Mangrove zooplankton of North Queensland, Australia I. Plankton community structure and environment

Measurements of plankton community structure and trophic resources potentially available to planktonic copepods were made in the mangrove estuaries of six rivers in Northeastern Australia. The Pascoe, Claudie, Lockhart, McIvor and Daintree Rivers represent wet tropical systems on Cape York, whereas the Haughton River estuary has restricted freshwater inflow because of a drier climate and freshwater diversion for agriculture. The Haughton River was sampled approximately monthly between October 1992 and May 1994, and had a mean abundance of zooplankton > 37 mu m of 200 l(-1) (range 60-500 l(-1)). The Cape York rivers were sampled infrequently, and zooplankton abundances ranged between 0.4 and 1400 l(-1). The zooplankton of all rivers was dominated by copepods, particularly representatives of the genus Oithona which were characteristic of a distinct mangrove fauna. Physical forcing influenced the zooplankton of mangrove estuaries much more than the measured biological variables. The water column was characterised by high concentrations of particulate matter, up to 3.3 mg l(-1) C and 1.1 mg l(-1) N, of low food quality las indicated by the C:N ratio). Phytoplankton biomass las chlorophyll a) in all six rivers was on average four-fold greater than in neighbouring coastal waters (1.1-12.6 mu g l(-1)), and 25% of this chlorophyll a was derived from cells > 10 mu m, and thus potentially available to copepods. The degree of mixing, determined by the combination of tidal state and the extent of freshwater input, appears to drive both the quantity and quality of particulate material available to higher consumers and the distribution of zooplankton communities within mangrove estuaries.