Abstract
During the Austral summer of 2012, a large research cruise, ANT-XXVIII/3, investigated a transect of the Southern Ocean from Capetown, South Africa to Punta Arenas, Chile. Samples were taken from three iron regimes: a high nutrient, low chlorophyll, iron-
deficient region at 10 oE (st. 84), west to a transient bloom that was subject to sporadic iron input at 12 oW (st. 86 – st. 140) and near the iron-replete South Georgia Island area (st. 174).
Despite the iron-rich region of South Georgia Island having 1.6 times more integrated
Chla (77 mg/m2) than st. 84 (49 mg/m2) st. 174 had the lowest fluxes in POC (0.08 g/m2/d at 100 m) and BSi (0.15 g/m2/d at 100 m), while fluxes at station 84 were within the higher range of fluxes observed during the study (0.2 g/m2/d POC and 1.6 g/m2/d BSi, 100 m). Chla was much higher in the 12oW bloom with an average of 128 mg/m2 with the highest and lowest values appearing on opposite edges of the bloom; st. 87 (NW edge) had the lowest chla of any station with 60 mg/m2 but the highest fluxes (0.4 g/m2/d POC and 3.8 g/m2/d BSi at 100 m) while station 86 on the SE edge of the bloom had the highest Chla with 187 mg/m2 but fluxes near the average (0.2 g/m2/d POC and 0.5 g/m2/d BSi at 100 m).
These variations in surface production and export could be explained by 1) natural bloom progression, i.e. temporal decoupling of nutrient exhaustion, indicating the demise of
the bloom, and sedimentation from the surface layer, 2) vertical mixing at the periphery of the bloom which slows sedimentation by diluting the bloom and preventing aggregates from reaching critical mass, and/or, 3) zooplankton interactions including grazing, remineralization and repackaging. The results suggest zooplankton interactions are the driving force behind
variations in export and flux attenuation (particularly in the high biomass, low export South Georgia region), but are probably supported by both vertical mixing and temporal decoupling.
