Cycling of biogenic silica within the upper water column of the Ross Sea

Abstract

We determined the distributions of biogenic particulate silica and dissolved silicic acid in the upper 80 m over the continental shelf of the southern Ross Sea, Antarctica, during two occupations of an east-west transect at 76°30′S in mid-January and early February 1990. There was a persistent (at least 3½ weeks in duration) diatom bloom within a surface meltwater lens extending 100–150 km seaward from the edge of the receding pack ice, with biogenic silica concentrations frequently exceeding 20 μmol l −1 in the upper 15 m. There were also other significant maxima in biogenic silica, with concentrations greater than 7 μmol l −1 at distances of 250–500 km seaward of the ice edge. These maxima were apparently unrelated to meltwater effects. 30Si tracer experiments to measure the production and dissolution rates of biogenic silica as a function of depth within the upper 50 m at three stations within the ice-edge diatom bloom indicate that the specific production rate (i.e. the rate per unit of biogenic silica present) ranged from 0.05 to 0.12 day −1 ( 0.07–0.17 doublings day −1). The resulting vertically integrated silica production rates ranged from 27 to 50 mmol m −2 day −1, with a mean of 34. Vertically integrated rates of biogenic silica dissolution ranged from 16 to 28 mmol m −2 day −1, with a mean of 22, or 64% of the mean silica production rate. The mean resulting net rate of biogenic silica production within the bloom, 12 mmol m −2 day −1, leads to an estimated net annual silica production rate of 1.0 mol m −2 in the upper 50 m of the southwestern Ross Sea. A revised silica budget for the western Ross Sea, incorporating the most recent estimates of production, redissolution, accumulation in the sediments and efflux from the seabed, indicates that virtually all silica exported from the upper 50 m must reach the sea-floor unless advective transport from the east is significant.