Water samples are collected regularly from fixed sites throughout the Marine Park

s a part of its central purpose of protecting the ecosystems of the Great Barrier Reef, the Great Barrier Reef Marine Park Authority manages a variety of reef-based economic activities, including tourism, shipping and fishing. A diverse range of agricultural and mining activities also occur in the watersheds draining into the Great Barrier Reef. The concern is that these activities may cause long-term changes to water quality that will ultimately lead to the degradation of the inshore ecosystems of the Great Barrier Reef.

Terrestrial run-off is the largest source of nutrients and sediments to the Great Barrier Reef that is likely to be increased by human activities. Activities of concern are the increase in fertilisers (figure 1a) and increased clearing of natural vegetation for agriculture (figure 1b). Concern has been expressed that the movement of nutrients and eroded sediments from the adjacent land presents a serious threat to the complex ecosystem of the reef.

It is estimated that total nutrient input into the Great Barrier Reef has risen by about 30% in the last 140 years (Pulsford 1996). The modern increase in nutrient load discharge into reef waters has created a potential long-term threat to Great Barrier Reef ecosystems.

Elevated nutrient concentrations have been demonstrated to cause a range of impacts on coral communities, including decreased calcification rates, changes in coral composition, reduced recruitment rates and juvenile mortality (Tomascik and Sanders 1985; Morrisey 1988; Hoegh-Guldberg 1994; Muller-Parker et al. 1994; Ward and Harrison 1996). Increased nutrients and turbidity can also adversely affect seagrasses (Short et al. 1996) by causing a shading-induced reduction in seagrass photosynthesis (Walker and McComb 1992; Abal and Dennison 1996).

Why monitor changes in the phytoplankton (chlorophyll a)?

One of the key elements in the understanding and management of water quality within the Great Barrier Reef Marine Park is the establishment of a monitoring program to detect and quantify changes in water quality over time. The Great Barrier Reef Marine Park Authority’s long-term water quality monitoring program was established in 1992 to provide long-term data on trends and regional differences in the nutrient status of Great Barrier Reef waters.

A central objective of this monitoring program is to detect long-term changes in the quality of Great Barrier Reef waters, particularly as a result of nutrient input from the land (Brodie and Furnas 1992). Because dissolved nutrients are rapidly converted to particulate forms, which are in turn rapidly recycled (Furnas et al. 1997), measurement of chlorophyll a (the major algae pigment) concentration was chosen as a proxy indicator of nutrient status (Brodie et al. 1997). The monitoring, at a regional scale, of long-term changes in phytoplankton biomass (as chlorophyll a) can be used as a proxy indicator of land-based nutrient input to the Great Barrier Reef lagoon.

Figure 2. Location of transects in the Great Barrier Reef (QEPA = Queensland Environmental Protection Agency) click on map to enlarge it

Water samples are collected by personnel from a number of government and non-government agencies and organisations. Samples are collected at fixed sites at approximately monthly intervals along transects located throughout the Marine Park (figure 2). Measurements of salinity, turbidity and sea conditions are also made at the time of sampling.

Continued routine sampling will define regional ambient chlorophyll a concentrations, which can be used to benchmark future changes in the nutrient status of Great Barrier Reef waters.

Analysis of chlorophyll data collected during the program from inshore and offshore sites in the northern, central and southern Great Barrier Reef indicates that average chlorophyll a concentrations were significantly higher and more variable in nearshore waters than in samples collected further from the shore (figure 3 [format PDF, size: 612 kb]). Regionally, chlorophyll a concentrations were greatest in the Keppel Bay/Capricorn cluster. These high chlorophyll a concentrations were related to the presence of Trichodesmium aggregations, which were present in over 30% of all samples (Steven et al. 1998).

Shallow nearshore Great Barrier Reef waters are subject to river run-off, urban point-source discharges and wind-forced re-suspension of suspended sediment loads. Each of these factors contributes variable and irregular low-level increases of nutrients to the nearshore water column which, in turn, result in variable and irregular elevations in biological production in shallow waters.

In addition to cross-shelf variations in chlorophyll concentration, there are persistent and significant regional (quasi-latitudinal) differences in mean chlorophyll concentrations through the whole of the Great Barrier Reef (Brodie et al. 1997; Haynes et al. 1998; Brodie 1997; Brodie and Furnas 1996). These differences arise as a consequence of the diverse geographic structure of the Great Barrier Reef shelf at the regional scale and from the combination of marine, atmospheric and terrestrial nutrient sources to Great Barrier Reef waters (Furnas et al. 1997).

The 1993–1998 chlorophyll time series (figure 4) illustrates local variability in chlorophyll concentrations under non-disturbed conditions as concentrations of the phytoplankton are representative of the higher concentrations present inshore of the lagoon. The length of this time series, however, is too short to resolve long-term trends in phytoplankton biomass as a proxy for nutrient loading.

Broadscale, long-term chlorophyll a sampling in the Great Barrier Reef reveals significant regional differences in chlorophyll concentrations and variability over time. However, the time scale over which the Great Barrier Reef Marine Park Authority’s monitoring program has been carried out is too short to resolve the issue of large-scale eutrophication in Great Barrier Reef waters. Further sampling over longer time scales and correlation with adjacent land use and proximity of river mouths to position of transects and chlorophyll a concentrations may provide new information on the status of water quality in the Great Barrier Reef lagoon. Data from this project will ultimately help quantify latitudinal and cross-shelf trends, and allow closer correlation to be drawn between the quality of Great Barrier Reef waters and changing land-use practices.

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