Isotopic Niche Research Articles

Late Pleistocene megafauna extinction leads to missing pieces of ecological space in a North American mammal community.

The conservation status of large-bodied mammals is dire. Their decline has serious consequences because they have unique ecological roles not replicated by smaller-bodied animals. Here, we use the fossil record of the megafauna extinction at the terminal Pleistocene to explore the consequences of past biodiversity loss. We characterize the isotopic and body-size niche of a mammal community in Texas before and after the event to assess the influence on the ecology and ecological interactions of surviving species (>1 kg). Preextinction, a variety of C4 grazers, C3 browsers, and mixed feeders existed, similar to modern African savannas, with likely specialization among the two sabertooth species for juvenile grazers. Postextinction, body size and isotopic niche space were lost, and the δ13C and δ15N values of some survivors shifted. We see mesocarnivore release within the Felidae: the jaguar, now an apex carnivore, moved into the specialized isotopic niche previously occupied by extinct cats. Puma, previously absent, became common and lynx shifted toward consuming more C4-based resources. Lagomorphs were the only herbivores to shift toward C4 resources. Body size changes from the Pleistocene to Holocene were species-specific, with some animals (deer, hare) becoming significantly larger and others smaller (bison, rabbits) or exhibiting no change to climate shifts or biodiversity loss. Overall, the Holocene body-size-isotopic niche was drastically reduced and considerable ecological complexity lost. We conclude biodiversity loss led to reorganization of survivors and many "missing pieces" within our community; without intervention, the loss of Earth's remaining ecosystems that support megafauna will likely suffer the same fate.

Svalbard reindeer winter diets: Long-term dietary shifts to graminoids in response to a changing climate.

Arctic ecosystems are changing dramatically with warmer and wetter conditions resulting in complex interactions between herbivores and their forage. We investigated how Svalbard reindeer (Rangifer tarandus platyrhynchus) modify their late winter diets in response to long-term trends and interannual variation in forage availability and accessibility. By reconstructing their diets and foraging niches over a 17-year period (1995-2012) using serum δ13 C and δ15 N values, we found strong support for a temporal increase in the proportions of graminoids in the diets with a concurrent decline in the contributions of mosses. This dietary shift corresponds with graminoid abundance increases in the region and was associated with increases in population density, warmer summer temperatures and more frequent rain-on-snow (ROS) in winter. In addition, the variance in isotopic niche positions, breadths, and overlaps also supported a temporal shift in the foraging niche and a dietary response to extreme ROS events. Our long-term study highlights the mechanisms by which winter and summer climate changes cascade through vegetation shifts and herbivore population dynamics to alter the foraging niche of Svalbard reindeer. Although it has been anticipated that climate changes in the Svalbard region of the Arctic would be detrimental to this unique ungulate, our study suggests that environmental change is in a phase where conditions are improving for this subspecies at the northernmost edge of the Rangifer distribution.

Contrasting Trophic Niche and Resource Use Dynamics Across Multiple American Horseshoe Crab (Limulus polyphemus) Populations and Age Groups

AbstractHorseshoe crabs (Limulus polyphemus) exhibit ontogenetic and geographic variability in migratory patterns, yet the implications of movement on their resource use dynamics remain poorly understood. Here, we evaluate horseshoe crab resource use across ontogeny (instars 14–18 and adults), sex, seasons, and between two spatially distinct populations that exhibit different migratory patterns. Multi-tissue stable isotope analyses of carbon, nitrogen, and sulfur were used to examine (1) ontogenetic and sex-specific variability in the reliance of resource pools; (2) variation in isotopic niche breadth within and among populations; and (3) seasonal variability in resource use within juvenile life stages. We found evidence for subtle ontogenetic shifts in resource use, with increased contributions of phytoplankton with ontogeny, but all age groups predominantly relied on sedimentary organic matter (SOM) pools. Sex-specific differences in resource use were observed with SOM serving as the predominant source assimilated in males, while females exhibited a higher dependence on phytoplankton resource pools, indicating foraging or spatial preferences may be different between sexes. Low trophic niche overlap between adult and juvenile horseshoe crabs was evident, likely arising from differences in migratory patterns and size constraints. Within juveniles, no seasonal differences in resource use were observed, indicating that dietary patterns may remain static across temporal scales, consistent with limited dispersal of that life stage. Spatial differences in resource use were, however, observed between adult crabs likely reflecting the migratory strategies of different populations. Our results are consistent with previous evidence that horseshoe crabs are dietary generalists but provide novel insights into the linkages between movement and trophic patterns.

