Trophodynamics of Southern Ocean pteropods on the southern Kerguelen Plateau

Abstract Pteropods are a group of small marine gastropods that are highly sensitive to multiple stressors associated with climate change. Their trophic ecology is not well studied, with most research having focused primarily on the effects of ocean acidification on their fragile, aragonite shells. Stable isotopes analysis coupled with isotope‐based Bayesian niche metrics is useful for characterizing the trophic structure of biological assemblages. These approaches have not been implemented for pteropod assemblages. We used isotope‐based Bayesian niche metrics to investigate the trophic relationships of three co‐occurring pteropod species, with distinct feeding behaviors, sampled from the Southern Kerguelen Plateau area in the Indian Sector of the Southern Ocean—a biologically and economically important but poorly studied region. Two of these species were gymnosomes (shell‐less pteropods), which are traditionally regarded as specialist predators on other pteropods, and the third species was a thecosome (shelled pteropod), which are typically generalist omnivores. For each species, we aimed to understand (a) variability and overlap among isotopic niches; and (b) whether there was a relationship between body size and trophic position. Observed isotopic niche areas were broadest for gymnosomes, especially Clione limacina antarctica, whose observed isotopic niche area was wider than expected on both δ13C and δ15N value axes. We also found that trophic position significantly increased with increasing body length for Spongiobranchaea australis. We found no indication of a dietary shift toward increased trophic position with increasing body size for Clio pyramidata f. sulcata. Trophic positions ranged from 2.8 to 3.5, revealing an assemblage composed of both primary and secondary consumer behaviors. This study provides a comprehensive comparative analysis on trophodynamics in Southern Ocean pteropod species, and supports previous studies using gut content, fatty acid and stable isotope analyses. Combined, our results illustrate differences in intraspecific trophic behavior that may be attributed to differential feeding strategies at species level.


