Tropical Meromictic Lakes: Specifics of Meromixis and Case Studies of Lakes Tanganyika, Malawi, and Matano

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This chapter discusses the specifics of meromixis in lakes at low latitudes. The lack of strong seasonality, year-round high temperatures, and high irradiance levels create conditions that are rarely manifested in temperate lakes. For example, while in temperate lakes the absence of vertical mixing (meromixis) requires a strong salinity gradient to stabilize the water column against thermal convection, deep tropical lakes may remain stable without it, with deep waters ventilated by slow basin-scale circulation. Similarly to temperate meromictic lakes, tropical lakes accumulate nutrients in their anoxic monimolimnia, but the role of those deep nutrient sources for primary production becomes significantly greater. Primary production is higher on a given nutrient base than in temperate lakes. Evaporation, rates of vertical water exchange, and wind patterns become important factors for the stability of stratification and nutrient fluxes, while responses to climate change may be variable and complicated. Focusing primarily on deep (large) tropical lakes, we first review the general features of meromixis in such environments, including physical, chemical, biological, and microbiological aspects. We then discuss more detailed examples from the East African lakes Tanganyika and Malawi and the Indonesian Lake Matano, followed by a brief mentioning of several other tropical meromictic lakes.