The benthic macroinvertebrate community of the upper reaches of an Australian estuary that undergoes marked seasonal changes in hydrology

Benthic macroinvertebrates were collected from the substrata of nearshore, shallow waters (depths <1 m) and offshore, deeper waters (depths 2·5–5 m) of three regions in the saline lower reaches of the tributary rivers that comprise the upper Swan Estuary in each season between winter 1995 and autumn 1997. Freshwater discharge in the two tributary rivers varied markedly during both years and consequently salinities in the upper estuary ranged from <1‰ in winter to >30‰ in autumn. The most abundant species, the galeommatid bivalve Arthritica semen , contributed >25% to the total numbers of macroinvertebrates in both shallow and deeper waters. The continuous reproduction, rapid growth, short life cycle and tolerance to a wide range of salinities, that characterize this species, represent ideal adaptations for life in a seasonally very variable environment. The overall number of species and density of benthic macroinvertebrates were typically greater in shallow than deeper waters. Species composition also differed significantly between water depths. The nereidid polychaete Ceratonereis aequisetis and mytilid bivalve Xenostrobus securis , which feed on plant material, were relatively far more abundant in shallow than deeper waters, whereas the reverse was true for the spionid polychaete Prionospio cirrifera and sabellid polychaete Desdemona ornata , which feed on the detrital material that settles out in deep waters. Heavy freshwater discharge in winter 1996 was accompanied by very marked changes in the richness, density and composition of species in shallow waters, but not in those of deeper waters where scouring would have been less pronounced. The species compositions in both the shallow and deeper waters of each region usually differed significantly among the seasons of each year and between the corresponding seasons in the different years. Region within the estuary had little or no influence on the species richness, density and faunal composition of benthic macroinvertebrates, even though the regions were located in two different tributary rivers. The above results demonstrate that, within each region, certain species were particularly successful in the first year, while others were more abundant in the second year, which in turn suggests that the benthic macroinvertebrate fauna of each region is responding to the same fine‐scale differences between environmental conditions in the two years. Copyright © 2001 John Wiley & Sons, Ltd.