Evidence for an increasing incidence and severity of Harmful Algal Blooms in the southern Benguela region

Harmful Algal Blooms (HABs) may lead to catastrophic mortality overa rangeof trophic levels and impact on fisheries, local species' populations, conservation management and the health of both livestock and humans. Consequently, any increase in frequency and/or toxicity of these events is of concern. Recently this concern has been realized, with reported increases in the frequency of HABs from all continents except Antarctica. This reported rise is supported by data from the Benguela coast of western South Africa, where, since 1930, there has been a significant increase in the frequency of HABs and a slight increase in their average severity. There has been a sixfold increase in the number of HABs per decade since the 1960s, with the period 1990-2005 experiencing the greatest number of blooms, as well as the most severe in terms of associated mortality. The recent occurrence of previously unrecorded HAB-causing species in this region may go some way to explaining this trend, and further implies that the increase is unlikely to diminish in the near future.