Vertical distribution of rotifers on the Upper Parana river floodplain:: the role of thermal stratification and chlorophyll-a

The degree of vertical aggregation by rotifers was quantified in different environments of the Upper Parana river floodplain. The regression coefficient of the relationship between the logarithm of variance and the logarithm of mean density was used as a criterion to evaluate the pattern of vertical distribution of the populations. The lake showed the highest degree of vertical aggregation by the populations. Thermal stratification was the possible causal process. In the river, a high degree of aggregation was observed. Correlation of the mean rotifer density with chlorophyll-a concentration indicated that, in this unstratified environment, the degree of aggregation may be dependent on the food concentration. Therefore, several processes in addition to the frequently studied hydrodynamic ones may promote aggregations of rotifers in floodplain environments.