Seasonal dynamics of zooplankton resource use revealed by carbon amino acid stable isotope values

Reliable tracer techniques are fundamental to retrieving accurate information on consumer dietary resource use in dynamic ecosystems. Analysis of delta C-13 values in essential amino acids has shown great promise in effectively capturing consumer food sources, since essential amino acids are not synthesized by heterotrophic organisms but instead routed directly from dietary sources to consumers, resulting in negligible C-13 trophic discrimination. We assessed seasonal feeding patterns of a widespread key copepod species (Acartia spp.) in the northern Baltic proper using complementary approaches: bulk delta C-13 and delta N-15 values, delta C-13 values of essential amino acids, and quantitative phytoplankton taxonomic data. Our results showed distinct differences between Acartia and seston delta C-13 essential amino acid values measured at weekly to monthly sampling intervals, which indicated that Acartia preferentially utilized specific dietary resources that comprised only parts of the total phytoplankton composition (varying from 19.7% to 81.4% during the season). Results also indicated that care should be taken when inferring trophic position from bulk stable isotopes when consumers are highly selective, since isotope values of seston may not accurately reflect consumer specific diet resource uptake. Analysis of delta C-13 values in essential amino acids was shown to be a promising tool to accurately trace consumer resource use in complex natural systems.