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The Australian Coastal Zone and Global Change: Research Needs

Australian Academy of Science, Becker House, Canberra. Friday 16 December 1994

LOICZ-JGOFS interactions: carbon fluxes in the continental margins - continued

Given the interests of JGOFS and LOICZ, there is to some extent a natural division of labour in the continental margins (Overhead 5).

LOICZ will provide information on catchments and terrestrial inputs into the margins. LOICZ, as opposed to JGOFS, is much more likely to be concerned with the role of benthic ecosystems such as coral reefs and mangroves. LOICZ will be interested in the potential impacts of pollution on those systems and on carbon cycling in the margins, and with impacts of resource exploitation. In general, JGOFS' interests are more off-shore. JGOFS field activities are more likely to be focused on the outer shelf and slope. JGOFS can contribute information about off-shore boundary conditions and forcing to LOICZ.

The science of common interest to both programs includes the process understanding of nutrient and carbon cycling in the water column, on both the shelf and slope, and of carbon and nitrogen cycling through sediments. The measurement protocols that have been developed in JGOFS and will be developed in LOICZ need to be exchanged and integrated. Both have a strong interest in remote sensing, especially in sea-surface temperature and ocean colour, and in the modelling of carbon and nutrient cycling.

The task team identified a number of important classes of continental margins (refer Overhead 6).

OH 6: important continental margins

Overhead 6

These include areas with a large sediment input, such as the East China Sea and the Bay of Bengal; areas with strong coastal upwelling; areas of cyclonic eddy upwelling in western boundary currents. The report singled out, as Patrick Holligan has already mentioned, the shelf area around the South-East Asian archipelago, which has high sediment input, high rates of primary production, and potentially affects the ocean carbon cycle through its influence on the properties of the water in the Indian Ocean and the Pacific-Indian Ocean through-flow.

Finally, the task team, at the time that the report was written, looked at a number of regional studies that were underway or planned in the near future and that could be considered as a contribution to joint JGOFS-LOICZ field activities (Overhead 7).

OH 7: regional studies

Overhead 7

They mentioned in particular the studies in the West and North Pacific or the East Asian area: the East China Sea, Yellow Sea, South China Sea. A number of countries there have field programs under way. A smaller number of participants were identified at the time in the South-East Asian archipelago. I think that is an area that will need boosting where Australia can contribute. In the report Australia and New Zealand were singled out separately as a South Central Pacific contribution.

Let me turn then to Australia and look briefly at the kind of JGOFS-LOICZ interaction we should look for in Australia. To set the scene, Overhead 8 (not available) shows a typically large-scale JGOFS look at the continental margin in Australia. This is actually a mean chlorophyll pigment image for the Australian region, based on the Coastal Zone Colour Scanner data collected from 1978 to 1986. There are a few general points to note immediately. One is that much of subtropical Australia is bathed by oligotrophic ocean waters. The water coloured blue and purple here has chlorophyll concentrations down around 0.1 mg m-3, or less.

The broad continental shelves across the north of Australia stand out in particular as having elevated chlorophyll levels, with chlorophyll concentrations up around 0.5 to 1.0 mg m-3. Even around the rest of Australia there is elevated chlorophyll on the continental shelf. South of Australia, we see higher oceanic chlorophyll concentrations associated with temperate - as opposed to subtropical - water masses, and the frontal region in the Subtropical Convergence.

Overhead 9 (not available) is based on an analysis of the seasonal cycle of chlorophyll in those eight years of Coastal Zone Colour Scanner data. This figure shows the phase of the seasonal cycle in chlorophyll around Australia; ie the month of the peak chlorophyll in the seasonal cycle. If you look closely at this figure you will see some very interesting broad patterns in the off-shore ocean; the oligotrophic oceans are fairly similar on both coasts. But you can also see very interesting contrasts between the off-shore ocean and the shelf systems in many places. There seems to be a common seasonal pattern along the shelf, right around northern Australia, which is quite distinct from the areas further off-shore. You can also see some interesting structure across the Great Australian Bight and around Tasmania.

It is important to realise that over much of the Australian coastline, these CZCS data are virtually the only information we have on seasonal cycles in chlorophyll. There are a number of sites where excellent short-term studies have been conducted, and a few sites with long-term monitoring records. Still, we have a long way to go to establish anything like a national database of the kind required for LOICZ.

Id like to briefly review some of the key processes involved in the interaction between the off-shore oceans, the shelf and slope, and terrestrial inputs, on a regional basis around Australia. In the north-east, we have the Great Barrier Reef along the shelf, with the coral reefs storing calcium carbonate. There is current concern about inputs of nutrients from agricultural production and run-off. We also have evidence of nutrient input onto the shelf from continental shelf waves.

