Harbours, estuaries and coast 2009/10
Key points:
• The ecological health of Porirua Harbour is gradually deteriorating, primarily due to sediment and nutrient inputs.
• Sedimentation is the main risk for Waikanae, Hutt and Whareama estuaries – the Whareama Estuary has very muddy and poorly oxygenated sediments that are not ideal for plants and animals.
• Initial monitoring suggests the ecological health of Lake Onoke is “moderate”, despite elevated levels of phosphorus in the lakebed’s sediments.
• Stormwater runoff from catchments where coal tar was used in road sealing may be the main source of harmful polycyclic aromatic hydrocarbons found at elevated levels in Wellington Harbour sediments.
What happened in 2009/10?
Estuary monitoring
Prolific growths of macroalgae – mainly sea lettuce (Ulva) and Enteromorpha – at the southern end of the Onepoto Arm of Porirua Harbour in January 2010. Thick growths like this reduce oxygen reaching the underlying sediment and the rotting leaves can produce large quantities of hydrogen sulphide, a foul-smelling and noxious gas.
Ecological monitoring was undertaken at one or more intertidal locations in Porirua Harbour and the Waikanae, Hutt and Whareama estuaries during January 2010. The monitoring targets key issues that threaten estuarine health such as sedimentation, eutrophication (excessive nutrient enrichment) and toxicity from heavy metals and other contaminants.
The monitoring included an assessment of sediment grain size and chemistry, mapping of intertidal algal cover, and identification of the types and numbers of sediment-dwelling fauna. In addition, sedimentation plates were buried in the estuaries. The depth of each sediment plate is measured each year, allowing us to determine the rate of sedimentation.
Collecting a sediment core from one of the monitoring sites in Porirua Harbour. At each site, sediment samples are collected for laboratory testing and the algae and animals living in and on the sediment are identified and counted.
Four intertidal sites were sampled in Porirua Harbour: two in the Onepoto Arm and two in the Pauatahanui Arm. The results of some indicators differ between sites, with sedimentation rates and nuisance algal cover highest in the upper reaches of both arms. Overall, the dominant intertidal habitat in Porirua Harbour is considered to be in a “moderate condition”. However, after three years of annual monitoring the sites show signs – such as increasing mud content and declining oxygen levels in the sediment – that suggest the harbour may be on the border-line of a shift towards excessive muddiness and nutrient enrichment. Ongoing management and monitoring of sediment and nutrients entering the harbour from residential and roading development, urban stormwater and agricultural runoff is required.
The 2009/10 sampling in the Waikanae, Hutt and Whareama estuaries found low levels of nutrients and contaminants in the intertidal sediments. However, sedimentation appears to be an issue in all three estuaries. In the Whareama Estuary, based on two years of measurements, the average sedimentation rate is 6–7 mm/year. The estuary’s sediments also have a high mud content, which translates to a poorly oxygenated habitat suited to few invertebrate species.
Lake Onoke monitoring
In August 2009 Greater Wellington started monitoring water quality in Lake Onoke on the South Wairarapa coast. An initial assessment in September 2007 found the lake was in a relatively poor condition and at risk from further degradation from a range of factors, including high inputs of nutrients, sediment and pathogens from land use intensification. With the Ruamahanga River discharging into the lake at its northeastern end, Lake Onoke is the receiving coastal water body for all nutrients, sediments and contaminants discharged to water in the Wairarapa Valley.
As well as the collection of monthly water samples, in January 2010 we collected samples of sediment and of animals living in and on the lake bottom from two locations. One sampling site was intertidal (exposed at low tide) and near the centre of the lake. The sediments at this site were predominantly sandy and looked to be well oxygenated – favourable conditions for most bottom-dwelling animals. In all, eight different species were found, with the crustacean Paracorophium excavatum the most abundant.
The second sampling site was subtidal and near the western margin of the lake. The sediments at this site were extremely muddy and anoxic (low oxygen), resulting in fewer species of bottom-dwelling animals being present at this site than at the intertidal site.
Chemical analyses revealed low levels of organic carbon and metals in the sediments at the intertidal site, but nitrogen and phosphorus levels were slightly and moderately elevated respectively, and DDT was detected at low levels in two samples. Slightly higher levels of nutrients and some metals were found in sediments at the subtidal site as compared with the intertidal site, but still well below national sediment quality guidelines.
Collecting sediment samples from the intertidal sampling site (left) and one of the sediment cores collected from a subtidal location in Lake Onoke (right). The sediments were black near the surface, indicating oxygen levels are low. Low oxygen conditions are not ideal for most sediment dwelling animals
Staff using an echosounder (sonar) during a bathymetric survey in Lake Wairarapa in February. Lake Onoke was also surveyed. A bathymetric survey enables us to measure and map the amount of sediment that has built up over time on the lake bed.
Wellington Harbour – organic contaminants
In early 2009/10 we received the results of further laboratory testing of sediment samples collected from Wellington Harbour in late 2006. These tests sought to help identify the potential source(s) of polycyclic aromatic hydrocarbons (PAHs) – contaminants derived from the combustion of organic matter – found at elevated levels in the sediments of Lambton Basin and Evans Bay. The results, while not conclusive, suggest that the principal source may be stormwater runoff from catchments where coal tar was used in the past as a binder for sealing roads. Further testing is required to confirm this and may be considered in 2011 when a second round of sediment contaminant and ecological monitoring is undertaken in Wellington Harbour.
What is Greater Wellington doing?
• Regularly monitoring microbiological water quality at 74 coastal sites (see the Recreational water quality report card) and nutrient levels in Lake Onoke.
• Monitoring sediment quality and ecological health in sensitive estuarine and harbour environments.
• Working with Porirua City Council, Wellington City Council and a range of other organisations and community groups to develop 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: Only water should go down stormwater drains because they lead directly to streams and coastal waters. Clean paint brushes in the laundry sink, wash your car in a car wash or use the lawn if you can, and take household chemicals and waste oil to collection points at the landfill.
• Join a coastal care group and get planting – see www.gw.govt.nz/takecare. Coastal plantings can help protect the shoreline from erosion and filter out sediment.
More information
The information on this card is a summary of the 2009/10 annual coastal monitoring report, which is available at www.gw.govt.nz/envreports
If you would like to know more about the health of our coastal environments, visit our website or contact us:
Phone: 04 384 5708 (Wellington office) or 06 378 2484 (Masterton office)
Email: environmentalscience@gw.govt.nz