Why the pampean lakes are quite different? The influence of external and internal factors

GABELLONE, N A; ARDOHAIN, DIEGO MARTÍN; BENITEZ, HERNÁN; CASCO, MARÍA ADELA; CANO, MARÍA GABRIELA; MAC DONAGH MARÍA ELICIA; RUIZ, GONZALO; SOLARI LÍA CRISTINA; CLAPS, MARÍA CRISTINA

Punta del Este, Uruguay

Congreso; VI Shallow Lakes Congress: Structure and Function of World Shallow Lakes; 2008

Universidad de la República Faculty of Sciences

A great number of shallow lakes associated to the Salado River basin present dissimilar limnological characteristics related to their connectivity degree, geomorphological features, eutrophic degree, and the human activities. Intrannual and interannual differences are associated to the combined action of external factors (hydrometeorological components) and internal biological factors (e.g. presence of different macrophytes, fishes) that promote important consequences on the structure and dynamic of the environments. Local exceptional rainfalls are relevant in all of these environments increasing notably the hydrometric level. One important biological factor is the location of the emerged macrophytes because defines the percentage of open water zone. These lakes can also be permanent of sporadic colonized by submerged macrophytes. These factors promote significant changes of nutrient concentrations, of suspended solids and conductivity with evident response of the plankton community with modifications of its structure and its temporal and spatial dynamic. As result of investigations performed in pampean shallow lakes, it is possible define various periods related to hydrometeorological factors. In a lake that is connected directly with the river, it was possible define three phases (drought – connectivity phase and isolation phase) according to the discharge of the Salado River. The lake is highly dynamic, including its biological and physical conditions. The system supports extreme conditions and, despite being an open system during most of the year, it responds to lentic characteristics when is isolated. In a lake located at the most distant sector of a connected system to the Salado River it was possible verified isolated and flooding phases with important changes of the hydrometric level. The presence of submerged macrophytes occurred after extend periods of isolation and scarce rainfall that promoted low hydrometric levels and the period finished in occasion of the connection with the Salado River. Marked differences in conductivity, dissolved polyphenols, suspended solids, composition and density of plankton were observed during each particular situation. In the Pampa plain, it is common the presence of seepage lakes. These environments showed marked differences related to the permanent or sporadic presence of submerged macrophytes and the human activities developed in the surroundings. In one of these environments, it was possible observed different phases whereas in other one the limnological conditions unchanged associated to the stable low hydrometric level and constant human perturbations. The complexity of the planktonic community and the influence of the mentioned external factors prevent the establishment of a general scheme about the structure and function of pampean shallow lakes. The high number of planktonic species gives them possibilities of varied responses against to external and internal factors.