STRUCTURE OF MACROBENTHIC COMMUNITIES IN RELATION TO HISTORICAL PATTERNS OF DREDGING IN THE BRISBANE RIVER ESTUARY G.A. SKILLETER Skilleter, G.A. 2002 07 30: Structure of macrobenthic communities in relation to historical patterns of dredging in the Brisbane River estuary. Proceedings of the Royal Society of Queensland 110: 75-83. Brisbane. ISSN 0080-469X. The composition of the benthic assemblages found along the margins of the Brisbane River estuary was examined to determine whether there were any relationships with the historical patterns of extractive dredging in the river. Three categories of treatment were used: control zones which had never been used for extractive dredging; recently dredged zones which had been dredged in the previous 2 years; and previously dredged zones which had been dredged within 3-5 years. A total of 3,113 animals was collected from 64 benthic grab samples, with the fauna dominated by spionid polychaetes (lower reaches) and tanaid crustaceans (upper reaches). The composition of the benthic assemblages in the up- and down-river sections was significantly different from each other, so analyses comparing the three treatments were done separately for the two sections. Multivariate and univariate analyses indicated that there were significant differences among the different zones in the numbers and types of animals found there, but these differences were not correlated with the dredge history for the zones. In each case, zones with a similar dredge history were as different from each other as they were from zones with a different historical pattern of dredging. p Brisbane River, Queensland, dredging, benthic, macrobenthic. G.A. Skilleter, Department of Zoology and Entomology, University of Queensland, St Lucia 4072, Australia (e-mail: gskilleter@zoology.uq.edu.au); accepted 1 September 2000. The Brisbane River is the largest river flowing into Moreton Bay, SE Queensland, with a total catchment of 13,500 km 2 . There is a long history, beginning in 1862, of dredging in the Brisbane River, for navigational purposes (McLeod, 1978) and the development of the Brisbane port facilities (Greenwood, 1993). There has also been substantial use of resources within the Brisbane River by extractive industries. Most of the sand and gravel for Brisbane’s rapidly growing building industry was taken from the Brisbane and Pine Rivers (O’Flynn & Thornton, 1990; Stephens, 1992). Erskine (1990) indicated that the continued use of the Brisbane River as a ‘subaqueous mine’ could lead to significant environmental impacts in several ways, including: (i) increased turbidity above background levels; (ii) lowering of bed levels with concomitant effects such as bank erosion; and (iii) changes to the tidal prism and tidal velocities within the river. Erskine (1990) also correctly noted, however, that any effects of dredging for the extractive industries could not be separated from effects due to dredging for navigational purposes. Despite the long history of dredging in the region, there have been no focussed studies on the effects of dredging on estuarine biota in the Brisbane River, although there have been several descriptive studies of the benthic and pelagic communities there and in other nearby systems (reviewed by Davie, 1990). The primary aim of this study was to determine whether the patterns of structure and composition of macrobenthic communities could be related to the historical patterns of extractive dredging in the Brisbane River, a first step in an examination of the effects of human disturbance on the estuarine biota. The focus of the study was on the fauna in the areas immediately adjacent to the main channel rather than in the areas being dredged as it has been well-established that dredging generally causes the complete loss of benthic organisms from an area which has been dredged (e.g. ICES, 1975; de Groot, 1986; Nichols et al., 1990). MATERIALS AND METHODS STUDY SITES. The Brisbane River was divided into contiguous zones, each ca. 300m in length, determined after examination of the historical patterns of extractive dredging in the river (Fig. 1). These zones corresponded to those used by the local government when issuing dredging permits. The information provided in the dredge history included data on the period over which the dredging had been done and estimates of the tonnage of material removed from each zone on an annual basis. The dredge history covered all