First Evaluation of the Role of Salp Fecal Pellets on Iron Biogeochemistry

10 páginas, 4 tablas, 2 figuras.-- his is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY)

Planktonic grazers such as salps may have a dominant role in iron (Fe) cycling in surface waters of the Southern Ocean (SO). Salps have high ingestion rates and egest large, fast sinking fecal pellets (FPs) that potentially contribute to the vertical flux of carbon. In this study, we determined the impact of FPs from Salpa thompsoni, the most abundant salp in the SO, on Fe biogeochemistry. During the Polarstern expedition ANT-XXVII/3, salps were sampled from a large diatom bloom area in the Atlantic sector of the SO. Extensive work on carbon export and salp FPs export at the sampling location had shown that salps were a minor component of zooplankton and were responsible for only a 0.2% consumption of the daily primary production. Furthermore, at 100 m, export efficiency of salp FPs was ~2–3 fold higher than that of the bulk of sinking particulate organic carbon (POC). After collection, salps were maintained in 200 μm screened seawater and their FPs were collected for further experiments. To investigate whether the FPs release Fe and/or Fe-binding ligands into the filtered seawater (FSW) under different experimental conditions, they were either incubated in the dark or under full sunlight at in situ temperatures for 24 h, or placed into the dark after a freeze/thaw treatment. We observed that none of the treatments caused release of dissolved Fe (dFe) or strong Fe ligands from the salp FPs. However, humic-substance like (HS-like) compounds, weak Fe ligands, were released at a rate of 8.2 ± 4.7 μg HS-like FP−1 d−1. Although the Fe content per salp FP was high at 0.33 ± 0.02 nmol dFe FP−1, the small contribution of salps to the zooplankton pool resulted in an estimated dFe export flux of 11.3 nmol Fe m−2 d−1 at 300 m. Since salp FPs showed an export efficiency at 100 m well above that shown by the bulk of sinking POC, our results suggest that in those areas of the SO where salps play a major role in the grazing of primary production, they could be actively contributing to the depletion of the dFe pool in surface water

DC was funded by the Swiss National Science Foundation (PP00P2_138955), LN by the UTS Chancellor Fellowship and CH by the Australian Research Council (Discovery Project DP1092892 and LIEF grand LE0989539) and the Swiss National Science Foundation (PP00P2_138955). LL and JS participation was funded by the MINECO of Spain (Grant CGL2010-11846-E) and the Government of the Balearic Islands (Grant AAEE083/09). ST was funded by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the priority programme “Antarctic Research with comparative investigations in Arctic ice areas,” project TR 899/2 as well as the Helmholtz Impulse Fond (Helmholtz Young Investigators Group EcoTrace). MI was funded by the Helmholtz Association for the Helmholtz Young Investigator Group SeaPump

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