Importance of biotopes of a neotropical river for ichthyoplankton: patterns in biotope association, community structure and spatial distribution

The environmental heterogeneity of the biotopes that compound the mosaic of the rivers is essential for the development of the initial stages of fish life. In this sense, we evaluated the abundance and composition of ichthyoplankton community from different biotopes of the Ibicuí River and its relation with environmental variables, between October 2019 and January 2020. The ichthyoplankton sampling was performed in the Ibicuí River basin, in four biotopes: main river (Ibicuí River), tributary river (Ibirocá River), mouth and lake. Larvae from twenty-nine species were identified, including some threatened with extinction. Eggs and larvae were found in all biotopes, however eggs were more abundant in the main river and larvae in the lake. This last biotope was characterized by the greater presence of larvae in the post-flexion stage and sedentary species. The tributary river had a greater presence of larvae of migratory species. Eggs and yolk-sac larvae were positively correlated with the water speed and river width, while the most developed larvae were correlated with electrical conductivity and zooplankton. Our study revealed that the abundance and diversity of the ichthyoplankton were heterogeneously distributed among the biotopes, where hydrogeomorphological and limnological characteristics enable the emergence of distinct ichthyoplankton assemblages along the river gradient.