Food selection by calanoid copepods in the euphotic layer of the Gotland Sea (Baltic Proper) during mass occurrence of N-2-fixing cyanobacteria

Food selection by 2 dominant calanoid copepods, Acartia sp. and Temora long icornis, was studied during mass occurrence of N-2-mixing 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 analysi s of marker carotenoids by HPLC (high-performance Liquid chromatography), a nd secondly the analysis of delta(15)N signals of copepods by mass spectrom etry. The first method provides a 'snapshot' of autotrophic material ingest ed; the second method summarises a longer period, and gives evidence that a certain food source is not only ingested but also assimilated. In 1994, ma ss occurrence of cyanobacteria showed a higher concentration in the euphoti c layer than 1993 (97 mu g C l(-1) in 1994, 57 mu g C l(-1) in 1993), which was reflected in higher food uptake of N-2-fixing cyanobacteria in 1994. T he average relative amount of myxoxanthophyll, the specific carotenoid of N -2-fixing cyanobacteria, in the copepod guts showed high values in 1994 (Ac artia sp. 37 %, T. longicomis 41%) and low values in 1993 (1% for both cope pods). The low delta(15)N values of both Acartia sp. and T. longicornis in 1994 (9 parts per thousand) compared to those in 1993 (10.5 parts per thous and) support the results of HPLC analyses, because Nz-fixing cyanobacteria have a lower delta(15)N (average 0.7 parts per thousand) than eukaryotic ph ytoplankton (average 12 parts per thousand). The low delta(15)N values in 1 994 indicate that N-2-fixing cyanobacteria were not only ingested but also assimilated by the copepods to a higher extent in 1994 than 1993.