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December 1997 NewsletterLOW ISLES SEAGRASS MEADOWS: 70 YEARS AFTER BRITISH REEF EXPEDITIONNo coral island in the world has been so intensively studied over so long a time as Low Isles. In 1928-1929, the Great Barrier Reef Committee collaborated with the Royal Society of London to co-ordinate the first major biological expedition to the Great Barrier Reef and used Low Isles as a base for scientific studies. The expedition spent a full year investigating Low Isles' coral reefs, seagrasses, mangroves and cay. They reported a rich and typical coral reef system despite its proximity to the mainland and freshwater run-off. The structure and ecology of Low Isles was re-examined in 1954 then again - but less intensively - in 1965 and 1973. In July/August 1997, members of the Seagrass Ecology Group from the Northern Fisheries Centre in Cairns conducted a survey of Low Isles' seagrass meadows. They were assisted by staff from Department of Environment, Port Douglas and Low Isles. The purpose of the survey was to map the distribution and abundance of Low Isles seagrass communities, and report any observed changes in the reef-top and anchorage habitats at Low Isles over the last 70 years. The team, supported by the CRC Reef Research Centre Ltd, examined 526 sites on the Low Isles reef flat, around Woody Isles, in the anchorage, and waters to 15 m (on the leeward side). At each site, position was recorded to 3 metres using dGPS, and seagrass above-ground biomass, seagrass species composition, sediment depth, and other benthos were recorded. A Geographic Information System (GIS) for Low Isles has now been established, including information on seagrass abundance and distribution, benthic communities and bathymetry. New aerial photographs of Low Isles were also taken and rectified onto the GIS to aid in interpretation. This is the first time seagrasses of Low Isles have been accurately mapped. The extensive inventory of seagrasses from the present survey, reports nine species (one species previously undescribed) at Low Isles. Seagrasses are present over much of the reef flat, anchorage and in deeper waters on the leeward side of the reef. The reef flat is extensively covered with Thalassia hemprichii/Halodule uninervis meadows on sand/shell substrates. Abundant seagrass meadows were found to a depth of 15 m, mainly dominated by Halophila tricostata and Halophila decipiens. From our interpretation of the 1928-1929 survey findings, it appears that the distribution of the most common seagrasses (Thalassia hemprichii and Halophila ovalis) on the reef flat has changed little, however seagrass has not been reported in the Low Isles anchorage prior to this 1997 survey. Today, Low Isles reef shows substantial ecological degradation, with many of its earlier rich coral communities largely dead and overgrown with soft coral and algae. Popular belief is that intensive agriculture in the region is a contributing factor to this decline. Growth and distribution patterns of seagrasses in response to ambient nutrient levels may throw light onto the regional impacts of agricultural land-use at reefs such as Low Isles. Analysis and interpretation of the current survey results is continuing and a report of findings is expected early 1998.
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