Effects of secondarily treated sewage effluent on intertidal macroalgal recruitment processes

Abstract

Effluent is discharged below the low-water mark at Boags Rocks, Victoria, Australia, at an average rate of 437 × 106 L day–1. Three study sites following a gradient of pollution from high (at Boags Rocks) to intermediate (Cape Schanck) to unpolluted (Cheviot Beach) were chosen for the main experiments. Surveys of the algal assemblages were conducted in spring and summer and showed the absence of the pre-discharge dominant Hormosira banksii at Boags Rocks and an abundance of turf-forming and ephemeral species at the two polluted sites. There was no evidence that the treated sewage detrimentally affected either the availability of propagules (asexual spores, gametes, zygotes or fragments) or macroalgal recruitment to artificial or natural substrata. Opportunistic genera such as UIva and Enteromorpha showed very high recruitment and propagule densities in the water column at polluted sites, apparently benefiting from the increased nutrient loads. Investigation of the number of H. banksii zygotes in water samples from various habitats showed very limited dispersibility for this species. The small dispersal shadow of H. banksii, combined with the environmental pressures placed on establishing zygotes, would severely limit the re-establishment of this species at polluted sites, even if given suitable conditions.