Inter-individual variability of early life stages of a model marine invertebrate with a bi-phasic life cycle is shaped by contrasting oceanographic conditions

Inter-individual variability plays a key role in species resilience. This, however, is difficult to assess in marine invertebrates with complex life cycles due to the inherent difficulty of sampling individuals in oceanic environments throughout their ontogeny. This study monitored the effect of contrasting oceanographic conditions, namely downwelling and upwelling, on the inter-variability of embryos and megalopae (the final larval stage) of the model brachyuran crab Carcinus maenas. We assessed i) the heterogeneity of energetic reserves, biomass, elemental composition, and isotopic niche in these early life stages, and ii) the effect of oceanographic conditions (up- and downwelling) on inter-individual variability. Indeed, organisms developing during downwelling consistently exhibited a higher heterogeneity than those developing during upwelling. While this finding suggests a higher resilience during downwelling, the overall condition of individuals was better during upwelling (i.e., higher C:N), irrespective of the developmental stage. Altogether, our data suggests that trophic history experienced under contrasting oceanographic conditions shape the plasticity of C. maenas populations and cascades over different life stages.

Effects of urbanization on the trophic niche of the brown anole, a widespread invasive lizard

Urbanization is one of the most dramatic forms of environmental change with the potential to alter trophic relationships among organisms in cities. In this study, we combine gut-content and stable isotope analyses of brown anole diets with field sampling of arthropods to assess whether and how urbanization-induced variation of basal resources influence trophic niche variation of anole populations inhabiting urban, suburban, and natural habitats. We found clear evidence for trophic niche variation across the urbanization gradient. Stable isotope analyses revealed that urban and suburban anoles had strongly elevated δ13C levels and slightly lower δ15N levels compared to anoles from natural forests— results consistent with increased reliance on grasses across the urbanization gradient. Isotopic niche widths were also greater in urban habitats. However, our analysis of anole diets revealed no difference in the diversity of prey consumed—despite urban habitats having the highest diversity of available prey compared to suburban and natural forest habitats. Overall, our results suggest that much of the observed variation in food web structure results from a shift in the diversity of basal resource pools across the urbanization gradient. The broader effects of a shift towards grass-dominated food webs in urban habitats remains to be understood.

Predatory fish diets shift towards an invasive mullet in a traditional Hawaiian aquaculture system

AbstractTraditional Hawaiian fishponds, called loko iʻa, are a low‐impact and culturally important aquaculture system that historically produced significant fish yields. To better understand the structure of the contemporary food web of the restored Heʻeia fishpond, a mark‐recapture experiment was conducted to estimate the population abundance of the three dominant predatory fishes, and gut content, genetic barcoding, and stable isotope analyses were used to assess their diets. Great barracuda Sphyraena barracuda had the largest estimated population at 190 individuals, followed by 89 giant trevally Caranx ignobilis and 19 bluefin trevally Caranx melampygus. The three species were caught throughout the pond, with C. ignobilis concentrated in the deeper northwest area near a mangrove island. Diets of the three species consisted mainly of nearshore, estuarine fishes, and crustaceans. Prey taxonomic resolution was greatly improved with the use of genetic barcoding since most fish were too degraded to be visually identified. Trophic level calculations and isotopic niche breadth comparisons indicated that C. ignobilis and S. barracuda occupied similar ecological niches in the fishpond, and stable isotope mixing models revealed that their long‐term diets were mostly comprised glass shrimp (Palaemon sp.), mangrove swimming crab (Thalamita crenata), and an invasive Australian mullet (Osteomugil engeli). The native mullet (Mugil cephalus), a primary food fish species from the fishpond, was not identified in the diets of the predatory fishes in this study nor in a previous diet study decades ago prior to the introduction of Australian mullet. The inclusion of the invasive Australian mullet in the contemporary diets of the predatory fishes demonstrates a shift towards the newly available prey which may benefit the native mullet populations. These findings improve the understanding of food web dynamics in contemporary Hawaiian fishponds and provide an example of native predatory fishes consuming an invasive species as part of their diets.

