Influence of seasonal variability and salinity gradients on benthic invertebrate assemblages in tropical and subtropical Australian estuaries

I examine the influences of three key factors (habitat type, climatic regimes, floods and associated salinity regime changes) on the structure of benthic invertebrate assemblages of tropical estuaries. I compared the habitat-specific invertebrate assemblages of the full spectrum of habitat types available in one region of Deluge Inlet, a near-pristine estuary. There were distinct faunal assemblages associated with particular habitat types that related to sediment size and presence/absence of seagrass. The results of this study laid the foundation for the following studies in establishing the need to sample a single habitat type to facilitate spatial comparisons. Consequently, I selected bare sand for subsequent sampling because this substrate type was available in all estuaries and estuary regions studied. I then examined the influenced of climatic regimes (wet tropics, dry tropics and dry sub-tropics) on estuarine benthic invertebrate assemblages. The benthic assemblage in wet tropic estuaries differed from assemblages in dry sub-tropic estuaries, with many taxa only recorded in wet tropics estuaries and vice versa. There was little overlap of species between climatic regimes, particularly in upstream transitional zones (i.e. areas at the estuary/freshwater interface subject to substantial seasonal variation in physical conditions). However, a common pattern seen in all ten estuaries studied was distinct differences in faunal assemblages between the mouth and upstream transitional assemblages. In the third part of this thesis I examined the effect of wet-season floods and how changes in salinity regimes along the downstream gradient of estuaries affected invertebrate assemblages. Following the seasonal flood, upstream assemblages underwent radical restructuring ranging from the total loss of taxa from some sites to significant decreases in diversity and abundances at other sites. The recovery of faunal assemblages after the flood varied between sites and appeared to be mainly a function of the location of the site in relation to freshwater input (i.e., sites closest to freshwater input were more adversely affected than sites further away from the source of the freshwater input). Clearly, although flooding is a natural occurrence in tropical estuaries, it can have a profound effect on macrobenthic invertebrate assemblages by greatly reducing and/or eliminating taxa, particularly in the upstream reaches at times of low salinity. These studies provide detailed understanding of benthic assemblages in a range of North Queensland estuaries, of the differences in mouth and transitional zones and of how assemblages respond and recover after a flood. Understanding these patterns and processes is essential in order to understand the ecological functioning of the systems and as a precursor to effective management