Environmental effects

The amount of pollutants that are transferred from land into the oceans, and the effect that they will have on coastal marine ecosystems, varies depending not only on the amount of waste actually released, and the type of waste, but also on the characteristics of the marine environment affected. As one would expect, transfer rates are much worse at times of more extreme wet weather, when more water is moving from the land to the sea. 

Whether or not land-sourced pollutants remain in the coastal marine area depends on a number of factors, in particular the geographical makeup of the area and the winds and currents experienced by the locality. In harbours and estuaries, the amount of mixing that occurs and the time the inflowing water is retained in the area, are important. Shallow inlets are more prone to retain sediment and pollutants than deeper ones, and estuaries and narrow inlets are more prone to retaining material than wide exposed beaches, which are subject to stronger currents that can carry inflowing substances out to sea. Many river estuaries have natural sea barriers broken only in high flows. Sediment and pollution retention behind these barriers is high.

New Zealand’s particular geographic makeup means that there are large stretches of coastline that do tend to trap sediment and pollutants. As the Ministry for the Environment’s 1997 State of the Environment report notes: “Much of the coastline is made up of river-fed estuaries whose wide, shallow waters are permanently protected from ocean waves by sand or shingle bars or offshore islands” 2990  Where there are high river flows, the fresh water may pass along an open coast, floating over seawater for some distance before it is mixed with the seawater. 

Cumulative impacts

Cumulative effects are referred to in the definition of “effect” in section 3 of the RMA as effects that can build up over time or occur in combination with other effects. The term refers to the impact of a number of contaminants or pollutants on a particular ecological system, which in some cases may be greater than the sum of their individual impact. It also incorporates the concept of flow-on impacts. Pollutants are dangerous due to their cumulative effect, as chemicals tend to have poor solubility, their effects are long term and they can travel long distances regionally. 2991  Toxic chemicals can enter marine food chains, and can accumulate to high levels in top predators. Changes in the marine environment from land-based contaminants can be the result of a single large event, or many small ongoing events. These are often unnoticed and occur over a long time.


An increase in the rate that sediment is transferred into the receiving environment can bring about profound changes in the marine ecosystem. An excess of sediment can reduce water depths, water coverage and change the type of sediment found on the sea floor (such as the size of the particles). In addition, when excess sediment enters the receiving environment it can become suspended in the water, reducing light levels and the ability of marine creatures to feed, and affecting the clarity of the water and its turbidity (cloudiness of a fluid generally invisible to the naked eye). Research being undertaken by NIWA, suggests that severe impacts can result from fine sediment being suspended in the water column, because of the way the particles interfere with light penetration of the water (or light attenuation). 2992  This can lead to impacts such as the inability of visual predators to see prey.

Sediment can bring about changes in vegetation, be detrimental to fish habitats, degrade spawning areas and deplete invertebrate populations. These impacts can flow through the food web, particularly if a particular life stage of a species is affected. As most sediment is deposited in intertidal zones, it is the species that live in these areas that are often most affected by excess sedimentation. 2993  This is particularly significant, because healthy intertidal zones support a particularly high diversity of species in comparison with other parts of the coastal area. For example, the Kaipara Harbour is the biggest estuary in the Southern Hemisphere, and where most of the snapper on the west coast of the North Island originate. Sediment from the catchment threatens to overwhelm horse mussel beds and seagrass meadows, which are nursery grounds for snapper. 2994

Some of the main environmental impacts from sediment suspended in the water or settling on the seafloor are identified in the figure below.




  • Decreases oxygen concentrations and changes sediment properties such as grain size, chlorophyll a and organic matter content 2999
  • Clogs the gills of fish, making it harder for them to breathe
  • Interferes with food intake and potentially affects growth and condition of fish and shellfish
  • Reduces visibility, making it harder for some fish to find food
  • Negatively affects microphytes or benthic primary producers, an important food source for many macrofaunal species
  • Smothers shellfish beds and cause cockles, pipi, mussels and scallops to die
  • Causes seafloor plants to die off
  • Reduces the hard surface area for kelp spores or baby paua to attach to


  • Reduces the food that is available for rocky reef ecosystems and larger animals
  • Reduces the growth and extent of kelp forests
  • Damages the entire productivity of the rocky reef ecosystem
  • Damages sensitive habitats that are important for part of a species life-stage, such as nursery or juvenile areas
  • Reduces the abundance of many biologically important fish species
  • Reduces the stocks of recreationally and commercially important fisheries species (such as paua and crayfish)

Tasman Bay in the South Island regularly suffers from murky seabed conditions which are believed to have come from sediment carried into the marine area by floods of the Motueka River, which then gets re-suspended in the water by tidal currents in the bay. In addition, on-going trawling and dredging re-suspends the sediment. Research has found that scallops on the seafloor temporarily stop feeding when in highly turbid water, whereas those in clearer water in the same vicinity continue to feed. In addition, the growth of scallops was shown to be adversely affected by high levels of suspended sediments over the long term. 3001