Ecological groups and isotopic niches of earthworms

Earthworms are generally categorized into three ecological groups. This categorization and its two refined versions have been widely used in studies focusing on earthworm community structure and biological invasions, as well as the effects of earthworms on vegetation, soil properties, carbon and nitrogen cycling, and ecosystem functions. We revisited this categorization system by analyzing the stable isotopes of carbon and nitrogen (13C and 15N) from 10 earthworm species commonly found in arable fields and deciduous forests in temperate North America. We showed that up to nine isotopic niches utilized by earthworms can be distinguished in the two-dimensional space delimited by δ13C and δ15N. Rather than forming distinct categories, isotopic niches of different ecological groups form a continuum, ranging from species feeding on leaf litter to those feeding on highly processed soil organic matter. Along this continuum, the three “traditional” groups are recognizable, and species of the same ecological group can exhibit considerable isotopic niche differentiation and resource partitioning. These results, together with field observations on native Lumbricidae in Europe, suggest that, despite their convenience, there are limitations to the commonly used ecological categories. We propose to incorporate stable isotopes as functional traits in studying earthworm ecology and the linkage between earthworm communities and ecosystem functions. Moreover, to fill existing knowledge gaps, the isotopic approach needs to be further expanded to native earthworm communities around the world, particularly outside Western Europe and North America, and to earthworm families and major taxonomic groups other than Lumbricidae, such as Glossoscolecidae, Acanthodrilidae, and Megascolecidae.

Coastal hypoxia reduces trophic resource coupling and alters niche characteristics of an ecologically dominant omnivore

Energy and biomass move through a variety of trophic pathways in coastal ecosystems, with pelagic and benthic pathways often dominating food web dynamics. Consumers that couple these pathways play important structuring roles in these food webs by integrating spatially disconnected or asynchronous production pathways. The objective of this study was to quantify benthic and pelagic trophic pathway contributions to the diet of the mysid Neomysis americana in the Choptank and Patuxent rivers (tributaries of Chesapeake Bay) and determine if the relative contribution of these two trophic pathways differs between ecosystems with different environmental, physical, and watershed features. This mysid species undergoes daily vertical migrations throughout the water column and it was hypothesized that hypoxia (< 2 mg/L) in deep-water habitats of the Patuxent River (but not the Choptank) could reduce the importance of the benthic pathway for mysids in the Patuxent River relative to the Choptank River. Mysids and trophic resources were collected in each river during the summers of 2018 and 2019 and analyzed for carbon and nitrogen stable isotope composition and tissue stoichiometry (C:N). Stable-isotope based estimates of benthic trophic pathway contribution, trophic position, and isotopic niche area (proxy for trophic niche) and C:N composition of N. americana were compared within and between tributaries. Overall, mysids in the Patuxent River assimilated less benthic material, realized lower trophic positions, and had lower C:N ratios (proxy for lipid content) than Choptank River mysids. No consistent pattern in trophic niche area was observed although niche area (and pelagic trophic pathway contribution) of Choptank River mysids increased during a high river discharge event. This study provides evidence that hypoxic conditions in the Patuxent reduced the strength of benthic-pelagic coupling by N. americana and, further, is associated with lower trophic position and a reduced stoichiometric marker of lipid storage. In addition to their role as omnivorous consumers, N. americana are a dominant prey taxon for many predators, underscoring the importance of this species to local food webs and the potential for bottom-up effects on the dynamics of coastal ecosystems in response to changes in their trophic ecology.

