CYCLING OF BIOGENIC SILICA WITHIN THE UPPER WATER COLUMN OF THE ROSS SEA

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-degrees-30'S in mid-January and early February 1990. There was a persistent (at least 3 1/2 weeks in duration) diatom bloom within a surface meltwater lens extending 100- 1 50 km seaward from the edge of the receding pack ice, with biogenic silica concentrations frequently exceeding 20-mu-mol l-1 in the upper 15 m. There were also other significant maxima in biogenic silica, with concentrations greater than 7-mu-mol l-1 at distances of 250-500 km seaward of the ice edge. These maxima were apparently unrelated to meltwater effects. Si-30 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 sea-bed, indicates that virtually all silica exported from the upper 50 m must reach the sea-floor unless advective transport from the east is significant.