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Slow Recovery at Moa Point

Monitoring of the marine environment at Moa Point, on Wellington's south coast, has shown that recovery after the closure of the 100-year-old sewerage outfall in June 1998 took longer than initially thought.

A study by FRST post-doctorate fellow Dr Karyne Rogers has shown that the gut and tissue of shellfish was affected by sewage for some considerable time after the closure. Rogers, an isotope geochemist, has been monitoring the site to investigate the extent of sewage pollution of flora and fauna, both before and after the outfall was closed following the commissioning of a new treatment plant at the point last year.

The project has profited by the use of a newly installed mass spectrometer at Rogers' host organisation, the Institute of Geological and Nuclear Sciences, in Lower Hutt. This machine measures the extent of sewage in biota (flora and fauna) from the sewage-contaminated environment, and compares it with data from nearby non-polluted sites. The mass spectrometer analyses stable carbon and nitrogen isotope signatures of the flora and fauna.

"Monitoring of Moa Point after the outfall closure showed that while the water cleared, the sewage isotopic signature was still present in the gut and tissue of the shellfish, suggesting that recovery would take longer than initially anticipated," Rogers says.

However, she says the diversity of seaweed is returning to normal levels.

Before closure of the outfall, biological diversity was fairly low at Moa Point, represented mostly by limpets, blue mussels and ulva seaweed. Ulva is a green seaweed typically found in areas with high nitrogen input. Immediately after closure, the ulva levels declined rapidly as the large limpet population chewed through the available ulva supply, which was not renewed as quickly because of the lack of nutrients from the sewage.

"Today, biological diversity of seaweed is returning, and the balance in the food chain has stabilised," Rogers says. "The isotopic signatures of the limpets and mussels has returned to similar levels of the control sites."

However, she says the extent of other pollutants associated with sewage sites, such as heavy metals, has not been investigated, and therefore Moa Point might not be fully recovered.