Improving stable isotope assessments of inter- and intra-species variation in coral reef fish trophic strategies.

Fish have one of the highest occurrences of individual specialization in trophic strategies among Eukaryotes. Yet, few studies characterize this variation during trophic niche analysis, limiting our understanding of aquatic food web dynamics. Stable isotope analysis (SIA) with advanced Bayesian statistics is one way to incorporate this individual trophic variation when quantifying niche size. However, studies using SIA to investigate trophodynamics have mostly focused on species‐ or guild‐level (i.e., assumed similar trophic strategy) analyses in settings where source isotopes are well‐resolved. These parameters are uncommon in an ecological context. Here, we use Stable Isotope Bayesian Ellipses in R (SIBER) to investigate cross‐guild trophodynamics of 11 reef fish species within an oceanic atoll. We compared two‐ (δ 15N and δ 13C) versus three‐dimensional (δ 15N, δ 13C, and δ 34S) reconstructions of isotopic niche space for interpreting guild‐, species‐, and individual‐level trophic strategies. Reef fish isotope compositions varied significantly among, but also within, guilds. Individuals of the same species did not cluster together based on their isotope values, suggesting within‐species specializations. Furthermore, while two‐dimensional isotopic niches helped differentiate reef fish resource use, niche overlap among species was exceptionally high. The addition of δ 34S and the generation of three‐dimensional isotopic niches were needed to further characterize their isotopic niches and better evaluate potential trophic strategies. These data suggest that δ 34S may reveal fluctuations in resource availability, which are not detectable using only δ 15N and δ 13C. We recommend that researchers include δ 34S in future aquatic food web studies.

Human food subsidies drive individual specialization and intrapopulation dietary differences in a generalist predator

AbstractGeneralist species can exhibit individual specialization (IS), where individuals adopt specialized foraging behaviors not attributable to age, sex, or social dominance. While IS increases with the diversity of available foraging resources (ecological opportunity), the potential impact of human food subsidies on ecological opportunity is unknown. We quantified the isotopic niche width of Steller's Jays (Cyanocitta stelleri), a synanthropic predator and dietary generalist, across a gradient of human land‐use categories ranging from subsidized (park campgrounds) to unsubsidized (forest interior) habitats in a protected area in California. We asked (1) if isotopic niche width was better predicted by individual foraging behavior than sex, social dominance, or habitat category (indicating IS) and (2) if degree of IS exhibited by jays increased with the availability of human food subsidies. We characterized the isotopic niche space occupied by individual jays relative to the population using carbon (δ13C) and nitrogen (δ15N) isotope values. Using linear models, we found that jay diet variation was best explained by individual behavior and habitat use rather than by sex or social dominance. While the degree of IS increased with the availability of human foods, individual jays exhibited a variety of foraging behaviors in all habitats that were segregated by foraging stratum and use of human foods. Individual diet also determined the degree of specialization, as jays that specialized on human foods had the narrowest niche width regardless of habitat. Management efforts targeted at generalist wildlife exhibiting large degrees of IS should therefore account for the impact of food subsidies on foraging behavior. Approaches that involve aversive conditioning may fail where highly specialized individuals are unlikely to encounter treatments; therefore, we recommend simultaneous efforts to limit human food subsidies (e.g., information campaigns and improved containment of human food and food waste), and deployment across the largest possible extent of foraging microhabitats and substrates in order to ensure exposure of whole populations to emetic eggs.

