they can not only contribute to maintaining the iron pool in the euphotic zone (i.e. true recycling, when iron released in waste originates in the euphotic zone), but also introduce new iron, e.g. released by sperm whales feeding at depth [START_REF] Lavery | Iron defecation by sperm whales stimulates carbon export in the Southern Ocean[END_REF]. Finally, predators tend to form significant aggregations in space and time, which can support "hot spots" and "hot moments" of biochemical cycling [START_REF] Atkinson | Consumer-driven nutrient dynamics in freshwater ecosystems: from individuals to ecosystems[END_REF][START_REF] Mcinturf | Vectors with autonomy: what distinguishes animal-mediated nutrient transport from abiotic vectors?[END_REF][START_REF] Subalusky | Context dependency of animal resource subsidies[END_REF].

These functional traits distinguish them from other biological or physical iron vectors [START_REF] Mcinturf | Vectors with autonomy: what distinguishes animal-mediated nutrient transport from abiotic vectors?[END_REF][START_REF] Subalusky | Context dependency of animal resource subsidies[END_REF]. Fully underpinning the drivers of food-web structure and productivity requires a thorough understanding of all the processes involved, including of biologically-mediated nutrient transfers, despite their complexity due to their local and transient scale.

To provide a better understanding of the role of megafauna in biologically-mediated nutrient transfers, we investigated the contribution of four resident species of pack-ice seals: the crabeater seal Lobodon carcinophaga, the Weddell seal Leptonychotes weddelli, the leopard seal Hydrurga leptonyx and the Ross seal Ommatophoca rossii to iron transfers in the SO system. As top predators, they may impact the structure and the biomass of the SO ecosystem through predation [START_REF] Krause | Novel foraging strategies observed in a growing leopard seal (Hydrurga leptonyx) population at Livingston Island, Antarctic Peninsula[END_REF][START_REF] Goetz | Temporal variation in isotopic composition and diet of Weddell seals in the western Ross Sea[END_REF][START_REF] Staniland | Long term movements and activity patterns of an Antarctic marine apex predator: The leopard seal[END_REF]).

However, their role as vectors of limiting micronutrients has received little attention despite iron concentrations in their faeces (950.8 ± 148.9 mg.Fe.kg -1 dry weight [START_REF] Wing | Penguins and seals transport limiting nutrients between offshore pelagic and coastal regions of Antarctica under changing sea ice[END_REF])) in line with that of whales and seabirds [START_REF] Nicol | Southern Ocean iron fertilization by baleen whales and Antarctic krill[END_REF][START_REF] Wing | Penguins and seals transport limiting nutrients between offshore pelagic and coastal regions of Antarctica under changing sea ice[END_REF].

As a basis for investigating their influence on the functioning of SO ecosystem through biological iron cycling, we used a bioenergetic model linking metabolism, diet and population estimates of pack-ice seals. We estimate iron release in faeces at individual and population levels, and we compare seal-mediated iron releases with those of other SO megafauna taxa (whales and penguins). Finally, we discuss the different iron transfers pack-ice seals may operate and highlight functional characteristics relevant to ecosystem functioning. While we recognise that iron is not the only essential micronutrient released in pack-ice seal faeces (e.g. cobalt Co and manganese Mn, see [START_REF] Wing | Penguins and seals transport limiting nutrients between offshore pelagic and coastal regions of Antarctica under changing sea ice[END_REF]), we restricted our study to iron given its importance in the SO ecosystem functioning and in overall ocean biogeochemical cycling.

Material & Methods

Estimating Fe release using a bioenergetic model

We used a bioenergetic model of prey biomass consumption to estimate iron consumption and egestion of individual seals. Daily energetic needs were estimated from basal metabolic rate (BMR, in kJ.day -1 ) using [START_REF] Kleiber | The Fire of Life. An Introduction to Animal Energetics[END_REF] formula (previously applied to marine mammal populations, e.g. [START_REF] Roman | The whale pump: Marine mammals enhance primary productivity in a coastal basin[END_REF], [START_REF] Spitz | Prey consumption by cetaceans reveals the importance of energy-rich food webs in the Bay of Biscay[END_REF]) and including the additional cost of daily activities through the species-specific metabolic index β [START_REF] Spitz | Cost of living dictates what whales, dolphins and porpoises eat: the importance of prey quality on predator foraging strategies[END_REF][START_REF] Spitz | Prey consumption by cetaceans reveals the importance of energy-rich food webs in the Bay of Biscay[END_REF].

For an individual i:

𝐴𝐷𝑀𝑅 ! = 𝛽 ! × 𝐵𝑀𝑅 ! = 𝛽 ! × 293.1 × 𝐵𝑀 ! " # ⁄ (1)
with ADMR the seal's average daily metabolic rate (in kJ.day -1 ) and BM its body mass (in kg).

We then estimated the daily ration R (in kg.day -1 ) needed for each individual to meet its ADMR, given the mean energy content of its diet (E, in kJ.kg -1 fresh weight) and the digestive assimilation efficiency (AE):

𝑅 ! = %&'( ! %) ×) ! (2)
Using the average iron concentration in the diet (xFe, in mg.kg -1 fresh weight), we estimated the daily iron consumption and thus the daily amount of iron released (Fed,i, in mg.day -1 ) given the iron release rate rFe:

𝐹𝑒 ,,! = 𝑅 ! × 𝑥 ./ ! × 𝑟 ./ (3)
We then extended this daily individual rate to an annual population rate using population abundance estimate A. We finally obtained the total amount of iron released by each species' population in the SO (Fetot, in t.yr -1 ):

𝐹𝑒 010 = 𝐹𝑒 ,,! × 365 × 𝐴 × 1 /2 (4)
Setting model parameters

Body mass BM

The average seals' body mass BM (in kg) for each species were taken from ranges found in the literature (Table 1). BM was set up as a random variable following a normal distribution with the above-mentioned mean value and a standard deviation of 20 %. Only adults were considered for our calculations.

Species-specific metabolic multiplier β

The metabolic index β is a species-specific indicator of the "cost of living": it accounts for the cost of activities and metabolic efficiency [START_REF] Spitz | Cost of living dictates what whales, dolphins and porpoises eat: the importance of prey quality on predator foraging strategies[END_REF]. Studies focusing on the metabolism of pack-ice seals (e.g. [START_REF] Castellini | Metabolic Rates of Freely Diving Weddell Seals: Correlations With Oxygen Stores, Swim Velocity and Diving Duration[END_REF], [START_REF] Renouf | Seasonal variation in the metabolic rate of harp seals: Unexpected energetic economy in the cold ocean[END_REF]) suggest they may have a relatively low metabolism. Based on what is known of the metabolism of polar seals and estimated Activity Metabolic Rates of Weddell seals [START_REF] Williams | A killer appetite: Metabolic consequences of carnivory in marine mammals[END_REF][START_REF] Hoelzel | Sealing, whaling and krill fishing in the Southern Ocean: past and possible future effects on catch regulations[END_REF], we considered β as a random variable with a mean value of 2 ± 0.5 for Weddell, Ross and crabeater seals and 3 ± 0.5 for leopard seals, and set minimum and maximum values using a truncated normal distribution to exclude non-physiologically viable values (Table 1). As this factor accounts for the cost of activities averaged over the individual year cycle (i.e. including breeding and moulting period), we did not include any fasting period in the model.

