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 marine ecosystems stay healthy. 

For hundreds of years people have used the ocean to gather food and resources, for recreation, and as a dumping ground. It is only recently that people have become aware of the importance of the ocean in supporting life and affecting climate.  

We now know that it’s not only our activities in marine areas that affect life in the sea, but also the things we do on land.

As kaitiaki/guardians of the sea we must build our understanding of 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 – 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. However, with better fishing equipment, larger ships, new tracking technology and growing numbers of people, many fish stocks around the world have been over-fished. 

Some types of fishing can have other negative impacts. For example, 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.


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. 

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 entire system suffers. These nutrients usually come from fertilisers used in intensive land farming, 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. 


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 creatures living in the mud. 

Too much sedimant makes the water cloudy. This kills aquatic plants such as seagrass, which can no longer make food through photosynthesis. 

Introduced/invasive species

Invasive species are a threat to our marine environment. It is not always easy to monitor or stop the introduction of unwanted marine organisms. Visiting ships may introduce them accidentally. They can also arrive ‘naturally’, eg Myrtle rust was blown over the sea from Australia.

Not all invasive species will spread or even survive, but if they get established they may be very hard or impossible to remove, and may out compete native species.

Climate change and ocean acidification

Climate change can lead to more severe storms which will increase the amount of sediment flowing into the sea, changing light and other conditions.

The oceans also absorb heat. A slight increase in sea temperatures can have a large impact on marine ecosystems and the organisms living within them.

Human activity has lead to an increase in carbon dioxide levels in the air. The sea absorbs some of this CO2. More CO2 in the sea leads to an increase in acidity. Even though this increase is small it can affect sea creatures, especially those that have a shell which is 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
  • 4WD on beaches can harm plants and shore bird nests.

Ready for a quiz?

Threats to Marine Ecosystems activity


Audio Māori keywords: 

How do you think the marin environment can be protected from pest plants and animals?

Healthy Ecosystems

Ecosystem Based Management


It is not only our activities in marine areas that affect life in the sea, but also the things we do on land. Image: LEARNZ.

When soils erode, sediments are washed into waterways. When forests are removed more sediment washes into waterways and can smother plants and animals. Image: NIWA.

Stormwater drains into rivers and the sea so it is important to only let rain enter these drains. Image: NIWA.

Dredging can harm marine ecosystems and remove habitat for marine organisms. Image: Sustainable Seas Challenge.

Rubbish which can harm marine life can end up in the sea and washed up on beaches. Image: Hedgehog House.

The Rena container ship came aground on the Astrolabe off the coast of Tauranga, causing a major oil spill. Accidents such as this can have a devastating impact on marine areas. Image: New Zealand Defence Force.