A recurrent coastal sediment plume (RCP) is an episodic event in the southern basin of Lake Michigan that typically coincides with the spring diatom bloom. Strong winter storm activity during El Ni¿o conditions in 1998 resulted in a large and intense RCP event. Consistently higher values of the light-saturated rate of photosynthesis, PmaxB, were observed in spring 1998 compared to 1999 and 2000. Higher values of PmaxB in 1998 appeared to be related to increased availability of phosphorus, as evidenced by significant correlations of PmaxB with soluble reactive phosphorus (SRP). Light-saturated growth rates were also significantly correlated with SRP concentrations. These findings were consistent the view that the RCP was a source of enrichment. However, incubation experiments involving lake water enriched with sediments showed relatively small increases in growth and photosynthetic parameters, while enrichments with river water exhibited elevated rates. This result, along with increased levels of river discharge in 1998 and high levels of dissolved phosphorus in river water, supported the view that riverine inputs rather than the RCP were responsible for the higher photosynthetic parameters and growth seen for coastal margin assemblages. Despite the higher levels of PmaxB in 1998, model analyses revealed that reduced light availability resulting from the intense RCP event constrained phytoplankton growth rates and primary production during this season and apparently suppressed the development of a typical spring bloom. These findings indicate a potential for reduced ecosystem productivity in response to extreme storm events, the frequency of which may increase with projected long-term climate changes.