Stocks and fluxes of biogenic silica in the western oligotrophic equatorial Pacific

Abstract

During a cruise along the 165°E meridian (6°N–15°S), distributions of biogenic silica (BSi) and of chlorophyll a were studied. BSi ranged between 10 and 60 nmol L−1 in the nitrate‐depleted surface layer. A deep BSi maximum (DBSiM) layer was located at the base of the photic layer. The high mesoscale variability in BSi distribution is explained as resulting from a combination of physical and chemical forcings. For studying the response of the tropical/equatorial ecosystem to physical forcing, BSi is a more sensitive tracer than chlorophyll a. Daily variations of stocks and fluxes of biogenic silica in the 0–340 m layer were measured at a drifting station during a 5 day experiment at 0°S, 167°E. Displacement of the pycnocline occurred at the drifting station, allowing injection of new nutrients in the photic layer to which the ecosystem reacted by an increase of BSi in the DBSiM layer. We hypothesize that this BSi increase is explained by increased diatom growth (at a rate of 0.6 doublings d−1). At this drifting station, sediment traps were deployed below the photic layer (at 125, 175, 240, and 340 m depths) to estimate export fluxes of BSi. They ranged between 60 μmol m−2 d−1 at 125 m and 180 μmol m−2 d−1 at 340 m. Compared to a BSi production of 960 μmol m−2 d−1 measured 1 week later during the same cruise in similar oligotrophic equatorial waters, such low export fluxes suggest that dissolution of BSi might be high in the warm equatorial Pacific waters.