Distribution and growth of protists and mesozooplankton during a bloom of Chrysochromulina spp. (Prymnesiophyceae, Prymnesiales)

Abstract

In the spring of 1992 a surface bloom of Chrysochromulina spp. occurred in Danish waters. At the peak of the bloom it reached a density of 5 × 10⁴ cells ml⁻¹ and covered approximately 10000 km². Vertical community structure was studied at six stations situated in the centre of the bloom. The biomass of grazers accounted for an average of 15% of the phytoplankton biomass and was dominated by heterotrophic dinoflagellates and copepods. Ciliates were almost excluded from the plankton, although the bloom consisted of nanoflagellates, which ciliates usually ingest. This may be explained by cells of the dominant Chrysochromulina spp. being covered with spines, making it too large for ciliates' to feed upon. In spite of the high grazer biomass, loss of phytoplankton biomass due to grazing was low, corresponding to about 2–3% of the phytoplankton biomass per day. No mortality was observed in the mesozooplankton during incubation with bloom water. However, about 50 tons of dead rainbow trout, Oncorhynchus mykiss Wallenbaum, were observed in association with the bloom. The fish kills occurred primarily during the early phase of the bloom.