Impacts of large-bodied crustaceans on the microbial loop

Abstract

We conducted a mesocosm experiment to assess the impacts of large-bodied crustaceans on microbial communities. Three alien crustacean species (Daphnia pulex, Simocephalus vetulus and Macrocyclops albidus) were collected from the regional species pool and added to mesocosms that were filled with water from a eutrophic lake (Masurian Lake District, Poland). We then analysed chemical (total phosphorus and nitrogen concentrations) and biological (algae, bacteria, nanoflagellates, ciliates, rotifers, crustaceans) parameters over the course of the 40 day experiment. Alien crustacean species constituted 59–88% of the total crustacean biomass throughout the experiment. The final biomass of bacteria and copepods were not affected by the addition of alien species. However, rotifer and native cladoceran biomass tended to be lower while nanoflagellate and ciliate biomass were higher in mesocosms with alien species. Our research suggests that the large-bodied crustaceans altered the structure of the microbial loop. In the control, nanoflagellates were likely the main consumers of bacteria and thus constituted the main link between bacteria and higher trophic levels. In the mesocosms with large-bodied crustaceans, protists were likely not important as bacterial grazers because of strong top-down control of nanoflagellates by crustaceans. Combined, our results provide evidence that alien large-bodied crustaceans can significantly impact the microbial loop.