Water quality, phytoplankton composition and microcystin concentrations in Kisumu Bay (Kenya) of Lake Victoria after a prolonged water hyacinth infestation period

Abstract

The production of phytoplankton (algal) toxins and their control is of concern because of the need to reduce their negative impacts on water quality and facilitate effective management of algal blooms. The present study was conducted between September 2017 to May 2018, focusing on Kisumu Bay in the Kenyan portion of Lake Victoria, in order to establish the magnitude of potential impacts on phytoplankton composition and microcystin following a prolonged presence of water hyacinth coverage between 2013 and 2018 within the gulf, with an estimated coverage range varying between 644 and 1224 ha. Triplicate samples of physico-chemical parameters, nutrients, phytoplankton, chlorophyll-a and algal toxins (N = 88) were collected at eleven sampling sites to determine their spatio-temporal variability. The main identified algal taxa comprised Cyanophyceae, Bacillariophyceae, Chlorophyceae, Euglenophyceae, Zygnematophyceae and Dinophyceae. The most dominant algal species were Microcystis aeruginosa (25%), Merismopedia spp. (23%) and Anabaena flos-aquae (16%). Enzyme-linked immunosorbent assay (ELISA) technique was used to determine microcystin (MC) toxins in the water. Mean MC-LR and MC-YR concentrations were significantly correlated (R² = 0.972), exceeding WHO standards at three sampling sites (Coca Cola, 2.84 ± 4.76; Kisumu pier, 1.78 ± 1.87; Midpoint, 1.44 ± 2.71 μg/L MC–LR). There were significant temporal variations (p < .05) in the SRP, TN, NO₃-N, NO₂-N, NH₄-N, SiO₂-Si, MC-LR, MC-YR, dissolved oxygen (DO), total dissolved and suspended solids (TDS; TSS), turbidity, electrical conductivity, Secchi depth, temperature and pH levels. The water depth, TP and DO also vary spatially. The nitrogen to phosphorus concentration ratios differed from the expected N:P ratio of 16:1, indicating a highly eutrophic status. The disproportionate ratio of total phosphorus and total nitrogen in the bay may be responsible for the enhanced cyanobacterial blooms it exhibits. The results of the present study provide useful information and data for formulating regulations for water quality management.