Diatom production in the offshore waters of Lake Michigan is limited by silica supplies in late summer and can be predicted from the seasonal disappearance of silica from the trophogenic zone. Biogenic silica fluxes obtained from sediment trap collections were compared with fluxes predicted from the silica disappearance model. Measured and predicted fluxes were in good agreement for offshore stations, but at stations closer to shore measured fluxes were 2-4 times greater than predicted fluxes. Sediment trap and water chemistry data show that little of the diatom production is dissolved in the water column. Therefore, greater than predicted fluxes at nearshore locations were attributed to new silica supplied from upwellings, tributary inputs, and recycling and resuspension in nearshore sediments. (Copyright (c) American Society of Limnology and Oceanography, Inc. 1984.)