High levels of sediment and toxic contamination in Wellington Harbour, New Zealand

Problems

• High levels of sediment and toxic contamination in Wellington Harbour

  Te Whanganui-a-Tara/Wellington Harbour inlet of Cook Strait indenting southern North Island, New Zealand. The almost circular harbour. Most of its extent, the bay is one of the world’s finest natural harbours. The Hutt River enters it from the north, and to the south a deep passageway joins it with Cook Strait. 
  
  Wellington Harbour is a deep, largely subtidal estuary (floodable mouth of a river). Average harbour depth is between 10 and 30 metres, so it acts as a natural settling area and has a largely muddy bed. Tidal action largely determines water circulation and the harbour is relatively well flushed by clean seawater on each tide. 
  Wellington's coastal waters are home to marine mammals such as the common dolphin and orca. Wellington Harbour has special features including a giant kelp forest, a population of rare sponge, at least 100 different species of algae (seaweed), sea horses, crayfish and paua. 
  Every four years, data is analysed from over twenty strategically placed monitoring sites, including several control sites in the depths of the harbour. The findings look to understand the impact of human activity and subsequent sediment contamination.
  
  Two key findings are the high amounts of muddy sediments at all sites barring Evan’s Bay and high concentrations of copper, lead, and legacy concentrations of mercury at some sites.
  “This hasn’t happened overnight, rather over decades or longer” said Megan Melidonis, GW Senior Environmental Scientist, referring to the metal concentrations.
  
  “Left untouched this industrial legacy often has limited effect on the marine environment, but when disturbances shift sediment, toxic effects can be felt both within localised bottom dwelling animal communities and within the wider aquatic environment.
  
  Such a disruption of the natural food chain may deter visiting marine life such as dolphins, orcas and seabirds but may also affect important aquatic habitats including recently discovered beds of red seaweed, an important habitat in Wellington Harbour that provides refuge for an abundance of sea life including the elusive decorator crab”.
  
  Potential degradation of the harbour is not confined to industrial sites as monitoring areas further afield are vulnerable to sediment, nutrients and contaminants from rivers and paved areas.
  Being a deep basin harbour, Te Whanganui-a-Tara encourages the retention of fine sediment, while the activity of waves and wind fetch over its great expanse can shift sediment and associated contaminants from point source stormwater discharges, industrial sites, and areas focused around port activities.
  Wellington Water are required to monitor sites within Wellington Harbour as a condition of the consent granted to them to allow discharges to the marine environment.
  
  “With numerous outfalls around the Harbour, we not only need to be aware of what’s entering our harbours through stormwater but also ensuring we’re not inadvertently contributing to the deterioration of our marine and coastal environments” said Melidonis, referring also to over 700 litter items recorded in an area the length of a rugby field, during quarterly Litter Intelligence surveys at 11 sites around Te Whanganui-a-Tara coastline.
  “Common household rubbish from plastic bags through to more unusual items like bikes and scooters are occasionally found, with our Harbours patrol vessel also pulling out odd bits of discarded household items from time to time”. 
  
  Wellington Harbour is a nationally significant harbour and port environment valued for the range of ecosystem goods and services it provides. Like other coastal environments surrounded by urban areas, the harbour receives inputs of sediment, nutrients and other contaminants from the surrounding catchments. Such contaminants have the potential to adversely impact on the health and function of its ecosystem.