Trophodynamics and life-cycle strategies of the copepods Temora longicornis and Acartia longiremis in the Central Baltic Sea

Trophodynamics and life-cycle strategies of the copepods Temora longicornis and Acartia longiremis in the Baltic Sea were studied, focussing on the seasonal progression of (i) fatty acid (FA) markers in seston and females, (ii) the copepods condition as well as (iii) their stage compositions. Trophic marker diversity and high levels of 18:1(n9) in copepods and seston indicated omnivory for both species. Significant inter-specific and seasonal differences in markers reflected different trophic niches of both species and a seasonal succession in diet composition. Storage lipids of A. longiremis were characterized by polyunsaturated C18-FAs, and those of T. longicornis by 16:1(n7), 20:5(n3) and 22:6(n3). Diatoms were important for T. longicornis at times of peak egg production or lipid accumulation. The preference for heterotrophic prey over diatoms of A. longiremis probably reflects a prevailing ambush-feeding strategy. Triacylglycerol reserves in T. longicornis increased in autumn and probably served as a buffer against starvation during winter. Reproductive activities were low during winter and copepodite stages developed slowly. All A. longiremis copepodids were present during winter, but no lipid accumulation was observed for overwintering. Such constantly low lipid levels suggest insufficient food supply, the absence of triggers for the induction of wax ester anabolism or a population-specific life-history trait, which does not rely on winter energy reserves, but active foraging in the winter season.