| INTRODUC TI ON
Human-driven climate change is causing significant chemical and physical changes to global oceans, leading to demonstrable direct and indirect ecological impacts to Southern Ocean assemblages across all trophic levels (Constable et al., 2014). Among these changes is ocean acidification, which causes dissolution of shell-secreting organisms (Fabry, Seibel, Feely, & Orr, 2008;Orr et al., 2005). Acidification and warming of polar waters is also expected to lead to changing size and composition of phytoplankton and microbial communities, which could have severe bottom-up effects on marine food webs (Davidson et al., 2016). Many shell-secreting organisms rely on these basal food sources, and if the availability of these sources is altered, it is unclear how this will impact grazing organisms, be it through range redistribution, in situ adaptation, or localized extinction.
Despite their importance, our knowledge of pteropod trophodynamics is very patchy, particularly in the Southern Ocean, and based almost entirely on visual analysis of their stomach contents. Co-occurring Southern Ocean pteropods could provide a unique model community to understand how different functional traits (e.g., feeding structures, body size) correlate with the trophic structure of communities consisting of both monophagous specialists and generalists.
The niche concept has been popular for over 100 years as it is a useful way of considering how different organisms fit into communities and ecosystems, and the roles they play (Grinnell, 1917;Sexton, Montiel, Shay, Stephens, & Slatyer, 2017). Several different versions of the niche concept have been introduced over time that focus on different aspects of species' ecologies including traits, trophic interactions (resource use), and environmental (habitat) requirements (Elton, 1927;Hutchinson, 1957). Over the last decade, stable isotopes have gained popularity as an approach for quantifying the trophic niche (thus being most closely analogous to the Eltonian niche, and the resource axes of the Hutchinsonian niche) (Newsome, del Rio, Bearhop, & Phillips, 2007). This approach is based on the detectable average enrichments of 3-5‰ and ~1‰ in the isotopic ratios for nitrogen and carbon, respectively, per tropic level (Post, 2002).
Measuring space occupied by consumers δ 13 C and δ 15 N enables insight into both trophic and ecological niches, as it provides both an index of the basal resources from which consumers derive their energy, and the trophic position at which they feed, respectively (Fink, Reichwaldt, Harrod, & Rossberg, 2012). As for the latter, δ 15 N values of predators rarely overlap with those of their prey, and are thus effective at estimating trophic position (Vanderklift & Ponsard, 2003).
The extent of stable isotopic values representing all resources used by a consumer is commonly used as a proxy for trophic and ecological niche (Bearhop, Adams, Waldron, Fuller, & Macleod, 2004;Jackson, Inger, Parnell, & Bearhop, 2011). Bearhop et al. (2004) first inferred the qualitative description of the trophic niche breadth by using δ 13 C and δ 15 N values of consumers and food source tissues, which was followed by the development of quantitative metrics to measure niche width, species spacing, density, overlapping, and clustering from the extent and spread of isotopic data points (Jackson et al., 2011;Layman, Quattrochi, Peyer, & Allgeier, 2007;Swanson et al., 2014). It is often assumed that smaller isotopic niches are associated with trophic specialists due to a low diversity of dietary resources, that is, consequently translated to small ranges in isotopic composition (Bolnick et al., 2003).
The niche variation hypothesis (Van Valen, 1965) proposed that broader, population-level dietary niches are associated with greater individual-level resource use, in relation to populations possessing narrower niches. Assuming the niche variation hypothesis, our objective was to measure trophodynamic variability in sympatric pteropod species in an under-surveyed region on the southern Kerguelen Plateau.
Here, we enlisted prior knowledge of gymnosome and thecosome pteropod feeding preferences (Lalli & Gilmer, 1989) and predicted that specialist gymnosomes will exhibit narrower isotopic niche areas than generalist thecosomes, with little variability over space and time scales.
Based on studies using nitrogen stable isotopes and a range of biomass size-spectra to show positive relationships between trophic position and body size (France, Chandler, & Peters, 1998;Jennings, Pinnegar, Polunin, & Warr, 2002), we assumed that larger individuals will have higher trophic positions than smaller individuals due increasing ability to access larger, higher trophic-level prey with increasing body size. The effects of climate change are expected to cause shifts in the trophodynamics of many polar organisms (Gutt et al., 2015), including predictions made that may see narrowing of niche areas due to decreased prey diversity, an increase in niche overlapping due to shifts in species distribution (Bas et al., 2019), and an increase in trophic positions of key marine predators due to expected diet shifts toward higher trophic prey (Tarroux, Lowther, Lydersen, & Kovacs, 2016). The trophic niches estimated here provide a benchmark for pteropods and a piece of the puzzle for ongoing efforts to understand trophic structuring of pelagic communities and ecosystems more generally. Size-fractioned particulate organic matter (POM) was retrieved from the ship's underway water supply (~5 m depth) via large volume sequential filtration, prescreened using an upstream 47 mm diameter, 1 mm filter mesh for zooplankton removal, and collected onto 25 mm diameter Sterlitech silver membrane (pore size = 1.2 µm) and

| Study area and sampling
Nitex filters (pore size = 210 µm). Two particle size fractions were analyzed as "large" >210 µm and "small" <210 µm, predominantly phytoplankton. The large fraction on Nitex filters was cleanly transferred to silver membranes for analysis. Filters were acidified and air dried at 60°C prior to stable isotopes analysis (SIA).

| SIA sample preparation
Individual pteropods were rinsed in filtered seawater and weighed prior to drying in an oven for 24 hr at 60°C. While previous research recommends acidifying thecosomatous pteropods to remove carbonate material that would otherwise bias stable isotopes results (Pomerleau, Winkler, Sastri, Nelson, & Williams, 2014), the pressure from the RMT8 sampling method completely stripped entire shells from most shelled pteropods, and we did not need to acid-treat our samples prior to further analyses as shell fragments could be easily removed using forceps. Individual dry weights were measured and samples were ground to powder using an agate mortar and pestle, then weighed into tin cups in preparation for isotopic analysis. Since lipids may bias carbon isotope ratios (Post et al., 2007), a pteropodspecific correction offset value determined by Weldrick, Trebilco, and Swadling (2019) was used to correct bulk samples. POM sample filters were acidified dried for 48 hr at 60°C prior subsampling 5 mm diameter aliquots into silver capsules (Sercon SC0037).

