Factors affecting the structure and functioning of shallow Pampean lakes

Abstract

The structure and function of the Pampa plain lakes are explained, in large part, by its history, geomorphology, climate, nutrient rich soils drainages, and by the modifications produced by man in his use of land and water. They can be synthetically characterized as plain lakes, very shallow, polimyctic, eutrophic or hypertrophic, and with water renewal time and salinity highly fluctuating. This conjunction of external characteristics explains the high biomass of each one of its biotic communities. A synchronous analysis, during the peak of the growth season, permits us to discriminate three types within the large lakes of permanent character: lakes with relatively low phytoplankton biomass and abundant development of the rooted macrophyte, lakes with abundant development of the phytoplankton but scarce macrophyte development, and lakes with high inorganic turbidity, scarce aquatic macrophyte abundance, and low phytoplankton biomass. The first type coincides with what usually is known as "clear" lakes, of relatively transparent waters and high relative abundance of large piscivorous fishes. The second coincides with green, "turbid" lakes, of low water transparency, and usually with high abundance of visual planktivorous fishes. An increased relative abundance of filter feeding planktivores is noticeable in lakes impacted by organic waste discharges. The third type encompasses lakes clearly limited in their productivity by light availability, and would be the result of direct human action on their drainage basins. The relative abundance of turbid lakes is generally greater in zones with higher intensity of land use. However, the high annual and year-to-year variability of the Pampean landscape is seen reflected in the operation of the lake ecosystem. Large part of the lakes goes through a seasonal phase of clear waters characterized by a high abundance of cladocerans of greater size that ends when reaches its peak of abundance the class 0+ of planktivores. The complex interaction between climatic factors and land use also would explain the non-periodic changes in the typology of a given lake. For permanent lakes, this phenomenon would be intensified with a decrease in lake surface area.