The trophic significance of Phaeocystis blooms

Abstract

Both colonies and solitary cells of the prymnesiophyte Phaeocystis are ingested by a wide array of marine vertebrates. Grazers include protozoa, bivalves, amphipods, euphausiids and many copepod species. Common fish species such as mackerel ( Scomber scombrus) and flounder ( Pleuronectus flesus) have also been reported to feed upon Phaeocystis. While qualitative observations on the potential trophic significance of Phaeocystis exist for a long time, attempts to quantify feeding rates on Phaeocystis have begun only during the last decade. In feeding experiments, copepods have received most attention. Many copepod species appear to consume Phaeocystis colonies and solitary cells in the North Atlantic and in polar seas. Ingestion rates are highly variable depending on size and physiological state of Phaeocystis as well as on copepod species. Within the same species, large variations in individual feeding rates have been reported from different areas and investigators. When Phaeocystis co-occurs with larger amounts of diatoms, the latter seem to be preferred by some copepod species while others do not select against Phaeocystis. At present it is unclear whether this is primarily due to unsuitable size of Phaeocystis or because it is poor quality food. The relative nutritive value of Phaeocystis single cells and colonies should be investigated in more detail. Feeding of protozoa on Phaeocystis has been little studied. Ciliates and the giant dinoflagellate Noctiluca are known to consume Phaeocystis solitary cells. Protozoa might efficiently control Phaeocystis blooms during their initial phases when the share of solitary cells relative to total Phaeocystis biomass is higher than during later stages of the bloom. By switching their food preference towards heterotrophic food, copepods might benefit from enhanced protozoan biomass during Phaeocystis blooms. The potential succession of protozoan and metazoan grazers in the course of a Phaeocystis bloom awaits further research.