Harbours, estuaries and beaches 2008/09

Key points:

• Concentrations of heavy metals, particularly zinc, remain elevated in sediments from the subtidal basin of the Onepoto Arm of Porirua Harbour.

• Porirua Harbour is moderately eutrophic (nutrient rich), with the January 2009 survey showing nuisance algae covering more than 50 per cent of some of the ‘mudflat’ areas exposed at low tide.

• There is clear evidence of stormwater-derived contamination in the sediments of Porirua Harbour, and in the sediments in the beds of streams that discharge into the harbour.

• The ecological condition of the Whareama Estuary is “fair” to “good” – the very muddy and poorly oxygenated sediments are not ideal for plants and animals.

• Castlepoint Beach is in good ecological condition.

What happened in 2008/09?

Porirua Harbour sediment quality and ecological health monitoring

In late 2008 we carried out our third survey of sediment quality in Porirua Harbour. Samples of harbour floor sediment and benthic fauna (animals that live on or in the sediment) were collected from five subtidal sites (always covered by water) – three in the Pauatahanui Arm and two in the Onepoto Arm.

Results of heavy metal tests on the sediment samples mirrored those from earlier surveys in 2004 and 2005 – concentrations of copper, lead and zinc in subtidal sediments of the Onepoto Arm remain above some “alert” or “early warning” guidelines. Concentrations of other metals are currently below guideline levels in this arm, as are the concentrations of all metals in the subtidal sediments of the Pauatahanui Arm. As well as higher sediment metal concentrations, the Onepoto Arm monitoring sites are muddier and more enriched, contributing to a lower diversity of benthic fauna than that found at Pauatahanui Arm sites.

 

 

 

 

 

 

 

 

 

 

 

In January, ecological surveys were undertaken at two intertidal sites (areas exposed at low tide) in each arm of Porirua Harbour. This was the second of a proposed series of three to four annual assessments to establish an ecological “baseline”. The intertidal monitoring programme is broader in scope than the subtidal monitoring – as well as sediment toxicity, it investigates sedimentation and nutrient enrichment (eutrophication), key issues in estuarine environments.

This year’s results indicate heavy metal toxicity is not an issue at any of the intertidal monitoring sites and, after one year, sedimentation rates at most sites are low. However, similar to last year, sediment nutrient concentrations and the depth of the oxygenated surface sediment layer indicate that both arms of the harbour are moderately enriched. One sign of this is the presence of nuisance growths of sea lettuce and other algae. Around one third of the intertidal habitat in the Onepoto Arm and one tenth of the intertidal habitat in the Pautahanui Arm had greater than 50 per cent coverage of algae, resulting in localised nuisance conditions (rotting algae and poorly oxygenated and sulphide-rich sediments).

Porirua Harbour sediment “hotspot” assessment

In February, together with Porirua City Council, we carried out some additional testing of contaminants in Porirua Harbour sediments, targeting intertidal areas close to contaminant sources such as stormwater outfalls and stream mouths. The sampling focused mainly on the surface sediments from the southernmost end of the Onepoto Arm, in the area between the Porirua Stream channel and several large stormwater outfalls draining parts of Porirua City. Sediment samples were also taken near the mouths of Onepoto Stream, Browns Stream and Duck Creek and from the beds of the lower reaches of Porirua, Kenepuru and Onepoto streams.

The sampling results show clear evidence of stormwater-derived contamination in parts of Porirua Harbour and in the sediments from the beds of streams that discharge into the harbour.

We found:

• Most samples exceeded sediment quality guidelines for one or more persistent contaminants, especially zinc (all samples from sites in the Onepoto Arm) and the pesticide DDT (all intertidal sediment and streambed samples).

• Concentrations of lead and polycyclic aromatic hydrocarbons were above guideline values in some sediment samples taken from near the Semple Street stormwater outfall and the Onepoto Stream, and intertidal sediments beside the Porirua Rowing Club and the mouth of Browns Stream.

In most cases, sediment contaminant concentrations only exceeded “early warning” guidelines. This indicates that there is an opportunity to implement actions to limit the extent of degradation and prevent adverse environmental effects from occurring.

 

 

 

 

 

Whareama Estuary and Castlepoint Beach monitoring

Ecological assessments of Whareama Estuary and Castlepoint Beach on Wairarapa’s east coast were carried out for a second time in January. Based on the National Estuary Monitoring Protocol, they included an assessment of sediment grain size and chemistry, and sediment dwelling plants and animals. The key findings were similar to those from last year’s initial surveys:

• Whareama Estuary: overall the intertidal habitat is generally in “fair” to “good” condition despite measurements from sediment plates buried in the estuary during the 2008 survey revealing a high sedimentation rate (average 14.5 mm over a year). Excessive inputs of sediment are largely a natural phenomenon given the erosion-prone mudstone soils in the catchment. While the very muddy and poorly oxygenated sediments are a concern (they create poor conditions for plants and animals), there was a slight improvement in the diversity of the estuary’s biological community over the year.

• Castlepoint Beach: the intertidal habitat is generally in “good” condition. The beach sediments consist of well-oxygenated sands and support animals commonly found in exposed low-nutrient beach environments, such as crustaceans and beetles.

What is Greater Wellington doing?

• Regularly monitoring microbiological water quality at 77 coastal sites (see the Recreational water quality report card) and designing a water quality monitoring programme for Lake Onoke, a shallow brackish lagoon on Wairarapa’s south coast.

• Monitoring sediment quality and ecological health in sensitive estuarine and harbour environments, particularly those likely to be affected by urban stormwater.

• Assisting with funding Victoria University postgraduate research comparing the biodiversity values associated with restored and non-restored coastal dune systems in the Wellington region.

• Working closely with the Porirua City Council and other stakeholders to put in place an action plan by June 2011 to address various environmental issues in Porirua Harbour and its catchment.

What can you do?

• Save the drain for rain: Rubbish, paint, oil or any other waste that enters the stormwater drain in the gutter outside your house go directly to streams or the coast. Paints (oil and water-based) and thinners are toxic to aquatic life, and discolour streams and coastal water.

• Secure your recycling and put your rubbish in the bin: On windy days, if not properly secured, recycling and rubbish left for local council collection can be blown into the coastal environment. This spoils the use of our beaches for recreation and may harm bird and marine life.

More information

The information on this card is a summary of the more detailed 2008/09 annual coastal monitoring report.

If you would like to know more about the coast, contact:

The Team Leader, Environmental Science
Phone: 04 384 5708 (Wellington office)
E-mail: environmentalscience@gw.govt.nz