Assessing the trophic ecology of three sympatric squid in the marine ecosystem off the Patagonian Shelf by combining stomach content and stable isotopic analyses

Squid species are important components of the Southern Atlantic Ocean ecosystems, as they prey on a wide range of crustaceans, fish and cephalopods. As a result of this trophic interaction and their high abundance, they are considered reliable indicators of energy transfer and biomass in the food web. We identified Illex argentinus, Doryteuthis gahi and Onykia ingens as the most important squid species interacting on the Patagonian shelf, and used isotope analysis and stomach content identification to assess the feeding ecology and interaction of these squids in the ecosystem. Our results describe trophic interactions by direct predation of O. ingens and I. argentinus on D. gahi, and a trophic overlap of the three squid, and indicate a higher trophic level and differences in the foraging areas for mature and maturing D. gahi inferred through δ15N and δ13C concentrations. These differences were related to the segregation and different habitat of large mature D. gahi and suggest a food enrichment of C and N based on feeding sources other than those used by small maturing D. gahi and I. argentinus and O. ingens.Versión del editor1,484

Application of Nitrogen and Carbon Stable Isotopes (δ15Ν and δ13C) to Quantify Food Chain Length and Trophic Structure

Increasingly, stable isotope ratios of nitrogen (delta N-15) and carbon (delta C-13) are used to quantify trophic structure, though relatively few studies have tested accuracy of isotopic structural measures. For laboratory-raised and wild-collected plant-invertebrate food chains spanning four trophic levels we estimated nitrogen range (NR) using delta N-15, and carbon range (CR) using delta C-13, which are used to quantify food chain length and breadth of trophic resources respectively. Across a range of known food chain lengths we examined how NR and CR changed within and between food chains. Our isotopic estimates of structure are robust because they were calculated using resampling procedures that propagate variance in sample means through to quantified uncertainty in final estimates. To identify origins of uncertainty in estimates of NR and CR, we additionally examined variation in discrimination (which is change in delta N-15 ordelta C-13 from source to consumer) between trophic levels and among food chains. delta N-15 discrimination showed significant enrichment, while variation in enrichment was species and system specific, ranged broadly (1.4‰ to 3.3‰), and importantly, propagated variation to subsequent estimates of NR. However, NR proved robust to such variation and distinguished food chain length well, though some overlap between longer food chains infers a need for awareness of such limitations. delta C-13 discrimination was inconsistent; generally no change or small significant enrichment was observed. Consequently, estimates of CR changed little with increasing food chain length, showing the potential utility of delta C-13 as a tracer of energy pathways. This study serves as a robust test of isotopic quantification of food chain structure, and given global estimates of aquatic food chains approximate four trophic levels while many food chains include invertebrates, our use of four trophic level plant-invertebrate food chains makes our findings relevant for a majority of ecological systems

Interpreting ancient food practices:Stable isotope and molecular analyses of visible and absorbed residues from a year-long cooking experiment

Chemical analyses of carbonized and absorbed organic residues from archaeological ceramic cooking vessels can provide a unique window into the culinary cultures of ancient people, resource use, and environmental effects by identifying ingredients used in ancient meals. However, it remains uncertain whether recovered organic residues represent only the final foodstuffs prepared or are the accumulation of various cooking events within the same vessel. To assess this, we cooked seven mixtures of C3 and C4 foodstuffs in unglazed pots once per week for one year, then changed recipes between pots for the final cooking events. We conducted bulk stable-isotope analysis and lipid residue analysis on the charred food macro-remains, carbonized thin layer organic patina residues and absorbed lipids over the course of the experiment. Our results indicate that: (1) the composition of charred macro-remains represent the final foodstuffs cooked within vessels, (2) thin-layer patina residues represent a mixture of previous cooking events with bias towards the final product(s) cooked in the pot, and (3) absorbed lipid residues are developed over a number of cooking events and are replaced slowly over time, with little evidence of the final recipe ingredients

Resolving the Trophic Relations of Cryptic Species: An Example Using Stable Isotope Analysis of Dolphin Teeth

