Profiles of silicic acid, chlorophyll a, biogenic silica, and lithogenic silica concentrations and the rate of silica production were obtained from 7 depths in the upper 100 to 200 m at 8 stations in the western Sargasso Sea between May 7 and 18, 1989. Stations were distributed from the southern edge of the Gulf Stream to about 400 km south of Bermuda. An additional set of profiles was obtained at Stn S near Bermuda on March 28, 1989. Silicic acid concentrations in the euphotic zone were generally between 0.6 and 0.9 mu M. Biogenic silica concentrations ranged from 7 to 1400 nmol Si l(-1) with concentrations <50 mmol Si l(-1) being typical of stations south of 35 degrees N well away from the Gulf Stream and its eddies. Specific production rates of biogenic silica (V-b) averaged 0.16 d(-1) (range: 0.01 to 0.67 d(-1)) cor responding to an average doubling time for the diatoms of 4.2 d. Values of V-b were lowest between 32 and 34 degrees N with higher values observed both to the north and south. The doubling times of the diatom assemblages at stations south of 35 degrees N (2 to 20 d), away from the Gulf Stream and its eddies, were in the same range as those reported for the picophytoplankton in the Sargasso Sea suggesting that diatoms can grow as fast as the more numerous picophytoplankton in oligotrophic oceans. Subsurface maxima in biogenic silica concentration, chlorophyll biomass and silica production rates were observed near the top of the nitracline at 7 of the 9 stations. Integrated silica production rates between the surface and the 0.1% light depth were generally between 0.2 and 0.7 mmol Si m(-2) d(-1) with an average of 47% of that production occurring within the nitracline. If diatoms taking up silicic acid within the nitracline also utilize nitrate, they may account for between 0.56 and 0.84 mol C m(-2) yr(-1) of new production which is 16 to 24% of the estimated annual total new production for the region.
