Benthos associated with two New Zealand coastal outfalls

Abstract

The benthic ecological impacts of 2 New Zealand ocean outfalls were studied. They are the country's largest (discharging 0.8 m³ s^‐1) at Hastings, and longest‐established (operating since 1965 and with a present discharge of 0.28 m³ s^‐1) at Gisbome. Both discharge essentially biodegradable domestic and food‐processing‐industry wastes. Samples were collected about each outfall and analysed for particle size, oil and grease, readily oxidisable carbon (ROC), total Kjeldahl nitrogen (TKN), volatile solids, and macrofauna. Changes in the sediments indicated that the effluents contained inorganic paniculate material in the mud/very fine sand size range. Peaks in concentrations of oil and grease, ROC, TKN, and volatile solids occurred at the diffusers. Numbers of taxa were low near the outfalls, but increased with distance away. Numbers of individuals were also low at the outfalls, increasing within 100 m, before dropping to normal levels. No grossly polluted zone, devoid of macrofauna, was found; however, it appeared that a “polluted” zone occurred within 200 m of the diffusers and a “transitional” zone extended to between 800 and 1600 m from the Gisborne outfall, and 400 and 1600 m at Hastings. Multivariate classification and ordination indicated that over the area sampled, the discharges were the main factor shaping the patterns of community structure.

Key words:

• outfalls
• sewage effluent
• pollution
• benthos
• sediments
• Hawke Bay
• Poverty Bay