Response of planktonic diatoms to eutrophication in Nyanza Gulf of Lake Victoria, Kenya

Abstract

In recent decades, the planktonic diatom community in central Lake Victoria changed from dominance of Aulacoseira species (mainly African endemic A. agassizii, A. nyassensis) to dominance of long needle shaped Nitzschia (often referred to as N. acicularis). This shift was referred to be caused by eutrophication, resulting in increased growth of diatom taxa with reduced requirement of silica in open water. In contrast, in the large shallow Nyanza Gulf A. granulata and A. granulata var. angustissima remained abundant, probably because of increased turbidity and nutrient availability. In this study, from July 2017 to July 2018, the planktonic diatom community composition was investigated monthly at three sampling stations in Nyanza Gulf, i.e. West Gulf (WG, located close to the open basin via Mbita channel), Mid Gulf (MG, central part of the Gulf) and East Gulf (EG, located most closely to Kisumu bay). A significant gradient in water quality conditions was observed along distance of 60 km from the main basin, i.e. the water transparency decreased from 0.8–1.2 m to 0.2–0.5 m while suspended solids increased from 3.3–9.8 mg/L to 12–63.8 mg/L. In total 19 common diatom morphospecies were documented and related to the gradient in trophic state. At WG, A. granulata and A. granulata var. angustissima formed diatom blooms or co-occurred with long needle shaped Nitzschia species such as N. kavirondoensis, N. lacustris, N. nyassensis, N. rusingae, Synedra cunningtonii, as well as Cyclostephanos malawiensis, C. damasii and Urosolenia victoriae indicating more oligotrophic conditions closer to the main basin. In the inner Gulf A. granulata (var. angustissima) and N. cf. fonticola var. pelagica possibly living epiphytically on the colony-forming cyanobacterium Microcystis aeruginosa dominated. In contrast to N. cf. fonticola var. pelagica, however, Aulacoseira spp. biovolume was found negatively related to Microcystis biovolume implying its suppression under Microcystis bloom conditions.