There are now concerns that coral-like bryozoan colonies further north of the Motueka River, which are important snapper and terakihi nursery habitats, are under increasing stress from the sedimentation. With the seabed in the area becoming largely covered in sediment, there are few hard surfaces for the bryozoans to settle on. Ultimately this is reducing the overall productivity of the tarakihi and snapper fisheries as well as decreasing the area’s biodiversity. 3002

Heavy metals and other dissolved pollutants

Many heavy metals and other pollutants find their way into the marine area, from roads, from industrial and agricultural activity, and from domestic households. Such toxic substances have also been used directly in the marine environment. In the 1950s, ‘booster’ agents such as DDT (dichloro-diphenyl-trichloroethanecan), organomercurial, organolead and arsenical compounds were used against antifouling organisms.

In particular, heavy metals, chemicals, hormones, dioxins, organochlorines, PCBs and DDT cause problems. The problem may not just be mortality of marine organisms, but disruption of growth, or reproduction, which may occur at much lower than lethal concentrations. Only a small proportion of heavy metal and pollutant contaminants are dissolved in the water column; the majority remain attached to particles of sediment, which settle in the inner parts of harbours and estuaries.  In these areas, where wave and current movements are likely to be too weak to move the polluted sediments around, they can build up to toxic levels. 3003

Scientific studies since the 1950s have shown that a high body burden of organochlorine pesticides (such as DDT) in animals can cause ecological damage, including abnormalities in the reproduction and development of fish-eating birds. Concerns have also been voiced about the impact that such chemicals are having on some marine mammals, including that they may be damaging their health, affecting their fertility, reproduction, immune system and growth. 3004

DDT was used in the 1950s and 1960s to control agricultural pests. During this period, it was applied over wide areas of land, often be being mixed into fertilisers and top-dressed onto pastures. The use of DDT in New Zealand was restricted in the 1970s and banned in 1989. Today, the residual DDT concentrations in the country’s pasture soils are relatively low. 3006 However, there is increasing evidence that DDT from the soils is still washing into the marine environment and having a negative impact on the marine ecosystem. The near-shore habitat of New Zealand’s threatened Hector’s dolphins has exposed this species to a variety of pollutants and contaminants such as organochlorines and heavy metals. Hector’s dolphin tissue has been found to contain high levels of organochlorines such as DDT, PCBs and dioxins. 3007  The DDT levels detected in Hector’s dolphins are very high on a global scale and second only to the highly contaminated Canadian beluga whales. 3008

Excess nutrients - nitrogen-nitrate, phosphorus, ammonia

Nutrients, in particular nitrogen and phosphorus, occur naturally in aquatic ecosystems and are essential elements for growth. This is why they are used in large quantities in fertilizer to promote grass growth on agricultural pasture. However, where excess amounts of them reach the catchment from man-made sources, they can upset the balance of the ecosystem and cause serious problems.

When there are excess nutrients in the water, they serve to fertilise microscopic algae, leading to abnormal increases in their growth and subsequent decay. Their decay stimulates the aerobic (oxygen consuming) bacteria, which in turn causes severe reductions in water quality and oxygen levels in the water, potentially leading to the deaths of fish and other marine life. When the plants decay, the nutrients are released back into the water, stimulating algal growth and allowing the cycle to start again.  This phenomenon is known as eutrophication. The consequent reduction in oxygen levels can result in ‘dead zones’, such as occur at Chesapeake Bay in the USA during mid-summer periods. 3009

The Firth of Thames’ nitrogen overload

In 2012 the Waikato Regional Council published a report into the water quality of 17 rivers in the Hauraki region using data gathered between 2000 and 2009. This showed that the Piako, Waitoa and Waihou rivers were in poor to intermediate condition, being oxygen-depleted and murky. The report showed these rivers had concentrations of nitrogen and phosphorus five to seven times above guidelines. 3011  As a consequence, the Firth of Thames is considered to be probably more productive today than it was in previous times, but water quality (in terms of nutrient levels, colour and clarity) is likely to have diminished. 3012  More recently long-term monitoring has shown evidence of oxygen depletion as well as nutrient runoff-driven acidification of the seawater in the Firth of Thames. 3013

The 2014 State of the Hauraki Gulf Report confirmed that riverine inputs of nitrogen to the Firth of Thames now exceed loads from oceanic sources, and that the nitrogen loads entering the Firth of Thames from Hauraki Plains rivers are significant and increasing. The report noted that this is consistent with the plains having one of the highest stocking rates for cows in the country and that a recent study on changing land use suggests nitrogen loads will continue to increase to 2020. There is concerned voiced in this report that long term studies indicate nitrogen inputs cause “seasonal elevation in carbon dioxide, corresponding sags in oxygen and increasing acidification”. 3014

Bacteria and viruses

Animal and human faecal matter contains large numbers of bacteria, which are produced by the gut.  Although many are harmless, some of these bacteria can cause serious health problems in humans if they reach water used for recreation and shellfish gathering. Shellfish contaminated with sewage may contain pathogens such as norovirus, hepatitis A, ShigellaVibrio and Salmonella. These can cause dysentery, gastro-enteritis or other illnesses within a few hours or days. 3015

Coliform bacteria are common in human and animal faecal waste – but can come from other sources. The amount of coliforms per hundred millilitres of water is used to indicate the likely presence of faecal matter in water. A more precise test for animal and human waste is that for a particular coliform, Escherichia coli, which comes from warm-blooded animals alone. Viruses can also be found in human and animal faecal matter and the main class is the enteric viruses, which cause illnesses such as polio, meningitis and hepatitis. 

