State of the Gulf report (2020)

Sedimentation

Sediment is ranked the 3rd highest threat (equal with bottom trawling) to Aotearoa’s marine habitats (after ocean acidification and global warming). (MacDiarmid et al, 2010). It is a serious pollutant that degrades our coastal habitats and smothers marine life. Land activities, such as forestry, farming, mining, draining of wetlands and urban development, have greatly increased the amount of sediment that enters our waterways and harbours. Sedimentation rates in the Waikato over the past 100 years were around 100 times those of pre-human times.

This has led to major changes in our coastal marine communities, such as the widespread loss of shellfish beds. High levels of suspended sediments prevent life-supporting light from reaching seaweeds and seagrass, damages the gills of fish, and stops filter-feeders such as shellfish and sponges from feeding efficiently. This sediment eventually settles on the seafloor, where it smothers marine life, resulting in the loss of mud-sensitive species such as tuangi and pipi, and the increase in mud-tolerant worms. 

Suspended sediment in our waters, the muddiness of our estuaries, and the health of intertidal animal communities around the Marine Park are monitored by Councils. Site health is graded from ‘extremely good’ to ‘unhealthy with low resilience’ depending on the number and type of animals present, as different communities are present on sandy shores versus muddy shores. 

[Project note, The Waiheke Island coast is not currently monitored.] 

Pollution

Toxic Chemicals - Human activities generate a variety of toxic heavy metals and other compounds that are used in the coastal environment (e.g., anti-fouling paints, oils and fuels), or which enter coastal waters through spills, run-off and discharges. The main causes of contamination in the Marine Park are urbanisation, historical mine activity, and agriculture. Major spills sometimes have immediate and catastrophic effects. These tend to be obvious and localised. Contaminant loads from individual sources such as port, industrial and mine activities, marinas and landfills, can also be very high and persistent, causing localised impacts.

But many contaminants come from small sources scattered throughout catchments, which combine to produce large loads that affect broad areas in harbours and estuaries.  Contaminants commonly bind to sediments and other particles, which settle out and accumulate on the seabed. Elevated contaminant concentrations in coastal sediments affect the survival, reproduction and/or behaviour of animals that live on the seabed, and may cause flow-on effects on other parts of the ecosystem, natural character or amenity values. Māori are particularly concerned about effects on the mauri of the coastal areas, and the health, abundance and safety of kai moana for consumption.

Primary contaminants of concern are the heavy metals, copper and zinc, with lead and mercury of secondary concern. Other contaminants may also accumulate, including new contaminants that are constantly emerging. Environmental scientists are struggling to keep pace with the rapidly increasing list.

Nutrients - these sustain the growth of microscopic algae, seaweeds and the other marine plants that form the base of the ocean food chain. In the Hauraki Gulf / Tīkapa Moana / Te Moananui-ā-Toi, nutrients come from the surrounding ocean, are recycled from the seabed, and washed in from the land. We increase nutrient loads to the Marine Park through our wastewater discharges, use of fertilisers, and the effluent produced by our livestock. Fish farming may be a significant source of nutrients in the future. 
Slight increases in nutrients can promote healthy plant growth. The extra energy and matter produced flows up the food chain, supporting greater numbers of fish, birds and other sea creatures. But problems occur if nutrient levels get too high. Then, microalgae and nuisance seaweed blooms can occur. Microalgae blooms reduce water clarity and light levels, which can stunt the growth of seaweed and seagrass. When the microalgae or seaweed dies, decomposing bacteria can reduce oxygen levels in the water to harmful levels. On the shore, rotting seaweed can create an unsightly, smelly mess.  Nitrogen is generally considered to have the greatest effect on marine water quality, but phosphorus is also a key nutrient of concerns. By far, the largest source of nutrients produced through our activities are the rivers draining the Hauraki Plains. Estimates from 2016 indicate that Waihōu and Piako rivers contribute around 97% of the nutrient load to the southern Firth of Thames, with agricultural sources (particularly dairy farming) estimated to account for 73% of the nitrogen and 41% of the phosphorus loads.