The trophic state of the very shallow Pampean lakes (Argentina)

The Pampa Plain lakes are shallow, polimyctic, eutrophic or hypertrophic, and highly fluctuating in water renewal time and salinity. Their structure and function are explained by its history, geomorphology, climate, nutrient rich soils drainages, and by man-produced modifications in land and water use. These characteristics explain, the high biomass of its biotic communities. A synchronous analysis, during the peak of the growth season, permits the discrimination of two types within the large lakes of permanent character: a) with relatively low phytoplankton biomass and abundant development Of aquatic macrophyte, and b) with abundant development of phytoplankton but scarce macrophyte development. The first type coincides with what is known as "clear" lakes, of waters relatively transparent and high abundance of large piscivorous fishes. Most of them are in the Pampa grasslands. The second type coincides with green, "turbid" lakes, with high abundance of visual planktivorous fishes. An increased relative abundance of filter feeding planktivores is noticeable in "highly turbid" lakes impacted by organic waste discharges. The shallow lakes were not completely "clear" or "turbid". Rather, a continuous distribution between definitely "clear" and "turbid" lake states was found. The relative abundance of "turbid" lakes is greater in zones with higher intensity of land use. The high temporal variability for the Pampa landscape is seen reflected in the operation of the lake ecosystem. The interaction of climatic factors and land use also would explain the non-periodic changes in the typology of a given lake.