Effects of a native New Zealand freshwater mussel on zooplankton assemblages, including non-native Daphnia: a mesocosm experiment

Abstract

Few studies have examined the effects of freshwater mussels on zooplankton in their native regions. We undertook an outdoor mesocosm-scale experiment to examine the effects of Echyridella menziesii, an endemic New Zealand mussel, on zooplankton composition. Experiments were undertaken in 125-L cylindrical drums that were randomly separated into the following three groups: seven control tanks with no mussels, seven treatment tanks with five mussels, and seven treatment tanks with 10 mussels each. Tanks were seeded with algae and zooplankton from a variety of small local water bodies, and monitored on Days 0, 5, 14 and 42. ANOSIM indicated that zooplankton composition among control and treatment tanks differed from Day 5 onward. Small, planktonic rotifers were commonly found to be most greatly affected by the presence of freshwater mussels. Benthic rotifers and larger crustacean zooplankton were typically unaffected or favoured when mussels were present. Overall, our results are consistent with removal of zooplankton by filtration, rather than competition for resources, being the primary driver of mussel influence on zooplankton community composition.