MEPS 191:243-250 (1999)  -  doi:10.3354/meps191243

ABSTRACT: Food selection by 2 dominant calanoid copepods, Acartia sp. and Temora longicornis, was studied during mass occurrence of N₂-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 N₂-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 δ¹⁵N 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 N₂-fixing cyanobacteria in 1994. The average relative amount of myxoxanthophyll, the specific carotenoid of N₂-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 δ¹⁵N values of both Acartia sp. and T. longicornis in 1994 (9o/oo) compared to those in 1993 (10.5o/oo) support the results of HPLC analyses, because N₂-fixing cyanobacteria have a lower δ¹⁵N (average 0.7o/oo) than eukaryotic phytoplankton (average 12o/oo). The low δ¹⁵N values in 1994 indicate that N₂-fixing cyanobacteria were not only ingested but also assimilated by the copepods to a higher extent in 1994 than 1993.

KEY WORDS: Selective feeding · N₂-fixing cyanobacteria · Acartia sp. · Temora longicornis · Baltic Sea