Assessment of Planktonic Algae Community Structure, Physico and Chemical Characteristics, and Microcystin Concentrations in Kisumu Bay, Lake Victoria, Kenya

Abstract

Studies on algal community structure and diversity in relation to their impact on water quality and ecosystem function are important in informing policy on water quality management. Consequently, studies were conducted in Kisumu Bay of Lake Victoria to establish factors that determine phytoplankton community structure, in relation to their toxins and impact on water quality and ecosystem health from September 2017 to May 2018. Triplicate samples of physico-chemical Parameters, Nutrients, Phytoplankton, Chlorophyll-a and algal toxins were collected at eleven sampling stations georeferenced by Geographical Positioning Systems (GPS) model number 35 C to help show their spatial and temporal variability. Enumeration and identification of phytoplankton was carried out using Axioinvert 35 inverted microscope at 400x magnification. Nutrients concentrations were analyzed using standard procedures described in APHA 2017. Physico-chemical parameters: Dissolved Oxygen (DO), Temperature, Conductivity and pH were measured using YSI multiparameter probe model No. 650). Other physical chemical parameters: Turbidity, Total Suspended Solids, Total hardness, alkalinity and Secchi depth were measured as described in APHA (2014). Chlorophyll a concentration was determined by filtration of water samples followed by cold extraction in ethanol then by determination of their optical densities at 665nm and 750 nm. Microcystin algal toxins were analyzed using Elisa Kit Model No. 357 C. Algal biodiversity was assessed using Shannon Wiener, Simpson and species richness evenness diversity indices. Spatial and temporal differences of all the parameters were tested using analysis of variance followed by F-test at p < 0.05. Other Statistical analysis were performed using the Minitab version 17 Inc. software for windows to determine significant differences followed by post hoc test to identify pairs of samples which had significant differences. The main algal taxa identified were: Cyanophyceae, Bacillariophyce, Chlorophyceae, Euglenophyceae, Zygnematophycea and Dinophceae. The most dominant algal species were Microcystis aeruginosa (25.44 %), Merismopedia spp (23.49 %) and Anabaena flos-aquae (16.06 %). Five Microcystin toxins were identified namely MC-LR, MC-YR, MC-LA, MCRR and MC-dmLR. Concentrations of two of the toxins namely MCLR and MCRY exceeded WHO acceptable standards at two sampling stations of which they were significantly correlated. Cocacola had a mean of 2.360 ± 4.41A MgL-1 MCLR and pier 1.100 ± 1.88A MgL-1 respectively. There were significant difference in chlorophyll a, temperature, dissolved oxygen, conductivity, pH and Secchi depth among different sampling stations (ANOVA; p < 0.05). However, no significant differences were observed in nutrients concentrations measured in Mid Bay. The total phosphate concentration to total nitrogen concentration ratios (TP : TN) for all the eleven sampling stations differed from the expected TP : TN Ratio of 1:16 and did depict a highly eutrophic environment. Showing situation where effluents with high, nutrient concentrations especially phosphates are discharged into bay. The disproportionate ratio of total phosphate and total nitrogen into the Bay may be responsible for the enhanced cyanobacterial blooms. This result will be useful to scientists, managers and authorities of the water sector in formulating regulations for management of water quality