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

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