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

Abstract

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.

Keywords:

• bay morphometry
• eutrophication
• Microcystis
• Anabaena

Acknowledgements

We are grateful to Gerold Winkler and Regina Brandstätter for their continuous assistance. Two anonymous reviewers provided comments that substantially improved the current manuscript. We are thankful to Tanzania Fisheries Research Institute for their cooperation during sample collection. We are indebted to Eugen Rott (Institute for Botany, University of Innsbruck, Austria) for assisting in identification of various phytoplankton species and Dr. I. Kimirei (TAFIRI-Kigoma) and Mr. C. Mashafi (TAFIRI Mwanza) for their valuable assistance during figure constructions.

Funding

We are grateful to the Netherlands Fellowship Programme for funding this research work and the International Postgraduate Course of Limnology (IPGL) administered by the Austrian Academy of Sciences for financial support. The data analysis was financially supported by the Austrian Science Fund (P24070). This study is a contribution to the EU network “European Cooperation in Science and Technology” (COST) Action ES1105 ‘CYANOCOST’ (http://cyanocost.com).