Uncovering the trophic relationship between Themisto gaudichaudii and Salpa thompsoni in the Antarctic Polar Frontal Zone

Abstract

Trophic dynamics of 2 abundant macrozooplankton species Salpa thompsoni and Themisto gaudichaudii were studied during the austral summer at 2 locations near the Antarctic Polar Front with contrasting low and high chlorophyll a (chl a) concentrations. Compound-specific stable isotope analysis, complemented by gut content and bulk isotope analyses, were used to investigate trophic interactions between species, and to assess their trophic positions in the pelagic food web. The results of the compound-specific stable isotope analysis placed S. thompsoni at the second trophic level and approx. 1 trophic level below T. gaudichaudii. Two forms of T. gaudi chaudii appeared to feed at different trophic levels, with T. gaudichaudii bispinosa feeding at a higher trophic level (∼3.3) than T. gaudichaudii compressa (∼2.8). Isotope data coupled with gut content analysis indicated a regular consumption of salps in both areas, although a higher contribution of gelatinous prey was encountered in a chl a poor area. The food web baseline values (bulk δ13C) varied regionally, highlighting 2 independent food webs albeit with a similar trophic structure. Overall, our findings suggested that in areas where S. thompsoni is highly abundant, T. gaudi chaudii may be a significant predator of this species.