Understanding the foraging ecology and diet of animals can play a crucial role in conservation of a species. This is particularly true where species are cryptic and coexist in environments where observing feeding behaviour directly is difficult. Here we present the first information on the foraging ecology of a recently identified species of dolphin (Southern Australian bottlenose dolphin (SABD)) and comparisons to the common bottlenose dolphin (CBD) in Victoria, Australia, using stable isotope analysis of teeth. Stable isotope signatures differed significantly between SABD and CBD for both δ13C (−14.4‰ vs. −15.5‰ respectively) and δ15N (15.9‰ vs. 15.0‰ respectively), suggesting that the two species forage in different areas and consume different prey. This finding supports genetic and morphological data indicating that SABD are distinct from CBD. In Victoria, the SABD is divided into two distinct populations, one in the large drowned river system of Port Phillip Bay and the other in a series of coastal lakes and lagoons called the Gippsland Lakes. Within the SABD species, population differences were apparent. The Port Phillip Bay population displayed a significantly higher δ15N than the Gippsland Lakes population (17.0‰ vs. 15.5‰), suggesting that the Port Phillip Bay population may feed at a higher trophic level - a result which is supported by analysis of local food chains. Important future work is required to further understand the foraging ecology and diet of this newly described, endemic, and potentially endangered species of dolphin

Temporal and spatial variability in stable isotope ratios of SPM link to local hydrography and longer term SPM averages suggest heavy dependence of mussels on nearshore production

Temporal changes in hydrography affect suspended particulate matter (SPM) composition and distribution in coastal systems, potentially influencing the diets of suspension feeders. Temporal variation in SPM and in the diet of the mussel Perna perna, were investigated using stable isotope analysis. The δ13C and δ15 N ratios of SPM, mussels and macroalgae were determined monthly, with SPM samples collected along a 10 km onshore–offshore transect, over 14 months at Kenton-on-Sea, on the south coast of South Africa. Clear nearshore (0 km) to offshore (10 km) carbon depletion gradients were seen in SPM during all months and extended for 50 km offshore on one occasion. Carbon enrichment of coastal SPM in winter (June–August 2004 and May 2005) indicated temporal changes in the nearshore detrital pool, presumably reflecting changes in macroalgal detritus, linked to local changes in coastal hydrography and algal seasonality. Nitrogen patterns were less clear, with SPM enrichment seen between July and October 2004 from 0 to 10 km. Nearshore SPM demonstrated cyclical patterns in carbon over 24-h periods that correlated closely with tidal cycles and mussel carbon signatures, sampled monthly, demonstrated fluctuations that could not be correlated to seasonal or monthly changes in SPM. Macroalgae showed extreme variability in isotopic signatures, with no discernable patterns. IsoSource mixing models indicated over 50% reliance of mussel tissue on nearshore carbon, highlighting the importance of nearshore SPM in mussel diet. Overall, carbon variation in SPM at both large and small temporal scales can be related to hydrographic processes, but is masked in mussels by long-term isotope integration

Dental calculus and isotopes provide direct evidence of fish and plant consumption in Mesolithic Mediterranean

In this contribution we dismantle the perceived role of marine resources and plant foods in the subsistence economy of Holocene foragers of the Central Mediterranean using a combination of dental calculus and stable isotope analyses. The discovery of fish scales and flesh fragments, starch granules and other plant and animal micro-debris in the dental calculus of a Mesolithic forager dated to the end of the 8th millenium BC and buried in the Vlakno Cave on Dugi Otok Island in the Croatian Archipelago demonstrates that marine resources were regularly consumed by the individual together with a variety of plant foods. Since previous stable isotope data in the Eastern Adriatic and the Mediterranean region emphasises that terrestrial-based resources contributed mainly to Mesolithic diets in the Mediterranean Basin, our results provide an alternative view of the dietary habits of Mesolithic foragers in the Mediterranean region based on a combination of novel methodologies and data

Longer and less overlapping food webs in anthropogenically disturbed marine ecosystems: confirmations from the past

