No abstract

Studies in the Great Barrier Reef Lagoon (GBRL) show that Trichodesmium (OsciUatoria) spp. fix significant amounts of atmospheric nitrogen and that the loads of 'new' nitrogen introduced by 7'richodesm.um spp. are at least of the same order as those entering via riverine discharge. The elevated growth of other genera of phytoplankton and hence eutrophication in some sections of the GBRL could now be largely driven by the in situ production of this 'new' nitrogen. 7'richodesmium is more prolific in the Central GBRL than in the Northern GBRL and there is evidence that the concentrations of Trichodesmium spp. have increased since the 1928-29 Great Barrier Reef Expedition to Low Isles. It is hypothesised that this increase has resulted from increases in river borne nutrients that would promote N fixation (e.g. phosphorus, iron and dissolved organic matter). It is estimated that the loads of such nutrients have increased several fold since the development of the coastal catchments of Queensland.

Long-term monitoring data show that hard coral cover on the Great Barrier Reef (GBR) has reduced by [70 % over the past century. Although authorities and many marine scientists were in denial for many years, it is now widely accepted that this reduction is largely attributable to the chronic state of eutrophication that exists throughout most of the GBR. Some reefs in the far northern GBR where the annual mean chlorophyll a (Chl a) is in the lower range of the proposed Eutrophication Threshold Concentration for Chl a (*0.2-0.3 mg m ) will need to be decreased to *0.2 mg m -3 for sustaining coral reef communities.

The results from the multimillion dollar Enrichment of Nutrients on Coral Reefs Experiment (ENCORE) on One Tree Island Reef (OTIR) suggest that increased nutrient loads to coral reefs will have little or no effect on the algal growth rates and, hence, on the associated effects that increased algal growth might have on the functioning and stability of coral reefs. However, a comparison of the concentrations of nutrients within the OTIR lagoon with the proposed nutrient threshold concentrations (NTC) for coral reefs suggests that all sites, including the control sites, were saturated with nutrients during ENCORE, and, hence, one would not expect to get any differences between treatments in the algal-growth related measurements. Thus, ENCORE results provide strong support for the proposed NTCs and support the ecological principle that algal productivity and, consequently, the functioning of coral reefs are sensitive to small changes in the background concentrations of nutrients. The principal conclusion of ENCORE, namely that the addition of nutrients did not cause the "pristine" OTIR to convert from coral communities to algal dominated reefs, is contrary to the fact that there was prolific macroalgal growth on the walls and crests of the experimental microatolls by the end of ENCORE.

Significant acetylene reduction and therefore N 2 fixation was observed for Lyngbya majuscula only during dark periods, which suggests that oxygenic photosynthesis and N 2 fixation are incompatible processes for this species. Results from a series of batch and continuous-flow-culture reactor studies showed that the specific growth rate and N 2 fixation rate of L. majuscula increased with phosphate (P-PO 4 ) concentration up to a maximum value and thereafter remained constant. The P-PO 4 concentrations corresponding to the maximum N 2 fixation and maximum growth rates werẽ 0.27 and ~0.18 µM respectively and these values are denoted as the saturation values for N 2 fixation and growth respectively. Regular monitoring studies in Moreton Bay, Queensland, show that concentrations of P-PO 4 generally exceed these saturation values over a large portion of the Bay and therefore, the growth of the bloom-forming L. majuscula is potentially maximised throughout much of the Bay by the elevated P-PO 4 concentrations. Results from other studies suggest that the elevated P-PO 4 concentrations in the Bay can be largely attributed to discharges from waste-water treatment plants (WWTPs), and thus it is proposed that the control of the growth of L. majuscula in Moreton Bay will require a significant reduction in the P load from the WWTP discharges. If the current strategy of N load reduction for these discharges is maintained in the absence of substantial P load reduction, it is hypothesised that the growth of L. majuscula and other diazotrophs in Moreton Bay will increase in the future.