Diatom growth and productivity in an oligo-trophic midocean gyre: A 3-yr record from the Sargasso Sea near Bermuda

Abstract

We measured the rate of biogenic silica production in the upper 160 m at the Bermuda Atlantic Time-series Study (BATS) site in the western Sargasso Sea on 12 cruises between August 1991 and May 1994 using a 30Si tracer method. Previously initiated time series of particulate silica concentrations and the gravitational flux of siliceous particles were extended to produce an October 1988-December 1994 time series for biogenic silica concentrations and August 1991-December 1994 time series for lithogenic silica concentrations and fluxes of biogenic and lithogenie silica. Specific rates of biogenic silica production during nonbloom periods indicate active diatom growth, with a mean doubling time of 4.5 d for biogenic silica. The annual silica production rate is conservatively estimated to be 240 mmol Si m-2, which corresponds to a diatom productivity of -20-45 g C m-2 yr-I, -15-25% of the mean annual primary productivity. Comparison of silica production rates with sinking fluxes from the upper 150 m (26-53 mmol Si m-2 yr-I) indicates that -80% of the silica produced in the euphotic zone dissolves in the upper 150 m. Sinking of diatoms and their remains can account for -30% of the particulate organic carbon export at the BATS site on an annual basis and nearly 100% during the annual winter/spring diatom bloom. Atmospheric dust inputs each summer, indicated by July-August maxima in the concentration and vertical flux of lithogenic silica, do not stimulate either biogenic silica production or primary productivity, implying that iron availability limits neither diatom growth nor total primary production in the western‘ Sargasso Sea. Most of the primary production in the oceans occurs in midocean gyres and other oligotrophic systems-a seemingly contradictory situation that results from those systems’ enormous spatial extent (e.g. Ryther 1969). The phytoplankton in the midocean gyres is comprised mostly of small, nonsiliceous forms, with <l p,rn prokaryotes (e.g.