Understanding Fish Assemblage Structure in Lentic Ecosystems: A Comparison of Natural Lakes and Reservoirs in Iowa, USA

Reservoirs are often managed similarly to natural lakes because they are assumed to be functionally comparable.  However, direct comparisons of fish assemblage-environment relationships between these two ecosystems are rare and this assumption has not been adequately evaluated.  We investigated associations of fish assemblage structure from 45 natural lakes and reservoirs in Iowa, USA.  Fish sampling was conducted with benthic trawls, modified-fyke nets, and night electrofishing.  Increased species diversity in reservoirs was most strongly related to morphometric characteristics (i.e., larger surface area, increased depth); whereas, fewer species were observed in natural lakes with low water clarity and high suspended solids.  Fish assemblage structure between natural lakes and reservoirs was consistently dissimilar for all sampling methods.  Structuring of species composition in reservoirs was correlated with a variety of limnological and physical characteristics, but was largely dependent on the sampling method.  In contrast, trophic structure of fishes in reservoirs was weakly associated with the environmental factors evaluated and was similar to fish species structure of natural lakes.  Fish trophic composition of natural lakes was related to waterbody size, but was consistent among sampling methods.  Overall, distinct differences in fish assemblage structure were observed between natural and artificial lentic ecosystems.  Our results emphasize the need to consider waterbody origin (i.e., natural or artificial) on fish assemblage characterization and subsequent inferences made from environmental correlations.