The distribution of biogenic and lithogenic silica and the composition of particulate organic matter in the Scotia sea and the Drake passage during autumn 1987

Abstract

Suspended matter was collected from surface, deep and bottom waters during the ANTIPROD 3 cruise (autumn 1987) at six stations located in the Scotia Sea and analysed for biogenic silica (BSi), lithogenic silica (LSi), particulate organic carbon (POC) and nitrogen (PON). The composition of suspended particulate matter also was determined in the photic layer at 10 stations situated in the Scotia Sea, the Drake Passage, the Bransfield Strait and near the ice edge. While nutrient levelsin surface waters were always high, phytoplankton biomass was typically quite low during this season, as demonstrated by the low concentrations of chlorophyll a (<0.3 mg m −3), BSi (<1 μmol l −1), POC (<1 μmol l −1). Nevertheless enhanced concentrations were measured in the vicinity of the Polar Front in the Scotia Sea (Chl a = 1.06 mg m −3, BSi = 2.32 μmol l −1) and within a diatom-dominated phytoplankton patch (Chl a = 0.48 mg m −, BSi = 4.24 μmol l −1) in the Drake Passage, within the area influenced by the Antarctic Divergence. In surface waters the contribution of lithogenic to total particulate silica was generally lower than 20%. However in the vicinity of icebergs, the Antarctica coast, as well as in areas receiving eolian inputs in the northern part of the study area, the proportion of LSi was much higher (rising to 89%). Concentrations of both biogenic and lithogenic silica were uniformly low in Warm Deep Water (<0.2 μmol l- −1). The dramatic enhancement of their concentrations in bottom waters (where BSi reached levels as high as 2.6 μmol l −1) is interpreted as resulting mainly from local resuspension due to bottom currents. The mean POC/PON mole ratio in the surface layer was 6.4±0.3, but increased to 10.1±0.6 in the deep and bottom water, which suggests that the nitrogen recycling is more rapid than that of carbon in Antarctic deep waters. BSi/POC mole ratios in surface waters exhibited values two to three times higher than typically found in pure diatom cultures. These results are consistent with previous observations in other sectors of the Southern Ocean showing that the biogenic particulate material is unusually rich in silica.