Description: eddy located along the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. Mixed layer (ML) waters were characterized by high nitrate (~20 μM), low dissolved iron (DFe ~0.2 nM) and low silicate concentrations (below 1 μM) restricting diatom growth. Upon initial fertilization, chlorophyll-a doubled during the first two weeks and stabilized thereafter, despite a second fertilization on day 21, due to an increase in grazing pressure. Biomass at the different trophic levels was mostly comprised of small autotrophic flagellates, the large copepod Calanus simillimus and the amphipod Themisto gaudichaudii. The downward flux of particulate material comprised mainly copepod fecal pellets that were remineralized in the upper 150 m of the water column with no significant deeper export. showed a greater variability (ranging from 0.3 to 1.3 nM) without a clear vertical pattern. Particulate iron concentrations (measured after 2 months at pH 1.4) decreased with time and showed a vertical pattern that indicated an important non-biogenic component at the bottom of the mixed layer. In order to assess the contribution of copepod grazing to iron cycling we used two different approaches: first, we measured for the first time in a field experiment copepod fecal pellet concentrations in the water column together with the iron content per pellet, and second, we devised a novel analytical scheme based on a two-step leaching protocol to estimate the contribution of copepod fecal pellets to particulate iron in the water column. Analysis of the iron content of isolated fecal pellets from C. simillimus showed that after the second fertilization, the iron content per fecal pellet was ~5 fold higher if the copepod had been captured in fertilized waters. We defined a new fraction termed leachable iron (pH 2.0) in 48 h (LFe48h) that, for the conditions during LOHAFEX, was shown to be an excellent proxy for the concentration of iron contained in copepod fecal pellets. We observed that, as a result of the second fertilization, iron accumulated in copepod fecal pellets and remained high at one third of the total iron stock in the upper 80 m. We hypothesize that our observations are due to a combination of two biological processes. First, phagotrophy of iron colloids freshly formed after the second fertilization by the predominant flagellate community resulted in higher Fe:C ratios per cell that, via grazing, lead to iron enrichment in copepod fecal pellets in fertilized waters. Second, copepod coprophagy could explain the rapid recycling of particulate iron in the upper 100–150 m, the accumulation of LFe48h in the upper 80 m after the second fertilization and provided the iron required for the maintenance of the LOHAFEX bloom for many weeks. Our results provide the first quantitative evidence of the major ecological relevance of copepods and their fecal products in the cycling of iron in silicate depleted areas of the Southern Ocean.

Description: In the vast Low Nutrient Low-Chlorophyll (LNLC) Ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be one order of magnitude greater over short timescales. The short turnover time of atmospheric Fe and N supply (〈1 month for nitrate) further supports deposition being an important source of nutrients in LNLC regions. Yet, the extent to which atmospheric inputs are impacting biological activity and modifying the carbon balance in oligotrophic environments has not been constrained. Here, we quantify and compare the biogeochemical impacts of atmospheric deposition in LNLC regions using both a compilation of experimental data and model outputs. A metadata-analysis of recently conducted field and laboratory bioassay experiments reveals complex responses, and the overall impact is not a simple “fertilization effect of increasing phytoplankton biomass” as observed in HNLC regions. Although phytoplankton growth may be enhanced, increases in bacterial activity and respiration result in weakening of biological carbon sequestration. The application of models using climatological or time-averaged non-synoptic deposition rates produced responses that were generally much lower than observed in the bioassay experiments. We demonstrate that experimental data and model outputs show better agreement on short timescale (days to weeks) when strong synoptic pulse of aerosols deposition, similar in magnitude to those observed in the field and introduced in bioassay experiments, is superimposed over the mean atmospheric deposition fields. These results suggest that atmospheric impacts in LNLC regions have been underestimated by models, at least at daily to weekly timescales, as they typically overlook large synoptic variations in atmospheric deposition and associated nutrient and particle inputs. Inclusion of the large synoptic variability of atmospheric input, and improved representation and parameterization of key processes that respond to atmospheric deposition, is required to better constrain impacts in ocean biogeochemical models. This is critical for understanding and prediction of current and future functioning of LNLC regions and their contribution to the global carbon cycle.

