The effect of marginal ice-edge dynamics on production and export in the Southern Ocean along 170°W

Abstract

We present a synthesis of the rates of gross, new, net and primary production along with particulate organic carbon (POC) flux at 100 m from four cruises along 1701W in the Southern Ocean. Concurrent satellite pigment data, a primary productivity model, and a nitrate mass balance are used to extrapolate daily production estimates in space and time to seasonal and annual rates. From this analysis, we gain a better understanding of the timing, magnitude and impact of the phytoplankton blooms in this region. One of the dominant features with respect to plankton biomass is the association of high chlorophyll levels with the retreat of the sea ice which begins in October just south of the Polar Front. Our primary production model and satellite pigment data suggest higher production and flux levels characterize the marginal ice zone than previous estimates. Elevated rates of new production in ice impacted regions are restricted to relatively short periods following retreat of the ice edge. Export is found to lag the onset of production by up to 1 month. The ratio of POC flux at 100 m to primary production when averaged over the entire season is quite high, increasing from 15% to 25% in the Subantarctic zones to 35-40% near the Polar Front and as high as 50-65% in the southernmost stations, just north of the Ross Sea gyre. Comparisons of phytoplankton community structure and Fe stress indicators suggest that blooms at the Polar Front are initially dominated by large centric diatoms, but are replaced by smaller pennate diatom and non-diatom species as Fe levels decrease. Further south, where Fe levels are never as high and large diatoms are not found, we still observe relatively high biomass and elevated production rates and POC fluxes during the short growing season. r

High particulate organic carbon export during the decline of a vast diatom bloom in the Atlantic sector of the Southern Ocean

“…stations 139 and C136) but high F Th evidenced a temporal decoupling between production and export. Buesseler et al (2003Buesseler et al ( , 2001a, Rutgers van der Loeff et al (2011) and Henson et al (2015) showed that export lags production by up to 1-2 months in the Southern Ocean, which is a time frame compatible with the present study.…”

“…The export efficiency was around 10% according to the ST, which is common in the open ocean (Buesseler, 1998), whereas it was around 25% considering the SWST method. Besides these differences, the export efficiency was always r 50% for the ST and SWST methods, contrasting with export efficiencies 450% reported during blooms at high latitudes, mostly characterized by large diatoms (Buesseler, 1998;Smetacek et al, 2012) and a combination of Phaeocystis and diatoms (Buesseler et al, 2003;Poulton et al, 2007). Other Southern Ocean studies Planchon et al, 2015) have also reported export efficiencies of $ 10% during not yet declining diatom blooms, where low POC export fluxes were measured (7-8 mmol C m À 2 d À 1 ).…”

High biomass, low export regimes in the Southern Ocean

“…In contrast, previous occupations of our sites along 170°W during the US JGOFS program between 1996-1998 showed higher maximum POC fluxes in the Antarctic compared to the Subantarctic at 100 m (Buesseler et al 2001) and 1000 m (Honjo et al 2000), and similar shallow and deep fluxes at both locations when averaged over the growing season (Buesseler et al 2003). Frontal/Antarctic Zones were fairly similar (Honjo et al 2000;Trull et al 2001).…”

An investigation of basin-scale controls on upper ocean export and remineralization

“…Previous 234 Th studies that traversed MIZs or partially ice-covered regions in the Southern Ocean (Buesseler et al, 2003;Rutgers van der Loeff et al, 2002;Savoye et al, 2004) and the Arctic (Amiel and Cochran, 2008;Cai et al, 2010;Ma et al, 2005) have suggested that processes at the ice-water boundary could lead to excesses and/or deficits of 234 Th with respect to 238 U that are atypical compared to those observed at low latitudes. In some cases, only excesses have been observed with no deficits (e.g.…”