Influência do pulso de inundação sobre variáveis limnológicas de um lago de várzea da Amazônia Central, lago Catalão

Abstract

Lake Catalão is a small water body lying on the floodplain of the Solimões River, next to its confluence with the Negro River, and therefore subjected to the seasonal water level fluctuations by both river systems. The aim of this study was to determine the spacial and temporal variation of some limnological parameters along the lake basin, in order to evaluate the role of river hydrology on the dynamics and functioning of the system. Sampling was undertaken four times during the hydrological year, representing each of the four distinct phases (low, rising, high and falling water periods) from November 2004 through August 2005. The following parameters were evalueted: transparency, temperature, dissolved oxygen, conductivity, pH, total alkalinity, carbon dioxide, seston, major cations, nutrients (phosphate, ammonium, nitrate, nitrite and silicate), total pigments and primary productivity by phytoplankton and its relationship with irradiance. The results indicated the presence of spacial variability and a strong seasonal pattern driven by the fluctuating river stage (or flood pulse). However, internal processes occurring in the lake (e.g., decomposition) seems to have a strong influence on the temporal distribution of some parameters, as such, dissolved oxygen, conductivity, total alkalinity, major cations and nutrients. Planktonic primary production and biomass showed lower rates at the falling and low water levels (<0-140 mgC/m3/h) characteristic of oligothrophic to mesotrphic systems, and higher at the rising and high water stage (100-380 mgC/m3/h) characteristic of mesotrophic to eutrophic systems. The seasonal difference in the photosynthetic rates seems to be related to the seasonal differences in the incoming radiation. The analysis of the data showed that phytoplanktonic photosythesis at lake Catalão reached its highest level at light intensities ca. 60% of the photosynthetically active radiation (PAR) reaching the water surface, further suggesting the occurrence of photoinhibition.