Nutrient and chlorophyll concentrations
in the Great Barrier Reef lagoon
TEMPORAL TRENDS

---

Jon Brodie

o determine the impact of human activities, particularly land-use practices, on the water quality of the Great Barrier Reef shelf, it is essential to define naturally occurring concentrations of nutrients and particulates in Great Barrier Reef waters. Natural variability, the sources of nutrients and sediments and the in situ processes whereby nutrients affect water quality and Great Barrier Reef ecosystems also need to be understood.

As part of the oceanographic studies carried out by the Australian Institute of Marine Science (AIMS) within the Great Barrier Reef region over the last 20 years, a large body of nutrient and other water quality related information has been collected. While most of these data were not collected explicitly to address water quality issues, they have the largest temporal and spatial coverage of any data set and the advantage of having been collected and analysed in a consistent manner. The trends summarised in this article are derived from these data, which were collected mostly by the AIMS Biological Oceanography Group between 1976 and the present. In addition, chlorophyll data from the Great Barrier Reef Marine Park Authority's long-term chlorophyll monitoring network (to be published as a Research Publication shortly - Steven et al., in press) is also included.

Much of the raw data have been published in data reports. Miles Furnas and co-workers at AIMS and the Great Barrier Reef Marine Park Authority (GBRMPA) have now summarised and analysed the data and two recent papers summarise spatial and temporal trends in the data (Furnas and Brodie 1996; Brodie et al., in press). Details of sampling procedures and chemical analysis methods are given in the specific reports. With minor changes, sampling practices, sample handling and analytical methods have been stable throughout the period. The figures in this article have been adapted from the papers cited above.

Figure 1. Mean water column chlorophyll concentrations measured in the central Great Barrier Reef lagoon (18°S - 19°30'S) between December 1975 and March 1996 (see text for details) - summer cumulative mean, -- winter cumulative mean

Mean concentrations shown in figure 1 vary over a wide range from ~0.1 to 1.3 µg/L. Stations directly affected by cyclonic disturbance (flood plumes, extensive sediment resuspension) were excluded from the analysis. These have measured chlorophyll concentrations of up to 20 µg/L (Brodie and Furnas 1996). Seasonal patterns in the data are apparent with the long-term mean summer value of 0.45 µg/L significantly different to the winter value of 0.33 µg/L. The seasonal regressions against time, shown in the figure, yield slopes not significantly different from zero. This analysis does not then support a case for an increase in chlorophyll concentrations (e.g. Bell 1992) and hence phytoplankton biomass, in the outer water of the central Great Barrier Reef over the last 20 years. The means are lower than, but close to, values of chlorophyll (0.4-1.0 µg/L) associated with eutrophication and reduced coral growth in Barbados (Tomascik and Sanders 1985). The mean concentrations are approximately twice those measured (< 0.2 µg/L) in the adjacent Coral Sea in the East Australian Current.

Figure 2. Changes in the concentrations of nitrogen and phosphorus species in coastal () and offshore () areas in the Cairns region between 1989 and 1995

Figure 3. Changes in the concentrations of water quality parameters in coastal () and offshore () areas in the Cairns region between 1989 and 1995 (PSU equivalent to o/oo)

The concentrations show distinct seasonal (between cruise) variability, particularly for dissolved inorganic species, with particulate and dissolved organic nitrogen the most stable parameters. Concentrations over this relatively short time series lack any overall temporal trend. Some peaks are clearly related to run-off events, as indicated by lowered salinity, notably in early 1991. Nitrate and suspended sediment concentrations are considerably below (almost one order of magnitude) those measured along the Barbados eutrophication gradient. Mean phosphate concentrations, however, are of similar order.

References Bell, P.R.F. 1992, Eutrophication and coral reefs - some examples in the Great Barrier Reef lagoon, Water Research 26: 553-568. Brodie, J.E. and Furnas, M.J. 1996, Cyclones, river flood plumes and natural water quality extremes in the central Great Barrier Reef, pp. 367-374, in HM Hunter, AG Eyles and GE Rayment (eds), Downstream Effects of Land Use, A National Conference on Downstream Effects of Land Use, held in Rockhampton, Queensland, Australia, 26-28 April 1995, Dept of Natural Resources, Queensland. Brodie, J.E., Furnas, M., Steven, A.D.L., Trott, L.A., Pantus, F. and Wright, M. in press, Monitoring chlorophyll in the Great Barrier Reef lagoon: Trends and variability, in Proceedings of the 8th International Coral Reef Symposium, Panama, June, 1996. Furnas, M.J. and Brodie, J.E. 1996, Current status of nutrient levels and other water quality parameters in the Great Barrier Reef, pp. 9-21, in HM Hunter, AG Eyles and GE Rayment (eds), Downstream Effects of Land Use, A National Conference on Downstream Effects of Land Use, held in Rockhampton, Queensland, Australia, 26-28 April 1995, Dept of Natural Resources, Queensland. Steven, A.D.L., Trott, L., Pantus, F. and Brooks, D. in press, Great Barrier Reef Water Quality Monitoring Network: Status Report 1, Research Publication Series, Great Barrier Reef Marine Park Authority, Townsville. Tomascik, T. and Sanders, F. 1985, Effects of eutrophication on reef-building corals: Growth rate of the reef building coral Monastrea annularis, Marine Biology 87: 143-155.

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