Seagrass Overview - Catherine Collier, Michelle Waycott, Len McKenzie, Michelle Devlin

Seagrass meadows occur in estuarine, reef and deepwater habitats, and cover approximately 13% of the GBRWHA: 6,000 square kilometres of shallow seagrass habitat and a further 40,000 square kilometres of deepwater seagrass habitat3. They are the only flowering plants (angiosperms) to colonise the sea, have internal veins, true roots, and produce flowers and seeds, and are more closely related to lilies and gingers than to true grasses1. They are found in the coastal waters around every continent except Antarctica, are the main diet of dugongs and turtles and provide habitat and nursery grounds for many marine animals, such as fish, lobsters and prawns, species of which are commercially exploited1.

Seagrass meadows also have a significant role as substrate stabilisers and nutrient sinks, buffering or filtering nutrient and chemical inputs to the marine environment. Their uptake of phosphorous and nitrogen from coastal run-off is fundamental to the maintenance of marine water quality1,2. The difficulty is that seagrasses are in decline, due to poor water quality and a succession of extreme weather years; a trend has significant ramifications for our coastal environment. Seagrasses are naturally dynamic – it is the time frames of recovery that is potentially changing and the current recovery rate is relatively slow.

A group led by Dr Catherine Collier of JCU presented on this issue; trends and risks, and current NERP research. Dr Collier told the meeting that reductions in light associated with flood plumes were more likely to cause seagrass loss than the associated reductions in salinity. Field studies showed that change in seagrass abundance was most pronounced at more turbid sites, and threshold light levels for such declines were established. This evidence was in line with that reported during the drafting of the Scientific Consensus Statement: increases in dissolved nutrients leading to algal blooms, or chronic or pulsed increases in suspended sediments leading to increased turbidity, both of which serve to reduce the amount of light reaching the seagrass4. As well as establishing threshold light levels Dr Collier has been investigating early warning indicators for seagrass decline, with carbon:nitrogen (C:N) ratios showing the most promise, and is assessing impacts of ocean acidification and pesticide exposure.

1  http://www.seagrasswatch.org/seagrass.html
2  http://www.seagrasswatch.org/Info_centre/education/Seagrass_Educators_Handbook.pdf
3  http://www.seagrasswatch.org/Info_centre/Publications/pdf/SORR_SEAGRASS_June06.pdf
Chin, A., March 2005, ‘Seagrasses’ in Chin. A (ed) State of the Great Barrier Reef On-line, Great Barrier Reef Marine Park Authority, Townsville.
4  Brodie, J., Waterhouse, J., Schaffelke, B., Johnson, J.E., Kroon, F., Thorburn, P., Rolfe, J., Lewis, S., Warne, M., Fabricius, K., McKenzie, L., Devlin, M., 2013. Reef Water Quality Scientific Consensus Statement 2013. Department of the Premier and Cabinet, Queensland Government, Brisbane.

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