Isotopic niches of Silver Carp and two native planktivores in a large reservoir

Human-mediated biological invasions can greatly reduce native species diversity. More than 138 nonnative fishes have been introduced to the United States. Many of these nonnative species can alter ecosystem dynamics and compete with native species. The planktivorous Silver Carp (Hypophthalmichthys molitrix Valenciennes in Cuvier and Valenciennes, 1844) is a nonnative species of concern that is currently expanding its range across the Greater Mississippi River Basin in the Midwestern United States. The objective of our study was to determine the degree of potential competition among invasive Silver Carp and the native planktivores, Gizzard Shad (Dorosoma cepedianum Lesueur, 1818) and Threadfin Shad (Dorosoma petenense Günther, 1867), in Kentucky Lake, a large reservoir located in western Kentucky and Tennessee. We used data derived from N and C isotopes to model isotopic niches, calculate quantitative metrics to describe variability within and among isotopic niches and trophic positions, and qualitatively assess seasonal shifts in trophic positions. Our results suggest that in the spring, older (≥1 y old) Silver Carp share resources with older and young-of-year (YOY; <1 y old) Gizzard Shad, whereas YOY Silver Carp only share resources with YOY Gizzard Shad. Additionally, our analyses showed that older Silver Carp and older Gizzard Shad share resources in the summer. In contrast, we found no overlap in resource use between YOY Threadfin Shad and YOY or older Silver Carp. Furthermore, we provide evidence of intraspecific ontogenetic shifts in isotopic niche area and overlap between invasive and native fishes (measured by sample-size corrected standard ellipse area). If resources become limiting, invasive Silver Carp may compete with multiple life stages of Gizzard Shad in Kentucky Lake and alter foodweb dynamics.

Dominance of autochthonous trophic base in northeast Asian stream food webs pre- and post-monsoon

Spatial and temporal variability in the trophic base of food webs is an inevitable ecological attribute in aquatic ecosystems with the role of aquatic and terrestrial sources varying from system to system. Despite high algal biomass, an apparent increase in terrestrial detritus during the East Asian summer monsoon would be expected to alter the dominant food web base from the pre- to post-monsoon period. The δ13C and δ15N values of common aquatic invertebrates and fish were compared with those of their potential sources of organic matter across the upstream channel and lacustrine habitats during the pre- and post-monsoon periods in the Se-oak Stream system of Korea. The purpose was to identify the variability in trophic structure (niche and diversity) at the community level and to quantify the contribution of terrestrial and aquatic carbon sources to the food web base. The δ13C and δ15N values of the major potential organic matter sources differentiated allochthonous leaf litter and riparian vegetation from autochthonous biofilm and seston, but with no significant spatial and temporal changes. The δ13C values of consumers fell within a narrower range than that of organic matter sources, consistent with the instream biofilm and seston values. However, the consumer δ15N values spanned a broad range, increasing from invertebrates to fish. The resultant niche overlaps of consumers in the isotopic niche space between seasons and locations, their narrower horizontal niche breadths compared to organic matter sources, and the isotope mixing-model estimates indicated that aquatic production was the primary source of organic matter supporting the aquatic food webs. Overall, the results revealed a consistent dominance of autochthonous trophic bases in both lotic and lentic food webs before and after the summer monsoon. Our findings, which highlight the trophic importance of aquatic algal production facilitated by anthropogenic nutrients in this shallow, low-turbidity, and well-illuminated stream food web, may provide insight into ecosystem processes that counteract potential terrestrial-aquatic links to influence food web dynamics based on natural nutrient sources in deep, highly turbid, or shaded stream counterparts.

Too Close for Comfort? Isotopic Niche Segregation in New Zealand's Odontocetes.