Assimilation efficiency AE

Experimental studies on captive pinnipeds measured AE ranging between 79 and 96.8 % (Ashwell-Erickson and Elsner 1981; [START_REF] Keiver | Metabolizable energy requirements for maintenance and faecal and urinary losses of juvenile harp seals (Phoca groenlandica)[END_REF][START_REF] Ronald | Energy requirements for maintenance and faecal and urinary losses of the grey seal (Halichoerus grypus)[END_REF][START_REF] Mårtensson | Digestibility of krill (Euphausia superba and Thysanoessa sp) in minke whales (Balaenoptera acutorostrata) and crabeater seals (Lobodon carcinophagus)[END_REF].

We applied a truncated normal distribution around the mean value of 0.9 with a standard deviation of 0.05 and lower and upper bounds at 0.80 and 0.95 (Table 1).

Iron release rate rFe

Adult mammals (excluding pregnant and lactating females) release most consumed micronutrients -including iron -through faeces, with release rates ≥ 90% [START_REF] Candela | Iron Absorption by Humans and Swine from Fe(III)-EDTA. Further Studies[END_REF].

Bioenergetic models that estimated iron egestion for cetaceans used an iron release rate of 0.85 [START_REF] Lavery | Iron defecation by sperm whales stimulates carbon export in the Southern Ocean[END_REF] or of 0.7 -0.9 with a base value at 0.8 (Ratnarajah et al. 2016a). We considered rFe as a random variable following a uniform distribution with a minimum of 0.70 and a maximum of 0.9 (Table 1).

Population abundance A

The largest survey on abundance estimates of the four species of pack-ice seals in the SO was the Antarctic Pack-Ice Seal (APIS) program survey, initiated by the Scientific Committee on Antarctic Research (SCAR) in 1999/2000 [START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF]. It used a unified distance sampling protocol based on the counting of hauled-out animals and satellite tagging studying haul-out behaviours, and had the broadest spatial coverage in the SO. We applied a truncated normal distribution using mean values and intervals from the APIS survey [START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF] to simulate variability in the abundance parameter (Table 1).

Average energy E and iron content xFe of the diet

We described the diet of each species using five functional prey groups: krill, fish, cephalopod, 2). For Weddell and leopard seals, we made small adjustments to the diets in [START_REF] Mccormack | Alternative energy pathways in Southern Ocean food webs: Insights from a balanced model of Prydz Bay, Antarctica[END_REF] to adapt to the functional prey groups we used, and to better match descriptions of diets reported in the literature that differed from those in [START_REF] Mccormack | Alternative energy pathways in Southern Ocean food webs: Insights from a balanced model of Prydz Bay, Antarctica[END_REF]. Krill was included as part of the diet of Weddell seals, in small proportions (see [START_REF] Casaux | The diet of the Weddell seal Leptonychotes weddelli at Harmony point, South Shetland Islands[END_REF], [START_REF] Lake | Regional, temporal and fine-scale spatial variation in Weddell seal diet at four coastal locations in east Antarctica[END_REF]), and the percentage of consumption of cephalopods of leopard seals was lowered whereas that of krill and penguins was increased (see [START_REF] Hall-Aspland | Summer diet of leopard seals (Hydrurga leptonyx) in Prydz Bay, Eastern Antarctica[END_REF], [START_REF] Casaux | Diet of the Leopard Seal Hydrurga leptonyx at the Danco Coast, Antarctic Peninsula[END_REF][START_REF] Forcada | Modelling predation by transient leopard seals for an ecosystem-based management of Southern Ocean fisheries[END_REF]) (Table 2).

To determine an average energy content and iron concentration per prey group (Epg and xFe,pg, with pg being the prey group index), we compiled analytical data of 205 prey samples from 9 studies and 300 prey samples from 17 studies, respectively (see Table 2 for references). We then used these values and the percentage of prey groups in diets (in weight, Wpg) to determine the mean energy content (E) and the mean iron concentration (xFe) in each species' diet (Table 1,2):

𝐸 = ∑ 𝑊 34 × 𝐸 34 34 (5) 𝑥 ./ = ∑ 𝑊 34 × 𝑥 ./,34 34 (6)
Diets vary individually, seasonally and spatially, sometimes significantly depending on species. The energy content and iron concentration of their prey also vary depending on life stage, season, location or body part (e.g. [START_REF] Yamamoto | Tissue distribution of heavy metals in Weddell seals (Leptonychotes weddellii)[END_REF], Jerez et al. (2013b), [START_REF] Kim | Monthly variations in the nutritional composition of Antarctic krill Euphausia Superba[END_REF], [START_REF] Ruck | Regional differences in quality of krill and fish as prey along the Western Antarctic Peninsula[END_REF]). We set up a normally distributed standard deviation of 20% around the calculated mean values (E and xFe) to account for the intrinsic variability in the composition of both prey items and diets of individuals.

Differences between levels of iron release

Dimensions of parameters and outputs (n = 1 e5 ) resulting from the Monte Carlo simulations precluded the use of standard statistical tests of significance when comparing outputs for two species (at both daily individual and annual population levels). Instead, we assessed unilateral binary relations calculating the proportion of values from one group superior to the other group. We considered a difference to be significant if this proportion, called p, was ≥ 95% or ≤ 5%. To express results in a way comparable to classic statistical tests (i.e. significant difference when p-value ≤ 0.05), we either reported p directly if it was ≤ 5%, or reported (1 -p) when it was ≥ 95%, adjusting for the direction of the binary relation of interest. For example, if the result of a test looking at difference in iron released between species A and B was 0.99, we considered iron release by species A to be significantly greater than iron release by species B with p = 0.01.

In addition, levels of iron release by pack-ice seals were compared with that reported in the literature for other S0 megafauna taxa and species (large whales and penguins).

Sensitivity analysis

We conducted a sensitivity analysis to quantify how the sources of uncertainty in the model parameters affected the uncertainty of the final output, and identify which parameters were more important in influencing this uncertainty [START_REF] Saltelli | Baleen whale prey consumption based on high-resolution foraging measurements[END_REF]. We adopted a global approach, in which the effect of a factor on the output can be estimated when all other inputs are varying. It enables the identification of interactions, and does not require an additive or linear model [START_REF] Cariboni | The role of sensitivity analysis in ecological modelling[END_REF]). We opted for the Sobol method, a variance-based approach where the variance of the output can be decomposed into the contributions imputable to each input factor. We used R package "sensitivity" (version 1.27.0) with function "soboltSalt", and computed first-order (with respect to each input factor individually) and total (inclusive of interactions between input factors) sensitivity indices for all parameters of the model.

Code to reproduce the full analysis in R is available on Github (https://github.com/Lolasan/FeSthOpinn.git).

Results

Statistics associated to all parameters of the model as a result of Monte-Carlo simulations (Table 1) and calculations from the bioenergetic model (equations 1-6, Table 2) are available in Online Resource 1. The average daily metabolic rates of pack-ice seals ranged from 28 072 kJ.d 3). This order remains for individual daily iron consumption and release rates, so that an individual leopard seal is estimated to consume and release ~ 9 times more iron per day than a crabeater seal (p < 0.01) (Fig. 1).

Annual estimates at the population level show that crabeater seals are the main contributors to iron release (151 [60 -315] t.Fe.yr -1 , significantly above the other three species with p < 0.01). They are followed by Weddell seals (44 [13 -106] t.Fe.yr -1 , significantly above leopard and Ross seals with p < 0.03). The contribution of leopard seals (9 [2 -23] t.Fe.yr -1 ) is then not significantly higher than that of Ross seals (3 [1;8] t.Fe.yr -1 , p = 0.09) (Fig. 1, Table 3).

