“Iron fertilization enhanced net community production but not downward particle flux … “


From recent article “Iron fertilization enhanced net community production but not downward particle flux during the Southern Ocean iron fertilization experiment LOHAFEX”, by P. Martin, M. Rutgers van der Loeff, N. Cassar, P. Vandromme, F. d’Ovidio, L. Stemmann, R. Rengarajan, M. Soares, H. E. González, F. Ebersbach, R. S. Lampitt, R. Sanders, B. A. Barnett, V. Smetacek, S. W. A. Naqv.

Abstract

A closed eddy core in the Atlantic Subantarctic Southern Ocean was fertilized twice with two tons of iron (as FeSO4) to test whether iron addition enhances downward particle flux into the deep ocean. The ~300 km2 fertilized patch was occupied for 39 d. Chlorophyll-a and primary productivity doubled after fertilization, and photosynthetic quantum yield (FV/FM) increased from 0.33 to ≥0.40. Silicic acid was at limiting concentrations (<2 µmol L-1), diatoms contributed <10% of phytoplankton biomass, and copepods exerted high grazing pressure. This is the first study of downward particle flux out of an artificially-fertilized bloom with very low diatom biomass. Net community production (NCP) inside the patch estimated from O2:Ar ratios averaged 21 mmol POC m−2 d−1, probably ±20%. Export flux at 100 m calculated from234Th profiles remained constant inside the patch (∼6.3 mmol POC m−2 d−1) and was similar to unfertilized adjacent waters. The difference between NCP and 234Th-derived export partly accumulated in the mixed layer, and was partly remineralized between the mixed layer and 100 m. Fluxes caught in neutrally buoyant sediment traps at 200 m and 450 m inside and outside the patch were mostly <1.1 mmol POC m−2 d−1, predominantly of fecal material, and did not increase upon fertilization. Our data thus indicate intense flux attenuation between 100 and 200 m, and probably between the mixed layer and 100 m. We attribute the lack of fertilization-induced export to silicon-limitation of diatoms and reprocessing of sinking particles by detritus feeders. Our data are consistent with the view that nitrate-rich but silicate-deficient waters are not poised for enhanced particle export upon iron addition.

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