Lake Victoria: The Changing Lake

Abstract

Water quality monito ring activities were carried out on physical - chemical parameters, water chemistry and biotic indices at selected littoral and pelagic stations along north - south and east - west transects over an annual cycle between 2000 and 2005. The activities were aimed a t collecting baseline information and data for use to define the current lake conditions and make a water quality assessment of the lake in relation to nutrient/ pollutant loadings as a basis for future monitoring surveys. As much as possible current condi tions of the lake were compared to pa st observations where field sites coincided with those of historical studies. Littoral stations (0 - 20m) showed higher temperatures, pH, turbidities and electrical conductivity while Secchi depths and dissolved oxygen w ere lower compared to pelagic ones. Deeper pelagic sites experience thermal stratification, leading to strong differences between surface and deep water layers especially in terms of temperature and dissolved oxygen except for June through August during wh ich the lake achieves complete mixing in all stations . On average, littoral stations have higher total nitrogen, chlorophyll, organic matter as well as total particulate carbon. Phytoplankton production and biomass in shallow inshore sites is light - limited due to self shading and the latter can therefore sustain higher algal biomasses because of limited mixing depths . Nitrogen often limits algal growth except for C yanobacteria that can fix atmospheric nitrogen to meet their N limitations. The higher algal b iomass accounts for the higher TN concentrations, chlorophyll and organic matter at littoral sites. During thermal stratification, TP and SRP concentrations are comparable in littoral and pelagic sites while during and just after mixing, both fractions of P are higher in the littoral areas. Regional Lake Victoria Environment Report Water Quality and Ecosystems Component Lake Victoria Environment al Management Project 63 Continuous excess SRP and the ability of C yanobacteria to fix atmospheric nitrogen create nutrient saturated conditions that result in light limitation for algal growth. Annual rainfall is characterised by two peaks in t he stratification seasons 2 and 3 and appears to be the main source of nutrient enrichment in the lake, particularly TP. Decomposition of organic matter during the stratification period results into release of TP and Si into deeper waters where they build up to much greater concentration than in surface waters. TN reaches annual minimum during the deep mixing period because of N - poor deep waters. Chlorophyll concentrations also reach their annual minimum at this time as deep mixing imposes strong light limi tation on algal growth and nitrogen fixation especially in pelagic areas . Comparisons with historical records show that the lake today is warmer than in the 1960s but with lower oxygen and pH in the deeper waters. Thus deep water respiration rates, oxygen consumption and CO 2 are higher in the 2000s largely attributed to the eu trophication of the lake. Oxygen depletion in deeper waters d uring thermal stratification has led to loss of habitat for fish and other biota. Chlorophyll levels, phosphorus concentra tions and electrical conductivity in both littoral and pelagic stations have increased compared to records of the 1960s. On the other hand nitrogen concentrations around marginal bays and gulfs have not shown a marked increase compared to historical record s of the 1960s and 1970s. Dissolved silicon in the pelagic areas has decreased 5 - 20 times compared to historical records as eutrophication has increased Si demand by diatoms relative to rather constant supply from the catchment. In line with increased alga l turbidities, Secchi depths have decreased at least two - fold since the 1960s. The need for lake wide regular water quality surveys and basin developments in support of appropriate management interventions can not be overemphasized at present in order to d etermine whether the lake is continuing along the same trends over time documented in this report.