| Stable isotopes analysis
Bulk carbon and nitrogen stable isotope ratios were obtained using an automated Elementar vario PYRO cube analyser interfaced with a continuous flow IsoPrime100 isotope ratio mass spectrometer for pteropods, and a Thermo Scientific Flash 2000HT analyser coupled with a Thermo Fisher Delta V Plus mass spectrometer through a Thermo Fisher Conflo IV for POM. For pteropods, SIA was performed at the Central Science Laboratory (CSL), University of Tasmania (Sandy Bay, Tasmania), and for POM, at the Australian Nuclear Science and Technology Organisation (ANSTO, Lucas Heights, Sydney). Isotopic ratios were expressed in delta (δ) notation and reported as parts per thousand (‰) deviations from conventional certified isotopic reference standards, Vienna Pee Dee Belemnite (for carbon) and atmospheric air (for nitrogen) (DeNiro & Epstein, 1978). At CSL, laboratory working standards of sulfanilamide were repeated every 6th sample for both isotopes to measure instrument stability and precision. Average standard deviations on F I G U R E 1 Relative abundance (size of circle, ind. 1,000 m −3 ) of pteropod species sampled from RMT8 plankton nets during the K-Axis voyage. Some gymnosome individuals were unable to be identified to species level and were not used in SIA. Large-fraction (triangles) and small-fraction (diamonds) are also featured here. Oceanographic features and front locations (dashed lines) determined by Bestley et al. (2018) include the southern ACC front (sACCf), the Southern Boundary (SB), the Antarctic Slope Front, and the Fawn Trough Current. Upper and lower dotted lines are October and January average sea ice extent, respectively triplicate measurements were 0.15‰ and 0.19‰ for δ 13 C and δ 15 N values of pteropods, respectively. At ANSTO, POM isotopic data are reported relative to IAEA secondary certified standards with a standard error of analysis to 1 standard deviation (SD) measured at ±0.3 mil. The carbon and nitrogen percentages, by weight, were converted to atomic C:N ratios.

| Statistical analysis
Data analyses were conducted using the statistical software R, version 3.4.0 (R Development Core Team, 2017). We used Shapiro-Wilk's and Levene's tests on isotopic values (δ 13 C and δ 15 N) to assess normality and homogeneity of variance, respectively. A multivariate analysis of variance (MANOVA; using the MANOVA function in base R) was also used to test for spatial (latitude, longitude, and site depth) and species effects (independent variables) on bulk isotopic values (dependent variables). Simple linear regressions were used to test the relationships between each isotopic values (δ 13 C and δ 15 N) and time, latitude, longitude, and site depth, as well as relationships between body size and trophic position (lm function, base R). Intraspecific comparisons of niche metrics could not be conducted by site as sample sizes of gymnosome species were lower than empirically determined recommended sizes (Syväranta, Lensu, Marjomäki, Oksanen, & Jones, 2013).

