Abstract:
Eutrophication and chemical pollution are among the most serious threats facing coastal marine systems. Understanding how land use affects the coastal environment is necessary to maintain and improve the health of the environment despite increasing anthropogenic stress. Zooplankton communities are suggested to be among the most sensitive indicators of environmental change and this potential was explored within the Hauraki Gulf, Auckland, New Zealand. Four estuaries were selected with predominantly rural or urban land uses within the catchment area, reflecting differences in heavy metal concentrations, and variables associated with eutrophication (elevated nitrogen, phosphorus, turbidity, and chlorophyll α). Samples were obtained from each site in four months representative of seasonal changes over summer, autumn, winter, and spring. Significant differences in zooplankton abundance, and community composition were observed between sites and between seasons. Canonical analysis of principal coordinates (CAP) based on Bray-Curtis similarity matrices were conducted to separately visualise differences in the zooplankton community between sites among seasons, and between seasons among sites. Spearman rank correlations were performed to characterise the species, and environmental variables that may be contributing to these differences. Between site differences were most strongly correlated with total phosphorus, turbidity, and the degree of heavy metal contamination. The taxa that most strongly contributed to these patterns were the copepods Temora turbinata, Corycaeus aucklandicus, Labidocera cervi, and Oithona sp. Seasonal differences were most strongly correlated with variations in dissolved oxygen (also a proxy for temperature), salinity, and total nitrogen concentration. The taxa that responded more strongly to seasonal variability include the copepods Paracalanus indicus, Euterpina acutifrons and other Chaetognatha, Cladocerans and Hydromedusae. Overall, the zooplankton community composition was not substantially different from historic records within the Hauraki Gulf obtained by Jillett in 1963-1965. The successful differentiation of zooplankton communities between sites attributable to variation in nutrient enrichment and urban contaminants suggests that zooplankton communities are a suitable bio-monitoring tool worthy of further investigation. It is recommended that future studies are conducted at the lowest level of taxonomic resolution possible as higher level aggregations or subsets of taxa do not generate as strong a signal. Establishing a zooplankton monitoring programme would contribute to the maintenance, and improvement of the ecological health of the Hauraki Gulf in the face of increasing environmental stress.