Responses of zooplankton assemblages to environmental variability among brackish coastal ponds, New Zealand

Abstract

Coastal brackish waters, including estuaries, lagoons and coastal ponds, experience fluctuations in physicochemical properties, particularly in salinity, due to the differential forcing of marine and freshwater inflows. Most research concerning brackish waters is skewed towards lagoons and estuaries, while relatively little exists on small coastal ponds. This study examined variability in zooplankton community composition relative to environmental conditions among brackish coastal ponds in Auckland, New Zealand. Eight coastal stormwater ponds, four freshwater ponds, and four marine sites were selected to represent a wide salinity gradient and sampled for zooplankton and environmental variables. Marked spatial and temporal variability in environmental conditions was observed among the coastal ponds. Salinity of the ponds was, on average, lower in winter, influenced by higher rainfall, while in summer reduced rainfall led to significantly lower freshwater inputs. Zooplankton communities within the coastal ponds were a mix of those in freshwater and marine sites, along with unique species not observed in each of the adjoining ecosystems. Salinity was the most important factor driving zooplankton community composition among sites, as widely reported for estuaries and lagoons. The degree of temporal change in salinity within ponds also affected community composition, with euryhaline copepods such as Sulcanus conflictus and Mesochra parva most abundant at sites with larger seasonal salinity changes. Salinity increases due to sea-level rises may lead to the disappearance of species that are less tolerant to salinity, with a move to dominance by euryhaline species.