Together, these four species release 208 [104 -378] t.Fe.yr -1 through their faeces (Fig. 2). This is equivalent to the current contribution of populations of SO humpback whales (221 [144 -394] t.Fe.yr -1 ) or the iron released in guano of Chinstrap, Adelie and Gentoo penguins populations in a year (169 t.Fe.yr -1 ) (Fig. 2 and references therein, Online Resource 2). It is also [193 -590] t.Fe.yr -1 ), respectively.

Sobol sensitivity indices show that population abundance A is the most influential model parameter, followed by the mean energy content of the diet E, the mean iron concentration of the diet xFe, the metabolic multiplier β and individual body mass BM (Fig. 3).

In contrast, the assimilation efficiency AE and iron release rate rFe have very little influence on the model output. Estimates from (4), originally calculated for the reproduction period only, have been raised to annual estimates using a simple cross product. Central point represents mean estimates and lower and upper bar the 2.5 and 97.5 percentiles, respectively. Percentiles were not reported with estimates in (1), ( 2) and (4). Detailed numbers are provided in Online Resource 2

Discussion

Although ice seals play an important role in trophic networks of the SO, their role in recycling iron, and the cascading effects this may have on ecosystem structure and productivity, are still unknown. Using the best available data on population abundance, diets, metabolism and prey composition, we provided the first estimates of total iron released in faeces of pack-ice seals inhabiting the SO. Based on these quantitative results, we can assess the relative contribution of the four species to the biological cycling of this key limiting nutrient at individual and population levels, and within the broader community of SO air-breathing megafauna.

Knowledge of the ecology of these pack-ice seals allows us to go further and discuss functional aspects of their contribution.

Uncertainties

As in any modelling exercise, our results carry uncertainties linked to the inherent variability of the system we are trying to model, and to biases in the model structure. While the uncertainty associated with each parameter can be considered individually, they were combined in our model through the use of Monte-Carlo simulations. Most of the intrinsic variability and uncertainty was inferred. We may have under-or over-estimated them, but we leaned towards the conservative side.

The most influential parameter was population abundance estimate, as expected given the size of the populations and the large confidence intervals associated with each estimate.

Global abundance estimates for highly mobile and marine predators are challenging to obtain, even more so given the spatial extent, remote location and climatic conditions of the SO. The data used here are from the more recent large-scale coordinated effort to estimate the abundance of these pack-ice seals throughout their range [START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF]). Even though it was in the early 2000s, it can be considered the best available data.

The mean energy content and iron concentration in the seals' diet also have a significant influence on our final model outputs. As mentioned earlier, seal diets vary individually, seasonally and spatially. In addition, prey energy content and iron concentrations also vary with life stage, season, location or body part [START_REF] Kim | Monthly variations in the nutritional composition of Antarctic krill Euphausia Superba[END_REF][START_REF] Ruck | Regional differences in quality of krill and fish as prey along the Western Antarctic Peninsula[END_REF]Ratnarajah et al. 2016b). For example, we could not directly account for the belly-biting behaviours of leopard seals that tend to consume only muscle and viscera when preying on pinnipeds or penguins (D. Krause, personal comm.). Liver, part of the viscera, highly concentrates iron, but we only considered the muscle iron concentration as a conservative choice. As we applied a relatively high standard deviation to these parameters to account for all these interconnected sources of variability, they influence the output of our model the most after population abundance.

The individual body mass BM, the metabolic cost of activities multiplier β, the assimilation efficiency AE and iron release rate rFe carry little uncertainty compared to the other parameters. BM, β and AE are well known for pinnipeds (e.g. [START_REF] Hoelzel | Sealing, whaling and krill fishing in the Southern Ocean: past and possible future effects on catch regulations[END_REF]; [START_REF] Trites | Prey Consumption[END_REF]). Furthermore, some of our intermediary output parameters, such as daily prey intake, including as a percent of body mass (Table 3), are within the range of those previously published for pinnipeds (e.g. [START_REF] Hoelzel | Sealing, whaling and krill fishing in the Southern Ocean: past and possible future effects on catch regulations[END_REF]; [START_REF] Trites | Prey Consumption[END_REF]; [START_REF] Nilssen | Diet and prey consumption of grey seals ( Halichoerus grypus ) in Norway[END_REF]). This provides confidence in our model outputs (Table 3 andOnline Resource 1). rFe has in contrast never been measured on pinnipeds, but as mammals, adult seals (excluding pregnant and lactating females, see [START_REF] Shero | Iron mobilization during lactation reduces oxygen stores in a diving mammal[END_REF]) are expected to defaecate any iron that they do not require to maintain their body homeostasis [START_REF] Candela | Iron Absorption by Humans and Swine from Fe(III)-EDTA. Further Studies[END_REF][START_REF] Ratnarajah | The biogeochemical role of baleen whales and krill in Southern Ocean nutrient cycling[END_REF]). Its small influence on the final output variability was expected, as it is close to one.

Finally, most of these uncertainties, except for the partial consumption of specific prey species, were also included in previously published models on air-breathing megafauna taxa [START_REF] Otero | Seabird colonies as important global drivers in the nitrogen and phosphorus cycles[END_REF][START_REF] Ratnarajah | Pelagic Iron Recycling in the Southern Ocean: Exploring the Contribution of Marine Animals[END_REF], which justifies comparisons between their results

and ours for contributions to the global SO system.

Quantitative contribution of pack-ice seals to total iron biological cycling in the

SO

Daily individual iron release

Daily amounts of iron released per individual differ between species (Table 3), due to different iron contents in their diet (linked to iron levels in their prey and consumption rate of each prey type) as well as intake levels and released biomass (linked to species' body mass and metabolic requirements). As long-lived top predators, pinnipeds and penguins have higher iron content than lower trophic level prey (e.g. zooplankton). This explains the high mean iron concentration in the diet of leopard seals (including fish and krill but also 21 and 6% of pinnipeds and penguins, respectively). Weddell and Ross seals prey mainly on medium trophic levels (fish and cephalopods) compared to crabeater seals, which feed almost exclusively on krill. In addition, crabeater and Ross seals are the smallest species, and they likely have lower metabolic requirements. As a result, individual crabeater and Ross seals release seven to nine times less iron daily than leopard seals and two to three times less iron than Weddell seals.

Iron release occurs as patchy, transient defecation events, limited to small spatial and temporal scales, and the response of primary producers to this iron supply also begins at these scales. Total faecal iron will affect local primary producers differently depending on its quantity and chemical form, on biochemical conditions (e.g. temperature, light, availability of other nutrients), and on the specific needs of local primary producer communities [START_REF] Allgeier | Animal pee in the sea: consumer-mediated nutrient dynamics in the world's changing oceans[END_REF]. However, as a general approach, individual daily iron release rates can provide an indication of the magnitude of the impact of iron fertilisation on local primary productivity (and hence, the trophic web), assuming that the faeces of each species have similar biochemical properties in terms of the speciation of the iron released and the time it remains in surface waters. Thus, our results suggest that the magnitude of the potential impact of leopard seal defecation is greater than that of crabeater, Weddell and Ross seals, given similar biochemical conditions.

