Carbon export production in the subantarctic zone and polar front zone south of Tasmania

Abstract

We studied the water column distribution of total 234Th in subantarctic (SAZ) and polar front (PFZ) zone systems south of Tasmania during mid-austral summer 2007. The objective was to assess whether the observed zonal differences in biomass in this sector translated into variability of the carbon export and sequestration potential, and to identify possible causes inducing this variability. This study is part of a broader investigation focusing on macro- and micronutrient availability controlling ecosystem functioning in this area. Surface deficits in 234Th activities were observed at every station. 234Th export fluxes calculated from the 234Th activity deficits assuming steady state conditions showed higher 234Th export fluxes in the western than eastern SAZ, both of which were higher than those in the PFZ. 234Th fluxes sampled by free-drifting IRS and PPS-3 sediment traps at 150 and 170 m during short-term deployments (∼ 6 days) at the three process stations were significantly lower than those obtained by the 234Th-deficit method. Possible reasons for this discrepancy are discussed. Carbon export fluxes were calculated based on the total 234Th fluxes ( 234Th deficit method) and the C:Th ratio for the >54 μm particle size fraction from the appropriate export depth. The >54 μm C:Th ratio was significantly lower in the eastern SAZ than the western SAZ or PFZ, resulting in carbon export fluxes that were lowest in the eastern SAZ. Overall, export fluxes range from 3.6±1.5 to 13.2±3.1 mmol C m −2 d −1. Carbon export fluxes are compared with gross primary production, new production and mesopelagic remineralization fluxes obtained by others during the same cruise. Contrary to expectations, we found higher export production in the PFZ and the western SAZ where biomass and dissolved Fe were lower than in the eastern SAZ. Significant differences in community structure of both primary producers and consumers likely contributed to this difference between the three regions. This pattern of higher shallow export continued into the mesopelagic, with lower remineralization efficiency in PFZ and SAZ-West compared to SAZ-East. Within the SAZ, east and west sites thus differed in their efficiency of carbon sequestration into the deep (>600 m) water column, with SAZ-West exceeding the sequestration capacity of SAZ-East. If the SAZ-East region can be used as an analog for a future climate warming scenario, our results suggest that export production at high latitudes could decrease, despite increased primary production.