Low Cobalt Inventories in the Amundsen and Ross Seas Driven by High Demand for Labile Cobalt Uptake Among Native Phytoplankton Communities
Abstract. Cobalt (Co) is a scarce but essential micronutrient for marine plankton in the Southern Ocean and coastal Antarctic seas where dissolved cobalt (dCo) concentrations can be extremely low. This study presents total dCo and labile dCo distributions measured via shipboard voltammetry in the Amundsen Sea, Ross Sea and Terra Nova Bay during the CICLOPS (Cobalamin and Iron Co-Limitation of Phytoplankton Species) expedition. A significantly smaller dCo inventory was observed during the 2017/2018 CICLOPS expedition compared to two 2005/2006 expeditions to the Ross Sea conducted over a decade earlier. The dCo inventory loss (~10–20 pM) was present in both the surface and deep ocean and was attributed to the loss of labile dCo, resulting in the near-complete complexation of dCo by strong ligands in the photic zone. A changing dCo inventory in Antarctic coastal seas could be driven by the alleviation of iron (Fe) limitation in coastal areas where the flux of Fe-rich sediments from melting ice shelves and deep sediment resuspension may have shifted the region towards vitamin B12 and/or zinc (Zn) limitation, both of which are likely to increase the demand for Co among marine plankton. High demand for Zn by phytoplankton can result in increased Co and cadmium (Cd) uptake because these metals often share the same metal uptake transporters. This study compared the magnitudes and ratios of Zn, Cd and Co uptake (ρ) across upper ocean profiles and observed order of magnitude uptake trends (ρZn > ρCd > ρCo) that paralleled the trace metal concentrations in seawater. High rates of Co and Zn uptake were observed throughout the region, and the speciation of available Co and Zn appeared to influence trends in dissolved metal : phosphate stoichiometry and uptake rates over depth. Multi-year loss of the dCo inventory throughout the water column may be explained by an increase in Co uptake into particulate organic matter (POM) and subsequent increased flux of Co into sediments via sinking and burial. This perturbation of the Southern Ocean Co biogeochemical cycle could signal changes in the nutrient limitation regimes, phytoplankton bloom composition, and carbon sequestration sink of the Southern Ocean.
Rebecca J. Chmiel et al.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2023-402', Randelle Bundy, 01 May 2023
- RC2: 'Comment on egusphere-2023-402', Neil Wyatt, 19 May 2023
Rebecca J. Chmiel et al.
Rebecca J. Chmiel et al.
Viewed (geographical distribution)
The manuscript titled, “Low Cobalt Inventories in the Amundsen and Ross Seas Driven by High Demand for Labile Cobalt Uptake Among Native Phytoplankton Communities” written by Rebecca J. Chmiel and coauthors, describes how dissolved cobalt concentrations in the Ross Sea were much lower in the 2017-2018 season compared to two previous expeditions a decade earlier. The differences in these observations were explored by examining dissolved cobalt (dCo), zinc and cadmium uptake rates, as well as dCo vs. phosphate relationships to gain insights into processes acting on the dCo pool. Overall, I really enjoyed reading this manuscript and thought it was very thought-provoking and thorough.
Most of my comments below are very minor and driven primarily by interest. My only more general question for the authors was whether they had explored potential differences in the water masses sampled during the CICLOPS and CORSACS expeditions, which may impact the deep dCo concentrations. For example, if the expeditions both appeared to sample Circumpolar Deep Water (CDW) and Antarctic Bottom Water (AABW) equally well, then that would strengthen the argument that the dCo inventory differences are primarily driven by differences in Co uptake by the phytoplankton community rather than changes in the water masses over the Amundsen shelf or in the Ross Sea. Perhaps related to this, I was also wondering if the authors have any evidence that the potential for Mn-oxidation might have changed over the 2007-2018 time period, perhaps due to a change in temperature? It seems like Mn-oxidation is low to non-existent in this region, but perhaps it would be something to think about for future work in this area and might have a significant impact on Co scavenging.
In general, I thought this was an excellent paper and it will be an exciting contribution to the field. Below are some additional very minor more specific comments.
Figure 1: Can you perhaps note broadly on this figure where the CORSACS cruises were? I realize it might clutter the figure to have all of the stations, but maybe just an outline of the regions that those cruises sampled?
Figure 4 and 7: How were the regression outliers selected?
Figure 6: I thought it was interesting that the CICLOPS expedition shows more of a scavenging signal for dCo compared to the CORSACS expedition. Any thoughts on why there might be those differences?
Figure 9: I really like this figure, the trends are very clear and it is really interesting.
Line 731-736: Perhaps split this into multiple sentences.
Figure 12: Is it possible to also plot an average of the dCo in the deep and surface box over time on top of the evolution of the pools? I thought it would be interesting to see if this dCo loss is a steady decrease or not, based on this model. It appears to be steady based on the trends in the deep and surface, but seeing the average plotted on top of this might be interesting.