Production, dissolution, accumulation, and potential export of biogenic silica in a Sargasso Sea mode‐water eddy

Abstract

Rates of biogenic silica (bSiO2) production, dissolution, accumulation, and potential bSiO2 export were determined in a Sargasso Sea mode‐water eddy (MWE) during two occupations in April 2007. Standing stocks and production rates of bSiO2 within the MWE were 15–20 mmol Si m−2 and 0.8–1.1 mmol Si m−2 d−1, respectively; these values were about six times those reported in April at the nearby Bermuda Atlantic Time‐series Study (BATS) site. The MWE also showed positive net bSiO2 production in the upper 140 m (> 0.5 mmol Si m−2 d−1 produced during both occupations), and thus a capacity for bSiO2 accumulation and export. Consistent with previous observations in eddies, the diatom community was dramatically different from that typically observed at the BATS site; diatom abundances were an order of magnitude higher, and larger forms (e.g., Chaetoceros sp.) were prominent. When accounting for both the loss of bSiO2 (between occupations) and the observed positive net bSiO2 production within the MWE, the estimated bSiO2 export rate (1.27 mmol Si m−2 d−1) was twenty times higher than the mean export flux measured at the BATS site with no bloom in progress. If similar processes occur in eddies of other types, then these mesoscale features may be quantitatively important in the regional silica balance, with a strong coupling between increased bSiO2 export and altered diatom community structure.