Silica production in the Srgasso Sea during spring 1989

Abstract

Profiles of silicic acid, chlorophyll a, biogenic silica, and lithogenicsilica concentrations and the rate of silica production were obtained from 7depths in the upper 100 to 200 m at B stations in the western Sargasso Seabetween May 7 and 18, 1989. Stations were distributed from the southern edgeof the Gulf Stream to about 400 km south of Bermuda. An additional set ofprofiles was obtained at Stn S near Bermuda on March 20, 1989. Silicic acidconcentrations in the euphotic zone were generally between 0.6 and 0.9 μM.Biogenic silica concentrations ranged from 7 to 1400 nmol Si l-1 withconcentrations < 50 nmol Si l-1 being typical of stations south of 35°Nwell away from the Gulf Stream and its eddies. Specific production rates ofbiogenic silica (V(b)) averaged 0.16 d-1 (range: 0.01 to 0.67 d-1)corresponding to an average doubling time for the diatoms of 4.2 d. Values ofV(b) were lowest between 32 and 34°N with higher values observed both to thenorth and south. The doubling times of the diatom assemblages at stationssouth of 35° N (2 to 20 d), away from the Gulf Stream and its eddies, werein the same range as those reported for the picophytoplankton in the SargassoSea suggesting that diatoms can grow as fast as the more numerouspicophytoplankton in oligotrophic oceans. Subsurface maxima in biogenicsilica concentration, chlorophyll biomass and silica production rates wereobserved near the top of the nitracline at 7 of the 9 stations. Integratedsilica production rates between the surface and the 0.1% light depth weregenerally between 0.2 and 0.7 mmol Si m-1 d-1 with an average of 47% ofthat production occurring within the nitracline. If diatoms taking up silicicacid within the nitracline also utilize nitrate, they may account for between0.56 and 0.84 mol C m-2 yr-1 of new production which is 16 to 24% of theestimated annual total new production for the region.