Stable isotope analysis (SIA) has been successfully applied to reconstruct diet and migration patterns of organisms, identify food-web structures and track flows of elemental matter within an ecosystem. Key to this approach is the principle of isotopic turnover, the fact that when in equilibrium with the local food web the stable isotopic signatures of animal tissues reflect those of their diets. When an animal switches to an isotopically different food source the isotopic composition of its tissues will change as a consequence of 2 processes. For growing animals isotopic turnover rate is regulated by simple dilution effects. However, turnover rates can be accelerated by the additional effect of tissue-specific maintenance metabolism i.e. the metabolic breakdown of old tissue synthesised during feeding on a previous diet and its subsequent replacement by tissue made from the new diet. The underlying mechanisms are still poorly understood and the relationship between diet and stable isotopic ratios in consumer tissues may be subject to variability wrought by differences in a range of factors, all related to metabolism. These include nutritional status, body size, diet quality, assimilation efficiency, excretion form and protein turnover. As a consequence, interpretations of diet based on SIA may be biased if variations in isotopic signatures among animal tissues and estimates of isotopic turnover are not understood for a given system. To account for these sources of variability the basic assumptions underlying the use of SIA need laboratory validation using controlled experiments. As isotopic turnover rates are specific to taxon, type of tissue analysed, ontogenetic stage and environmental conditions it is crucial that studies using SIA to provide information on diet and migration dynamics take specific turnover rates for the organism and system under study into account. The need for an understanding of trophic linkages is particularly imperative in the Antarctic, where impending changes in climate are predicted to bring about large-scale shifts in the environment and where the organisms themselves have a reduced ability to physiologically acclimate to those environmental changes.
The Antarctic marine environment is both one of the most stable and one of the most variable on Earth. Temperature variation is predominantly less than 3Ã‚Â°C, whilst primary productivity (phytoplankton bloom intensity) can vary seasonally by over 4 orders of magnitude. Research over the past four decades has shown that these conditions have resulted in a highly adapted, stenothermal fauna with a poor capacity to resist elevated temperatures. The Rothera Oceanographic and Biological Time-Series (RaTS) is a key component of the BAS Biodiversity Programme and has been collecting data on seasonal and interannual variability in reproductive biology and feeding activity of selected marine invertebrates since 1997. There now exists a good knowledge base of published studies focussed on key benthic invertebrates from the shallow-water habitats within Marguerite and Ryder Bay and whilst many of these organisms have been shown to have growth rates, reproductive cycles and metabolic demands that are closely related to environmental temperature and seasonal variability in food supply, there has been less focus on the trophic relationships and linkages within these benthic communities. Understanding carbon flow and trophic linkages in the context of a food-web is a fundamental requisite in determining future ecosystem-wide changes to community structure and function.
Whilst there exists a significant body of work related to food-web structure in Antarctic shallow-water benthic communities there has currently been no investigation into specific isotopic turnover rates for these often-endemic organisms with well documented metabolic and physiological adaptations to low temperature and variable food supply. Ongoing core BAS research is focussed on describing temporal variability in components of both the soft-sediment and hard substrata shallow-water ecosystems at Rothera. This research will provide crucial information on trophic linkages and carbon flow within these systems to complement the ongoing provision of data on diversity, density and biomass. This will allow for the development of a much more detailed ecosystem model.
The continuation of a field-sampling study, coupled with the SIA of samples held from 2015 onwards, will contribute a unique temporal element to the collaborative construction of a wider conceptual food-web model for the shallow coastal ecosystems of Marguerite Bay, Rothera. This research will (a) identify the main carbon sources utilised by the benthic fauna; (b) provide estimates of trophic position and (c) assess variation through time. The generality of the food web model will be assessed over a range of temporal and spatial scales by comparison between local sites and with published data on similar species from the east of the continent.
A long-term (18mth), diet-switch, laboratory experiment will be based at the BAS aquarium and controlled environment facility in Cambridge, to determine the isotopic turnover rate in the tissues of two functionally different key taxa, from the Western Peninsula. We propose a suspension feeder/grazer (cf. Laternula elliptica, Heterocucumis steinini etc) and a predator/scavenger (cf. Odontaster validus, Ophionotus victoriae etc), with a reported trophic position separation. We hypothesise that these species would have different turnover rates from each other and that their cold-adapted metabolism would result in different rates from those reported for temperate invertebrates from similar taxa. The influence of short-term food deprivation will also be investigated to infer the possible confounding effects of seasonal fasting in some species on the wider food-web.