Simple SummaryWe used carbon and nitrogen stable isotopes as ecological tracers to investigate isotopic niche overlap between 21 odontocete (toothed whale) species inhabiting neritic, mesopelagic, and bathypelagic waters. Results showed a clear niche separation for the bathypelagic Gray’s beaked whales (Mesoplodon grayi) and sperm whales (Physeter macrocephalus), but high isotopic niche overlap and potential interspecific competition for neritic and mesopelagic species. This study represents the first insights into the coexistence of odontocetes in a biodiverse hotspot and provides a critical baseline for a system already undergoing ecosystem changes via ocean warming and its subsequent effect on prey abundance and distribution.Species occurring in sympatry and relying on similar and limited resources may partition resource use to avoid overlap and interspecific competition. Aotearoa, New Zealand hosts an extraordinarily rich marine megafauna, including 50% of the world’s cetacean species. In this study, we used carbon and nitrogen stable isotopes as ecological tracers to investigate isotopic niche overlap between 21 odontocete (toothed whale) species inhabiting neritic, mesopelagic, and bathypelagic waters. Results showed a clear niche separation for the bathypelagic Gray’s beaked whales (Mesoplodon grayi) and sperm whales (Physeter macrocephalus), but high isotopic niche overlap and potential interspecific competition for neritic and mesopelagic species. For these species, competition could be reduced via temporal or finer-scale spatial segregation or differences in foraging behaviour. This study represents the first insights into the coexistence of odontocetes in a biodiverse hotspot. The data presented here provide a critical baseline to a system already ongoing ecosystem change via ocean warming and subsequent effects on prey abundance and distributions.

Interspecific and intraspecific comparison of the isotopic niche of shrimps targets of fishing in south-eastern Brazil

AbstractThe study compares the isotopic niche (δ13C–δ15N) of three species of penaeid shrimps (Xiphopenaeus kroyeri, Artemesia longinaris and Litopenaeus schmitti) that are targets of fishing in south-eastern Brazil. The two hypotheses raised are based on the niche theory, in which the coexistence between organisms that have similar dietary demands is possible due to trophic partitioning: (i) shrimp species that share the habitat have segregated isotopic niches; and (ii) stages of maturity and genders vary with the trophic habitat (δ13C) and/or trophic position (δ15N). In multispecific fishing grounds, the isotopic niches of the shrimp species were segregated or had low overlap, indicating trophic partitioning. The intraspecific comparison of δ13C and δ15N showed a similar trend in all species. For δ13C, the values were more enriched in adult individuals than in juveniles, but similar between males and females. For δ15N, adult individuals also had more enriched values, and males were more enriched than females. The results confirm the hypotheses raised, and the assumptions of niche theory apply to these penaeid shrimps.

Diet variation in a critically endangered marine predator revealed with stable isotope analysis.

Understanding the foraging ecology of animals gives insights into their trophic relationships and habitat use. We used stable isotope analysis to understand the foraging ecology of a critically endangered marine predator, the Māui dolphin. We analysed carbon and nitrogen isotope ratios of skin samples (n = 101) collected from 1993 to 2021 to investigate temporal changes in diet and niche space. Genetic monitoring associated each sample with a DNA profile which allowed us to assess individual and population level changes in diet. Potential prey and trophic level indicator samples were also collected (n = 166; 15 species) and incorporated in Bayesian mixing models to estimate importance of prey types to Māui dolphin diet. We found isotopic niche space had decreased over time, particularly since the 2008 implementation of a Marine Mammal Sanctuary. We observed a decreasing trend in ∂13C and ∂15N values, but this was not linear and several fluctuations in isotope values occurred over time. The largest variation in isotope values occurred during an El Niño event, suggesting that prey is influenced by climate-driven oceanographic variables. Mixing models indicated relative importance of prey remained constant since 2008. The isotopic variability observed here is not consistent with individual specialization, rather it occurs at the population level.

