Aquatic macroinvertebrates from littoral vegetated lakes in arid Patagonia: The influence of fish introduction on community structure

Wetlands in arid regions are characterized by their great heterogeneity, often including endemic species that contribute to regional aquatic biodiversity. In the Patagonian steppe, most lakes are naturally fishless and sustain a rich aquatic biodiversity, including endemics; however, many of these lakes have been stocked with Oncorhynchus mykiss (rainbow trout) during recent decades. The introduction of predatory fish into aquatic ecosystems naturally devoid of these organisms can produce profound alterations in the structure of the original communities and in the configuration of their biotic assemblages. Aquatic invertebrates play important roles in both the structure and functioning of freshwater ecosystems, including their contribution to global biodiversity and their cascading effects across ecosystem boundaries. The aim of this study was to assess the effect of introduced non-native fish on the pleustonic macroinvertebrate assemblage. Samples were taken from 10 shallow Patagonian lakes (five fishless and five stocked with fish) in two consecutive warm periods. Lakes with fish had higher conductivity and phosphate values. Higher inter-annual differences were also observed in lakes with fish for phytoplanktonic chlorophyll a, nitrate, phosphate, conductivity, and water temperature. The community structure (relative abundance of different taxa, species composition, and diversity index) was affected by the presence of rainbow trout, with the differences mainly driven by the relative abundance of Ostracoda, Hyalella, Chironomidae, and Haliplus species. This approach can be used to assess the required conservation actions in shallow Patagonian steppe lakes, which have been listed as priority areas for waterbird conservation. Specifically, this study provides a baseline for monitoring future trends of the macroinvertebrate communities and for evaluating conservation and management efforts over time.