Research: How will the NSW continental shelf ecosystem be impacted by climate change?
It slopes gradually to a point at around a depth of 200 m where the edge drops away suddenly to much deeper waters, forming the continental slope. The shelf is anywhere between 10 km to more than 200 km from the shore.
The NSW Shelf Model project used lots of environmental information to create a computerised model to help answer questions about the future of marine resources like commercial fisheries under climate change:
- Will current conservation strategies protect our natural resources?
- Will the ecological impacts of running our fisheries worsen with any changes to ocean currents?
- What if the NSW shelf ecosystem changes dramatically because of ocean currents?
- Will what we know to be the current environmental impacts of our fishing methods change?
Scientists included many variables in the model, so that the model's predictions could be as lifelike as possible. Variables included:
- distributions of habitats (including habitats associated with the sea floor and the water column);
- organisms;
- physical processes (for example current patterns, water masses, tidal exchange information); and
- human influences (our use of ocean resources, contaminant inputs and management zones).
Among the physical processes, scientists looked at satellite images of ocean sea surface temperatures and of the chlorophyll contained in the water.
Chlorophyll is the photosynthetic part in a plant. It absorbs sunlight and converts it to sugar during photosynthesis. By measuring chlorophyll, nutrients in the water column are measured because very tiny plant cells called phytoplankton absorb or eat up any available nutrients in the water column.
Examples of satellite sea surface temperature off northern NSW from 13-12-2002 (left) and satellite chlorophyll estimates (SeaWiFS) off southern NSW from 12-12-1997 (right). Both reveal the complex eddy structures generated by the East Australian Current in this region.
© CSIRO Marine. Reproduced with permission.
Source: Shelf Report1
Nutrients found so far out to sea come about by something called upwelling. Upwelling occurs because there is a temperature difference between the colder deep waters and the warmer near-surface waters. In some areas of the oceans, something happens to set this transfer of water in motion, for example winds blowing or ocean currents disturbing a parcel of water, and the water tries to correct the temperature difference: water from the sea floor replaces water in shallower waters, bringing with it sediments from the sea floor. The sediments from the sea floor of deep waters are typically rich in nutrients due to the sinking and settling of organic matter from surface waters to the sea floor.
Where there are nutrients in offshore areas, we can expect life. Phytoplankton absorb nutrients, plankton eat phytoplankton, small fish eat plankton and bigger fish eat smaller fish. This is called an aquatic food web.
If ocean currents and areas where upwelling occurs change from their present location, offshore fisheries will be affected. Currently, offshore fisherman have expansive fishing grounds from where they catch large amounts of fish for both Australian and international fish markets.
The information gained from these studies can help guide where productive fishing grounds have been newly created or lost due to changes in ocean current and upwelling patterns.
The model shows scientists that the ocean trawl fishery is already fully exploited. This means that despite any future changes in the climate and thus ocean circulation patterns, the continued sustained production of fish resources from that fishery cannot continue at its current rate. The model shows that, by reducing the total number of fish caught (reducing the fishing pressure), currently overfished predatory groups could recover, but with an accompanying decline in their prey. So while the bigger fish might recover, the numbers of fish that those bigger fish eat reduce in number.
Many more management scenarios can be played out using the model so that decisions can be made with some guidance about how the shelf ecosystem will respond.
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1Savina M, Forrest R, Scandol J et al., 'Ecologically sustainable development of the regional marine and estuarine resources of NSW: Modelling of the NSW continental shelf ecosystem', Report, CSIRO Wealth from Oceans Flagship, University of British Columbia, NSW Department of Primary Industries, March 2009.