BENITEZ Hernan Hugo
congresos y reuniones científicas
Zooplankton community in a shallow lake during two annual cycles characterized by contrast environmental conditions
Punta del Este, Uruguay
Congreso; VI Shallow Lakes Congress: Structure and Function of World Shallow Lakes; 2008
body { background: #FFFFFF; margin: 0px; padding: 0px; } Changes in the composition and dynamic of the zooplankton in shallow lakes are related to food availability, grazing ability, fish predation, the eutrophication process, alternation of turbid and clear water periods. The San Miguel del Monte Lake is located in the ?depressed Pampa? of Buenos Aires province, Argentina and belongs to an interconnected lake system with the Salado River. One stream flows into the lake from an extensive agricultural and cattle breeding area. This permanent affluent contributes with high concentrations of dissolved organic matter. A town, with 10,000 inhabitants, is located on the shore. Floodgates prevent the natural connection with Las Perdices Lake and control the water level. During floods of the Salado River, this system of interconnected lakes acts as a backwater with exchanges of water and organisms. The water level of the lake was influenced by local rainfall and also by rainfall from the headwaters of the river. Zooplankton and water samples were taken at four -week intervals during two annual cycles (1997-1998 and 2002-2003) along the major axis of the lake. A vertical profile was performed at the deepest sector of the lake. In the first period, three different situations were recognized in the first period (turbid, clear water and flooding). The presence of submerged macrophytes occurred after extend periods of isolation and scarce rainfall that promoted low hydrometric levels and the macrophytes period finished in occasion of the connection with the Salado River. The chlorophyll a as an estimation of phytoplankton biomass, ranged between 6-219 µg L-1 and the transparency was highest during the submerged macrophyte colonization (1 m). The mean zooplankton density (886 ind. L-1) was dominated by rotifers and small cladocerans as expected from the trophic status of the lake. The crustaceans, mainly copepods, dominated the mean total biomass (198 µg DW L-1) in concordance another similar lakes. During the period 2002-2003, the lake was unconnected with the Salado River and the hydrometric level was high (upper than 2 m). The transparency was low (0.15 - 0.50 m) related to the concentration of inorganic suspended solids. Turbidity and dissolved organic matter promoted low phytoplankton biomass (chlorophyll a: 0 ? 119 µg L-1) and density (two orders of magnitude lower than the recorded in the first period). The total mean zooplankton density (903 ind. L-1) and biomass (180 µg DW L-1) were similar to the recorded in the first annual cycle whereas the composition was markedly different. The testate amoebae and ciliates increased their importance in this turbid period. Species of Keratella were dominant in contrast to the predominance of Brachionus in the other sampling period. Epiphytic organisms were absent or scarce during this period. The principal component analysis (PCA) showed that the zooplankton structure responded consistently to seasonality, hydrometerological conditions, changes in physical and chemical characteristics, presence of submerged macrophytes, and food availability.