INTEGRATED ECOLOGICAL INTEGRITY ASSESSMENT OF LAKE SIMBI, A DEEP ALKALINE-SALINE LAKE IN WESTERN KENYA

Abstract

The resources and ecological services derived from lentic ecosystems are constantly threatened by climate change and unprecedented pressures from anthropogenic activities. This has led to alteration of their physical habitat, water quality, trophic state and aquatic biota creating a constant need to assess their integrity for management and conservation purposes. Lake Simbi National Bird Sanctuary is the least studied lake in Kenya despite being a bird haven with ecotourism value; gaps exist on the lake’s biota and other key ecological aspects such as the water quality dynamics, phytoplankton and the trophic status. To bridge this knowledge gap, the current study was employed in assessing Lake Simbi’s ecosystem integrity which has been brought into question by the recent decline in the numbers of the ―near-threatened‖ lesser flamingos unique only to this lake. The objective was to assess the ecological integrity based on the water quality dynamics phytoplankton indices, habitat quality and trophic state index (TSI). Sampling was done on a monthly basis for 6 months from December 2018 to May 2019 at six fixed stations systematically selected. Standard methods stipulated in APHA (2012) were used in determining levels of selected water quality parameters and phytoplankton characteristics. The Carlson’s and Nygaard’s trophic state indices were used in determining the trophic status of the lake. Foot-based lake habitat survey based on the European Union Water Framework Directive standards was conducted to determine the physical habitat quality of the lake. The spatial and temporal trends of the parameters were analyzed using Excel and SPSS software. Phytoplankton diversity was estimated using Shannon-Wiener, Simpson, evenness and richness indices. Descriptive and correlation statistics was done for all variables. ANOVA and Tukey test for comparisons of means were used to screen for significant differences (p < 0.05) among the study variables. The results indicated that the water quality of Lake Simbi is heavily polluted and unsafe for any domestic usage since almost all the basic water quality variables (DO, pH, TDS, alkalinity, hardness and turbidity) measured exceeded the maximum permissible limits set by both National Environmental Management Authority and the World Health Organization. A total of 84 phytoplankton species were identified comprising of Cyanophyceae (36), Chlorophyceae (25), Bacillariophyceae (11), Zygnematophyceae (4), Dinophyceae (3 ) and Euglenophyceae (3). Reduced abundance of the cyanobacterial species, especially Spirulina species on which flamingos feed and the subsequent dominance of the toxin producing Microcystis sp. might have contributed to the decline of the flamingo population. The Carlson’s and Nygaard’s trophic state indices both characterized the waters as hypereutrophic. The Redfield’s TN/TP ratios established that N was the limiting nutrient in Lake Simbi, also indicating its eutrophic nature. The Lake Habitat Survey indices of Lake Habitat Quality Assessment and Lake Habitat Modification Score collectively suggested that the Lake Simbi physical habitat is moderately pristine since its hydromorphology is moderately modified by the various pressures which are still operating at marginal scales. For most of the variables studied, significant temporal variations were realized while no significant spatial variations were observed. Generally, the poor ecosystem integrity of Lake Simbi can be attributed to the anthropogenic activities in its catchment and the changing climatic conditions. This pioneer study demonstrates that multivariate ecological indices can be effective in monitoring the ecological status of aquatic ecosystems in Kenya, hence should be adopted as a tool for informing decision-making for lake conservation and management purposes. Key words: Ecological integrity, Lake Habitat Survey, nutrients, physico-chemical, phytoplankton, trophic state index and Lake Simbi.