Moreover, these results imply that the role of pack-ice seals as iron recycling vectors may (i) vary between seasons and locations, following the variation in the diet of seals (e.g. [START_REF] Lake | Regional, temporal and fine-scale spatial variation in Weddell seal diet at four coastal locations in east Antarctica[END_REF]) and the variation of iron concentrations in their prey [START_REF] Kim | Monthly variations in the nutritional composition of Antarctic krill Euphausia Superba[END_REF][START_REF] Ruck | Regional differences in quality of krill and fish as prey along the Western Antarctic Peninsula[END_REF]), (ii) vary if these predators switch prey, for example in response to changes in prey availability or changes in their environment (e.g. [START_REF] Wing | Penguins and seals transport limiting nutrients between offshore pelagic and coastal regions of Antarctica under changing sea ice[END_REF]). Given the severe changes occurring in the SO [START_REF] Gutt | The Southern Ocean ecosystem under multiple climate change stresses -an integrated circumpolar assessment[END_REF], this should be considered when assessing SO ecosystem functioning [START_REF] Wing | Penguins and seals transport limiting nutrients between offshore pelagic and coastal regions of Antarctica under changing sea ice[END_REF].

Annual population iron release

Although individual crabeater seals release the least amount of iron daily, they contribute the most to iron release within the pack-ice seal community at the population level (73%, Table 3) because they are by far the most numerous [START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF]) (Table 1). Individual

Weddell seals release almost three times more iron than crabeater seals daily, but their population is the second largest [START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF]) (Table 1), so they contribute significantly to total pack-ice seal iron cycling (21%). Despite feeding on higher, iron-enriched trophic levels, the lower populations of leopard and Ross seals are only small contributors to the global biological cycling of iron in the entire SO (4 and 1% respectively).

Contribution of pack-ice seals to biological iron cycling within the air-breathing SO megafauna community

This study identifies pack-ice seals as major contributors to the biological supply of iron to the SO euphotic zone (Fig. 2). In several aspects, the role of pack-ice seals in iron biological transfers to the SO euphotic zone relates to that of penguins, rather than that of whales. Like penguins [START_REF] Sparaventi | Trace elements in Antarctic penguins and the potential role of guano as source of recycled metals in the Southern Ocean[END_REF], pack-ice seals are long-lived, year-round residents of the SO with abundant and widely distributed populations; they mostly prey on low (krill) or medium trophic levels (fish and cephalopods). Pack-ice seals are specific in that they are dependent on sea ice for parts of their cycle [START_REF] Shirihai | The Complete Guide to Antarctic Wildlife -Birds and Marine Mammals of the Antarctic Continent and the Southern Ocean, 2nd edn[END_REF], which only applies to Adelie penguins. The likely release of large amounts of waste products directly onto the sea ice is an important factor to consider in terms of ecosystem functioning. Sea ice retains nutrients and primary producer biomass (phytoplankton and algae) sometimes in greater amounts than the underlying euphotic zone, and its annual advance and retreat plays a great role in the SO ecosystem [START_REF] Massom | Antarctic sea ice change and variability -Physical and ecological implications[END_REF]. In spring, biological activity along the ice edge is boosted as the melting ice releases iron and algae when light conditions are favourable, which stimulates overall productivity [START_REF] Massom | Antarctic sea ice change and variability -Physical and ecological implications[END_REF]Lannuzel et al. 2016b). The dependence of pack-ice seals on sea ice also makes their spatial distribution relatively complementary to that of whales [START_REF] Laws | Seals and whales of the Southern Ocean[END_REF][START_REF] Hoelzel | Sealing, whaling and krill fishing in the Southern Ocean: past and possible future effects on catch regulations[END_REF]. Obviously, the scale at which whales, seals or penguins individually release iron is as different as their respective body mass range. Yet, baleen whales are migratory species, most individuals spending only the summer season in the SO and targeting highly productive areas. In addition, while some whale populations have recovered significantly from industrial whaling (e.g. humpbacks [START_REF] Pallin | High pregnancy rates in humpback whales (Megaptera novaeangliae) around the Western Antarctic Peninsula, evidence of a rapidly growing population[END_REF])), some others still struggle to recover (e.g. blue (Savoca et al. 2021)). In contrast, packice seals, like penguins, have never been subject to any high-level human exploitation (Hofman 2017). It is therefore possible that populations of pack-ice seals (and penguins) have taken on a comparatively larger role in the fertilization of the SO system than they used to.

While we provide another piece to the puzzle of the contribution of SO air-breathing megafauna to iron biological cycling, it is important to note that two important pinniped species are still missing: Antarctic fur seals Arctocephalus gazella and southern elephant seals Mirounga leonina, although they spend most of their annual cycle in SO waters. Like whales, their populations have been severely depleted due to exploitation of their blubber (elephant seals) and fur (Antarctic fur seals), with contrasting recovery successes between populations [START_REF] Laws | History and present status of southern elephant seal populations[END_REF][START_REF] Hucke-Gaete | Examining natural population growth from near extinction: The case of the Antarctic fur seal at the South Shetlands, Antarctica[END_REF]). Unlike pack-ice seals, these species are tied to ice-free land to breed and moult [START_REF] Shirihai | The Complete Guide to Antarctic Wildlife -Birds and Marine Mammals of the Antarctic Continent and the Southern Ocean, 2nd edn[END_REF]. They carry out nutrient transfers from sea to land, which significantly influence the structure of ecosystems around their colonies [START_REF] Bokhorst | Nitrogen inputs by marine vertebrates drive abundance and richness in Antarctic terrestrial ecosystems[END_REF]. They may also forage in waters around ice-free land for extended periods. As these waters tend to be richer in iron due to land weathering compared to pelagic waters or waters surrounding sea ice, the importance of these seals in stimulating primary productivity is likely to be different than that of pack-ice seals. Yet, their contribution to the biological cycling of iron in the SO has never been quantified and should be investigated in the future.

Functional characteristics of the contribution of pack-ice seals to SO biological iron cycling

Spatial segregation and aggregation

Dispersal ranges of crabeater, Weddell, leopard and Ross seals greatly overlap around the Antarctic continent (Fig. 4), but these four species do not display the same habitat preferences (Fig. 5). Thus, they may deposit their waste in areas with different inherent characteristics in terms of iron supply and demand of the community of primary producers. Crabeater and

