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 theANTIPROD 3 cruise (autumn 1987) at six stations located in the Scotia Sea and analysed forbiogenic 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 10stations situated in the Scotia Sea, the Drake Passage, the Bransfield Strait and near the ice edge.While nutrient levels in surface waters were always high, phytoplankton biomass was typicallyquite low during this season, as demonstrated by the low concentrations of chlorophyll a (<0.3 mgm-3), BSi (< 1/~mol 1-1), POC (<6/~mol 1-1) and PON (< 1/~mol I-1). Nevertheless enhancedconcentrations were measured in the vicinity of the Polar Front in the Scotia Sea (Chl a = 1.06 mgm -a, BSi = 2.32/~mol 1-1 ) and within a diatom-dominated phytoplankton patch (Chl a = 0.48 mgm-3; BSi = 4.24/~mol 1-1) in the Drake Passage, within the area influenced by the AntarcticDivergence. In surface waters the contribution of lithogenic to total particulate silica was generallylower than 20%. However in the vicinity of icebergs, the Antarctica coast, as well as in areasreceiving eolian inputs in the northern part of the study area, the proportion of LSi was muchhigher (rising to 89%). Concentrations of both biogenic and lithogenic silica were uniformly low inWarm Deep Water (<0.2/~mol 1-1). The dramatic enhancement of their concentrations in bottomwaters (where BSi reached levels as high as 2.6/~mol 1-1) is interpreted as resulting mainly fromlocal resuspension due to bottom currents. The mean POCJPON mole ratio in the surface layer was6.4 + 0.3, but increased to 10.1 + 0.6 in the deep and bottom water, which suggests that thenitrogen recycling is more rapid than that of carbon in Antarctic deep waters. BSi/POC mole ratiosin surface waters exhibited values two to three times higher than typically found in pure diatomcultures. These results are consistent with previous observations in other sectors of the SouthernOcean showing that the biogenic particulate material is unusually rich in sifica