Dieter Wolf-Gladrow

Samples from the upper 300 m along the 52°S band, south of the Antarctic Polar Front in the Atlantic sector of the Southern Ocean, including a 3‐week monitoring of a persistent bloom, were analyzed to advance our understanding of iron cycling and supply to recurrent phytoplankton blooms. We measured dissolved Fe (dFe, 48h ) targeting primarily detrital and fecal material. Particulate iron was partitioned into size classes to distinguish small, “slow” sinking particles (53 μm), while intermediate fractions were analyzed to investigate aggregation and export processes. Across all stations, dFe exhibited a consistent vertical structure, including a previously undescribed ubiquitous minimum below mixed and euphotic layers. The absence of pLFe ^48h in the 0.2–3 μm fraction indicates reduced aggregation and is consistent with minimal authigenic Fe formation. Instead, pLFe ^48h was concentrated in “slow” sinking particles, indicating surface entrainment. Biomass was negatively correlated with dFe but positively correlated with pLFe ^48h , linking bloom development to particulate iron availability rather than dFe. Overall, rapid surface recycling of “slow” sinking pLFe ^48h appeared to meet most of the daily iron demand during a prolonged diatom bloom. Near South Georgia Island, wind‐driven mobilization of iron‐rich material from the island snowpack and deposition into leeward waters likely supplied an additional iron source. By applying new protocols to characterize labile particulate iron, we reveal the elusive magnitude of the recycling dynamics of particulate iron in fecal and detrital materials sustaining Southern Ocean blooms.

In January 2009, on its way from Cape Town to South America (at around 49.5^∘S and 25^∘W), the German research vessel Polarstern entered a region with a dense cover of icebergs and broken-off chunks of ice up to a few meters in size, largely hidden in a thick fog, a result of the microclimate created by the large agglomeration of icebergs and broken-off glacial ice. Melting of glacial ice led to a cooling (by up to 8^∘C) and freshening (by up to 2.5 psu) of the surface ocean with perturbations reaching down to about 75 m depth. The observed surface ocean equilibrium fugacity of CO₂, fCO₂, dropped to values that were over 100 µatm lower than atmospheric fCO₂ values. Given the similar concentrations in chlorophyll a and the macronutrients nitrate and phosphate inside and outside the perturbed areas, influences of potential iron input from disintegrating icebergs on phytoplankton productivity and fCO₂ can be ruled out. The low fCO₂ values, compared to adjacent regions, can be attributed to thermodynamic effects, i.e. mostly increase of CO₂ solubility with decreasing temperature with a smaller contribution from the dilution due to freshwater inputs. Based on these observations, we consider the potential impact on atmospheric fCO₂ by the release of an armada of icebergs during Heinrich events.

Many biogeochemical processes change total alkalinity: this has been reported for carbonate precipitation and dissolution, uptake and release of various nitrogen‐containing compounds, uptake of phosphate, sulfate reduction combined with methane oxidation. However, the list is not exhaustive. Here we discuss additional processes, namely the uptake of Mg, K, Ca by phytoplankton, and calculate their contribution based on the explicit conservative expression for TA and an extended (compared to Redfield's C : N : P) stoichiometry of phytoplankton. These additional contributions are of the same order of magnitude as that of phosphate uptake and thus much smaller than the contribution from nitrate uptake and of opposite sign to the contribution by nitrate and phosphate uptake.