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Food selection by 2 dominant calanoid copepods, Acartia sp. and Temora longicornis, was studied during mass occurrence of N 2 -fixing cyanobacteria in June/July 1993 and 1994 in the Gotland Sea (Baltic Proper). The aim of this study was to assess the importance of N 2 -fixing cyanobacteria in the diet of calanoid copepods. Two different methods were used: firstly the analysis of marker carotenoids by HPLC (high-performance liquid chromatography), and secondly the analysis of δ 15 N signals of copepods by mass spectrometry. The first method provides a 'snapshot' of autotrophic material ingested; the second method summarises a longer period, and gives evidence that a certain food source is not only ingested but also assimilated. In 1994, mass occurrence of cyanobacteria showed a higher concentration in the euphotic layer than 1993 (97 μg C l -1 in 1994, 57 μg C l -1 in 1993), which was reflected in higher food uptake of N 2 -fixing cyanobacteria in 1994. The average relative amount of myxoxanthophyll, the specific carotenoid of N 2 -fixing cyanobacteria, in the copepod guts showed high values in 1994 (Acartia sp. 37 %, T. longicornis 41%) and low values in 1993 (1% for both copepods). The low δ 15 N values of both Acartia sp. and T longicornis in 1994 (9‰) compared to those in 1993 (10.5‰) support the results of HPLC analyses, because N 2 -fixing cyanobacteria have a lower δ 15 N (average 0.7‰) than eukaryotic phytoplankton (average 12‰). The low δ 15 N values in 1994 indicate that N 2 -fixing cyanobacteria were not only ingested but also assimilated by the copepods to a higher extent in 1994 than 1993.

Food selection by calanoid copepods in the euphotic layer of the Gotland Sea (Baltic Proper) during mass occurrence of N2-fixing cyanobacteria