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 Foundation for Research, Science and Technology post-doctorate fellow Karyne Rogers has shown that the gut
and tissue of shellfish was affected by sewage for some considerable time afterward the closure.
A new treatment plant was commissioned at Moa Point last year. Dr 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.
The project has profited by the use of a newly installed mass spectrometer at Dr 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," Dr Rogers says.
However, she says the diversity of seaweed is returning to normal levels.
Before closure of the outfall, the biological diversity was fairly low at Moa Point, with the main biota present as
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," Dr 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.