Trophic ecology of northern gannets Morus bassanus highlights the extent of isotopic niche overlap with other apex predators within the Bay of Biscay

Understanding the linkages within complex and evolving marine food webs is essential to comprehend marine ecosystem structure and dynamics. Carbon (δ13C) and nitrogen (δ15N) stable isotope signatures are recognized to be powerful descriptors of the trophic ecology and trophic relationships within marine communities. Apex predators such as seabirds can influence the structure of communities by predating in lower trophic levels. They also convey information both over a range of spatial and temporal scales due to their high mobility and longevity, respectively. For this reason, here, we studied the trophic ecology of northern gannets and the extent of niche overlap with other apex predators within the Bay of Biscay (North-East Atlantic), a key feeding area for numerous predatory species. Mixing models indicated that northern gannets fed primarily on European anchovy Engraulis encrasicolus and on a mixed group composed of European sardine Sardina pilchardus, European sprat Sprattus sprattus, juvenile hake Merluccius merluccius, Atlantic mackerel Scomber scombrus and Atlantic horse mackerel Trachurus trachurus. Within the apex predator community, the northern gannets trophic niche overlapped the most with common guillemots Uria aalge (15.58%, based on Stable Isotope Bayesian Ellipses (SIBER)) and overlapped to a lesser extent with cetacean species occupying shelf habitats (i.e., the harbour porpoise Phocoena phocoena (9.99%) and the short-beaked common dolphin Delphinus delphis (10.37%)). Overall, the overlap found was moderate indicating trophic similarities and dissimilarities between these species. Information on trophic interactions between co-existing species is necessary for holistic environmental management, particularly in areas where several megafauna species share their foraging grounds with fisheries activity.

Recovery of Macrobenthic Food Web on Rocky Shores Following the Hebei Spirit Oil Spill as Revealed by C and N Stable Isotopes

The impact of large-scale oil spills on organisms can lead to modifications of the food web structure. To assess the effects of the Hebei Spirit oil-spill accident on the trophic structure of the macrobenthic community on intertidal rocky shores along Taean Peninsula on the western coast of Republic of Korea 4 years after the Hebei Spirit oil spill, we analyzed carbon and nitrogen stable isotope ratios of macrobenthic consumers and their potential food sources in two heavily oil-impacted and one non-impacted sites. The results show no significant differences in isotopic ratios of feeding groups and their potential food sources between the polluted and reference sites, suggesting similar trophic structures given similar resource use by consumers. Similar isotopic niches and substantial overlap areas of feeding groups between the affected and reference sites suggest that the oil-impacted sites have re-achieved the trophic functions of the natural ecosystem. This study provides valuable information on the ecological processes of trophic recovery in coastal ecosystems impacted by oil spills.

Trophic Ecology of Deep-Sea Megafauna in the Ultra-Oligotrophic Southeastern Mediterranean Sea

The trophic ecology of fourteen species of demersal fishes and six species of demersal decapod crustaceans from the continental slope and rise of the Southeastern Mediterranean Sea (SEMS) was examined using stable isotope analysis. Mean δ13C values among fish species varied by ca. 4.0‰, from -20.85‰ (Macroramphosus scolopax) to -16.57‰ and -16.89‰ (Conger conger and Centrophorus granulosus), showing an enrichment in 13C as a function of depth (200 – 1400 m). Mean δ13C values of the crustaceans showed smaller variation, between -18.54‰ (Aristeus antennatus) and -16.38‰ (Polycheles typhlops). This suggests a shift from pelagic to regenerated benthic carbon sources with depth. Benthic carbon regeneration is further supported by the low benthic-pelagic POM-δ13C values, averaging -24.7 ± 1.2‰, and the mixing model results, presenting relatively low contribution of epipelagic POM to the deep-sea fauna. Mean δ15N values of fish and crustacean species ranged 7.91 ± 0.36‰ to 11.36 ± 0.39‰ and 5.96 ± 0.24‰ to 7.73 ± 0.46‰, respectively, resulting in trophic position estimates, occupying the third and the fourth trophic levels. Thus, despite the proximity to the more productive areas of the shelf, low number of trophic levels (TL~1.0) and narrow isotopic niche breadths (SEAC&amp;lt;1) were observed for demersal crustaceans (TL = 2.94 ± 0.18) and fishes (TL = 3.62 ± 0.31) in the study area – probably due to the ultra-oligotrophic state of the SEMS resulting in limited carbon sources. Our results, which provide the first trophic description of deep-sea megafauna in the SEMS, offer insight into the carbon sources and food web structure of deep-sea ecosystems in oligotrophic marginal seas, and can be further used in ecological modeling and support the sustainable management of marine resources in the deep Levantine Sea.