Weddell seals generally haul out on sea-ice throughout the year, while Ross and leopard seals spend more time in pelagic, ice-free waters outside of the breeding and moulting seasons [START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF]. Crabeater seals primarily use pack-ice [START_REF] Southwell | Taking account of dependent species in management of the Southern Ocean krill fishery: Estimating crabeater seal abundance off east Antarctica[END_REF][START_REF] Bengtson | Distribution, density, and abundance of pack-ice seals in the Amundsen and Ross Seas, Antarctica[END_REF] whereas Weddell seals are primarily bound to fast-ice and nearby pack-ice, relying on breathing holes to navigate between ice-floes [START_REF] Shirihai | The Complete Guide to Antarctic Wildlife -Birds and Marine Mammals of the Antarctic Continent and the Southern Ocean, 2nd edn[END_REF][START_REF] Thomas | Weddell Seal Leptonychotes weddellii[END_REF][START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF][START_REF] Davis | Classification and behavior of freeranging Weddell seal dives based on three-dimensional movements and video-recorded observations[END_REF][START_REF] Goetz | Temporal variation in isotopic composition and diet of Weddell seals in the western Ross Sea[END_REF]. Whether iron is released on packice or on fast-ice -and thus seals different spatial contributions -has different implications, given the central role of sea-ice in retaining and releasing iron in SO waters (Lannuzel et al. 2016b;[START_REF] Genovese | A proposed seasonal cycle of dissolved iron-binding ligands in Antarctic sea ice[END_REF]). Pack-ice, for instance, may grow and melt further offshore, releasing the iron it contains in more limited waters (Lannuzel et al. 2016a) than fast-ice melting in coastal, enriched waters. Ross seal is the most pelagic species, spending most of the year in open pelagic water and returning to pack-ice only for short periods of time to breed and moult [START_REF] Thomas | Ross Seal Ommatophoca rossii[END_REF][START_REF] Arcalis-Planas | Limited use of sea ice by the Ross seal ( Ommatophoca rossii), in Amundsen Sea, Antarctica, using telemetry and remote sensing data[END_REF][START_REF] Brault | Trophic position and foraging ecology of Ross, Weddell, and crabeater seals revealed by compound-specific isotope analysis[END_REF]. As Ross seals forage in low productivity, open ocean food web where conditions are particularly limiting, the local impact of the iron they release could be more significant. Habitat preferences of leopard seals are still poorly known [START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF]), but overall, they seem to prefer marginal pack-ice areas with extended periods spent in pelagic waters outside of the breeding and moulting seasons [START_REF] Bengtson | Distribution, density, and abundance of pack-ice seals in the Amundsen and Ross Seas, Antarctica[END_REF][START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF]. They can migrate further north than other species (especially between 90°W and 90°E, Fig. 4) and are regular visitors to subantarctic islands and southern continents [START_REF] Walker | Seasonal occurrence and diet of leopard seals (Hydrurga leptonyx) at Bird Island, South Georgia[END_REF][START_REF] Nordøy | Movements and dive behaviour of two leopard seals (Hydrurga leptonyx) off Queen Maud Land, Antarctica[END_REF][START_REF] Rogers | Leopard Seal Hydrurga leptonyx[END_REF][START_REF] Krause | Novel foraging strategies observed in a growing leopard seal (Hydrurga leptonyx) population at Livingston Island, Antarctic Peninsula[END_REF][START_REF] Staniland | Long term movements and activity patterns of an Antarctic marine apex predator: The leopard seal[END_REF]. They can display residency around islands for extended periods [START_REF] Walker | Seasonal occurrence and diet of leopard seals (Hydrurga leptonyx) at Bird Island, South Georgia[END_REF], and could thus contribute to nutrient transfers from sea to land.

Patterns of animal aggregation could also locally shape the iron supply rate, a key parameter controlling primary productivity [START_REF] Howarth | Nutrient limitation of net primary production in marine ecosystems[END_REF][START_REF] Mcinturf | Vectors with autonomy: what distinguishes animal-mediated nutrient transport from abiotic vectors?[END_REF]. Ross and leopard seals are mostly observed alone when hauled-out [START_REF] Bengtson | Distribution, density, and abundance of pack-ice seals in the Amundsen and Ross Seas, Antarctica[END_REF][START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF], while Weddell seals are most likely to form groups of up to 20 individuals on fastice [START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF]. Crabeater seals can form large groups on pack-ice [START_REF] Bengtson | Distribution, density, and abundance of pack-ice seals in the Amundsen and Ross Seas, Antarctica[END_REF][START_REF] Joiris | Summer social structure of crabeater seal Lobodon carcinophaga in the Amundsen Sea, Antarctica[END_REF] but are mostly observed alone or in pairs [START_REF] Gales | Do crabeater seals forage cooperatively? Deep[END_REF][START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF]. More noteworthy, they can form large aggregations of up to 350 individuals when foraging [START_REF] Gales | Do crabeater seals forage cooperatively? Deep[END_REF][START_REF] Gottfried | Cooperative foraging behaviour by crabeater seals (Lobodon carcinophaga) at Pleneau Island, Antarctic Peninsula[END_REF][START_REF] Joiris | Summer social structure of crabeater seal Lobodon carcinophaga in the Amundsen Sea, Antarctica[END_REF]. These four seal species could therefore affect iron supply at different spatial and temporal scales. In addition to these species-specific aggregation patterns, there may be some overlap between the four species, particularly in summer (Fig. 4) in highly productive areas (e.g. Ross and Weddell seas). These areas are also targeted by other megafauna taxa, i.e. penguin colonies and aggregations of feeding whales [START_REF] Herr | First evidence for fin whale migration into the Pacific from Antarctic feeding grounds at Elephant Island[END_REF]. As all individuals take advantage of the increased productivity, they synchronously participate in maintaining iron levels in the euphotic zone when conditions are favourable for primary producers. Until now, studies have focused on the role of single species or group of species from the same taxa in iron biological cycling in the SO. However, the localized and sequential aggregation of communities of air-breathing predators likely have a synergistic impact. The rapid recycling of iron they mediate on these hot spots and during these key periods could extend seasonal productivity by maintaining higher levels of bioavailable iron when compared to the influence of external inputs [START_REF] Boyd | Biotic and abiotic retention, recycling and remineralization of metals in the ocean[END_REF][START_REF] Mcinturf | Vectors with autonomy: what distinguishes animal-mediated nutrient transport from abiotic vectors?[END_REF]. While challenging, studying such complex biochemical transfers at community levels and at fine spatial and temporal scales would significantly further our understanding of SO ecosystems functioning.

Horizontal iron transfers

Pack-ice seals are highly mobile predators. They can travel several kilometres on a single dive (e.g. Weddell seals [START_REF] Thomas | Weddell Seal Leptonychotes weddellii[END_REF][START_REF] Davis | Classification and behavior of freeranging Weddell seal dives based on three-dimensional movements and video-recorded observations[END_REF])) and up to 11 500 km within 10 months (e.g. Ross seals [START_REF] Arcalis-Planas | Limited use of sea ice by the Ross seal ( Ommatophoca rossii), in Amundsen Sea, Antarctica, using telemetry and remote sensing data[END_REF]). Thus, they can transfer iron from where they forage to anywhere along their path after digestion, between habitats (fast-ice, pack-ice, open water) or within a single habitat (e.g. Weddell seals [START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF])) (Fig. 4 and5). As the nutrient dynamics associated to the relatively narrow and stationary fast-ice does not compare to that of the highly dynamic and constantly drifting pack-ice [START_REF] Massom | Antarctic sea ice change and variability -Physical and ecological implications[END_REF], the consequences of iron release in either of these habitats are likely to differ, and transfers between them can be of ecological importance. Defecation on sea-ice also makes iron return to surface waters entirely dependent on weather conditions and sea-ice melting. In the case of multi-year ice, it can be years.

