Trophic position of coexisting krill species: a stable isotope approach

Four krill species with overlapping functional biology coexist in Greenland waters. Here, we used stable isotopes to investigate and discuss their trophic role and mode of coexistence. Bulk carbon (delta C-13) and nitrogen (delta N-15) stable isotope analyses of Thysanoessa longicaudata, T. inermis, T. raschii and Meganyctiphanes norvegica sampled in June 2010 in Godthabsfjord, SW Greenland, revealed new insight into the species' trophic roles and positions. There was a general positive correlation between body length and trophic position. The largest species, M. norvegica, had the highest trophic position (TP [mean +/- SE] = 2.8 +/- 0.2) indicating carnivory, while T. inermis (TP = 2.4 +/- 0.3) had a more omnivorous diet. In turn, T. longicaudata and T. raschii (TP = 2.2 +/- 0.2) were herbivorous. Along the fjord, plankton composition affected trophic position. T. longicaudata was more omnivorous offshore than inshore, where it had the same trophic position as the baseline primary consumer Calanus spp. Similarly, T. raschii and T. inermis had higher trophic positions in the mouth of the fjord compared with the inner fjord. Regardless of spatial variations in potential food and the overlap in diet, typical of opportunistic species, body size appears as the key factor determining the role and position of krill in the food web.