| Niche dispersion metrics, trophic position, and body size
Interspecies isotopic niche widths were estimated using the Stable Isotope Bayesian Ellipses package in R (SIBER) version 2.1.3 (Jackson et al., 2011). SIBER calculates bivariate standard ellipses corresponding to isotopic niches, and standard ellipse areas (SEA) representing standard deviation of the data. Standard area ellipses corrected for sample sizes (SEAc) were also calculated (Jackson et al., 2011). Interspecies dietary niche area overlaps were calculated using nicheRover in R version 1.0 (Swanson et al., 2014), which uses a Bayesian framework to produce probable pairwise comparisons between the niche region of each species combination. The niche region of a species was defined at 95% and 99% probability that a species will be located within isotopic bivariate space; similarly, the niche overlap is defined at 95% and 99% probability that the niche region of one species will overlap with that of another.
We also calculated multiple probability estimates of the trophic positions of each pteropod species. Unlike the trophic level, which refers to categories of trophic modes based on integer values, the trophic position of a species is a continuous, numerical measure that represents the relative location of a particular species within a trophic hierarchy (Carscallen, Vandenberg, Lawson, Martinez, & Romanuk, 2012). Trophic positions were calculated for each pteropod species separately using the R package tRophicPosition version 0.7.5 (Quezada-Romegialli et al., 2018). This package calculates consumer trophic positions within a Bayesian framework based on the stable isotopes of both consumers and baselines as well as userspecified trophic enrichment factors (TEF). We adopted TEF values (±SE) of 0.5 ± 0.13‰ for carbon (∆δ 13 C values) and 2.3 ± 0.18‰ for nitrogen (∆δ 15 N values) based on those measured for similarly sized marine gastropods previously reported in the literature (McCutchan, Lewis, Kendall, & McGrath, 2003). We also converted δ 15 N values to trophic positions on specimens used to compare with body size.
Body lengths (mm) from a subset of intact pteropods were measured using ImageJ/Fiji software (Schindelin et al., 2012). Results from individuals sampled for SIA were plotted against δ 15 N values to determine the effect of body size on trophic position.
1,000 m −3 , respectively. Relative abundances were 89.6% for C. pyramidata, 4.8% for C. antarctica, 0.4% for S. australis, and 5.2% for gymnosomes unidentified to species level that were not used in the subsequent SIA (Figure 1). Latitudinal ranges were narrowest for S. australis (60.4°S-62.7°S), followed by C. antarctica (61.4°S-67.0°S), and C. pyramidata (57.7°S-67.0°S).  Table S1). δ 13 C values were strongly associated with spatial attributes, including latitude, longitude, and depth modeled together. Variation in δ 15 N values was primarily predicted by longitude and species. Linear regression models of species-specific relationships between each isotopic value versus spatio-temporal variables (date, depth, latitude, and longitude; Figures 2-5) showed statistically significant positive relationships for the following: C. pyramidata between δ 15 N and sampling dates and latitude (Figures 2 and 4, respectively), and between δ 13 C and longitude ( Figure 5); and C. antarctica between δ 15 N and δ 13 C and sampling dates (Figure 2). Statistically significant negative relationships were measured for the following: C. pyramidata between δ 13 C and sampling dates and longitude (Figures 2 and 5, respectively); and C. antarctica between δ 13 C and longitude ( Figure 5).
Probability niche overlap was high among all pteropod species

