Threats to the marine environment

Below is a list of the main threats to Sydney's marine environment. They all have had significant impacts on our marine ecosystems, causing the decline of many commercially and recreationally important species and even pushing some towards the brink of extinction. Marine food webs then become unbalanced, leading to follow-on effects.

Climate change and ocean acidification

The past decade has seen a lot of emphasis placed on the cause of rising CO2 levels in the atmosphere. The scientific consensus is that anthropogenic (human-caused) CO2 emissions are steadily warming the planet, and that the climate will change significantly as greenhouse gasses build up in the atmosphere. Less attention, however, has been given to the role that our oceans play in regulating the global climate. Despite their central and complex role in the climate system, the oceans remain on the periphery of climate research, policy-making, and protection.

The oceans absorb up to half of all human CO2 emissions. Their heat capacity is nearly 1000 times that of the atmosphere; by absorbing heat in the summer and releasing it in winter, they are instrumental in moderating our climate and ensure that changes happen gradually. Anthropogenic climate change is making the oceans warmer and more acidic, and if these processes are allowed to continue unchecked, they will drastically alter our climate and the marine ecosystem.

Furthermore ocean acidification has the potential to affect a wide range of species including fish and marine invertebrates, such as oysters which serve as an important source of food have economic value.

Destruction of vital habitat

Over-development and aquaculture – fish and shrimp farming – have been responsible for severe declines of productive habitats over the last several decades, particularly of mangrove forests, which protect coastlines by absorbing storm forces and supply nutrients vital to marine life. As these habitats disappear, both people and the environment suffer: marine animals lose both their food and shelter, leading to the collapse of food webs from the bottom up, and human settlements become increasingly exposed to the effects of storms and flooding.


Fertilizer runoff from farms and lawns is a huge problem for coastal areas. The extra nutrients cause eutrophication - flourishing of algal blooms that deplete the water's dissolved oxygen and suffocate other marine life. Eutrophication has created enormous dead zones in several parts of the world, including the Gulf of Mexico and the Baltic Sea.

Fishing pressure

Only a small fraction of the world’s oceans are protected within marine parks; far below what leading marine scientists recommend. Of the marine protected areas that have been established internationally, only a small proportion are achieving their management objectives; and internationally, the majority are open to fishing.

Invasive pest species

New threats, though, are emerging as climate change triggers a strengthening of the East Australian Current, bringing warmer waters – and new invasive species – further south. Rising carbon dioxide levels are also changing the chemistry of the sea enabling invasive species to move into new areas and forcing other species to migrate.

The Maria Island Marine Reserve of Tasmania is a great example of how marine parks can counteract the effects of invasive pest species. Created in 1991 to protect a representative range of marine habitats found on the east coast. Half of this area is fully protected and the rest allows recreational fishing. Maria Island has successfully shown that fully protected areas are critical to restoring species diversity and abundance in the marine environment particularly for heavily-fished species. Such rehabilitation can help control invasive species (such as urchins) and restore natural balance and resilience in an ecosystem.

Pollution from the water and land

One of the most common pollutants in our oceans is plastic. Plastic as we know it, really only came into mass production after WWI and in the relatively short amount of time that it’s been in use, has already spread to every corner of the globe. Estimates show that marine litter is now 60–80% plastic, reaching 90–95% in some areas.

It’s not known exactly how long it takes plastic to biodegrade. Estimates for plastic shopping bags range from 500 years to a 1,000 years or even longer. What we do know is that plastic is accumulating in vast quantities in our oceans. Since the 1950s, one billion tons of plastic have been discarded and may persist for hundreds or even thousands of years. Ocean currents collect the plastic into concentrated areas known as gyres resulting in vast areas of ocean being completely saturated with plastic.

Plastic is often mistaken for food by marine animals such as turtles (plastic bags look a lot like jellyfish), sea birds and even the oceans smallest feeders can be misled by tiny plastic fragments which are indistinguishable from plankton. Plastic can fill the digestive system of these animals causing them to starve. Scientists are also finding that plastic often acts as a carrier for other pollutants (such as persistent organic pollutants or POPs) making it even more harmful as it becomes extremely toxic to marine life.

All rights reserved National Parks Association of NSW Inc 2015

National Parks Association of NSW is a non-government conservation group that seeks to protect, connect and restore the integrity and diversity of natural systems in NSW and beyond, through national parks, marine sanctuaries and other means.