Are the abiotic and biotic characteristics of aquatic mesocosms representative of in situ conditions

There is debate about whether results from mesocosm studies should be extrapolated to natural ecosystems. Critics argue that the  abiotic and biotic characteristics of mesocosms do not represent the i n situ conditions of the habitats in which they are designed to  mimic. We addressed this idea by deploying mesocosms in two lakes of varying trophic status (mesotrophic and eutrophic). Water chemistry  was relatively similar between the mesocosms and the respective lakes. In contrast, biological variables including most phytoplankton,  nanoflagellates, ciliates, and zooplankton were lower in the mesocosms than they were in the respective lakes. There was  also a shift in zooplankton composition such that mesocosm assemblages diverged from pelagic to littoral communities. While our  results show that freshwater planktonic organisms can vary considerably between mesocosms and in situ habitats, we also suggest that  factors such as isolation from lake sediments, the lack of predators, and the temporal scale at which mesocosms are filled and sampled  should be considered when designing and interpreting mesocosm studies. Nevertheless, mesocosms are an important tool for developing  and testing general ecological hypotheses that cannot be achieved using other approaches alone.