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Threats to Marine Ecosystems

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People use and value marine areas differently. The use of marine areas needs to be well managed to make sure these ecosystems stay healthy.


Our activities at sea and on land effect marine ecosystems. Image: LEARNZ.

For hundreds of years people have used the sea to gather food and resources, for recreation, and as a dumping ground. Now people are becoming more aware of how important the ocean is for wildlife and its effect on climate.

We now know that it’s not only our activities at sea but also what we do on land that affects life in our oceans, because all rivers flow to the sea.

As kaitiaki/guardians of the sea we must learn more about our marine areas, so we can better protect them.

Many of the species and habitats found in our waters are under threat. The marine environment is affected by the following:

  • harvesting – of food and other resources
  • pollution from the land
  • sedimentation
  • introduction of marine pests
  • climate change and ocean acidification
  • urban development – eg reclaiming land
  • 4WD vehicles on beaches.


People living near the sea have probably always used the sea as a source of food. But with better fishing equipment, larger ships, new tracking technology and more people, over-fishing is a problem.

Some types of fishing such as dredging and trawling can cause damage to the sea floor and anything that lives there. Dredging and trawling can also catch other species (known as bycatch) that are then discarded.

In New Zealand, fisheries are managed by a system that sets limits on how many fish can be caught. Our fisheries are managed by MPI, the Ministry for Primary Industries.

Pollutants from land, including plastic can end up in the sea and washed up on beaches. Image: Hedgehog House.


Our oceans have been used as a dumping ground for all sorts of waste. People now realise that our oceans cannot survive if we keep polluting them. The health of our oceans also affects the health of people who gather kaimoana and use the sea for recreation.

Some marine pollution may be accidental, for example, oil spills caused by tanker accidents. Some may not be direct, when pollutants from land flows out to sea in stormwater drains and rivers.

Some effects may build up over time and toxins can build up in animals. In some parts of the world it is not safe to eat shellfish or fish because they have pollutants such as lead in them.


Ever heard the phrase, 'too much of a good thing?' Eutrophication is a good example of this. When marine systems have too many nutrients flowing into them, the whole system suffers. These nutrients usually come from fertilisers used on some farms, which are washed into the sea.

Extra nutrients in the sea can cause too much phytoplankton growth. When these blooms of phytoplankton have used up all the nutrients they die and are broken down by bacteria that use oxygen. This leads to a drop in oxygen levels, which kills other plants and animals.

Runoff from land has an impact on water quality. Image: NIWA.


Rain, rivers and streams wash sediment off the land and into estuaries and the sea. This can be sped up by clearing land for buildings and industry. Sediment can build up and smother the seabed, killing animals living in the mud.

Too much sediment makes the water cloudy. This kills aquatic plants such as seagrass, which can no longer make food from sunlight.

Loss of light and water clarity also affects visual predators, such as birds and some fish (such as snapper) who can’t find their prey.

Introduced/invasive species

Invasive species are a threat to marine areas. It is not always easy to monitor or stop the introduction of unwanted marine organisms.

Visiting ships may bring exotic species accidentally. They can also arrive ‘naturally’, eg Myrtle rust was blown over the sea from Australia. Not all introduced species will spread or even survive, but if they do, they may be hard or impossible to remove, and may out compete native species.

This diagram shows ten indicators for a warming world. How many of these indicators can be seen in New Zealand? Image: NOAA.

People, climate change and ocean acidification

Climate change can lead to bigger storms which will increase the amount of sediment flowing into the sea, changing light and other conditions. Oceans absorb heat. A small increase in sea temperatures can have a big impact on marine life. Oceans also absorb carbon dioxide leading to a small increase in acidity. This can also affect marine animals. 

Ready for a quiz? Try the Threats to Marine Ecosystems interactive activity.

Since 1955, the oceans have absorbed more than 90% of the excess heat from greenhouse gases. Even a small increase in sea temperatures can have a large impact on marine ecosystems and everything living within them.

As oceans heat they also expand (get bigger); this expansion has contributed to one-third of sea-level rise. Human activity has led to an increase in carbon dioxide levels in the air. The sea absorbs about a third of this CO2. More CO2 in the sea increases ocean acidification. Even though this increase is small it can affect sea creatures, especially those that have a shell which can be damaged by the increase in acidity.

Other impacts

The way people use the land and sea can have a negative impact, including:

  • Reclaiming land by drainage – for development which means marine plants and animals lose their home
  • Removing sand – for making things like concrete, and dredging channels, this also removes marine habitat
  • Rubbish dumping – polluting and burying marine areas
  • Aquaculture – waste from farms can affect the surrounding marine ecosystem
  • Sediment – from increased land use, removing forest and farming
  • 4WDs on beaches can harm plants and shore bird nests.

Audio Māori keywords:

How do you think the marine environment can be protected from invasive pest species?