Changes to a rocky shore following upgrades from secondary to tertiary sewage treatment

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Changes to the impacts of effluent from a large well-treated sewage discharge on a rocky shore in southern Australia were measured after it was upgraded from secondary to tertiary treatment. Two upgrades transformed a 13 year monitoring program into two BACI (Before, After, Control, Impact) experiments. A novel method was used to map changes to the distribution of space-occupying biota on six reefs on three occasions before, and 5 occasions after the first treatment upgrade. Two reefs were near (0–0.4 km) the outfall, two reefs were within the nutrient plume of the outfall (3.6–4.6 km) and 2 reefs were distant (7.5–14 km) from the outfall. The first upgrade resulted in a slow, but marked, increase in the dominant intertidal fucoid Hormosira banksii 3.6–4.6 km from the outfall, correlated with and probably ultimately driven by a decrease in dissolved inorganic nitrogen (DIN). Hormosira recovered or was always present on reefs where DIN was ≤2.0 μM, and never recovered on reefs where DIN was ≥2.5 μM, suggesting that large-scale ecological changes, associated with the loss of Hormosira, are resisted until a threshold mean DIN concentration of ∼2.0–2.5 μM DIN is reached.