Phytoplankton composition and microcystin concentrations in open and closed bays of Lake Victoria, Tanzania

This study was carried out in order to investigate the spatial variation of algal toxin (microcystin) concentrations along the shoreline of Lake Victoria. A total of 16 nearshore stations differing in connectivity to the main lake basin were categorized as either closed bays (ratio of bay area to bay opening <1) or open bays (ratio ≥1) and sampled during November and December 2009. Water samples were analyzed for total phosphorus, chlorophyll a, phytoplankton community composition and concentrations of microcystin. Open and closed bays were significantly different for phytoplankton abundance and composition: Average phytoplankton biovolume was higher for closed bays (45 mm3l−1 ± 11 SE) than open bays (5 ± 2 mm3l−1). Cyanobacterial biovolume (mainly Microcystis spp., Anabaena spp. and Planktolyngbya spp.) was also significantly higher in closed bays (82 ± 9% of total biovolume) than in open bays (44 ± 5%). In contrast, diatom biovolume was lower in closed bays (7 ± 1%) than in open bays (36 ± 6%). Microcystins were found only among sites from closed bays and concentrations ranged from 0.4 to 13 µg l−1 Microcystin-LR equivalent and coincided with high abundance of Microcystis spp. It is concluded that the level of water exchange from individual bays to the main basin is an important factor influencing eutrophication and microcystin production in nearshore habitats of Lake Victoria.

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