| D ISCUSS I ON
We hypothesized that Southern Ocean pteropods would reveal variability in isotopic niche areas that reflect a priori knowledge of species-specific feeding preferences, with specialists, the gymnosomes, exhibiting smaller niche areas than generalists, the thecosomes.
We expected narrow isotopic niches for gymnosomes possessing monophagous diets, consisting of a single species or genus, with C. antarctica preferring to prey on Limacina spp. and S. australis preying only on C. pyramidata. Instead, we measured broader than expected isotopic niche areas for gymnosomes C. antarctica and S. australis,  (Cummings, Buhl, Lee, Simpson, & Holmes, 2012). Below, we discuss factors that may explain these patterns, but note that distinguishing among these mechanisms would require additional sampling and analysis and would be a fruitful direction for future work.
The isotopic variation within pteropod species measured here suggests niche partitioning and could point to their distinctive functional traits as observed through anatomical structures associated with feeding. Thecosomes such as C. pyramidata particle feed on marine snow by deploying an external mucus web to capture suspended materials of all sizes, which are then transported to the mouth by ciliary pathways, where particle sorting and ingestion occur (Gilmer, 1974(Gilmer, , 1990. Gut content data from the Weddell Sea have revealed an omnivorous diet for C. pyramidata (Hopkins & Torres, 1989), corroborated by the broad range of δ 15 N values measured within the present study. In contrast to thecosomes, the feeding apparatus of gymnosomes varies among species. C. antarctica possess six buccal cones that capture prey, and a series of hooks and the radula remove and swallow the soft tissue whole from the shell (Conover & Lalli, 1972), whereas S. australis captures prey using two lateral arms, each possessing a series of suckers (Lalli & Gilmer, 1989). While both gymnosomes revealed broad δ 13 C value ranges, which closely matched that of C. pyramidata, the variable ranges in δ 15 N values displayed between each gymnosome may be a result of these species-specific feeding strategies.
Published δ 13 C-δ 15 N biplots featuring Southern Ocean pteropod species place S. australis trophically higher than any other pteropod species common to the region (Hunt et al., 2008; Table 1). By our estimates, S. australis was one trophic level above its prey C. pyramidata, however despite obtaining a similar outcome to previous isotopic work evaluating the interaction between these two species (Table 1), these results alone cannot confirm a direct trophic relationship without supplemented by other lines of evidence, such as direct observation, genetic, and/or gut content analyses (Nielsen, Clare, Hayden, Brett, & Kratina, 2018). More work is required to better understand prey size preference for this species. Meanwhile, C. pyramidata possessed a larger total niche area than those measured for gymnosomes, which likely reflecting omnivory.
We predicted narrow isotopic niche widths for both gymnosomes relative to that of C. pyramidata; however, our results demonstrate significant trophic diversity not only between orders but also between gymnosome species. For S. australis, this may be a function of latitude, as this least-abundant species was sampled within the narrowest F I G U R E 3 Linear relationships between δ 13 C (upper plots) and δ 15 N (lower plots) and depth (m). For C. antarctica: δ 13 C, R 2 = −0.05, p = 0.62, δ 15 N, R 2 = 0.14, p = 0.07; C. pyramidata: δ 13 C, R 2 = 0.006, p = 0.17, δ 15 N, R 2 = −0.006, p = 0.67; S. australis: δ 13 C, R 2 = −0.13, p = 0.60, δ 15 N, R 2 = −0.13, p = 0.59 latitudinal range; however, linear relationships did not reveal any statistically significant spatial correlations. We expected the isotopic niche width of both gymnosomes to be narrow because of a priori knowledge of their specialist feeding preference for thecosomes (Phleger, Nichols, & Virtue, 1997). However, the niche width of C. antarctica was significantly broader along the δ 15 N-axis than hypothesized, which might be a function of their wider geographical range within our voyage transect, relative to that of S. australis more commonly found north of the Polar Front (PF) and the northern extent of our sampling effort (Hunt et al., 2008).
The unexpectedly wide niche area of C. antarctica might be a function of either diet switching to other unrecorded food sources, and/or driven by nutritional stress. One study posited that

