Human impacts on marine environments

Throughout human existence we have relied on the oceans – for food, as a waste dump, for recreation, for economic opportunities and so on. However, it’s not only our activities in the marine environment that affect life in the sea – it’s also the things we do on land.

With more than half the world’s population now living within 100 kilometres of the coast, it’s not surprising that our activities are taking their toll. Human impacts have increased along with our rapid population growth, substantial developments in technology and significant changes in land use. Over-fishing, pollution and introduced species are affecting life in the sea – and New Zealand is no exception!

Fisheries

Humans living near the coast have probably always used the ocean as a source of food. However, with advances in fishing equipment, larger ships and new tracking technologies, many fish stocks around the world have reduced significantly. Fish stocks on continental shelf areas are now widely considered to be fully or over exploited. Aside from reducing fish stocks, unsustainable fishing practices can have other negative impacts on the marine environment. For example, some fishing techniques such as dredging and trawling can cause widespread damage to marine habitats and organisms living on the sea floor. These techniques also often capture non-target species (known as bycatch) that are then discarded.

In New Zealand, fisheries are managed by a quota system that sets catch limits for commercially important species and aims at sustainable management of our fish stocks. The Royal Forest and Bird Protection Society (NZ) publishes the Best Fish Guide to try and encourage us to make more sustainable choices when purchasing seafood. The list evaluates fish stocks and bycatch levels and the fishing methods used.

Pollution

Our oceans have long been used as an intentional dumping ground for all sorts of waste including sewage, industrial run-off and chemicals. In more recent times, policy changes in many countries have reflected the view that the ocean does not have an infinite capacity to absorb our waste. However, marine pollution remains a major problem and threatens life in the sea at all levels.

Some marine pollution may be accidental, for example, oil spills caused by tanker accidents. Some may be indirect, when pollutants from our communities flow out to sea via stormwater drains and rivers. Some effects may not be immediately obvious, for example, bioaccumulation – the process where levels of toxic chemicals in organisms increase as they eat each other at each successive trophic level in the food web.

All marine pollution has the potential to seriously damage marine habitats and life in the sea. Scientists are concerned that marine pollution places extra stress on organisms that are already threatened or endangered.

Eutrophication

Eutrophication is the result of a particular type of marine pollution. It is caused by the release of excess nutrients into coastal areas via streams and rivers. These nutrients come from fertilisers used in intensive farming practices on land. Additional nutrients in the sea can lead to excessive phytoplankton growth that results in ‘blooms’. When these large numbers of organisms die, the sharp increase in decomposition of the dead organisms by oxygen-using bacteria depletes oxygen levels. In some cases, this can result in the death by oxygen starvation of large numbers of other organisms such as fish.

Introduced species

Since the arrival of humans in New Zealand, introduced species in our terrestrial ecosystems have contributed to a significant loss of biodiversity. Introduced species also present a threat to our marine environment. It is not always easy to monitor or prevent the introduction of unwanted marine organisms, and visiting ships may introduce them accidentally on their hulls, in ballast water or on equipment.

Not all introduced species will spread or even survive, but once established, they may be difficult or impossible to remove. For example, the Japanese seaweed, wakame Undaria pinnatifida, which probably arrived in 1987, is now widespread. Scientists are still monitoring its impact on our native marine organisms.

In New Zealand, the Ministry of Primary Industries is responsible for providing border inspectors who manage risks from people, planes, vessels (like ships) and goods coming into the country. They also coordinate responses when new, harmful pests and diseases are detected in our country.

Ocean acidification

There is evidence to suggest that human activities have caused the amount of carbon dioxide in our atmosphere to rise dramatically. This impacts on the marine environment as the world’s oceans currently absorb as much as one-third of all CO₂ emissions in our atmosphere. This absorption of CO₂ causes the pH to decrease, resulting in the seawater becoming more acidic.

Scientists have long understood that an increase in carbon dioxide in the atmosphere will result in higher levels of dissolved CO₂ in seawater. However, a relatively recent discovery is that even small changes in water pH can have big impacts on marine biologyOcean acidification is a worldwide issue, but as CO₂ is more soluble in colder water, it is of particular concern in New Zealand’s temperate oceans.

It is difficult to predict the overall impact on the marine ecosystem but many scientists fear that ocean acidification has the potential to decrease marine biodiversity on a very large scale.

New Zealanders are aware that old ways of managing our seas are in need of a rethink. The Sustainable Seas National Science Challenge is tasked with helping New Zealand enhance the value of our marine resources while ensuring they are safeguarded for future generations.