An Introduction to the Limnology of Lake Chapala, Jalisco, Mexico

Abstract

This chapter describes some of the physical, chemical, and biological properties of Lake Chapala. It brings together descriptive data from several sources and provides a reference for the more specific papers, which follow. Lake Chapala, located at the western edge of the neo-volcanic zone of Mexico, is a tectonic lake of great age and is one of the oldest lakes in North America. At approximately 1000 km² it has the greatest area of any Mexican lake. Never the less, because of its’ age and because of water extraction, it is extremely shallow and, with a relative depth of < 0.02%, it is one of the most shallow of the world’s large lakes. The water budget consists of inputs principally from two sources, the Lerma river and direct surface rainfall. The outputs are to the city of Guadalajara’s domestic water supply, to the Santiago river and evaporation. The proportions of these input and output sources has changed dramatically in the past 25 years. The Lerma river’s input is reduced from approximately two-thirds of the total to approximately one-half. Output to the Santiago river has stopped as water demands by Guadalajara increased. The lake is elongate lying in a rift valley with an east to west orientation. Consequently wind action is principally along the long axis with the generation of large waves and complete mixing. The prevailing East to West winds generates currents capable of transporting materials entering via the Lerma river along the south shore with a return in the mid-lake. The east end, near the river delta, has extensive areas less than one-meter depth for much of the year (the annual rainy season — dry season depth change averages approximately 0.8 m). In attempts to lessen water loss to evaporation, significant areas have be isolated by dykes and converted to agricultural land. Sediments resuspended from the shallow area are transported along the south shore with return gyres evident in space images and photography. These suspended clays are a major forcing factor in determining the lake communities’ functions. The mean annual lake-wide Secchi depth is 0.5 m. This varies from 20 cm at the shallow east end to 70 cm at mid-lake. The mean coefficient of PAR extinction averages 2.3 m⁻¹. Transparency is rarely, and only in the shallowest regions, influenced by algae mass. Lake wide, inorganic suspended solids average 18 mg L⁻¹. Because of the shallowness and wind mixing, the lake does not seasonally stratify, but like other tropical lakes, can stratify on a daily basis with appropriate climatic conditions. The mean annual water temperature is approximately 22 °C with an annual seasonal range of only 21 to 23 °C. The water is moderately hard (TH = 150 mg L⁻¹) and alkaline (TA = 185 mg L⁻¹, pH = 8.7). Because of the soil properties of this volcanic region and the inputs of anthropogenic materials from the Lerma river and villages surrounding the lake, the total phosphorus concentrations are high (0.4–0.5 mg L⁻¹). Most of this (≈90%) is SRP. Total nitrogen is of a similar concentration (0.5–0.8 mg L⁻¹) and total inorganic nitrogen is 0.1–0.4 mg L⁻¹. With the exception of nitrogen, chemical elements follow a pattern of dilution with the rainy season. Nitrogen, with much supplied in rainwater, tends to increase at that time. The nature of the lake’s biota is less well known than that of the chemistry. Species lists are incomplete for most taxa. And the functional ecology of most communities is poorly understood. Undoubtedly the very low transparency is a major determinant of the structure and function of the biota. For example, macrophyte communities are limited to either floating or emergent forms. Cattail (Typha angustifolia) is abundant in the shallow waters of the eastern region. Water hyacinth (Eichornia crassipes) can be abundant with a cycle associated with water level fluctuations. Recently and in 1989, scientists from SIAPA studied phytoplankton taxonomy in relation to water treatment problems. The occurrence of cyanobacteria appears to have increased in the past decade. Perhaps because of the high nitrogen content of the water, cyanobacterial nitrogen fixation does not occur. Phytoplankton biomass as chlorophyll is generally low (<10 µg L⁻¹) over most of the lake but can be high (>25 µg L⁻¹) in shallow regions with a favorable light climate. Phytoplankton photosynthesis in the water column is low. Because of the light-limited phytoplankton photosynthesis, bacterioplankton production is unusually important is the formation of small grazable particles as the base of the food web. The lake’s zooplankton has been studied taxonomically, but poorly with regard to community function. The benthos is less well known either taxonomically or functionally. Little of the taxonomic data on zooplankton and benthos has been published in peer-reviewed literature. The lake’s fish community is rather well known taxonomically and from a fishery perspective. The importance of the fishery to the local economy has necessitated serious study of the abundance and distribution of harvested fish species. The unusual occurrence of the Chirostoma species flock has attracted evolutionary biologists for study of this taxon. The functional role of the fishes in the lake ecosystem has not been investigated, but study of niche partitioning in the Chirostoma flock is now underway. Opportunities abound for both descriptive and functional investigations of many lake communities.