The human exploitation of marine resources is characterised by the preferential removal of the largest species. Although this is expected to modify the structure of food webs, we have a relatively poor understanding of the potential consequences of such alteration. Here, we take advantage of a collection of ancient consumer tissues, using stable isotope analysis and SIBER to assess changes in the structure of coastal marine food webs in the South-western Atlantic through the second half of the Holocene as a result of the sequential exploitation of marine resources by hunter-gatherers, western sealers and modern fishermen. Samples were collected from shell middens and museums. Shells of both modern and archaeological intertidal herbivorous molluscs were used to reconstruct changes in the stable isotopic baseline, while modern and archaeological bones of the South American sea lion Otaria flavescens, South American fur seal Arctocephalus australis and Magellanic penguin Spheniscus magellanicus were used to analyse changes in the structure of the community of top predators. We found that ancient food webs were shorter, more redundant and more overlapping than current ones, both in northern-central Patagonia and southern Patagonia. These surprising results may be best explained by the huge impact of western sealing on pinnipeds during the fur trade period, rather than the impact of fishing on fish populations. As a consequence, the populations of pinnipeds at the end of the sealing period were likely well below the ecosystem's carrying capacity, which resulted in a release of intraspecific competition and a shift towards larger and higher trophic level prey. This in turn led to longer and less overlapping food webs

Evidence for Changes in Subsurface Circulation in the Late Eocene Equatorial Pacific from Radiolarian-Bound Nitrogen Isotope Values

Microfossil-bound organic matter represents an important archive of surface ocean environmental information. Sedimentary nitrogen (N) isotope reconstructions of surface nitrate consumption and nitrogen source changes are made using fossil diatom (autotrophs) and planktic foraminiferal (heterotrophs)-bound organic matter with success. However, because diatoms and planktic foraminifera are poorly preserved and sedimentary organic matter content is near zero during the late Eocene, our ability to examine nutrient dynamics across this important climate transition is limited. Here we present new data exploring the use of N isotope records from radiolarian tests. A comparison of surface ocean nitrate and core top bulk and radiolarian N isotope values (as δ15N) from the equatorial Pacific indicates that radiolarian-N records δ15N variability with fidelity but that a significant offset exists between bulk sedimentary and diatom δ15N values and those measured from radiolarians (~7.1 ± 1.1‰). A downcore profile of radiolarian δ15N values is compared to siliceous microfossil assemblage changes across the Eocene-Oligocene boundary. Average of radiolarian-bound δ15N values is 0.5 ± 2.0‰, which, when corrected using the offset derived from the modern surface samples, suggests that the mean nitrogen isotopic composition of the early Cenozoic eastern Pacific was not significantly different from today. The overall trend, of decreasing δ15N values with decreasing export productivity, is consistent with either a regional decline in pelagic denitrification or a large-scale change in nutrient sources to the eastern equatorial Pacific (EEP), both linked to the cooling climate and changing intermediate water circulation. Decreasing/low δ15N values cooccur with high radiolarian species turnover at ~35.5 and 34 Ma, suggestive of a significant ecological change in the EEP, consistent with cooling and water mass distribution changes. The preliminary results suggest that radiolarian-bound organic nitrogen represents another promising archive and underscores the fact that the different microfossil fractions must be separated to ensure robust results

Variability in Isotope Discrimination Factors in Coral Reef Fishes: Implications for Diet and Food Web Reconstruction

Interpretation of stable isotope ratios of carbon and nitrogen (δ13C and δ15N) is generally based on the assumption that with each trophic level there is a constant enrichment in the heavier isotope, leading to diet-tissue discrimination factors of 3.4‰ for 15N (ΔN) and ∼0.5‰ for 13C (ΔC). Diet-tissue discrimination factors determined from paired tissue and gut samples taken from 152 individuals from 26 fish species at Ningaloo Reef, Western Australia demonstrate a large amount of variability around constant values. While caution is necessary in using gut contents to represent diet due to the potential for high temporal variability, there were significant effects of trophic position and season that may also lead to variability in ΔN under natural conditions. Nitrogen enrichment increased significantly at higher trophic levels (higher tissue δ15N), with significantly higher ΔN in carnivorous species. Changes in diet led to significant changes in ΔN, but not tissue δ15N, between seasons for several species: Acanthurus triostegus, Chromis viridis, Parupeneus signatus and Pomacentrus moluccensis. These results confirm that the use of meta-analysis averages for ΔN is likely to be inappropriate for accurately determining diets and trophic relationships using tissue stable isotope ratios. Where feasible, discrimination factors should be directly quantified for each species and trophic link in question, acknowledging the potential for significant variation away from meta-analysis averages and, perhaps, controlled laboratory diets and conditions