This study was based on the hypothesis that different associations can be described as subsystems that interchange production by means or their mobile elements ; namely the fish. The Chascomús Lake was sampled at 19 sites over two years. Different habitats based on the presence or absence of vegetation and substrate type were determined. The association of fish to the different environments displayed seasonality. Some species can be considered opportunists, others are limited by some variables and a third group moves according to intrinsic behaviour. Species richness in higher in coastal environments. Diversity shows gradients that indicate true boundaries, though it never reaches saturation levels.