Description: The LOHAFEX iron fertilization experiment consisted in the fertilization of the closed core of a cyclonic eddy located south of the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. This eddy was characterized by high nitrate and low silicate concentrations. Despite a 2.5 fold increase of the chlorophyll-a (Chl-a) concentrations, the composition of the biological community did not change. Phytoplankton biomass was mostly formed by small autotrophic flagellates whereas zooplankton biomass was mostly comprised by the large copepod Calanus simillimus. Efficient recycling of copepod fecal pellets (the main component of the downward flux of organic matter) in the upper 100–150 m of the water column prevented any significant deep export of particulate organic carbon (POC). Before fertilization, dissolved iron (DFe) concentrations in the upper 200 m were low, but not depleted, at ~0.2 nM. High DFe concentrations appeared scattered from day 14 onwards as a result of the grazing activity. A second fertilization on day 21 had no significant effect on the DFe and Chl-a standing stocks. Work with unfiltered samples using different acidification protocols revealed that, by midway of LOHAFEX, rapid recycling of iron-replenished copepod fecal pellets explained the source of bioavailable iron that prolonged the duration of the bloom for many weeks. Here we present the evolution of the organic speciation of iron in the upper 200 m of the water column during LOHAFEX by a Competing Ligand Equilibrium method using voltammetry. During the first 12 days of the experiment, ligands of an affinity for iron similar to the ligands found before fertilization (logK′Fe′L~11.9) accumulated in fertilized waters mostly in the upper 80 m (from ~1 nM to ~2.5 nM). The restriction of ligand accumulation to the depth of Chl-a penetration points to exudation by the growing autotrophic population as the initial source of ligands. From day 5 onwards, we found in many samples a new class of ligands (L1) characterized by a significant higher conditional stability constant than the background complexation (logK′Fe′L1~12.9). During the middle section of the experiment (days 12 to 25) the accumulation of overall ligands and specifically L1, reached an upper limit in surface waters (at ~3 nM). Overall ligands and L1 accumulation was also observed below the mixed layer depth indicating that grazing was the process behind ligand release. During the last 10 days of the experiment ligands kept accumulating in deep waters but suffered a small decrease in the upper 50 m of the water column caused by the vanishing of L1. Ligand removal restricted to the euphotic layer was probably caused by photodegradation. A high correlation between [DFe] and [L1] suggested that recycled iron (released during grazing and copepod fecal pellet cycling) was in the form of FeL1 complexes. We hypothesize that the iron binding ligands released to the dissolved phase during LOHAFEX were mostly photosensitive intracellular ligands rapidly degraded in extracellular conditions (e.g.: pigments). Sloppy feeding by copepods and recycling of cells and cellular material in copepod fecal pellets caused the transfer of particulate ligands to the dissolved phase as zooplankton built up as a response to the blooming community.

Description: Nitrogen fixation by the diazotrophic community in the subtropical NE Atlantic, including the Canary Current region, was investigated during two oceanographic cruises, RODA I (summer 2006) and RODA II (winter 2007). The first evaluated the role of the Canary eddy fields in the growth of nitrogen-fixing Trichodesmium and their rates of nitrogen fixation and of the unscreened whole diazotrophic community. The second cruise was designed to look at the variability of these processes in the subtropical NE Atlantic study area that is influenced by atmospheric transport of African aerosols. During RODA I, significantly higher mean nitrogen fixation rates of unscreened whole surface water ( P 〈 0.05, t -test) were observed at oligotrophic far-field stations than in the nutrient-enriched eddies, but the pattern was not consistent with nitrogen fixation rates of 〉50 µm samples ( Trichodesmium ) integrated over depth. However, correlation analyses showed decreasing rates of nitrogen fixation rates of 〉50 µm samples ( Trichodesmium ) at stations with higher NH 4 + concentrations, suggesting that combined N seemed to be an important regulator affecting nitrogen fixation rates. Temperature was also correlated with the abundance of Trichodesmium. Iron (from dust deposition), which has been reported in previous studies to stimulate N 2 fixation, did not show any correlation with N 2 fixation rates or with the abundance of Trichodesmium .