Nutrients and Eutrophication in Lake Chapala

Abstract

The primary tributary to Lake Chapala is the Lerma river. Large quantities of domestic, agricultural, and industrial sewage from the entire Lerma-Chapala basin still flow untreated into the watershed and eventually into the lake, resulting in excessive inputs of phosphorus (P) and nitrogen (N), both known to cause eutrophication. Anthropogenic water demand from Lake Chapala surpasses the surface supply and groundwater recharge rate. This has resulted in a hydrologic imbalance in its basin. In addition, high nutrient concentrations in the lake have led to degraded water quality, resulting in growth of floating aquatic vegetation and blue green algae. Some studies establishing a P balance for the lake have been carried out, but none for an N balance. Spatial and temporal P and N distributions in the lake during the past 24 years are discussed in relation to hydrologic changes. A method based on the bathymetric features of the lake is proposed to estimate the amounts of total P and N. This method uses a digital elevation model (DEM) of the site and the water quality data monitored for 24 years in 28 lacustrine and 9 littoral monitoring stations. Many authors suggest 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. Determining the nutrient balance of any lake or reservoir constitutes a very important tool to understand its chemical and biological processes. Similarly, the knowledge of existing relationships between hydrological variables, nutrient concentrations, and primary productivity could strongly help to determine the most appropriate restoration and clean-up policies wherever eutrophication problems are present. The different relationships are discussed in this chapter.