Different mixing techniques in experimental mesocosms-does mixing affect plankton biomass and community composition? : Mixing techniques in mesocosms

Abstract

Over the past four decades, mesocosm studies have been successfully used for a wide range of applications and have provided a lot of information on trophic interactions and biogeochemical cycling of aquatic ecosys- tem. However, the setup of such mesocosms (e.g., dimensions and duration of experiments) needs to be adapt- ed to the relevant biological processes being investigated. Mixing of the water column is an important factor to be considered in mesocosm experiments because enclosing water in an artificial chamber always alters the mix- ing regime. Various approaches have been applied to generate mixing in experimental ecosystems, including pure mechanical mixing (e.g., using a disc), airlifts, bubbling with compressed air, and pumping. In this study, we tested different mixing techniques for outdoor mesocosms and their impact on plankton biomass and com- munity composition. We compared mesocosms mixed with a disc, an airlift-system, and bubbling, and used a nonactively mixed mesocosm as a control. We investigated phytoplankton, ciliate, and zooplankton commu- nities during a 19-d mesocosm experiment. Based on our results, we concluded that mechanical mixing with a disc was the most effective technique due to the undertow produced by lowering and lifting the disc. While no mixing technique affected seston biomass, zooplankton biomass was highest in the treatments mixed with the disc. The airlift treatments had the lowest relative share of small flagellates. However, no further differences in phytoplankton community composition occurred and no differences in zooplankton community composition existed between all actively mixed treatments.