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Benthic microalgal biomass was measured as sediment chlorophyll a at two open ocean beaches in southeastern North Carolina between December 2003 and December 2004, spanning beach renourishment projects in spring 2004 at both beaches. Sampling design included replicate sampling (n = 6) at three elevations in the surf zone along paired transects at paired renourished and unnourished (control) sites at each beach 17 times during the study period. Grain-size analyses of beach sediments and measurements of water temperature were also conducted. Sediment grain size, expressed as mean grain size and percentage of total sediments <500 pm diameter, significantly affected sediment chlorophyll a values. Renourishment drove a small and temporary but significant increase in sediment chlorophyll a at one beach, which was interpreted as an effect of source material but otherwise had no detectable negative effects. The grain-size effects reinforce the importance of efforts to match material used for renourishment with existing beach sediments because benthic microalgae likely form a significant portion of the base of the food chain in surf-zone communities. [ABSTRACT FROM AUTHOR]