Is geomorphic zonation a useful predictor of patterns of benthic infauna in intermittent estuaries in New South Wales, Australia?

Geomorphic classifications of estuaries are used to guide management. One classification recognizes four geomorphic zones within estuaries: marine tidal delta, central mud basin, fluvial delta, and riverine channel. It is assumed that the biota is controlled primarily by salinity and characteristics of sediments. By ignoring small-scale variability, the number of species and diversity are predicted to be homogeneous within each zone, but to decrease with increasing distance from the sea. Data from surveys of macrobenthos and meiobenthos in four intermittently closed coastal lakes in New South Wales, Australia, were used to test these predictions. Assemblages of macrobenthos and meiobenthos in different localities within marine tidal deltas were different and also differed from those in fluvial deltas. Several families of polychaetes, in addition to nematodes and oligochaetes, distinguished between macrobenthic assemblages. Assemblages of meiobenthos were distinguished mainly by nematodes, turbellarians, and copepods. Numbers of macrobenthic species decreased by more than 50% between localities near the mouth and those further upstream, even though they were located within the same geomorphic zone, before increasing to a maximum in fluvial deltas. For meiobenthos, similar trends were evident, but differences were smaller. Half of the macrobenthic taxa and all but one among the meiobenthos exhibited the predicted decrease in density with increasing distance from the sea. There were no consistent correlations with physical factors (median grain size, percent clay, organic content, or salinity) at the level of assemblages or of individual taxa. Physical classifications fail to predict patterns in benthos, possibly because they do not incorporate linkages between physical and ecological processes at appropriate spatial and temporal scales.