EFFECTS OF SECONDARILY TREATED SEWAGE EFFLUENT ON INTERTIDAL MACROALGAL RECRUITMENT PROCESSES

Effluent is discharged below the low-water mark at Boags Rocks, Victor ia, Australia, at an average rate of 437 x 10(6) L day(-1). Three stud y sites following a gradient of pollution from high (at Boags Rocks) t o intermediate (Cape Schanck) to unpolluted (Cheviot Beach) were chose n for the main experiments. Surveys of the algal assemblages were cond ucted in spring and summer and showed the absence of the pre-discharge dominant Hormosira banksii at Boags Rocks and an abundance of turf-fo rming and ephemeral species at the two polluted sites. There was no ev idence that the treated sewage detrimentally affected either the avail ability of propagules (asexual spores, gametes, zygotes or fragments) or macroalgal recruitment to artificial or natural substrata. Opportun istic genera such as Ulva and Enteromorpha showed very high recruitmen t and propagule densities in the water column at polluted sites, appar ently benefiting from the increased nutrient loads. Investigation of t he number of H. banksii zygotes in water samples from various habitats showed very limited dispersibility for this species. The small disper sal shadow of H. banksii, combined with the environmental pressures pl aced on establishing zygotes, would severely limit the re-establishmen t of this species at polluted sites, even if given suitable conditions .