In some parts of the Southern Ocean (SO), even though low surface concentrations of iron (Fe) and manganese (Mn) indicate FeMn co-limitation, we still lack an understanding on how Mn and Fe availability influences SO phytoplankton ecophysiology. Therefore, this study investigated the effects of Fe and Mn limitation alone as well as their combination on growth, photophysiology and particulate organic carbon production of the bloom-forming Antarctic diatom Chaetoceros debilis. Our results clearly show that growth, photochemical efficiency and carbon production of C. debilis were co-limited by Fe and Mn as highest values were only reached when both nutrients were provided. Even though Mn-deficient cells had higher photochemical efficiencies than Fe-limited ones, they, however, displayed similar low growth and POC production rates, indicating that Mn limitation alone drastically impeded the cell’s performance. These results demonstrate that similar to low Fe concentrations, low Mn availability inhibits growth and carbon production of C. debilis. As a result from different species-specific trace metal requirements, SO phytoplankton species distribution and productivity may therefore not solely depend on the input of Fe alone, but also critically on Mn acting together as important drivers of SO phytoplankton ecology and biogeochemistry.

Salp fecal pellets release more bioavailable iron to Southern Ocean phytoplankton than krill fecal pellets Highlights d Salps recycle iron in a more bioavailable form than krill d Per mol fecal pellet carbon, salps release more iron than krill d Possibly, salps increase the carbon fixation potential of the Southern Ocean

<p><strong>Abstract.</strong> Incubation experiments comprising Saharan dust additions were conducted in the tropical North Atlantic Ocean along an east-west transect at 12° N to study the phytoplankton response to nutrient release in oligotrophic seawater conditions. Experiments were performed at three stations (M1, M3, M4), mimicking wet and dry deposition of low and high amounts of Saharan dust deposition from two different dust sources (paleo-lake and sand dune). Dust particle sizes were adjusted to resemble dust that is naturally deposited over the ocean at the experiment sites. For wet dust deposition, the dust was pre-leached in acidified ‘artificial rainwater’ (H₂SO₄) for 16 to 24 hours, mimicking acid cloud processing at different pH values. Experiments were run up to eight days. Daily nutrient measurements of phosphate (PO₄^{3^&amp;minus;}), silicate (SiO₄^{4^&amp;minus;}), nitrate (NO_{3^&amp;minus;}) and cell abundances were performed in addition to measurements of concentrations of total dissolved iron (DFe), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) at the start and at the end of the experiments.</p><p> A significant initial increase and subsequent gradual decrease in PO₄^{3^&amp;minus;}, SiO₄^{4^&amp;minus;} and DFe concentrations were observed after wet dust deposition using high amounts of dust previously leached in low pH rain (H₂SO₄, pH = 2). Remarkably, the experiments showed no nutrient release (PO₄^{3^&amp;minus;}, SiO₄^{4^&amp;minus;} and DFe) from dry-dust addition and the NO_{3^&amp;minus;} concentrations remained unaffected in all (dry and wet) experiments. The prokaryotic cyanobacterium <i>Synechococcus</i> spp. was the most prominent picophytoplankton in all mixed layer experiments. After an initial increase in cell abundance, a subsequent decrease (at M1) or a slight increase (at M3) with similar temporal dynamics was observed for dry and wet dust deposition experiments. The POC concentrations increased in all experiments and showed similar high values after both dry and wet dust deposition treatments, even though wet dust deposition is considered to have a higher potential to introduce bioavailable nutrients (i.e. PO₄^{3^&amp;minus;}, SiO₄^{4^&amp;minus;} and DFe) into the otherwise nutrient-starved oligotrophic ocean. Our observations suggest that such nutrients may be more likely to favor the growth of the phytoplankton community when an additional N-source is also available. In addition to acting as a fertilizer, our results from both dry and wet dust deposition experiments suggest that Saharan dust particles might be incorporated into marine snow aggregates leading to similar high POC concentrations.</p>

Salp fecal pellets release more bioavailable iron to Southern Ocean phytoplankton than krill fecal pellets Highlights d Salps recycle iron in a more bioavailable form than krill d Per mol fecal pellet carbon, salps release more iron than krill d Possibly, salps increase the carbon fixation potential of the Southern Ocean