Evaluating the implications of recent filter-feeding Daphnia invasions for kākahi (Echyridella menziesii)

Abstract

Freshwater mussels function as key ecosystem engineers due to their highly efficient filter feeding and bioturbating abilities, which enhance water clarity and promote nutrient cycling. Although they play a crucial role in facilitating these ecosystem services, freshwater mussels globally are in decline. New Zealand’s native freshwater mussels, traditionally referred to as kakahi or kaeo, are no exception. On a global scale, freshwater mussels are impacted by a suite of anthropogenic stressors, including pollution, habitat loss and the establishment of non-indigenous species. However, factors contributing to mussel declines in New Zealand are less understood. Recent studies have suggested that non-native species may be a key contributing factor. At present, one-third of non-native invertebrate species that have established in New Zealand’s freshwater lakes are zooplankton; two of these are highly efficient filter-feeding Daphnia species. Various studies have demonstrated that Daphnia invasions can have serious ecological impacts in their receiving environments, as these organisms are capable of modifying zooplankton communities and limiting the availability of algal food resources for other organisms. New Zealand studies are yet to explore the possibility of non- native Daphnia competing with freshwater mussels for algal food resources.

In response to this hypothesis, my research aimed to; i) examine the effects freshwater mussels have on Daphnia via predation and/ or interference competition, and ii) investigate the effect of non-native Daphnia on kakahi, through exploitative competition for algal food resources. Controlled tank experiments were used to investigate whether New Zealand’s most widespread kakahi species (Echyridella menziesii) could prey on zooplankton. Two-hour predation trials were undertaken on two small native zooplankton species (Brachionus calyciflorus and Bosmina meridionalis), and two non-native cladoceran species (Daphnia pulex and Daphnia galeata). Kakahi were only found to remove statistically significant numbers of the small rotifer species B. calyciflorus (30.2%) and the large cladoceran D. pulex (1.7%). These findings indicate that kakahi are unable to remove ecologically significant numbers of either non-native Daphnia. Thus, Echyridella menziesii is not suitable to be used as a biomanipulation tool to remove non-indigenous Daphnia species from shallow lakes and ponds. Kakahi could remove moderate quantities of the small native zooplankton species B. calyciflorus (30.2%) over the two-hour period. This suggested that small zooplankton, particularly rotifer species, may function as an important food source for kakahi. Additional laboratory experiments were undertaken to test the algal removal capabilities of D. pulex and of adult and juvenile E. menziesii over a three-hour period. Both kakahi life stages were found to consume a broad range of algal taxa, including diatoms, green algae, and filamentous algae, which ranged in size (between 33.6 and 348.0 μm). Comparatively, D. pulex was unable to consume statistically significant numbers of the same algal taxa. Instead, D. pulex consumed smaller algal species and microbes. As such, my findings suggest a limited niche overlap between the two grazers and, therefore, D. pulex is unlikely to reduce algal food availability for kakahi. Due to these differences in algal removal, it is possible that these grazers could be used together as a biomanipulation tool to remove a wide size range of algal biomass in shallow lake systems.