Broad Whitefish (Coregonus nasus) isotopic niches: Stable isotopes reveal diverse foraging strategies and habitat use in Arctic Alaska.

Understanding the ecological niche of some fishes is complicated by their frequent use of a broad range of food resources and habitats across space and time. Little is known about Broad Whitefish (Coregonus nasus) ecological niches in Arctic landscapes even though they are an important subsistence species for Alaska's Indigenous communities. We investigated the foraging ecology and habitat use of Broad Whitefish via stable isotope analyses of muscle and liver tissue and otoliths from mature fish migrating in the Colville River within Arctic Alaska. The range of δ13C (-31.8- -21.9‰) and δ15N (6.6-13.1‰) across tissue types and among individuals overlapped with isotope values previously observed in Arctic lakes and rivers, estuaries, and nearshore marine habitat. The large range of δ18O (4.5-10.9‰) and δD (-237.6- -158.9‰) suggests fish utilized a broad spectrum of habitats across elevational and latitudinal gradients. Cluster analysis of muscle δ13C', δ15N, δ18O, and δD indicated that Broad Whitefish occupied four different foraging niches that relied on marine and land-based (i.e., freshwater and terrestrial) food sources to varying degrees. Most individuals had isotopic signatures representative of coastal freshwater habitat (Group 3; 25%) or coastal lagoon and delta habitat (Group 1; 57%), while individuals that mainly utilized inland freshwater (Group 4; 4%) and nearshore marine habitats (Group 2; 14%) represented smaller proportions. Otolith microchemistry confirmed that individuals with more enriched muscle tissue δ13C', δD, and δ18O tended to use marine habitats, while individuals that mainly used freshwater habitats had values that were less enriched. The isotopic niches identified here represent important foraging habitats utilized by Broad Whitefish. To preserve access to these diverse habitats it will be important to limit barriers along nearshore areas and reduce impacts like roads and climate change on natural flow regimes. Maintaining these diverse connected habitats will facilitate long-term population stability, buffering populations from future environmental and anthropogenic perturbations.

Shifts in the seasonal trophic ecology of larvae and juveniles of European hake (Merluccius merluccius): From the Galician upwelling system (NW Spain)

AbstractThe trophic ecology of European hake (Merluccius merluccius) larvae and early‐juveniles from the Iberian shelf off Galicia (NW Spain) are assessed by means of stable isotopes (SIA) and nutritional condition in conjunction with hydrobiological variations observed during the winter and summer season of 2012. Hake early‐juveniles (18–47 mm SL) showed higher δ15N than larvae (3–11 mm SL) together with the microzooplankton (55–200 μm) and mesozooplankton (&gt;200 μm) during both seasons. Low δ15N values and high variability in both zooplankton and hake larvae were found during winter coinciding with an unusually strong upwelling event. Inversely, high δ15N content in zooplankton in summer suggests strong nitrogen reutilization. However, hake larvae sampled in summer had a higher RNA:DNA ratio in line with higher Fulton condition factors than those sampled in winter. Higher δ13C values for microzooplankton and mesozooplankton and hake larvae in summer compared to late winter point to prey availability differences. The relationship between nitrogen and carbon stable isotopes versus size or weight suggests an ontogenetic shift in the diet of hake larvae. Early‐juveniles had a lower isotopic niche width compared to larvae in both late winter and summer, indicating a trophic specialization related to changes following settlement process from planktonic life to demersal habitat. Higher trophic specialization was observed in summer, which recorded a narrower isotopic niche and higher trophic position estimations.