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 c omprise the upper Swan Estuary in each season between winter 1995 and autum n 1997. Freshwater discharge in the two tributary rivers varied markedly du ring both years and consequently salinities in the upper estuary ranged fro m < 1 parts per thousand in winter to > 30 parts per thousand in autumn. Th e most abundant species, the galeommatid bivalve Arthritica semen, contribu ted > 25% to the total numbers of macroinvertebrates in both shallow and de eper waters. The continuous reproduction, rapid growth, short life cycle an d tolerance to a wide range of salinities, that characterize this species, represent ideal adaptations for life in a seasonally very variable environm ent. The overall number of species and density of benthic macroinvertebrate s 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 deepe r waters, whereas the reverse was true for the spionid polychaete Prionospi o cirrifera and sabellid polychaete Desdemona ornata, which feed on the det rital material that settles out in deep waters. Heavy freshwater discharge in winter 1996 was accompanied by very marked changes in the richness, dens ity and composition of species in shallow waters, but not in those of deepe r waters where scouring would have been less pronounced. The species compos itions in both the shallow and deeper waters of each region usually differe d significantly among the seasons of each year and between the correspondin g seasons in the different years. Region within the estuary had little or n o influence on the species richness, density and faunal composition of bent hic macroinvertebrates, even though the regions were located in two differe nt tributary rivers. The above results demonstrate that, within each region , certain species were particularly successful in the first year, while oth ers were more abundant in the second year, which in turn suggests that the benthic macroinvertebrate fauna of each region is responding to the same fi ne-scale differences between environmental conditions in the two years. Cop yright (C) 2001 John Wiley & Sons, Ltd.