Biological effects of the toxic algal bloom of February and March 1998 on the benthos of Wellington Harbour, New Zealand

Abstract

Wellington Harbour is a large (∼85km2), semi-enclosed temperate ecosystem. We used unpublished baseline data from 6 subtidal and 1 intertidal harbour stations to quantify the ecological effects of the naturally occurring toxic bloom of the naked dinoflagellate Karenia brevisulcata (Chang) G.Hansen & Moestrup during February and March 1998. Species richness decreased at 6 stations and increased at 1 station, numbers of individuals decreased at 4 of 4 stations, species diversity decreased at 5 of 5 stations, evenness decreased at 2 and increased at 2 stations, and biomass decreased at 2 of 3 stations. These data indicate that the K. brevisulcata bloom of late summer 1998 resulted in a mass die-off of the subtidal benthic invertebrates over a large area of Wellington Harbour, as well as of the flora and fauna at 1 intertidal location studied. Four major phyla (echinoderms, molluscs, polychaetes and crustaceans) were equally affected by the bloom, although different species within each phylum were differentially affected (regardless of phylum, small and shallow-burrowing individuals were most affected). Multivariate statistical analyses of these data identified 3 clusters composed of 4 subtidal stations which showed the highest level of impact by the bloom and were based on a spatial/environmental component (location in the northern harbour; greater depth; low current/wave energy) as well as a temporal component (pre- vs post-bloom). The remaining 3 subtidal stations were less affected by the bloom and each formed a separate cluster according to spatial/environmental rather than temporal effects. This event has permitted us to quantify, for the first time, the effects of a natural toxic bloom on a large-scale temperate harbour ecosystem and to relate the magnitude of these effects to the geographic and environmental properties of different locations within that ecosystem.