SUMMARY.1. Reduced total phosphorus concentrations in the summer which followed the addition of iron aluminium sulphate to White Lough failed to reduce significantly the phytoplankton. which continued to be dominated byOscitlatoria agardhiivar.isothrixSkuja.This species was present throughout the 4 years studied, forming over 50% of the algal volume in 80% of samples. In contrast, species which occurred principally in the summer months were found to be severely curtailed when sediment release of phosphorus was suppressed.2. The phosphorus: carotenoid ratio was used to assess the extent of phosphorus limitation because laboratory studies on O.agardhiivar.isothrixshowed that this ratio was a much beter indicator of cell phosphorus content than the phosphorus: chlorophyll a ratio. Reduced summer phosphorus concentrations in White Lough caused a transition from intermittent to continuous phosphorus limitation rather than a proportional reduction in the summer phytoplankton.3. Reduced autumn grazing pressure byDaphnia hyatinaLeydig allowed large algal populations to develop in the winters following phosphorus reduction despite a 50% decline in total phosphorus. The combination of increased winter phytoplankton and lower total phosphorus reduced soluble reactive phosphorus concentrations to less than 5,μg P l−1which in turn curtailed the spring diatom pul