Abstract The relative importance of the additions of iron (Fe), phosphorus (P) and the chelating agent EDTA in promoting the growth of Lyngbya majuscula in waters collected from the NW region of Moreton Bay was investigated using a series of continuous-flow growth studies. In addition, the possible impact of sewage/waste-water treatment plant (WWTP) discharges on the growth of L. majuscula was investigated in a series of batch and semi-continuous-flow growth studies. A preliminary study on the potential of phytoplankton growth in the receiving waters to affect the productivity of L. majuscula was also conducted. The results from the continuous-flow growth studies show that the growth rates of L. majuscula in the NW Moreton Bay waters were stimulated by the addition of EDTA alone but were not stimulated by the addition of P. The additions of P + EDTA, FeEDTA and P + FeEDTA did not result in higher growth rates than those obtained by the addition of EDTA alone. These results demonstrate that the productivity of L. majuscula in the NW Moreton Bay waters was not limited by P and that the addition of Fe did not affect the productivity. The stimulation effect of EDTA could be due to various reasons but we hypothesise that the principal reason for the stimulation is that EDTA increases the bioavailability of non-labile Fe species already present in the water. The results of the batch and semi-continuous-flow growth studies show that diluted (100:1) WWTP discharge water and the receiving waters impacted by WWTP discharges supported significant growth of L. majuscula and that the addition of EDTA to those waters increased the growth potential of L. majuscula. Also the growth of phytoplankton in a sample of the receiving waters impacted by WWTP discharges significantly reduced the growth rate of L. majuscula but the addition of EDTA restored the growth rate to near its maximum value. These results suggest that phytoplankton growth (and probably that of the associated bacterioplankton) could reduce the bioavailability of trace chemical factors needed for the growth of L. majuscula in Moreton Bay and in particular, could reduce the bioavailability of Fe. Overall the results support the hypothesis that growth of L. majuscula in Moreton Bay is often limited by the bioavailability of Fe and the principal reason for this is the lack of supply of suitable organic ligands/chelators, not the lack of Fe per se.
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