MEPS 281:27-35 (2004)  -  doi:10.3354/meps281027

Effects of phosphorus on the growth and nitrogen fixation rates of Lyngbya majuscula: implications for management in Moreton Bay, Queensland

Ibrahim Elmetri1,2,*, Peter R. F. Bell1

1Division of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia 2Present address: Institute of Technology and Engineering, Massey University, Turitea, New Zealand

ABSTRACT: Significant acetylene reduction and therefore N2 fixation was observed for Lyngbya majuscula only during dark periods, which suggests that oxygenic photosynthesis and N2 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 N2 fixation rate of L. majuscula increased with phosphate (P-PO4) concentration up to a maximum value and thereafter remained constant. The P-PO4 concentrations corresponding to the maximum N2 fixation and maximum growth rates were ~0.27 and ~0.18 µM respectively and these values are denoted as the saturation values for N2 fixation and growth respectively. Regular monitoring studies in Moreton Bay, Queensland, show that concentrations of P-PO4 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-PO4 concentrations. Results from other studies suggest that the elevated P-PO4 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.

KEY WORDS: Lyngbya majuscula · Nitrogen fixation · Phosphorus · Continuous culture · Growth kinetics · Moreton Bay

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