C. limacina diet may include other taxa when the preferred prey
Limacina species are limited. DNA-based approaches used to investigate prey within Arctic C. limacina stomach contents revealed alternative prey items, such as amphipods and calanoid copepods (Kallevik, 2013). Further, laboratory observations have demonstrated that C. limacina can survive between 260 and 365 days without prey due to an adaptive ability to reduce body size and metabolic rate as a consequence of efficient utilization of lipids and phospholipids stores (Böer, Graeve, & Kattner, 2006 Falk-Petersen, Sargent, Kwasniewski, Gulliksen, & Millar, 2001).
However, no studies have tested how these changes affect C. limacina isotopically. Among the numerous studies examining how a range of nutritional stresses affect isotopic compositions of various animals, enrichment in 15 N is the response most likely to confound interpretations of isotopic niches, particularly given that isotopic niche metrics rely on the assumption that interindividual variability in enrichment between consumer and prey is insignificant (Karlson, Reutgard, Garbaras, & Gorokhova, 2018;McCue & Pollock, 2008).
One experiment-based study on the amphipod Monoporeia affinis revealed wider niche widths among specimens exposed to nutrient and chemical stress relative to control animals (Karlson et al., 2018).
It is understood that increases in isotopic niche areas are a function of interindividual variability in the responses of consumers to stress exposure through changes in their metabolic pathways. Further laboratory testing is needed to understand how exposure to physiological stress, such as food limitation, can affect the magnitude of the isotopic compositions and niche metrics of C. antarctica, with special focus on the scale of individual. Furthermore, results of laboratory tests will need to be compared with in situ estimates of niche metrics taken from C. limacina sampled in years where prey species, L. helicina, were also abundant.
Recently, Henschke et al., (2015) employed isotopic niche analysis of co-occurring zooplankton and suspended POM assemblages from the Tasman Sea under three different oceanographic water types and found that omnivorous zooplankton became more carnivorous under low chlorophyll a conditions. They concluded that niche breadth among different zooplankton groups was a function of the responses of phytoplankton and POM to oceanographic conditions. Narrower niches have been observed among CO 2 -enriched hydrothermal vent communities relative to controls, whereby nearfuture predicted pCO 2 concentrations are directly associated with decreased higher trophic species diversity and simplified habitats (Agostini et al., 2018;Vizzini et al., 2017).
For C. pyramidata, we did not observe an increase in trophic position with body size that would signify a possible shifting from herbivory to omnivory. Positive correlations between C. pyramidata abundance and phytoplankton blooms have been linked to high ingestion rates, with pteropods contributing up to 53% to total grazing impact within the Spring ice edge while contributing only 13% to total biomass in the region (Pakhomov & Froneman, 2004). Gut content analyses from Weddell Sea specimens showed C. pyramidata f. sulcata diets primarily composed of diatoms; however, larger motile organisms such as tintinnids, foraminiferans, copepods, and polychaetes appeared in larger sized C. pyramidata, pointing to omnivorous feeding habits (Hopkins & Torres, 1989). Fatty acid profiles linked to crustacean biomarkers have also been measured in C. pyramidata from the Antarctic Peninsula (Phleger et al., 2001).
For gymnosomes, our results imply a continuous change in dietary preference throughout adult stages, which may vary spatially and depend on prey availability. A laboratory-based feeding experiment using Arctic C. limacina sampled in mid-summer measured low prey assimilation rates that increased toward the end of the experiment (Boissonnot et al., 2019). Results from their study suggested that sampling period overlapped with a seasonal period of low prey availability followed by high reproductive activity  Table S4), and laboratory-based feeding experiments have shown that adult C. antarctica rarely consume L.

Pteropod abundances throughout the Southern Ocean indicate
highly variable yet sometimes dominant densities in proportion to other zooplankton groups, contributing up to 93% of total macrozooplankton (Hunt et al., 2008), and strongly tied to El Niño events, sea ice retreat, and the presence of prey (Thibodeau, Steinberg, Stammerjohn, & Hauri, 2019). Our maximum abundance of 272 ind. 1,000 m −3 for C. pyramidata is within the range of 2-996 ind.
Linear regressions revealed a statistically significant positive relationship between δ 13 C and longitude for C. pyramidata, which may relate to the cluster of enriched values overlapping the same region (just north of the sACCf on the Plateau) we measured high abundances of this species, toward the eastern flank of the Plateau.
Particulate organic nitrogen (PON) measurements revealed enriched values along this region, indicating nitrate uptake which suggests new production was occurring here (Schallenberg et al., 2018).
However, we also detected a statistically significant negative relationship between δ 13 C and longitude for C. antarctica, whose highest abundances generally overlapped with those of C. pyramidata.
A weakly positive significant linear relationship was measured between δ 15 N and latitude for C. pyramidata, who displayed gradients of isotopic enrichment from deeper regions of the transect toward the coastlines and toward shallower areas toward Banzare Bank, at 60°S and 80°E. Regions along the transect where abundances were lowest (between 70°E to 80°E) overlapped with the lowest δ 13 C and δ 15 N values measured for all species, combined. Incidentally, this region, which covers the western area of the Plateau, also measured the most 15 N-depleted in PON, suggesting nitrogen recycling and ammonia uptake (Schallenberg et al., 2018).
F I G U R E 8 Bayesian estimates of trophic position created from 20,000 Markov Chain Monte Carlo iterations for each pteropod species TA B L E 1 Bulk stable isotopes (δ 13 C and δ 15 N) signatures (± SD) averaged over all sampling sites and species, as well as results from the same Southern Ocean species from other studies The pteropod species sampled from the southern extent of the Kerguelen Plateau are among the four common species regularly found south of the PF (Loeb & Santora, 2013 ing a 50% to 75% reduction in phytoplankton biomass and high sea ice cover which resulted in nutrient stress-related decreases in metabolic rates in both L. helicina antarctica and its monophagous predator C. antarctica (Seibel & Dierssen, 2003). In 1989, low densities were also recorded in the East Antarctic and may be attributed to low chlorophyll a biomass resulting from late winter sea-ice retreat (Hunt et al., 2008). Satellite observations for the region and period surveyed here exhibited comparatively low average chlorophyll a concentration, ranging from 0.45 to 0.55 mg/m 3 (see Figure S1 for monthly interannual time series (