Down the east coast of Australia we have a typical western boundary current system (the East Australia Current), with topographic upwelling where the current turns off-shore and upwelling associated with eddies,. In the south-east corner of the continent we have an area with a temperate rather than subtropical ecosystem with deep winter mixing, both on the shelf and off-shore. There is a confluence of an East Australia Current extension with the Leeuwin Current, which extends right across the Great Australian Bight and down the west coast of Tasmania at times, and their interaction with eddies spun off the Antarctic Circumpolar Current running across to the south.

Through the Great Australian Bight, there is evidence for wind-driven upwelling along the south-east corner of South Australia. We know that the Leeuwin Current flows along the shelf edge through the Bight. I have to say that at least my knowledge of the forcing of the shelf system through the Bight is very weak. There has been a fair amount of work done looking at the Leeuwin, interaction of eddies and impacts shelf ecosystems in the south-west of Western Australia. There is more work being done now on the North West Shelf and some evidence for seasonal patterns driven by upwelling, and/or mixing by internal tides. There is of course an important coral reef system there.

Finally, we have the interesting area of broad continental shelf across tropical Australia, characterised by strong seasonal run-off, monsoonal forcing, influences of cyclones on sediment resuspension, productivity, and benthic systems. This area is contiguous with the South-East Asian archipelago.

In terms of the JGOFS field programs that have been running or are planned around Australia, there has been quite a bit of work done already and more planned by the Australian Institute of Marine Science and collaborators, on the tropical area across the north of Australia and down the North West Shelf. There is, of course, other historical work done by other agencies, especially in the Gulf of Carpentaria, and ongoing research on the effect of nutrient run-off in the shelf system of the Great Barrier Reef. CSIRO has been doing some work on the slope system in Tasmania in the SE corner. I think there may be opportunities for more work in the East Australia Current region and some work planned by consortia in West Australia on the Leeuwin Current and eddies associated there. Of course, a key aspect for the continental margin studies is the work on sediments by the Australian Geological Survey Organisation and others, especially on the slope off Western Australian, with more work planned off southern Australia.

I would like to conclude just by coming back to a couple of the key scientific approaches or processes that are of interest in JGOFS-LOICZ interactions. One of them is remote sensing. There has been a lot of interest in LOICZ in the use of remote sensing to look at transport and exchange of material between the shelf and the off-shore. Overhead 10 (not available) shows an interesting example - a coastal zone colour scanner image of the area around Tasmania, apparently showing high chlorophyll along the shelf on both coasts, that seems to be entrained into eddies and pulled off into the Southern Ocean.

Of course, there is a lot of humic substance in the water that runs off the Tasmanian coast. So whether the chlorophyll seen by CZCS is really chlorophyll is still to be determined. There is a need for algorithm development as well as interpretation to take full advantage of remote sensing in the coastal zone.

Finally, carbon and nitrogen cycle modelling must be a key area for collaboration between JGOFS and LOICZ.

OH 11: C, N cyclone modelling

Overhead 11

The elements singled out in Overhead 11 include the need for good physical circulation models and, of course, sediment transport models. We will hear this afternoon about a new initiative on development of EEZ (exclusive economic zone) - scale models of ocean circulation for Australia. These can be used as a context for nesting the kind of local models needed to tackle LOICZ-JGOFS continental margin modelling.

Clearly, we need to get the processes involving nitrogen and carbon cycling in the water column and in the sediment right. I think a lot of those can be looked at in common with JGOFS. Based on some recent experience that we have had looking at nitrogen cycling in coastal embayments, I think denitrification in sediments will turn out to be a key process for understanding nitrogen cycling and the fate of terrestrial inputs in the margins.

There is a healthy and growing modelling effort in Australia at the local (estuarine/embayment) level, and at the ocean basin scale level. The challenge for JGOFS and LOICZ is again the scaling-up problem: how to get from the estuarine embayment models to shelf and slope scale models for the Australian region.

Professor Bruce Thom: Thank you very much, John. That has certainly brought us into the Australian arena with respect to research in hand as well as research needs. Of course, in this afternoon's session we will concentrate in sessions 3 and 4 with further examples of Australian research activity to try and look further at what direction we should be proceeding towards, but specifically keeping in mind the context provided for us both by Patrick and Stephan at the global level.

I am wondering if anybody has any questions or comments that they would like to direct at any of our three speakers in session 2 this morning. Does anybody have any points of clarification or other aspects they would like to raise?