Abstract :
[en] Marine food webs are complex networks of ecological interactions, but can essentially be summarized using two dimensions, leading to the traditional depiction of food webs as bi-dimensional diagrams. The horizontal dimension of these diagrams encompasses the diversity of producers sustaining the food web. Their vertical structure is dictated by the trophic position of the consumers (e.g. primary consumers, secondary consumers, omnivores, etc.) relative to the food web baseline. Trophic positions provide simple ways to assess organisms’ functional roles and estimate energy flow through ecological communities, while taking into account complex processes such as omnivory. For these reason, they have been a central element to isotope ecologists‘ toolboxes for decades. The most common way of estimating trophic position involves measuring bulk δ15N of consumer tissue and baseline item(s). However, applicability of this time-tested method can be impaired when actual food web baseline are hard or impossible to identify due to temporal mismatch, consumer physiology, or logistical constraints. Coastal Antarctica, with its remoteness, intense seasonality, and extreme conditions, provide a striking example of these limitations, leading to sometimes unrealistic estimations of trophic position.
Here, to circumvent this issue, we used compound-specific isotope analysis of amino acid δ15N to estimate trophic positions of 5 key-members of Antarctic zoobenthos: the sea urchin Sterechinus neumayeri, the sea star Odontaster validus, the nemertean worm Parborlasia corrugatus, the scallop Adamussium colbecki, and the anemone Isotealia antarctica. When compared with estimates calculated using the traditional bulk δ15N method, trophic position estimation through amino acid δ15N showed good correspondence for O. validus and P. corrugatus, but not for the other 3 taxa. Our results suggest that use of the traditional bulk method leads to slight underestimation of trophic position for S. neumayeri, strong underestimation for A. colbecki, and considerable overestimation for I. antarctica. Results also suggest that not all investigated seem to depend upon the same food web baseline. Overall, our findings confirm the interest of the more novel amino acid δ15N method to delineate energy fluxes in complex marine ecosystems with multiple primary producers and intricate trophic interactions, such as coastal Antarctica.
