Spatial distribution of soluble reactive silica (SRSi) in the Tanzanian waters of Lake Victoria and its implications for diatom productivity

Abstract

Soluble reactive silica (SRSi) concentrations and diatom abundance were determined during four surveys in the Tanzanian waters of Lake Victoria between 2005 and 2008. The SRSi concentrations increased towards offshore sites, while the opposite was true for diatom abundance. The water directly above the sediment had higher concentrations of SRSi (1.17 ± 0.68 mg l−1) than surface waters (0.79 ± 0.29 mg l−1). The SRSi concentrations in the lake were low compared to those in the period preceding the 1980s. Increased uptake, decreased dissolution and increased accumulation in the sediments are the major contributing factors. The high bottom and low surface concentrations of SRSi are accounted for by increased SRSi uptake by the highly abundant surface diatom communities and their increased sinking rate. The change in diatom species composition and abundance, leading to the dominance of robust and opportunistic species, is probably accelerated by the increased eutrophication and pollution of the lake water.