Do toxic algal blooms affect subtidal soft-bottom communities?

Abstract

An extensive bloom of the toxic flagellate Chrysochromuha polylepjs occurred over much of the Skagerrak and Kattegat area during May-June 1988 with severe effects on biota in shallow marine habitats. Results reported here are from annual grab-sampling of subtidal soft-bottom communities off the southwest coast of Norway during the period 1986 to 1991 at depths between 69 and 184 m, and describe changes In fauna associated with the toxic algal bloom. In the 3 years prior to the bloom the fauna at each station was persistent and only modest year-to-year changes in the number of species, abundance and species composition were observed. Samples collected 1 yr after the bloom event revealed a significant reduction in both number of species and individuals. The most obvious changes were found at sites situated in accumulation areas characterised by fine sediments indicating a possible connection between the quantity of sinking toxic material and the degree of faunal perturbation. A DCA ordination demonstrated a clear switch in faunal composition in the study area subsequent to the bloom, and a tendency of the fauna to return to the pre-bloom communities 2 yr after the bloom event. Principal species affected were moderately abundant oligochaetes, polychaetes, crustaceans, molluscs and sipunculids. Allocation of each species into the categories 'gaining' and 'losing' according to annual changes in abundance gave a good description of the community disturbance probably induced by the toxic alga. Other possible causes for the observed faunal perturbation may be changes in water temperature, food input, predation interference and changes in sediment composition, but none of these were found to give reliable, alternative explanations for the substantial changes observed in the soft-bottom communities subsequent to the toxic bloom.