| CON CLUDING REMARK S
While the Southern Ocean supports a range of diverse ecosystems, it is also among the most rapidly changing regions on a global scale, and it is an ongoing challenge to predict how these ecosystems and species will respond (Murphy et al., 2012). Climate-driven changes in ocean temperature, ocean frontal positions, seasonal ice cover, and CO 2 uptake are expected to significantly alter abundance, distribution, and trophodynamics of key polar organisms (Constable et al., 2014), including pteropods, as a response to changing metabolic fitness, prey availability and diversity, and competition. Given the complex spatial and temporal dynamics of the Southern Ocean, future research focused on understanding species responses due to human-induced physical and chemical changes need to be small scale and regionally comparative (Allan et al., 2013). Monitoring variation between pteropod species in their isotopic niches coupled with their regional abundance and distribution could play a key role in understanding oceanographic and ecological drivers of change, for instance, due to their intraspecific reliance on sea ice primary production (Jia et al., 2016), and close associations with ocean frontal zones and biogeochemical provinces (Burridge et al., 2017). Additionally, such work will need to account for the magnitude of phenotypic plasticity in pteropods, including if and how their metabolic responses to multiple stressors of climate change are driving variability in isotopic niches. The work presented here concludes that isotopic niches in Southern Ocean pteropod communities are not merely driven by diet diversity nor follow the niche variation hypothesis of Van Valen (1965). To our knowledge, the present study represents the first comprehensive assessment involving measurement and interpretation of isotopic niche widths among pteropod assemblages. Our findings highlight the utility of employing isotopic niche metrics and dispersion analyses to reconstruct the trophic structure of co-occurring Southern Ocean pteropods, and the need to expand this research to understand drivers of diet behavior.

ACK N OWLED G M ENTS
We thank the Associate Editor and two anonymous reviewers for comments and insights that greatly improved the quality of this manuscript. We are grateful for the master and crew as well as the scientific and technical teams aboard the RV Aurora Australis for valuable logistical support. We would also like to acknowledge Barbora Gallagher Fellowship. Some analyses and visualizations used in this paper were produced with data from the Giovanni online data system, developed and maintained by the NASA GES DISC.

CO N FLI C T O F I NTE R E S T
None declared.

AUTH O R CO NTR I B UTI O N S
CKW conceived ideas, designed methodology, gathered pteropod observations, conducted analyses, led the writing of the manuscript; RT and KMS contributed critically to all draft versions; DMD provided POM observations. All authors gave final approval for publication.

DATA ACCE SS I B I LIT Y
All isotopic signatures will be made available at Australian Antarctic Division Data Centre, https ://data.aad.gov.au/aadc/; R scripts can be accessed at https ://github.com/Cheva ldeMe r/Weldr ick_etal_ Ecolo gy_and_Evolu tion_2019.