Where levels of viruses or bacteria in coastal waters are relatively high, they can pose a risk to swimmers and other contact water users. They can also concentrate in shellfish, which eat by filtering food through the water, and can therefore be dangerous to humans who consume the shellfish. 

Organic material

Agricultural run-off, urban wastewater and industrial waste transfer organic material to the catchment. This can include faeces from humans and animals; proteins, vegetables and sugars from food preparation; and cleaning soaps. When effluent enters a healthy aquatic environment where there is plenty of oxygen dissolved in the water, aerobic (oxygen using) bacteria, which occur naturally in the water, will eat the organic material, using up oxygen in the water to do so. Consequently their numbers grow beyond what would normally be expected in a healthy aquatic ecosystem.

This use of oxygen can have very serious effects on the health of the ecosystem, as fish and plants need oxygen levels to be maintained in order to survive. Even an occasional reduction of the oxygen level can have the effect of removing the marine species that rely on a continuous supply at a high level. If all the oxygen is used up, anaerobic bacteria (non-oxygen-using bacteria) will take over, decomposing the waste material and producing gases such as methane hydrogen sulphide and carbon dioxide, making the water become toxic. 3016

Thus, the potential harm caused by organic waste can be measured by its potential to remove oxygen from the water. There is a standard measure known as the biochemical oxygen demand which is the amount of dissolved oxygen needed by aerobic bacteria to consume the waste (over a 5 day period where the water temperature is 20 degrees centigrade, measured in terms of the number of grams of organic material per cubic metre). Thus, if waste deposited in the catchment has a biochemical oxygen demand loading that is too high, too much oxygen will be removed from the water by the aerobic bacteria, jeopardising plant and animal life. 

The surface sediment of the seabed is an important ecological zone and the home of some specialised forms of life. It is normally oxygenated. Excess sediments and low oxygen in the water above can cause it to become anoxic and unable to support its normal inhabitants.

  1. Ministry for the Environment, 1997, 27

  2. http://www.niwa.co.nz/freshwater-and-estuaries/freshwater-and-estuaries-update/freshwater-update-55-october-2012/multiple-effects-of-fine-suspended

  3. Hauraki Gulf  Forum, 2004, 67

  4. Morrison M A, et al., 2009

  5. http://www.aucklandcity.govt.nz/council/documents/technicalpublications/TP211%20Effects%20of%20suspended%20sediment%20concentrations%20on%20suspension%20and%20deposit%20feeding%20marine%20macrofauna.pdf

  6. http://www.fish.govt.nz/en-nz/Environmental/Land-based+Effects+on+Fisheries+and+Ecosystems/Suspended+Sediment+and+Sedimentation.htm

  7. http://www.fish.govt.nz/en-nz/Environmental/Land-based+Effects+on+Fisheries+and+Ecosystems/Suspended+Sediment+and+Sedimentation.htm

  8. Hauraki Gulf Forum, 2004, 57

  9. https://www.mfe.govt.nz/issues/managing-environmental-risks/hazardous/about/organochlorines/what-are.html

  10. https://www.mfe.govt.nz/laws/trade/china/documents/pops-in-pastoral-farming-and-horticulture-part1.pdf

  11. http://www.fish.govt.nz/NR/rdonlyres/2C2AB64F-ABA8-4F6B-954F-BFE76C102C38/0/hectors_discussion_document_april_07.pdf

  12. http://www.fish.govt.nz/NR/rdonlyres/2C2AB64F-ABA8-4F6B-954F-BFE76C102C38/0/hectors_discussion_document_april_07.pdf

  13. http://resources.ccc.govt.nz/files/Water_2166_DissolvedInorganicNitrogenInEstuary-docs.pdf

  14. http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10801492

  15. Hauraki Gulf Forum, 2011

  16. http://www.seachange.org.nz/PageFiles/144/John%20Zeldis%20-%20Sea%20Change%20presentation%20to%20SWG3.pdf

  17. http://www.aucklandcouncil.govt.nz/EN/environmentwaste/naturalenvironment/Pages/stateofthehaurakigulf.aspx

  18. http://www.foodsmart.govt.nz/elibrary/collecting_shellfish_keeping.htm

  19. Ministry for the Environment, 2003 http://www.mfe.govt.nz/publications/waste/wastewater-mgmt-jun03/html/part1-section2.html

Last updated at 2:11PM on February 25, 2015