Difference in the foraging strategy between daphnia and copepods in Shirakoma-ike: evidence from ¹⁵N / ¹⁴N of amino acids

Abstract

Trophic position (TP) of the daphnia Daphnia longispina and the copepod Acanthodiaptomus pacificus in an oligotrophic lake, Shirakoma-ike, was investigated via stable nitrogen isotope composition (¹⁵N/¹⁴N) analysis of amino acids, to understand the foraging strategy of these two dominant freshwater zooplankton species in an oligotrophic lake. The surface water of this lake is frozen in winter, and the life cycle and trophic behavior are different between these two species: the daphnia is found only in spring-autumn, whereas the copepod is found in whole season. In the present study, we found that the TP is 2.1±0.0 for the daphnia in spring-autumn and 2.3±0.3 for the copepod in whole season. These results reveal strong herbivory for the daphnia compared to dietary plasticity and facultative omnivory for the copepod. The latter is suggested to feed on phytoplankton for spring and autumn (TP=2.1±0.0) and on both phytoplankton and heterotrophic microbes (e.g., protists and bacteria) for summer and winter (TP=2.6±0.0). The foraging strategy is thus different between daphnia and copepods in this lake. This finding may explain why the daphnia is absent whereas the copepod is present in the frozen lake in winter where primary production is limited.