Deep Chlorophyll Maxima (DCMs) are ubiquitous in low-latitude oceans, and of recognized biogeochemical and ecological importance. DCMs have been observed in the Southern Ocean, initially from ships and recently from profiling robotic floats, but with less understanding of their onset, duration, underlying drivers, or whether they are associated with enhanced biomass features. We report the characteristics of a DCM and DBM (Deep Biomass Maximum) in the Inter-Polar-Frontal-Zone (IPFZ) south of Australia from CTD profiles, shipboard-incubated samples, a towbody, and a BGC-ARGO float. The DCM and DBM were ~20 m thick and co-located with the nutricline, in the vicinity of a subsurface ammonium maximum characteristic of the IPFZ, but ~100 m shallower than the ferricline. Towbody transects demonstrated that the co-located DCM/DBM was broadly present across the IPFZ. Large healthy diatoms, with low iron requirements, resided within the DCM/DBM, and fixed up to 20 mmol C m-2 d-1. The BGC-ARGO float revealed the DCM/DBM persisted for >3 months. We propose a dual environmental mechanism to drive DCM/DBM formation and persistence within the IPFZ: sustained supply of both recycled iron within the subsurface ammonium maxima and upward silicate transport from depth. DCM/DBM cell-specific growth rates were considerably slower than those in the overlying mixed layer, implying that phytoplankton losses are also reduced, possibly as a result of heavily silicified diatom frustules. The light-limited seasonal termination of the observed DCM/DBM did not result in a ‘diatom dump’, rather ongoing diatom downward export occurred throughout its multi-month persistence.

A document by Oliver M. Medd. Click on the document to view its contents.

Dissolved strontium (Sr) concentrations from Southern Ocean water samples and Sr export fluxes from sediment trap moorings at 1000 m were used to assess particulate organic carbon (POC) export associated with Acantharia for 2010, 2018 and 2020. The dissolved Sr data revealed a prominent vertical gradient with lower surface Sr concentrations, depleted up to 1.4% relative to deep waters. A strong latitudinal surface gradient was observed, ranging from 87.3 µmol kg-1 near the northern end to 88.5 µmol kg-1 near the southern end of a transect through the Australian sector of the Southern Ocean. These findings highlight the significant role that Acantharia, which precipitate celestite (SrSO4), play in marine Sr cycling. Seasonal variability in Sr export fluxes can be large, particularly during intense events in summer, and reaches a maximum of 11.7 µmol kg-1, contributing up to 7% of the POC export flux. The coincidence of Sr flux with the second peak of POC export flux implies a potential association of Acantharia biomass with summertime productivity.

Phytoplankton indirectly influence the climate, through their role in the ocean biological carbon pump. Hence factors limiting phytoplankton growth directly impact the strength of the biological carbon pump and consequently climate. In the Southern Ocean, the subantarctic zone represents an important carbon sink, yet variables limiting phytoplankton growth are not fully constrained. Co-limitation by iron (Fe) and manganese (Mn) has recently been observed in the coastal and offshore Southern Ocean, but very few studies have focused on the subantarctic zone. In addition, no study has investigated the seasonal variability of Mn (co-)limitation of phytoplankton growth in the Southern Ocean. Using three shipboard bioassay experiments, we evaluated the seasonality of Fe and Mn co-limitation of subantarctic phytoplankton growth, south of Tasmania. We observed a strong seasonal variation in phytoplankton Fe limitation, and that the response of phytoplankton to Mn was subtle and thus readily masked by the responses to Fe. Combined addition of Fe and Mn enhanced carbon uptake of nanoeukaryotes in spring and microeukaryotes in summer while the addition of Mn alone stimulated the growth of picocyanobacteria in autumn. These results suggest the importance of Mn may vary seasonally and its control on phytoplankton growth may be associated with specific taxa.

In the Subantarctic sector of the Southern Ocean, vertical entrainment of dissolved iron (DFe) triggers the seasonal productivity cycle. However, diminishing physical supply of new Fe during the spring to summer transition rapidly drives epipelagic microbial communities to rely upon recycled DFe for growth. Hence, subpolar waters evolve seasonally from a high fe ratio system (i.e., [uptake of new Fe]/[uptake of new+recycled Fe]) to a low fe ratio system. Here, we tested how resident microbes within a cyclonic eddy respond to different Fe/ligand inputs which mimic entrained new DFe (Fe-NEW), diffusively-supplied regenerated DFe (Fe-REG), and a control with no addition of DFe (Fe-NO). After 6 days, 3.5 (Fe-NO, Fe-NEW) to 5-fold (Fe-REG) increases in Chl a were observed despite ~2.5-fold range between treatments of initial DFe. Marked differences were also evident in the proportion of in vitro DFe derived from recycling to sustain phytoplankton growth (Fe-REG, 30% recycled c.f. 70% Fe-NEW, 50% Fe-NO). This trend supports the concept that DFe/ligands released from subsurface particles are more bioavailable than new DFe collected at the same depth. This additional recycling may be mediated by bacteria. Indeed, by day 6 bacterial production (BP) was comparable between Fe-NO and Fe-NEW but~2 fold higher in Fe-REG. Interestingly, a preferential response of phytoplankton (haptophyte-dominated) relative to bacteria was also found in Fe-REG. In contrast, in Fe-NEW and Fe-NO the proportion of diatoms increased. Hence, different modes of Fe/ligand supply modify BP and Fe bioavailability to phytoplankton that may drive distinctive floristic shifts and biogeochemical signatures.

Chromium (Cr) has shown promise as a paleoceanographic proxy due to the redox-driven control of dissolved Cr concentrations ([Cr]) and stable isotope composition (δ53Cr). However, substantial uncertainties in the biogeochemical Cr cycle have limited its paleoproxy application to date. To improve the mechanistic understanding of Cr cycling in the modern ocean and strengthen its potential proxy applications, we present new data from regeneration incubations, bottom and sediment pore waters, and a compilation of intermediate and deep water data. While Cr removal and biological export from the surface ocean is associated with organic carbon export, the deep water release of dissolved Cr from sinking particles is not directly dependent on organic carbon respiration, as indicated by differing trends between Cr, oxygen utilization and the regeneration of organic-associated macronutrients (e.g. N, P). Pore water and bottom water data demonstrate that benthic Cr fluxes are locally important and may be significant globally. The pore water dissolved Cr flux at our CaCO3-rich site is likely driven by the re-release of Cr scavenged from the water column by sinking particles, with minor contributions from lithogenic phases. We argue this is consistent with the highest open ocean [Cr] to date being found in the water column below oxygen minimum zones, likely reflecting the release of scavenged Cr in deep waters or surface sediments. Chromium released from suspended particles and surface sediments follows the global δ53Cr–[Cr] array, supporting the proposed role of biological export and regeneration in shaping global Cr and δ53Cr distributions. Global intermediate and deep water [Cr], δ53Cr and Cr:macronutrient relationships are thus shaped by a synergy of circulation patterns, water mass mixing, a deep Cr regeneration cycle, and benthic Cr sources. A biogenic control on global Cr distributions indicates that sedimentary Cr records may reflect biogenic as well as O2-dependent processes, while more research is needed to assess sediment Cr record fidelity based on an active diagenetic cycle.