When combined with problems of over-fishing, and other disturbances, the changes humans are bringing to the marine environment can be extensive. The actual effects of these influences remain unquantified, however. For example, although researchers have found some alterations to food webs on inshore reefs - most likely due to excess nutrients from fertiliser run off - it remains impossible to estimate the exact amount of excess nutrients actually entering the coastal zone of the GBR.

In addition, understanding and minimising the impact of these human activities is complicated by other factors namely:

• basic ecological and physical oceanographic information is available, but the interrelationships between marine organisms and physical processes are poorly understood.
• the environment varies greatly through both time and space.
• links between the coastal habitats (i.e. mangroves and seagrass beds) and the food webs and energetic processes of the open ocean are unclear.
• the links between the land and the coastal zone and between the coastal zone and the shelf proper are poorly understood.

To answer some of these questions, researchers at the Australian Institute of Marine Science are studying two sites along the Great Barrier Reef shelf - an area off Cairns and another in the Herbert River region. Their research involves monitoring water column nutrient levels, conducting grazing experiments on phototrophic picoplankton, measuring water column respiration and a host of other measurements in order to examine the links between human activities and the effects they are having on marine food webs.

The two areas differ in the major human activities which have an impact on the regional ecosystems. Development and agriculture are both potentially influencing the marine environment in Cairns, whereas in the Herbert River region, agriculture is the primary source of environmental change.

Theoretically, food webs and nutrient cycles further offshore should be most affected in the Cairns region, where the Continental Shelf is relatively narrow and the Great Barrier Reef is in close vicinity. By contrast, researchers expect to see comparatively little impact on food webs and nutrient cycles beyond the Hinchinbrook Channel in the Herbert River region. Extensive mangrove systems trap nutrients and other land-based materials, and so the strongest evidence of change should be along-shore rather than offshore.

To add a further dimension to the study, researchers are also examining the Kings Sound in the Kimberley region of Western Australia. As a relatively pristine area with few mangroves and in the dry tropics, it differs a great deal from the other sites.

Each site consists of three transects, along which the scientists have established a number of monitoring stations. By taking a holistic approach to their research - working together to study the ecosystem as a whole rather than piecemeal projects in isolation - AIMS scientists intend to create a clearer picture of the complex interplay of life in the ocean.

The work is divided amongst four subgroups, each of which is studying a different aspect of the project.

Rivers and mangroves - This subgroup is focusing on the nutrient and trophic exchanges between rivers and adjacent coastal waters. They are also investigating primary production in mangroves and following the fate of its nutrients through the system.

Inshore and reef trophic processes - moving further offshore, this subgroup is focusing on the flux of materials into and within the inner shelf ecosystems. They are also studying the processing of organic matter in food webs and the biological response to materials introduced to the ecosystem naturally and by humans.

Pelagic-benthic coupling - Working to understand the links between activities on land and the open ocean, this subgroup is studying how nutrients introduced by humans influence the growth of phytoplankton and algae in the open ocean. Understanding the connections between the life on the ocean floor and the open ocean is also part of this sub-project. The researchers are also studying the exchange of nutrients and other materials between the shelf benthos and the overlying water column.

Physical processes and modelling - The work of this subgroup unifies the entire project by linking ecological processes to physical oceanographic processes, such as the exchange of nutrients and other materials between rivers and shelf waters. From this information, researchers are constructing a variety of computer models which can predict the amount of materials management authorities can safely allow into the environment before this results in serious damage to coastal ecosystems. These models will also allow for a deeper, more basic understanding of the physical and ecological connections - a perspective rarely understood for any shelf ecosystem in the world.

It has become clear to researchers that the energetics of coastal ecosystems are not divorced from the surrounding environment, and yet to date few, if any, large-scale programs from established marine organisations have examined these connections closely. An understanding of the links between the land and the sea, food webs and the physical environment, is vital for authorities working to protect ecosystems as fragile and valuable as the Great Barrier Reef.