ABSTRACT
ABSTRACT The world is experiencing a significant growth in human populations, and freshwater is fast becoming a scarce resource. Lakes represent a major source of such freshwater, but are coming under increasing pressure through exploitation for agriculture, industry, drinking water and hydroelectricity generation. This is particularly the case in tropical countries. This chapter presents a case study, Lake Chapala, the largest lake in Mexico. The primary tributary of Lake Chapala is the Lerma River. Much of the flow of this tributary is used in a non-conservative fashion upstream for agricultural and industrial purposes. Large quantities of domestic, agricultural, and industrial sewage from the entire Lerma-Chapala basin still flow untreated into the watershed and eventually to the lake, resulting in excessive inputs of phosphorus (P). High nutrient concentrations in the lake have led to degraded water quality, resulting in growths of floating aquatic vegetation of bulrush and water hyacinth, as well as blue-green algae. Spatial and
temporal P distributions in the lake during the past 24 years have been discussed in relation to the hydrologic changes. A method based on the bathymetric features of the lake estimates the amount of total P in the lake. It has been well established in the case of temperate lakes that the most effective long-term measure for the control of eutrophication in a water body is a reduction in the input of external nutrients. In tropical shallow lakes, internal recycling of phosphorus from sediments represents a significant long-term input. Determining the nutrient balance of any lake or reservoir constitutes a very important tool to understand its chemical and biological processes. Equally, the knowledge of existing relationships between hydrological variables, nutrient concentrations, and primary productivity can help in determining the best way of restoration where eutrophication problems are involved. These relationships are discussed in this chapter.
