Zooplankton are a valuable food source for both native and exotic freshwater fish species and play a key role in the survival of species during larval and juvenile phases. Fish can alter the abundance and community composition of zooplankton, via a number of mechanisms including sediment disturbance, predation and altering nutrient water chemistry. This study describes the impact of the common Carp (Cyprinus carpio) and Weather Loach (Misgurnus anguillicaudatus) on the abundance and composition of emerging zooplankton communities. Using replicated mesocosms, the experiment was designed so that the three main effects of Carp and Weather Loach on zooplankton (sediment disturbance, water chemistry and predation) could be studied individually and as a whole.
Total zooplankton abundances increased in the presence of both Carp and Weather Loach in comparison to the Control treatment. Zooplankton community composition was also altered in the presence of both Carp and Weather Loach in comparison to the Control treatment. Overall increases in abundance were driven by shifts towards rotifers which increased and declines in cladocerans. The cladoceran Moina micrura appeared to be particularly sensitive to Carp and Weather Loach. It was dominant in the Control treatment and absent in the presence of both fish.
Our findings show that Carp and Weather Loach can have profound impact on zooplankton communities. They impact both directly and indirectly, by predating larger cladocerans, altering emergence from sediments and potentially increasing abundances through altered nutrient water chemistry.