Abstract
The lowest biogenic silica production rates in the Southern Ocean (average of 2.59 mmol Si m-2 d-1) have been recorded in an area of heavy ice cover along a transect through the Weddell Sea from Joinville Island to Cap Norvegia (November-December 1990). The associated biomass was also very low (concentrations less-than-or-equal-to 0.6 mumol l-1 for biogenic silica and less-than-or-equal-to 0.8 mug l-1 for chlorophyll a). Based upon these direct measurements of biogenic silica production rates and other data available from the marginal ice zone and open ocean areas, we estimated the annual production of biogenic silica in the northern Weddell Sea to be 810 to 870 mmol m-2 yr-1. This leads to a revised estimate of the total annual biogenic silica production in the Southern Ocean of between 11 and 32 Tmol Si yr-1. Comparing our annual production estimate to previous estimates of vertical flux of opal in the Weddell Sea, we conclude that no more than 1 % of the silica produced annually by phytoplankton in the upper water column reaches a depth of 800 m. This is consistent with the general distribution of high accumulation rates of opal in Southern Ocean sediments which evidence an unexplained gap in the Weddell Sea. Thus, regarding the cycling of biogenic silica in the Southern Ocean, the Weddell Sea appears to be atypical.
Highlights
The major role of the Southern Ocean in the global production of biogenic silica (BSi) has been emphasized by many studies
Treguer & van Bennekom (1991) gave an estimate of 50 Tmol Si yr-' for the total annual production of BSi in the Southern Ocean, but large uncertainties remain mostly because of imprecisions in the estimates for several of the major subsystems that have been identified in this area (Treguer & Jacques 1992)
Much attention has been given to the silica cycle of the seasonal ice zone (SIZ) especially in the Weddell Sea
Summary
The major role of the Southern Ocean in the global production of biogenic silica (BSi) has been emphasized by many studies (review in Treguer & van Bennekom 1991). Treguer & van Bennekom (1991) gave an estimate of 50 Tmol Si yr-' for the total annual production of BSi in the Southern Ocean, but large uncertainties remain mostly because of imprecisions in the estimates for several of the major subsystems that have been identified in this area (Treguer & Jacques 1992). Much attention has been given to the silica cycle of the seasonal ice zone (SIZ) especially in the Weddell Sea. BSi standing stocks in that system have been reported for winter (Cota et al 1992),spring (Nelson et al 1987, Leynaert et al 1991, Qukguiner et al 1991, Treguer et al 1991), and late summer (Nelson et al 1989).Direct measurements of BSi production rates in the Weddell Sea have been reported only for spring (Queguiner et al 1991,Treguer et al 1991). 2 indirect estimates of seasonal BSi production have
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