Annual patterns, distribution and long-Term trends of Pseudo-nitzschia species in the northern Benguela upwelling system

Phytoplankton data obtained from a 14-year data set off the coast of Namibia were analysed. The temporal distribution of species belonging to the diatom genus, Pseudo-nitzschia, was examined using samples collected quasimonthly at onshore and offshore locations. This data set revealed that Pseudo-nitzschia blooms occurred sporadically along the central coast of Namibia, and cell concentrations frequently exceeded 200 000 cells L-1, with levels close to or exceeding 1 000 000 cells L-1 noted occasionally. Two relatively cold years (2005 and 2010) and three relatively warm years (2001, 2003 and 2011) were found to be exceptional for high biomass Pseudo-nitzschia blooms, so possible drivers (wind, temperature and nutrients) were evaluated. Pseudo-nitzschia species abundance was associated with moderate temperature (13-16°C) and nutrient conditions. Climatology showed an increase in Pseudo-nitzschia blooms during austral summer, reaching a maximum in May, June or July. These blooms seem to initiate following periods of high total phytoplankton concentrations. Wind was determined to play an important role in controlling these blooms. Low biomass Pseudo-nitzschia blooms were observed during months with high or irregular winds that coincided with periods of upwelling, whereas high biomass blooms were associated with the decrease of wind afterthe main wind peak at the end of summer.