Function analysis of a seepage lake during an annual cycle and its adjustment to the valid theoretical proposals

CASCO, M. A. .; CANO M. G,; SOLARI, L. C.; MAC DONAGH, M. E.; CLAPS, M. C.; GABELLONE N. A,

Punta del Este

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

An investigation was carried out in the Lacombe Lake (Buenos Aires, Argentina) during an annual cycle to know it’s dynamic in the frame of the current postulates. Some characteristics (climate temperate, marked seasonality and the seepage condition, low human impact) were crucial for its selection. Meteorological conditions, physical and chemicals parameters of water, plankton and attached algae (epipelic and epiphytic) communities, presence or absence of submerged and emerged macrophytes were analysed. According to the theory of alternative equilibrium state in shallow lakes, three states or phases were recognized in the Lacombe Lake: one turbid phase and two clear phases (with and without submerged macrophytes). The phytoplankton maximum was achieved in the turbid phase whereas the epipelic algae reached a maximum in the clear phase without macrophytes. The zooplankton showed markedly differences in density and species richness between initial phases (lowest values of density and richness) and final phase (clear without macrophytes). Taking into account the model of Goldsborough and Robinson (1996), Lake State (low transparency, high phytoplankton density, low epiphytic biomass and high nutrients concentration) and Open State (dominated by epiphyton biomass, unstable water column conditions, lower nutrient and phytoplankton chlorophyll concentrations and development of submerged macrophytes) were identified in correspondence with to the first two periods of the alternative equilibrium state. A transitional state coinciding with a clear phase without submerged macrophytes was recorded when the ecosystem was not driven into a new state, oscillating, instead, around an Open State condition. In both cases, the changes among the states can be attributed to the growth of macrophytes and exceptional rainfall, perturbations of relatively short duration, but extensive in time. Both conceptual interpretations are temporally coincident for the first two periods and explain weakly the situation observed during summer-autumn period. This last situation could be considered as a clear water phase with absence of submerged macrophytes or as one transitional period without the predominance of any particular algae community.