Abstract
The Nyanza Gulf is a large shallow bay of Lake Victoria suffering from eutrophication by human activities. In order to characterize the harmful algal bloom formation as a consequence of eutrophication, both spatially and seasonally, environmental conditions, phytoplankton community composition, and microcystin (MC) concentrations were investigated monthly from Kisumu Bay, and bimonthly from the center of the gulf, as well as quarterly from the Rusinga Channel and the main basin of Lake Victoria between July 2008 and September 2009. The sites located in Kisumu Bay and the central gulf were most strongly affected by eutrophication, including increased nutrient concentrations and phytoplankton growth. More than 90% of the samples obtained from the gulf were dominated by cyanobacteria, whereas diatoms only dominated in the samples obtained from Rusinga Channel and the main lake. In general, Microcystis accounted for the largest part (>50–90%) of cyanobacterial biovolume. MCs were found in 35 (54%) out of 65 samples and were detected throughout the study period in the gulf, but only in two out of eight samples from the Rusinga Channel and the main lake. A significant linear relationship between Microcystis biovolume and MC concentration was observed (n = 65, R 2 = 0.88, P < 0.001). The highest MC concentrations were recorded in Kisumu Bay between November and March (max. 81 μg l−1) when Microcystis showed max. biovolume (18 mm3 l−1 in November 2008). The results suggest that seasonal variability did not outweigh the spatial differences in phytoplankton composition and MC production, which is seasonally persistent in Kisumu Bay.Electronic supplementary materialThe online version of this article (doi:10.1007/s10750-012-1062-8) contains supplementary material, which is available to authorized users.
Highlights
During the last half century, Lake Victoria has undergone drastic changes in its water quality and biota, which have been attributed to eutrophication and the introduction of exotic species (Hecky, 1993; Verschuren et al, 2002)
The results suggest that seasonal variability did not outweigh the spatial differences in phytoplankton composition and MC production, which is seasonally persistent in Kisumu Bay
Reductions in total nitrogen (TN) concentrations were recorded from December to February, coinciding with the mats formed by the water hyacinth
Summary
During the last half century, Lake Victoria has undergone drastic changes in its water quality and biota, which have been attributed to eutrophication and the introduction of exotic species (Hecky, 1993; Verschuren et al, 2002). In other bays of Lake Victoria, significant dilution effects because of massive water exchange with the main basin have been invoked to explain the relatively low phytoplankton biovolume (Haande et al, 2011). These authors concluded that surface seiches might cause considerable daily water exchange in Murchison Bay (Uganda) and could explain why the increase in total phytoplankton biomass in consequence of nutrient enrichment is not detectable. Dilution effects from the main basin due to the strong seasonal SE winds were invoked to explain the low phytoplankton abundance in the Mwanza Gulf (Tanzania), near-shore nutrient enrichment was clearly visible (Sekadende et al, 2005). The sensitivity of near-shore areas to eutrophication depends on the connectivity to the main basin, which
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