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Land-based activities that put pressure on coastal marine habitats

Land-based pressures on coastal marine habitats come from water catchments – such as streams, rivers, and stormwater pipes. These waterways carry sediment and pollution from the land into the sea. In this section we describe the pressures from water catchments: sedimentation; run-off from roads, urban development, and agriculture (including heavy metal concentrations in estuaries); and wastewater pollution.

Other land-based pressures on coastal marine habitats come from various types of coastal engineering, such as infilling of estuaries and harbours to get rid of ‘clean’ fill or for building coastal infrastructure (reclamation), removal of sand and gravel for building roads and infrastructure, and building of raised roads, paths, and access ways for boats (causeways). We do not have the data on these pressures for this report.

Excess sediment in waterways can damage marine habitats

The transfer of sediment from land into waterways, and ultimately out to sea, is a natural process. It brings sand to our beaches and sediment to estuaries, creating habitats for wildlife. However, changes in land use such as harvesting forests and clearing land for farming and urban subdivisions can disrupt this natural process, causing excess sediment to enter waterways and eventually our harbours and estuaries.


A plume of suspended sediment in the Hoteo River flows into Kaipara Harbour. Increased sedimentation is a threat to coastal marine habitats. (Photo: Mark Pritchard, NIWA)

Sediment plumes can often be seen from the air at river and harbour mouths following heavy rain. New Zealand loses about 192 million tonnes of soil into waterways and the ocean every year. This is estimated to contribute about 1.5 percent to global sediment loss, despite New Zealand making up only 0.2 percent of the global land area (Syvitski et al, 2005; Walling, 2008).

In a 2012 assessment of human-based threats to New Zealand’s marine habitats, marine experts ranked increasing sedimentation as one of the top four threats to New Zealand’s marine habitats, on a par with seabed trawling, and outranked only by ocean acidification and warming (MacDiarmid et al, 2012a).

While it continues to be a present-day pressure, excess sedimentation is an outcome of fine sediments accumulating over decades in estuaries and coastal bays. Emerging evidence supports the following impacts from increased sedimentation of coastal waters:

  • direct effects on shellfish species such as cockles, pipi, and scallops, for example, by clogging their gills and reducing their filtering ability
  • reduction in survival rates of shellfish, for example, young pāua and kina
  • reduction in foraging abilities of finfish, for example, juvenile snapper
  • loss of important nursery habitats, for example, seagrass meadows and mussel beds (Morrison et al, 2009).

These effects on species and habitats do not occur in isolation from other pressures such as excess nutrients and pollution. The impact of sedimentation has implications for recreational and commercial fishing and Māori harvesting of traditional marine-based foods.

Run-off from urban development and agriculture can harm coastal ecosystems

Land-based activities, particularly agriculture, can cause excess nutrients to wash down waterways into estuaries and coastal waters. Having too many nutrients in the sea promotes the growth of algae and can lead to harmful algal blooms that can affect habitats and species (Ministry for Primary Industries, nd). An increase in nutrients can lead to a reduction of oxygen in water, which in turn can affect fish and other marine animals that depend on dissolved oxygen to survive.

The two main nutrients of concern are nitrogen and phosphorus. While data are available for some regions these are not comparable as a national dataset. Available data show levels of nitrogen and phosphorus that are highly variable across sites and from year to year. This is partly because the levels of nutrients are affected by the weather, for example, high rainfall and flooding flushes nutrients through waterways into the marine environment.

Improving these data as part of an overall indicator for coastal water quality is a priority for future reports (see Future reporting). For more information on water quality in local marine environments please refer to regional council websites.

For more detail see Environmental indicators Te taiao Aotearoa: Coastal and estuarine water quality, River water quality trends: phosphorous, and River water quality trends: nitrogen.

Heavy metals wash into waterways, estuaries, and harbours

Run-off from roads and other sources contains heavy metals such as lead, zinc, copper, and cadmium. Heavy metals are toxic to both animals and humans, even at low concentrations. They wash into waterways, estuaries, and harbours, especially at times of heavy rain. Cadmium is also a component of some fertilisers, and can be present in run-off from farms.

Heavy metals from run-off accumulate in the sediments of estuaries and can be taken up by organisms living in the sediment. They also build up in higher concentrations in species further up the food chain (bio-accumulate).

Regional and unitary councils monitor the concentrations of lead, cadmium, copper, and zinc in sediment in coastal environments and estuaries around New Zealand. Our data cover 10 of 16 regions: Auckland, Bay of Plenty, Canterbury, Gisborne, Hawke’s Bay, Marlborough, Northland, Otago, Waikato, and Wellington.

In the five years from 2010 to 2014:

  • 89 percent (copper), 92 percent (zinc), 94 percent (lead), and 95 percent (cadmium) of sites had concentrations of these heavy metals at levels that were unlikely to cause harm to seabed species
  • between 3 and 11 percent of sites had concentrations of at least one heavy metal at levels that can cause harm to seabed species – these sites were in estuaries or waterways in the regions of Auckland, Bay of Plenty, Gisborne, Marlborough, Northland, Waikato, and Wellington.
  • 1 percent of sites had concentrations of cadmium or zinc at levels where harm frequently occurs to seabed species – these sites were in estuaries or waterways in the regions of Auckland and Bay of Plenty for cadmium, and Gisborne and Northland for zinc.

A contributing factor to the low heavy-metal concentrations from 2010 to 2014 may be the low concentrations found at Canterbury sites. Liquefaction and other effects of the 2010 and 2011 Canterbury earthquakes shifted sediments around during this time, for example, overlaying existing sediment with new sediment. Heavy-metal concentrations decreased in many estuary sites in the Canterbury region over this period. None of the 18 Canterbury sites had heavy-metal concentrations above the threshold level at which harm can occur to seabed species.

For more detail see Environmental indicators Te taiao Aotearoa: Heavy metal load in sediment.

Pollution from sewage has improved since the 1990s

New Zealand’s sewage treatment improved in the 1990s. A small proportion (less than 10 percent) of sewage from households and industries still receives only basic treatment to remove solid, suspended waste before it is discharged directly into the ocean (Water New Zealand, 2016). Heavy rain events can overload sewage systems causing untreated sewage to leak into coastal seas.

Wastewater in stormwater pipes drains untreated into the sea, and can include pollutants such as plastic litter, animal excrement, cleaning products, corrosives such as battery acid, and heavy metals.

When people immerse themselves in seawater during activities such as swimming, diving, and surfing, they can risk ingesting pathogens (disease-causing microorganisms) through the mouth, nasal passages, or ears. Pathogens generally enter waterways through contaminated animal or human faecal matter.

Regional councils monitor popular swimming sites to assess the level of health risk for recreational activities such as swimming. However, the data are not included in this report as they are not intended to be nationally representative of the state of New Zealand’s coastal water quality, and are not comparable across all regions.

For information about your local swimming spots, see Land, Air, Water Aotearoa.