Zooplankton dynamics in mangrove and other nearshore habitats in tropical Australia

Abstract

Zooplankton was sampled in a mangrove forest, a mangrove drainage creek, the main- stream of a mangrove-dominated estuary, an adjacent seagrass flat and at an offshore (10 km) station in northeastern Australia every 6 wk between April 1985 and February 1986. Copepods, in particular Parvocalanus crassirostris, Paracalanus spp., several species of Oithona and Euterpina acutifrons, were numerically dominant. Community structure often differed amongst mangrove habitats, but mangrove habitats clustered separately from seagrass and bay habitats in classification analyses due to the abundance of meroplankton taxa, particularly invertebrate eggs and brachyuran zoea, in mangrove habitats. Total densities of zooplankton in mangrove and seagrass habitats (range of mean annual densities, 1.13 to 1.97 X 104 m-3) were always higher than in the offshore bay habitat (F = 0.88 X 104 m-3). Mangrove and seagrass habitats exhibited marked seasonality in densities of most taxa (for total zooplankton typically ca 2 to 6 X 104 m-3 in the wet season to ca 0.1 to 5 x 103 m-3 in the dry season). Generally, seasonality in most taxa did not correlate with water temperature, salinity, mangrove litter fall or fish predation, although fish may have a significant influence on brachyuran zoea. There were often significant tidal variations in densities in mangrove creeks; low-tide densities were usually lower than high-tide values. Comparisons of the density of major prey taxa of fish in seagrass and mangrove habitats gave only partial support to the hypothesis that mangroves are more important nursery sites for zooplankton-feeding juvenile fish because they are areas of greater food abundance; during the summer recruitment period of juvenile fish, brachyuran zoea, a major prey of fish, were an order of magnitude more abundant in mangrove habitats.