Copepod interactions with toxic and non-toxic cyanobacteria from the Gulf of Finland

Abstract

Recurrent blooms of toxic cyanobacteria dominate surface waters of the western Gulf of Finland in late summer, overlapping peak densities of two copepods, Acartia bifilosa (Giesbrecht) and Eurytemora affinis (Poppe). These cyanobacteria possess morphological and toxic properties that might affect planktic herbivory and impact secondary production. In summer 1994 clearance rates of the two copepods on the cyanobacteria Nodularia spumigena Mertens and Aphanizomenon flos-aquae Ralfs ex Bornet et Flahault were measured over a range of phytoplankton densities. The effects of the two cyanobacteria on copepod egg production were also investigated. Clearance rates of the two copepods feeding on cyanobacteria were substantially lower than the rates of copepods fed similar levels of cyanobacteria-free food, the flagellate Brachiomonas submarina Bohlin, or < 63 μm ambient phytoplankton assemblages. In addition, cyanobacteria-rich assemblages reduced egg production in both copepods, with rates declining from 15-22 to < 10 eggs (copepod d)-1. These results indicate that (1) copepod herbivory is substantially reduced in these cyanobacterial blooms and (2) copepod recruitment through reproduction should be very low in bloom-dominated areas, perhaps leading to shifts in community composition of the mesozooplankton.