Vertical iron transfers

To fully understand the role of megafauna in supporting primary production, the release of new and recycled iron in the euphotic zone need to be teased apart [START_REF] Lavery | Iron defecation by sperm whales stimulates carbon export in the Southern Ocean[END_REF][START_REF] Martin | Integral functions of marine vertebrates in the ocean carbon cycle and climate change mitigation[END_REF]). Recycled iron is initially assimilated by primary producers within the euphotic zone, transferred between trophic levels through grazing and predation, and returned in the euphotic zone to be available again to primary producers. Released iron is considered new when it originates from outside -or was transported out -of the euphotic zone, through trophic transfers and/or sedimentation. The four SO species of pack-ice seals feed on different prey (Fig. 5, Table 2). Consequently, they have different foraging strategies, and are likely to contribute differently to new versus recycled iron transfers in the water column. Leopard and crabeater seals are primarily shallow divers, foraging mostly between 10 to 50 m depths [START_REF] Shirihai | The Complete Guide to Antarctic Wildlife -Birds and Marine Mammals of the Antarctic Continent and the Southern Ocean, 2nd edn[END_REF][START_REF] Nordøy | Movements and dive behaviour of two leopard seals (Hydrurga leptonyx) off Queen Maud Land, Antarctica[END_REF][START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF]. They mostly operate horizontal transfers of recycled iron. Weddell and Ross seals, while exploiting different habitats (fast-ice and open pelagic areas, respectively), forage in deeper waters, between 100 and 300 m [START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF][START_REF] Davis | Classification and behavior of freeranging Weddell seal dives based on three-dimensional movements and video-recorded observations[END_REF]. Both prey on demersal to bathypelagic species of myctophids, to some extent [START_REF] Casaux | The diet of the Weddell seal Leptonychotes weddelli at Harmony point, South Shetland Islands[END_REF][START_REF] Casaux | The diet of the Weddell Seal Leptonychotes weddellii at the Danco Coast, Antarctic Peninsula[END_REF][START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF]. Ross seals prey largely on squid [START_REF] Southwell | A review of data on abundance, trends in abundance, habitat use and diet of ice-breeding seals in the Southern Ocean[END_REF] while Weddell seals are the only species known to forage in benthic habitats on benthopelagic fish (e.g. channichthyids) [START_REF] Casaux | The diet of the Weddell seal Leptonychotes weddelli at Harmony point, South Shetland Islands[END_REF][START_REF] Casaux | The diet of the Weddell Seal Leptonychotes weddellii at the Danco Coast, Antarctic Peninsula[END_REF]. Therefore, they operate vertical transfers of iron, benefiting organisms in the euphotic zone. Daily migration of prey between shallow and deep waters or prey feeding in benthic habitats may also indirectly contribute to these vertical transfers (e.g. [START_REF] Schmidt | Seabed foraging by Antarctic krill: Implications for stock assessment, bentho-pelagic coupling, and the vertical transfer of iron[END_REF]))

even when prey are captured in the euphotic zone, which adds another layer of complexity to these processes.

Trophic iron transfers and turnover rate

As previously mentioned, the trophic level of pack ice seals affects their individual daily iron release rates, but it also affects the turnover rate of the iron released. Nutrient turnover is the time between the initial uptake of a nutrient into living biomass and its re-release into the environment in a bioavailable form. The turnover rate of iron released by small, low trophic levels (e.g microbial community, phytoplankton grazers or secondary consumers) is thus faster than the turnover rate of iron released by animals feeding on higher trophic levels (e.g. top predators). However, a rapid nutrient turnover can support a greater primary production than a slower one, even at low concentrations (Benitez-Nelson and Buesseler 1999), so that turnover rate is another functional feature to consider in iron biological cycling. As predators, pack-ice seals (and other megafauna species) occupy a medium-to-high trophic level, which implies a lower contribution to iron biological cycling compared to small organisms (microzooplankton and heterotrophic microbial community) [START_REF] Maldonado | Southern Ocean biological iron cycling in the pre-whaling and present ecosystems[END_REF]). Yet, as megafauna species do not all feed at the same trophic level, the turnover rate of the iron they release is another functional trait that differentiates their relative contribution.

The four species of pack-ice seals all consume krill, although in different proportions (Table 2, Fig. 5). With its large biomass, low trophic level, circumpolar distribution and seasonal persistence, krill plays a key role in the biochemical iron cycling [START_REF] Nicol | Southern Ocean iron fertilization by baleen whales and Antarctic krill[END_REF][START_REF] Ratnarajah | The biogeochemical role of baleen whales and krill in Southern Ocean nutrient cycling[END_REF][START_REF] Ratnarajah | Pelagic Iron Recycling in the Southern Ocean: Exploring the Contribution of Marine Animals[END_REF][START_REF] Ratnarajah | Are Antarctic Krill a Previously Overlooked Source in the Marine Iron Cycle ?[END_REF]. The level of krill consumption can thus be an indicator of the impact of the seal species on iron turnover. Weddell and leopard seals have the highest trophic positions [START_REF] Zhao | Trophic interactions of Antarctic seals as determined by stable isotope signatures[END_REF][START_REF] Brault | Trophic position and foraging ecology of Ross, Weddell, and crabeater seals revealed by compound-specific isotope analysis[END_REF], although leopard seals feed on both large and long-lived predators and on small fish and krill. On average, the iron released in these two seal species faeces has thus a slower turnover rate than that of crabeater seals that feed primarily on krill [START_REF] Zhao | Trophic interactions of Antarctic seals as determined by stable isotope signatures[END_REF][START_REF] Brault | Trophic position and foraging ecology of Ross, Weddell, and crabeater seals revealed by compound-specific isotope analysis[END_REF]. In this respect, the role of crabeater seals in biotically-mediated iron transfers relates to that of baleen whales and krillfeeding penguin species. Thus, crabeater seals are not only the main contributors to total iron release among the pack-ice seal community, they also contribute to a more efficient iron cycling. This further highlights the significance of iron transfers operated by pack-ice seals in the functioning of SO ecosystems, and the central role of krill in the SO biochemical cycling of iron.

Limits and future directions

The complete elucidation and quantification of the role of marine top predators in supplying iron to the euphotic zone is a lengthy task given the numerous processes and parameters involved. Estimating how much total iron these predators release is an essential first step. Trace element analysis of scats from the four pack-ice seal species, collected from different locations and seasons, could help to validate our estimates. However, the amount of total iron estimated here to be released by pack-ice seals should not be assumed to be fully recycled, i.e. taken up by phytoplankton to be re-integrated into the trophic network. The fate of iron released in waste products, in terms of how much is retained in the euphotic zone and how much sinks, and the subsequent contribution of predators to primary productivity, likely depends on its chemical speciation. Iron in the faeces of pack-ice seals could be dissolved (labile or colloidal) or particulate, eventually bound to organic or inorganic ligands, and is probably in several of these forms (e.g. [START_REF] Ratnarajah | Physical speciation and solubility of iron from baleen whale faecal material[END_REF]). Yet, estimating the relative composition of the iron released in faeces by means of a bioenergetic model is today too speculative and is beyond the scope of this study. Further investigations into the chemical speciation of iron in these faeces would help to quantify its relative bioavailability to primary producers or heterotrophic bacteria, and thus to quantify the true contribution of these seals to overall productivity of the system.

