Iron partitioning during LOHAFEX: Copepod grazing as a major driver for iron recycling in the Southern Ocean

• Published in: Marine chemistry 2017-11, Vol.196, p.148-161
• Main Authors: Laglera, Luis M., Tovar-Sánchez, A., Iversen, M.H., González, H.E., Naik, H., Mangesh, G., Assmy, P., Klaas, C., Mazzocchi, M.G., Montresor, M., Naqvi, S.W.A., Smetacek, V., Wolf-Gladrow, D.A.
• Format: Article
• Language: English
• Subjects: Grazing; Iron cycling; Iron fertilization; Iron partitioning; LOHAFEX; Silicate depletion; Southern Ocean
• ISSN: 0304-4203; 1872-7581
• DOI: 10.1016/j.marchem.2017.08.011

The LOHAFEX iron fertilization experiment was conducted for 39days in the closed core of a cyclonic mesoscale eddy located along the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. Mixed layer (ML) waters were characterized by high nitrate (~20μM), low dissolved iron (DFe ~0.2nM) and low silicate concentrations (below 1μM) 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 150m of the water column with no significant deeper export. DFe concentrations in the upper 200m were not significantly affected by the two fertilizations but after day 14 showed a greater variability (ranging from 0.3 to 1.3nM) without a clear vertical pattern. Particulate iron concentrations (measured after 2months at pH1.4) 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 (pH2.0) in 48h (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 80m. We hypothesize that our observations are due to a combination of two biological processes. First, phagotrophy of iron colloids f