Silica production in the Sargasso Sea during spring 1989

Abstract

Profiles of silicic acid, chlorophyll a, biogenic silica, and lithogenic silica concentrations and the rate of sllica 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 distnbuted 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 Sillcic acid concentrations in the euphotic zone xvere generally between 0 6 and 0 9 pM Blogenic sillca concentrations ranged from 7 to 1400 nmol S1 1-' with concentratlons c 5 0 nmol S1 1-' being typical of stations south of 35" N well away from the Gulf Stream and its eddies Specific production rates of biogenic silica (V,) averaged 0 16 d-l (range 0 01 to 0 67 d-') colresponding to an average doubling time f o ~ the diatoms of 4 2 d Values of V, wele loxvest between 32 and 34"N with higher values observed both to the north and south The doubling times of the diatom assemblages at stations south of 35" N (2 to 20 d ) , away from the Gulf Stredm and ~t s e d d ~e s , were in the same range as those reported for the picophytoplankton In the Sargasso Sea s u g g e s t ~n g that diatoms can gioxv as fast as the more numerous plcophytoplankton in oligotrophic oceans Subsurface maxima in biogenic silica concentration, chlorophyll biomass and silica production rates were observed neai the top of the nitracl~ne at 7 of the 9 stations Integrated silica production rates between the surface and the 0 1 % light depth were generally betxveen 0 2 and 0 7 inmol Si md-l with an aveiage of 47 % of that production occurring within the nitracline If diatoms taking up silicic acid xvithin the nitracline also ut~lize nitrate, they may account for between 0 56 and 0 84 m01 C m-' yr.' of nexv production lvhich is 16 to 24 % of the estimated annual total new production for the region KEY WORDS Diatoms .