Moreover, we limited our calculation to the release of iron for its pivotal role as a primary limiting nutrient in the SO, and comparison between taxa can only be conducted on the basis of iron release. However, SO megafauna waste products contain a wide range of micronutrients associated with biochemical and physiological functions shared by most forms of life [START_REF] Ratnarajah | The biogeochemical role of baleen whales and krill in Southern Ocean nutrient cycling[END_REF][START_REF] Wing | Seabirds and marine mammals redistribute bioavailable iron in the Southern Ocean[END_REF][START_REF] Wing | Marine micronutrient vectors: Seabirds, marine mammals and fishes egest high concentrations of bioactive metals in the subantarctic island ecosystem[END_REF][START_REF] Wing | Penguins and seals transport limiting nutrients between offshore pelagic and coastal regions of Antarctica under changing sea ice[END_REF]Shatova et al. 2016;[START_REF] Sparaventi | Trace elements in Antarctic penguins and the potential role of guano as source of recycled metals in the Southern Ocean[END_REF]. The alleviation of a primary nutrient limitation or co-limitation can trigger new limiting effects by other nutrients [START_REF] Koch | Limitation by Fe, Zn, Co, and B12 results in similar physiological responses in two Antarctic phytoplankton species[END_REF]. Micronutrients released along with iron in seal, penguin and whale faeces could be of interest for primary producers, as suggested by the synergetic effect of guano addition compared to simple iron fertilization observed in lab experiments [START_REF] Shatova | Phytoplankton community structure is influenced by seabird guano enrichment in the Southern Ocean[END_REF]. Hence, the role of other micronutrients in productivity and community structure should not be overlooked [START_REF] Coale | Effects of iron, manganese, copper, and zinc enrichments on productivity and biomass in the subarctic Pacific[END_REF]Hassler et al. 2012b). The contribution of marine predators to nutrient transfers in SO ecosystems and trophic webs does not solely apply to iron, even though formally identifying and quantifying the impact of multiple nutrients leaching from animal products remains a complex task [START_REF] Subalusky | Context dependency of animal resource subsidies[END_REF]. Extending our knowledge of the complex SO ecosystem functioning and of the role of megafauna in fertilizing their own habitat would thus require to extend the present study to include synergistic impacts of several key nutrients.

It is also worth noting that, although not relevant to the study of processes regulating primary productivity, the proportion of nutrients being transferred towards deep waters may be important to the functioning of benthic ecosystems.

Conclusions

Our results provide quantitative and functional arguments to extend the role of pack-ice seals in ecosystem functioning from simple predators to significant actors in the biochemical cycling of iron, placing them alongside other megafauna taxa such as whales and penguins. Leopard, crabeater, Weddell and Ross seals all make different quantitative contributions in terms of iron released at individual and population levels. In addition to their different quantitative contributions, differences in the functional traits of the four species, such as habitat use, social and foraging behaviour, suggest that each species may contribute to maintaining contrasting biochemical conditions throughout the SO euphotic zone, with possible impacts in sustaining biodiversity in the ecosystems [START_REF] Marzetz | Linking primary producer diversity and food quality effects on herbivores: A biochemical perspective[END_REF]. These behavioural traits also distinguish these predators as active iron vectors compared to physical, abiotic and passive processes of iron supply [START_REF] Mcinturf | Vectors with autonomy: what distinguishes animal-mediated nutrient transport from abiotic vectors?[END_REF][START_REF] Subalusky | Context dependency of animal resource subsidies[END_REF], and are therefore important functional characteristics of their role in the biological cycling of iron in the SO.

  pinniped and penguins. For crabeater and Ross seals, we used diets described in McCormack et al. (2020) resulting from the compilation of dietary sample results from 21 studies over the SO, available in the SCAR Southern Ocean Diet and Energetic Database (Scientific Committee on Antarctic Research (SCAR) 2018) (Table

a

  [START_REF] Armstrong | Consumption of Antarctic krill by minke whales[END_REF];[START_REF] Martensson | Seasonal changes in energy density of prey of Northeast Atlantic seals and whales[END_REF];[START_REF] Kiørboe | Zooplankton body composition[END_REF]; Ruck et al. (2014) b Palmer Locarnini and Presley (1995); Caroli et al. (1998); Barbante et al. (2000); Nicol et al. (2010); Kim et al. (2014) c Tierney et al. (2002); Chouvelon et al. (2022b, a) d Honda et al. 1987; Goutte et al. 2015; Chouvelon et al. (2022b, a) e Croxall and Prince (1982); Chouvelon et al. (2022a) f Lourenço et al. (2009); Chouvelon et al. (2022a) g Forcada et al. (2009) h Yamamoto et al. (1987); Noda et al. (1995); Julshamn and Grahl-Nielsen (2000) i Forcada et al. (2009) j Honda et al. (1986);[START_REF] Szefer | Mercury and major essential elements in seals, penguins, and other representative fauna of the Antarctic[END_REF][START_REF] Yamamoto | Comparison of trace element concentrations in tissues of the chick and adult Adelie penguin[END_REF];Jerez et al. (2013b, a)

Fig. 1

 1 Fig.1Iron released in the faeces of Southern Ocean leopard, crabeater, Weddell and Ross seals. Individual, daily level (left panel, vertical axis in mg per day) and populational annual level (right panel, vertical axis in tonne per year), as estimated by means of a bioenergetic model. Central point represents mean estimates and lower and upper bar limit the 2.5 and 97.5 percentiles, respectively. Letters indicate the significance of the difference between species as assessed based on unilateral binary relations between estimates (see Methods): if two species are associated with different letters, the difference between the two estimates is significant (p < 0.05); if they share a common letter, it is not significant (p > 0.05)

Fig. 2

 2 Fig.2 Iron released by different Southern Ocean species of megafauna. Data is from literature and present study:(1) Lavery et al. (2010), (2) Lavery et al. (2014), (3) Savoca et al. (2021), (4)[START_REF] Sparaventi | Trace elements in Antarctic penguins and the potential role of guano as source of recycled metals in the Southern Ocean[END_REF]. The number of days during which species were considered present in the Southern Ocean and thus for the estimates (in the literature and present study) are indicated in brackets. Estimates from (4), originally calculated for the reproduction period only, have been raised to annual estimates using a simple cross product. Central point represents mean estimates and lower and upper bar the 2.5 and 97.5 percentiles, respectively. Percentiles were not reported with estimates in (1), (2) and (4). Detailed numbers are provided in Online Resource 2

Fig. 3

 3 Fig.3Estimates of first-order and total order Sobol sensitivity indices of the bioenergetic model parameters. The seven parameters input used to estimate iron released by leopard, crabeater, Weddell and Ross seals in the Southern Ocean included seal body mass, species-specific metabolic multiplier beta, mean energy content of the diet, mean iron (Fe) content of the diet, assimilation efficiency or assimilation rate, iron release rate and population abundance. Boxplots display the median with solid black line in each box, lower and upper hinges correspond to the 25 th and 75 th percentile, respectively; upper and lower whiskers extend respectively from the hinges to the largest and lowest values no further than 1.5 times the inter-quartile range, and data beyond the end of whiskers are plotted as points

Fig. 4

 4 Fig. 4 Dispersal and summer ranges of leopard, Weddell, crabeater and Ross seals in the Southern Ocean (adapted from Shirihai (2008), original map by FreeVectorMaps.com)

Fig. 5

 5 Fig. 5 Conceptual diagram showing functional traits of the contribution of leopard, crabeater, Weddell and Ross seals to iron (Fe) cycling in the Southern Ocean through a habitat segregation and horizontal transfers (blue triangles indicating relative time spent in habitats including fast-ice, pack ice and open pelagic waters for each species), b trophic transfers of iron operated through the consumption of prey at different trophic levels (green arrows) and c vertical transfers, operated through the consumption of prey in meso-benthopelagic zones and subsequent release of iron-rich waste back in the euphotic zone (red arrows)

Table 1

 1 Bioenergetic parameters' setting information, based on literature values. BM is individual body mass, β is a metabolic species-specific multiplier, AE is assimilation efficiency, E is the mean energy content of diet, xFe is the mean iron content of diet, rFe is iron release rate and A population abundance. Truncated normal parameters are indicated as follows:~ 𝑁(𝑚𝑒𝑎𝑛, 𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑑𝑒𝑣𝑖𝑎𝑡𝑖𝑜𝑛, [𝑙𝑜𝑤𝑒𝑟 𝑙𝑖𝑚𝑖𝑡 -𝑢𝑝𝑝𝑒𝑟 𝑙𝑖𝑚𝑖𝑡]); if only one number in brackets, it is the lower limit

Par. Leopard seal H. leptonyx Crabeater seal L. carcinophagus Weddell seal L. weddellii Ross seal O. rossii Ref.

