Sewage and the biota on seashores: Assessment of impact in relation to natural variability.

The three largest sewage outfalls serving Sydney, New South Wales, Australia are to be moved several kilometres offshore in 1991-2. Changes in their environmental impact due to these moves are likely, but detection relies on a good understanding of the present impacts on the natural environment. Biological surveys in 1986-8 of rocky intertidal sites located directly at the present cliff outfalls have been compared to several (≥2) other shores some distance away (which serve as controls). This technique of multiple control sites is recommended as a way of including natural variability in the data set. By comparing the community structure at the point of impact to the range among control sites, it is theoretically possible to be more sure of whether purported impacts represent a response out of the ordinary. Effects detected in this way include gross reductions in species diversity, with complete dominace of rocky shore biota by ephemeral green algae and the absence of animals. Zonation disappears close to outfalls. These striking effects were, however, very localised. Denudation experiments suggested great recruitment and productivity of these algae, but the community as a whole is held at an early stage of succession. Such clear alterations attributable to sewage pollution suggests that this ecosystem would be a good indicator of any changes occurring once outfalls are moved offshore. Continued monitoring including multiple control sites is recommended.