Adelaide Coastal Waters Study

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EP 1—Assessment of the effects of inputs to Adelaide's coastal waters on seagrass ecosystems and key biota

Issue: Seagrasses are being put under increased stress. How are they reacting?

Response: Ecological Processes Study

The Ecological Processes Study seeks to find out the things we don’t know about seagrasses and their way of living (their ecology) in the Adelaide coastal environment. Seagrass beds form habitats for a range of species. They have been selected for intensive investigation in recognition of their role as overall indicators of marine conditions. Changes in seagrass beds can be useful triggers for management actions. Seagrass remains the major marine habitat type and is common along the coast and in the Port Adelaide River and Barker Inlet.

Seagrass is the name given to any member of a group of flowering plants that grow underwater - some of them up to the low to mid water mark and some in deeper waters. (Despite its name, seagrass is not a kind of grass, although it resembles it in general appearance). Since the 1970s, the South Australian Coast Protection Board (CPB), and more recently the South Australian Environment Protection Authority (EPA), have been concerned about the losses of seagrasses and the possible effects of the loss, both on biological processes and on physical processes such as sand movement. Some areas that used to be dominated by seagrasses are now dominated by algae, particularly Giffordia, a kind of brown seaweed, and Ulva, also known as ‘sea lettuce’. The reduction of seagrass and its partial replacement with seaweed is a clear indicator of decline in water quality and the effects of nutrient enrichment.

Some basic questions remain to be answered before effective management strategies can be devised:

• What inputs are affecting the important combinations of species that include seagrasses and other plants, fish and other animals, in the Adelaide coastal waters?
• Why do seagrasses struggle to recolonise once they have suffered an initial loss?
• How does the nutrient cycle (of nitrogen, phosphorus, carbon, for instance) operate in seagrass ecosystems?
• Does the cycle vary from place to place or situation to situation?
• What are the existing and tolerable concentrations of metals and organic chemicals in the life cycles of these biota?

The teams will be looking for correlations between the conditions in various parts of the Gulf and changes in the seagrass ecosystems. Observations and experiments by divers will complement observations and experiments done in large seawater tanks (called ‘mesocosms’) containing replicas of the coastal waters environment. Factors to be investigated include turbidity, nutrients, toxic substances, low salinity and sedimentation. Stormwater, sewage and industrial discharges are common sources of these factors.

Just because a toxic substance can be detected by chemical analysis of seawater, it does not necessarily mean it will affect sea creatures. The substance may not be ‘bioavailable’ (that is, it may not be able to be absorbed by plants and/or animals). Conversely, even very small, possibly unmeasurable, concentrations of some substances can stop a species breeding effectively, or damage some other part of its life cycle, for example, reproduction. Therefore, ecotoxicological testing will complement the chemical analyses of water contaminants. The research team will measure the bioavailability of contaminants to selected organisms by examining their effects on the organisms at a range of concentrations — that is, via ‘bioassays’. Bioassays will provide data on the effects of mixtures of pollutants and environmental variables such as turbidity, salinity and dissolved oxygen. There will be bioassays on several sensitive species because their responses to the mixtures may differ.

Rather than just watch more of the seagrass ecosystems disappear, the team will also find ‘biomarkers’ in various marine plants or animals, as early warning indicators of toxic effects. Monitoring of biomarkers (particular biochemical indicators) which change as a result of exposure to a substance should also show up the effects of management actions that reduce the loads of nutrients, toxicants or freshwater discharged into the sea.

The knowledge generated from this task will be valuable for the management of the Adelaide marine environment. At present, too little is known about the complex coupled processes (physical, chemical and biological) that operate there. Although other coastal seagrass systems have already been studied (for example, off Brisbane, Melbourne and Perth), the Adelaide coastal waters and seafloor are different and the ecosystem is thought to function differently as a result. Previous studies have indicated that seafloor sediment is both a source of and a sink for toxic substances. In the Adelaide Coastal Waters Study, divers will survey the seafloor and measure the way seafloor communities (plants and animals) vary from place to place and through time. Then the teams will try and correlate the variations with concentrations of nutrients, heavy metals and organic compounds measured in the sediments of the seafloor. The third prong of this ‘triad’ approach will use amphipods (small prawn-like crustaceans) to show up toxicity in the same sediments. The triad approach is a recognised method for making an integrated assessment of sediment quality in the marine environment.