Weak ‘top-down’ control of dinoflagellate growth in the coastal Skagerrak

Effects of planktivorous fish (Sprattus sprartus), copepods (mainly Oithona sp.) and benthic filter feeders (Mytilus edulis) on the biomass and species composition of a natural marine plankton community, dominated by dinoflagellates, was studied experimentally in September 1988. Plastic 300 I cylinders were filled with Skagerrak surface water, filtered through a 90 μm nylon net in order to remove mesozooplankton. Copepods were added in three different concentrations (0, 1× and 6× the ambient density) with and without nutrient additions (nitrate, silicate and phosphate). In two nutrient-enriched cylinders with added copepods (1× and 6×), 5 cm sprats were also added (10 per cylinder). In two nutrient-enriched cylinders without added copepods. blue mussels (20 per cylinder. 3 cm length) were introduced. Chlorophyll a. inorganic nutrients and densities of phytoplankton, ciliates and copepods were measured throughout the experimental period of 10 days. Gyrodinium aureolum increased in cell numbers in all but the mussel cylinders, while the other dominating dinoflagellate, Ceratium furca, decreased. This decrease was not due to grazing, but probably a container artefact. Diatoms, Rhizosolenia fragilissinia, Leptocylindrus danicus and Chaetoceros spp., increased slowly in treatments without fish. In the fish treatments, diatoms had appreciably higher growth rates and cell numbers, probably due to predation by fish on the copepods. Gyrodiniun aureolun also had a somewhat higher growth rate in the presence of fish. although this was probably not due to reduction of copepod grazing. The results show that planktivorous fish can affect the biomass and species composition of natural phytoplankton communities in marine waters, but that the effect on the late summer-autumn dinoflagellate dominated community is weak. Blue mussels are efficient grazers of the whole phytoplankton community and are not particularly selective with respect to either size or species.