A Review of the impact of climate change on African lake productivity
Odada, E O
Langenberg, V T
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The African Great Lakes are renowned as one of the world's most important aquatic ecosystems from the point of view of water resources, food security, socio-economic activities, biodiversity and carbon cycling. The livelihoods and dietary needs of an estimated 50 million people are to 2 great deal dependent on the fisheries resources that these lakes provide. We utilised historical and current information on the meteorological, physical-chemical, biological and ecological properties of the 10 largest East African lakes in the region from lake Turkana to lake Kariba and carried out an interbasin ecosystem functioning analyses in order to gain new insights on the main climatic drivers impacting the mechanisms securing the biological base for fish production now and in the future. The lakes displayed a range of tropic states largely depending on the natural characteristics of their basins and catchments. Over the last 50 years this range broadened due to increased 111.1 trient inflows resulting from expanding urbanization and agriculture. Primary production rates and dynamics between the lakes vanes considerably, hut. in most lakes these arc related to a clear seasonality in stratification strength, nutrient availability and plankton abundance that in rur n arc largely governed by the seasonal meteorological and hydrodyriamical conditions on the lakes. Vertical stratification, which isolates nutrients from the euphotic zone, and mixing depth, which govern underwater tight conditions and the availability of internal nutrients for epilimnetic production arc key variables in setting overall productivity levels. Stratified tropical lakes, with density gradients over a narrow range of relatively high temperature, show to be most sensitive 1.0 climate change. Small variations in climate there cause drastic changes in the stratification dynamics and associated phenomena (mixing, entrainment and internal wave action). On overall, we show that even moderate climate warming may significantly increase the whole water column resistance to wind-driven mixing and hamper the lakes productivity by reducing internal nutrient loading. Increased water column stability can trigger a cascade of events that further affect the carrying capacity of the lakes, e.g., epilimnetic aligotrophication, confinement of reduced matter building up in deeper waters, hampered oxygen renewal, increasing hypoxia events and loss of fish and their habitats, shift from palatable green algae and heavv-walled diatoms to weakly silicified smaller diatoms, piceplarikton and. Diazotrophic Cyanobacteria. Recent reports from the central part of the region studied describe a decline in wind stress and surface cooling by radiative and cooling losses, both mechanisms that largely determine the degree of deep mixing. Climate change projections show a further increase in temperature and intensification in meteorological variation. If true, then the profound seasonal mixing characteristics for most lakes in the region may shift more permanently towards a condition with shallower and decreased mixing. We have seen that stronger density stratification situated closer to the surface may more easily lead to reduced water quality, destabilised plankton production dynamics, and changes in the food resources for fish. If prolonged, such a situation will have far-reaching consequences for fish production, food security, livelihoods but also trade. The foodwebs and production patterns of a growing number of African Great Lakes are going through major changes. A lowered carrying capacity has not yet been clearly demonstrated. Increased nutrient influxes from shore communities and increasing fishing pressures probably mask the apparent changes in drivers determining the biological basis for fish production. However, this compensation could fail when stocks collapse, increased external inputs further strengthen stratified conditions and eutrophication becomes too strong. We stress that to secure the continued functioning of the coupled social environmental systems in the African lake region we must invoke 1) close monitoring of current trends in environmental change of Africa's lake basins in order to govern the health and productivity of these ecosystems and 2) new, strategic, interdisciplinary research that can deliver the lake communities the knowledge and supporting information necessary to assess the changes they are facing from climate warming, and to understand how they may effectively mitigate unwanted changes but also cope with the change that we cannot manage.