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Management Concerns related to Climate Change

In chapter 1 on environmental aspects of eutrophication, hydrological factors were discussed with respect to nutrient loading, residence times and flushing rates. The importance of thermal stratification within lakes and reservoirs for the distribution of nutrients was also noted. These factors are ultimately controlled by climate, i.e., precipitation, evaporation, ambient temperatures and the heat budget of the lake or reservoir.

Predicted changes in the world’s climate need to be considered in the planning and management of lakes and reservoirs. The IPCC (Intergovernmental Panel on Climate Change) Second Assessment of Climate Change predicts global temperature increases of between 0.8oC and 4.5oC over the next 50 to 80 years. Continental subhumid regions, such as the savannas of Africa, are likely to experience greater warming and increased aridity. This will result in increased lake temperatures, greater evaporation and altered thermal properties of lakes and reservoirs, tending towards stronger stratification. These effects will have significant impacts on water quality, in general, and eutrophication, in particular.

Changes in precipitation and enhanced evaporation could have profound effects on some lakes and reservoirs. In the early decades of the 20th century the outflow from Lake Malawi ceased for a period of seven years; much of Lake Victoria appears to have been converted into a grassland some four thousand years ago. Lake Chilwa in Malawi displays a six-year cycle of lake level fluctuations, which often leads to the drying of the lake. This cycle corresponds to precipitation variability in the region. Furthermore, these studies show that under the present climate, a number of large lakes and wetlands have a delicate balance between inflow and outflow. Therefore, evaporative increases could result in much reduced outflow.

Climate-induced changes could also have significant ecological impacts on freshwater ecosystems. For example, reduced outflow would increase the residence period of lakes and reservoirs, thus changing nutrient loading characteristics in favor of eutrophication. Further, increased temperatures may result in increased productivity, increasing sedimentation rates of organic material. This sedimentation could increase biological oxygen demand.

Increased temperatures cause increased density difference per unit change in temperature. In low to mid latitudes, this may prolong stratification, as well as strengthen the stability of stratification itself. Higher water temperatures would also mean reduced oxygen solubility. In eutrophic and mesotrophic water bodies, this leads to prolonged periods of oxygen depletion in the hypolimnion. A combination of increased evaporation and increased urban wastewater discharge to lakes and reservoirs could significantly change water chemistry, especially in closed-basin lakes. These observations require the manager of eutrophication to design management programs that take into consideration possible climate change impacts.

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