SIMS

Image of Seagrass
Photo: GiGi Buretta
© GiGi Buretta

Despite its high conservation status and importance to commercial and recreational fisheries, seagrass is still declining in Australian waters, and researchers from the University of Technology, Sydney, are investigating why. Nutrient pollution is a leading cause of seagrass mortality because it promotes phytoplankton blooms and epiphyte growth which reduces the light available for seagrass photosynthesis.

These same epiphytes also provide food for small grazing invertebrates, which are the major source of nutrition for commercially and recreationally important fish, such as bream and tarwhine. The vitality of our seagrass systems are therefore governed by a series of hierarchical resource-consumer interactions (i.e., seagrass epiphytes - small invertebrate grazers - juvenile fish - large predatory fish), called a trophic cascade.

"From an ecological point of view, increases in nutrient could actually enhance our fisheries resources, says Dr Brendan Kelaher, from the Department of Environmental Sciences, UTS.

"However, too much nutrient is certain to be catastrophic, as it will bring about the demise of the seagrass via epiphyte overgrowth."

Maintaining healthy and productive seagrass ecosystems therefore requires a balance between the bottom-up force of nutrient enrichment and the top down force of grazing and predation. Understanding the impacts of nutrient enrichment and over-fishing is necessary for effective estuarine management, however little is know about these issues in Australian systems. Our research is providing a synthetic understanding of these issues by experimentally investigating a trophic cascade that links nutrient pollution, seagrass epiphytes and invertebrate grazers to commercially-important juvenile fishes. The outcomes of this work will enable accurate prediction of impacts of future nutrient loads on the growth and survivorship of seagrass and associated fish species, which will be invaluable for developing policy for water quality guidelines that conserve seagrasses and near-shore fisheries.