	BM	~ 𝑁(350, 0.2)	~ 𝑁(200, 0.2)	~ 𝑁(300, 0.2)	~𝑁(160, 0.2)	a
	(kg)
	β	~𝑁(3, 0.5, [2 -3.5]) ~𝑁(2, 0.5, [1.5 -3]) ~𝑁(2, 0.5, [1.5 -3]) ~𝑁(2, 0.5, [1.5 -3])	b
	AE	[0.80 -0.95]) ~𝑁(0.9, 0.05,	[0.80 -0.95]) ~𝑁(0.9, 0.05,	[0.80 -0.95]) ~𝑁(0.9, 0.05,	[0.80 -0.95]) ~𝑁(0.9, 0.05,	c
	E (kJ.k g -1 )	~𝑁(12.1, 0.2 × 12.1, [0.2 × 12.1])	~𝑁(5.1, 0.2 × 5.1, [0.2 × 5.1])	~𝑁(5.4, 0.2 × 5.4, [0.2 × 5.4])	~𝑁(4.8, 0.2 × 4.8, [0.2 × 4.8])	See Eq. (5-6) & Table 2)
						See
		~𝑁(165,	~𝑁(10.1,	~𝑁(22.6,	~𝑁(15.6,	Eq. (5-
	xFe	0.2 × 165,	0.2 × 10.1,	0.2 × 22.6,	0.2 × 15.6,	6) & Table
		[0.2 × 165])	[0.2 × 10.1])	[0.2 × 22.6])	[0.2 × 15.6])	2)
	rFe	~𝑈(0.7, 0.9)	~𝑈(0.7, 0.9)	~𝑈(0.7, 0.9)	~𝑈(0.7, 0.9)	d
		~𝑁(35 500,	~𝑁(5 869 400,	~𝑁(633 000,	~𝑁(78 500,
	A (No.)	[10 900 -22 925,	[3 699 400 -1 229 325,	[221 000 -278 500,	[39 400 -47 950,	e
		102 600])	8 616 700])	1 335 000])	231 200])
	a Bryden and Erickson (1976); Laws (1977); Skinner and Klages (1994); Trites and Pauly (1998); Van Den Hoff
	et al. (2003); Williams and Yeates (2004); McDonald et al. (2008); Shirihai (2008); Bengtson (2009);
	Castellini et al. (2009); Forcada et al. (2009); Rogers (2009); Thomas and Rogers (2009)
	b Castellini et al. (1992); Renouf and Gales (1994); Williams et al. (2001); Hoelzel (2009)
	c Ashwell-Erickson and Elsner (1981); Keiver et al. (1984); Ronald et al. (1984); Martensson et al. (1994)
	d Candela et al. (1984); Lavery et al. (2010); Ratnarajah et al. (2016a)
	e Southwell et al. (2012)

Table 2

 2 Data and literature sources (References) used to estimate the energy content (Epg) and iron (Fe) concentration (xFe,pg) of the functional prey groups (pg) used to describe the diet of each pack-ice seal species, and diets as established with percentage of consumption (in weight) of each prey group

		Mean
	Prey	energy
	group	content
	pg	Epg
		(kJ.

g -1 ) Mean iron content xFe, pg (mg.kg -1 wet weight) % in diet (in weight) References Leopard seal H. leptonyx Crabeater seal L. carcinopha gus Weddell seal L. weddellii Ross seal O. rossii

	Krill	5.1	11.3	30	98.5	2	35	[energy] a [Fe] b
	Fish	5.5	24.3	42	0.5	90	30	[energy] c [Fe] d
	Cephalop ods	3.8	14.3	1	1	8	35	[energy] e [Fe] f
	Pinniped (muscle)	10	194.9	21	0	0	0	[energy] g [Fe] h
	Penguins (muscle)	10	180.4	6	0	0	0	[energy] i [Fe] j

Table 3

 3 Means and 95% confidence intervals [in brackets] for metabolic requirements, prey consumption, iron ingestion and release parameters estimated from our bioenergetic model for the four Southern Ocean species of pack-ice seals

	Species	Leopard seal	Crabeater seal	Weddell seal	Ross seal
	Average daily metabolic rate (kJ.d -1 )	68 151 [43 135 -95 014]	33 143 [20 962 -49 213]	44 587 [27 344 -66 535]	28 072 [17 853 -41 449]
	Daily food ration (kg.d -1 )	12 [6 -20]	8 [4 -14]	10 [5 -18]	7 [4 -12]
	% of biomass
	consumed per	3 [2 -6]	4 [2 -7]	3 [2 -6]	4 [2 -7]
	day
	Iron
	concentration in
	the diet xFe	67 [41 -93]	11 [7 -16]	23 [14 -32]	16 [10 -23]
	(mg.kg -1 wet
	weight)

Daily individual iron consumption rate (mg.day -1 )

		773 [349 -1460]	87 [39 -170]	226 [99 -442]	112 [50 -217]
	Daily individual
	iron release rate	618 [275 -1180]	70 [31 -137]	181 [78 -357]	90 [39 -176]
	(mg.day -1 )
	Population
	annual iron	9 [2 -23]	151 [60 -315]	44 [13 -106]	3 [1 -8]
	release (t.yr -1 )

  Table3). They require from 7[4 -12] kg of prey daily for Ross seals to 12 [6 -20] kg for leopard seals, which represents 3 to 4%[2 -7] of seals' individual body mass for all 4 species.The diet of a leopard seal is the most concentrated in iron(67 [41 -93] mg.Fe.kg -1 wet weight), followed by that of a Weddell seal(23 [14 -32] mg.Fe.kg -1 wet weight), a Ross

-1 (95% confidence interval -hereafter [CI 95%], i.e. [17 853 -41 449]) for Ross seals (the smallest species) to 68 151 [43 135 -95 014] kJ.d -1 for leopard seals (the largest and most metabolically active species) (seal (16 [10 -23] mg.Fe.kg -1 wet weight) and a crabeater seal (11 [7 -16] mg.Fe.kg -1 wet weight) (Table

Acknowledgments

We wish to thank Sarah Kienle and Douglas J. Krause for helpful discussion about the predatory behaviour of leopard seals, and Matthew S. Savoca for kindly providing details of estimates of iron release by Southern Ocean baleen whales.

Funding

This work was supported by the French Ministry for Ecology ("Ministère de la Transition Ecologique et Solidaire/Direction de l'Eau et de la Biodiversité -MTES/DEB").

Data and code availability

No original data were used nor generated for this study which mobilised data from the literature for the setting of parameters in our bioenergetic model. References are provided in the manuscript (Methods and Table 1 and2). Statistics of estimates used to generate Figure 1 are provided in Table 1 and ESM 1, with additional information on estimates for other input or output parameters of the model in the latter. Statistics for Sobol sensivity index reported

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1007/s00300-023-03198-6.

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Author contributions

LG, TJDD and JS designed the study; LG performed the data analysis with contributions of JS and TJDD on parameter settings; LG, JS and TJDD wrote the manuscript.

on Figure 3

Declarations

Competing interests The authors disclose no conflict of interest.