Measured and predicted fluxes of biogenic silica in Lake Michigan’

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 2-3 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. The good agreement between measured and predicted fluxes in offshore waters shows that biogenic silica fluxes may be useful in determining the collecting efficiency of sediment traps. Lake Michigan provides an ideal system for the study of seasonal dynamics of the silica cycle. The annual seasonal change in concentration of soluble silica is 1.3 mg’ liter-’ (Rousar 1973)-the largest seasonal disappearance among the five Laurentian Great Lakes (Schelske et al. 1983). Lake Huron and Lake Superior have larger concentrations of soluble silica than Lake Michigan but, because phosphorus concentrations are lower, there is less silica disappearance or diatom production than in Lake Michigan. Because of excessive phosphorus inputs, epilimnetic diatom production in Lake Michigan is limited by available supplies of silica during summer stratification (Schelske and Stoermer 197 1). The supply of silica for diatom growth is maintained mainly by rccycling (dissolution) of biogenic silica (Conway et al. 1977), with ~5% of the silica required annually for diatom production being supplied from tributary inputs (Parker et al. 1977). Only one paper has been ’ This is Contribution 356, Great Lakes Research Division (University of Michigan), and Contribution 320, Great Lakes Environmental Research Laboratory (NOAA). The research was supported in part on US. EPA grants R-806294 and R-804503 to C.L.S. published on biogenic silica fluxes obtained from sediment trap collections in the Great Lakes (Parker et al. 1977). Silica disappearance resulting from biological processes has been studied indirectly in several marine and coastal systems (see Paasche and Ostergrcn 1980). However, according to Paasche and Ostergren (1980) there are few quantitative data on the subsequent fate of silica in the water column, due to the lack of an analytical method for biogenic silica in seawater. We present here data on biogenic silica fluxes from two independent types of field experiments and compare these data with predicted fluxes calculated from a silica disappearance model (Schclske 1975). Independent estimates of biogenic silica fluxes were obtained from sediment trap collections and from observed changes in total silica concentrations (soluble plus biogenic) in the trophogcnic zone. Measured fluxes from sediment traps agreed well with predicted and calculated fluxes for offshore stations; however, at stations closer to shore and the Grand River, biogenic silica fluxes were 2-3 times greater than predicted by the disappearance model. Our hypothesis is that the increased flux at the nearshore sta-