High biomass, low export regimes in the Southern Ocean

Abstract

2 AbstractThis paper investigates ballasting and remineralization controls of carbon sedimentation in the twilight zone (100-1000 m) of the Southern Ocean. Size-fractionated (<1 μm, 1-51 μm, >51 μm) suspended particulate matter was collected by large volume in-situ filtration from the upper 1000 m in the Subantarctic (55°S, 172°W) and Antarctic (66°S, 172°W) zones of the Southern Ocean during the Southern Ocean Iron Experiment (SOFeX) in January-February 2002.Particles were analyzed for major chemical constituents (POC, P, biogenic Si, CaCO 3 ), and digital and SEM image analyses of particles were used to aid in the interpretation of the chemical profiles.Twilight zone waters at 66°S in the Antarctic had a steeper decrease in POC with depth than at 55°S in the Subantarctic, with lower POC concentrations in all size fractions at 66°S than at 55°S, despite up to an order of magnitude higher POC in surface waters at 66°S. The decay length scale of >51 μm POC was significantly shorter in the upper twilight zone at 66°S ( e =26 m) compared to 55°S ( e =81 m).Particles in the carbonate-producing 55°S did not have higher excess densities than particles from the diatom-dominated 66°S, indicating that there was no direct ballast effect that accounted for deeper POC penetration at 55°S. An indirect ballast effect due to differences in particle packaging and porosities cannot be ruled out, however, as aggregate porosities were high (~97%) and variable.Image analyses point to the importance of particle loss rates from zooplankton grazing and remineralization as determining factors for the difference in twilight zone POC 3 concentrations at 55°S and 66°S, with stronger and more focused shallow remineralization at 66°S. At 66°S, an abundance of large (several mm long) fecal pellets from the surface to 150 m, and almost total removal of large aggregates by 200 m, reflected the actions of a single or few zooplankton species capable of grazing diatoms in the euphotic zone, coupled with a more diverse particle feeding zooplankton community immediately below.Surface waters with high biomass levels and high proportion of biomass in the large size fraction were associated with low particle loading at depth, with all indications implying conditions of low export. The 66°S region exhibits this "High Biomass, Low Export" (HBLE) condition, with very high >51 μm POC concentrations at the surface (~2.1 μM POC), but low concentrations below 200 m (<0.07 μM POC). The 66°S region remained HBLE after iron fertilization. Iron addition at 55°S caused a ten fold increase in >51 μm biomass concentrations in the euphotic zone, bringing surface POC concentrations to levels found at 66°S (~3.8 μM), and a concurrent decrease in POC concentrations below 200 m. The 55°S region, which began with moderate levels of biomass and stronger particle export, transitioned to being HBLE after iron fertilization. We propose that iron addition to already HBLE waters will not cause mass sedimentation events. The stability of an iron-induced HBLE condition is unknow...

Controls on mesopelagic particle fluxes in the Sub-Antarctic and Polar Frontal Zones in the Southern Ocean south of Australia in summer—Perspectives from free-drifting sediment traps

“…4 and 5): some were relatively compact and consisted of agglomerated faecal pellets, while others contained less distinct material, such as loose faecal matter, phyto detritus, or feeding structures or exopolymeric polysaccharide particles. Our observations are very similar to particles collected from the PFZ south of New Zealand using large volume in-situ filtration system (MULVFS) by Lam and Bishop (2007), who found intact faecal pellets as well as aggregates of faecal matter and marine snow. They concluded that the material was heavily processed by zooplankton-a view which we share based on our observations of large numbers of aggregates containing at least some faecal matter.…”

“…Recently, the term high biomass, low export condition (HBLE) has been suggested for settings like we found at P1 (Lam and Bishop, 2007). According to these authors the most important factors determining POC export during the Southern Ocean Iron Experiment (SOFeX) were biological processes that affected the fragmentation and remineralisation of large particles-coinciding with the view that faecal-aggregates usually are the main contributors to POC flux as previous studies have indicated (e.g.…”

Early spring mesopelagic carbon remineralization and transfer efficiency in the naturally iron-fertilized Kerguelen area

“…These conditions suggested that phytoplankton biomass had accumulated in the surface waters without significant export at that point, or that C had been channelled to higher trophic levels as suggested by Christaki et al (2014). Note that a negative relationship between primary productivity and surface carbon export efficiency has already been reported from previous studies in the Southern Ocean (Lam et al, 2007;Morris et al, 2007;Savoye et al, 2008;Jacquet et al, 2011a, b). Among possible explanations for the occurrence of highproductivity low export efficiency regimes in high-latitude systems Maiti et al (2013) mentioned differences in trophic structure, grazing intensity, recycling efficiency, high bacterial activity or increase in DOC export, but the exact reason remain unclear.…”

Strong contribution of diatom resting spores to deep-sea carbon transfer in naturally iron-fertilized waters downstream of South Georgia

“…Maiti et al (2013) demonstrated an inverse relationship between primary production and export efficiency in the Southern Ocean, contrasting with the positive relationship between primary production and export prevailing in most of the global ocean (Laws et al, 2011). These so called high biomass, low export regimes (HBLE, Lam and Bishop, 2007;Lam et al, 2011) appear to be characterized by enhanced heterotrophic microbial activity Christaki et al, 2014) and intense reprocessing and fragmentation of particulate matter by zooplankton. On the Kerguelen Plateau for example, net community production (NCP) was estimated to be 6.6 mol m À 2 yr À 1 (Jouandet et al, 2008), although only a small fraction of this NCP was exported out of the mixed layer (Rembauville et al, 2015b).…”