Zooplankton community structure and copepod egg production in coastal waters of the central Great Barrier Reef

A 20 month study of zooplankton at three stations located across the transition zone between coastal and offshore waters in the central Great Barrier Reef lagoon was undertaken.The abundance and egg production rate of constituents of the zooplankton assemblage characteristic of the coastal zone rapidly increase subsequent to events such as flooding and upwelling. Sampling spanned two summer monsoonal seasons, the first of which (1990-91) was very wet. The second monsoonal season (1991-92) was very dry and was characterised by intrusive upwelling events from the Coral Sea. Chlorophyll a concentrations did not rise in the wet year, probably because of light limitation, but did rise as a result of upwelling. Terrestrial run-off in the wet year had a greater apparent effect on zooplankton abundance patterns than did upwelling in the dry year, except where coastal trapping allowed sufficient time for increases in zooplankton abundance to occur. Egg production rates by the copepods Acrocalanus gibber and Acrocalanus gracilis showed haphazard spatial differences. Nitrogen-specific egg production ranged between 0.03 and 0.21/day for Acrocalanus gibber, and between 0.13 and 0.41/day for Acrocalanus gracilis. The egg production rate by Acrocalanus gibber was food limited for most of the year and showed a poor correlation with temperature.
This research was undertaken to examine the effects of the interaction between coastally trapped water and offshore water on zooplankton community structure and production.