Te Awarua-o-Porirua Harbour subtidal sediment quality monitoring Technical Report
- Author
- Oliver, M.D., Conwell, C.
- Year
- 2010
- Journal / Source
- Greater Wellington Regional Council, Publication No. GW/ESCI-T- 14/110.
- Keywords
- Porirua Harbour, Onepoto Arm, Pauatahanui Inlet, benthic ecology, sediment quality, contaminants, heavy metals, environmental monitoring, Wellington region, coast, subtidal
- Summary
- Contaminants in urban stormwater discharges have been identified as a potential medium to long-term risk to the health of the marine organisms living in our harbours, largely through the accumulation of these contaminants in the subtidal sediments. This report presents the results of the fourth survey of sediment quality and benthic community health at five subtidal sites in Te Awarua-o-Porirua Harbour (Porirua Harbour). These sites were sampled on 9 November and 2 December 2010. Consistent with the results of the 2004, 2005 and 2008 surveys, concentrations of total copper, lead and zinc exceed nationally recognised ‘early warning’ (ie, ARC ERC1 or ANZECC2 ISQG-Low) sediment quality guidelines in the subtidal sediments of the Onepoto Arm of Porirua Harbour. Mercury concentrations are approaching guideline trigger levels, but are otherwise, along with the other five metals analysed, currently below guideline levels in both the Onepoto and Pauatahanui arms of the harbour. Fifty-eight species of benthic fauna were identified in the samples collected from Porirua Harbour in 2010. Of these, 54 species were found in samples taken from the Pauatahanui Arm and 29 species were identified in samples collected from the Onepoto Arm. Polychaete worms (27 species), crustaceans (14 species) and bivalve molluscs (7 species) were the most abundant groups amongst the fauna collected. Five species were found that had not been recorded in any of the previous surveys. The biomass at sites in the Onepoto Arm was dominated by the shrimp-like tanaidaceans and the bivalve Cyclomactra ovata. The bivalves Cyclomactra ovate and Linucula hartvigiana, and the sea cucumber, Paracaudina chilensis were dominant members of the biomass at Pauatahanui Arm sites. Our analyses indicate that the combination of higher heavy metal, mud and organic content is influencing the structure or ‘health’ of the benthic invertebrate communities at some sites within the harbour. We are unable to isolate which of these variables are driving the changes though we can conclude that sites in the Pauatahanui Arm are of higher ‘environmental quality’ and support more diverse invertebrate communities. After having now completed four surveys of subtidal sediment quality and in the absence of any substantial environmental changes, a comprehensive review of the Porirua Harbour subtidal sediment quality monitoring programme is recommended. This review should take into account the specific aims of the programme, survey frequency, contaminants to be sampled and the key issues identified from Greater Wellington Regional Council’s intertidal monitoring programme of Porirua Harbour, particularly the elevated rates of sedimentation, declining levels of sediment oxygenation and increasing sediment mud content.