In summer 1997-98 a toxic bloom of the blue-green algae Anabaena circinalis and Anabaena spiroides occurred in the Canning River near Perth, WA, contaminating the river and precipitating river closure to the public for bathing and fishing. We execute a simple analysis to determine the important nutrient fluctuations a) leading to the instigation of the bloom and b) leading to bloom termination, in the context of ongoing remediation efforts including artificial oxygenation of the river. We conclude that even at the extremely high maximum concentrations observed (0.25 mg chlorophyll L-1), the cyanophytes had taken up only 15% of the P available to them from sediment efflux during summer anoxia. Evidence suggests the vast majority of the released P (3970 mg m-2) was either left unassimilated in bottom waters or taken up by non-photosynthetic bacteria in bottom waters, and that these waters were isolated from the Anabaena spp. bloom by strong temperature stratification. This means that any disruption of stratification by oxygen addition intended as a remediation measure risks creating greater problems of water quality by bringing huge bottom water P stores to the surface with no bacterial sink. Early seasonal oxygenation and / or full prevention of anoxic sediment P release would help cap nutrient release locally and avoid this problem.