Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.

We characterized the effect of an inshore-offshore gradient in Fe in the northeast subarctic Pacific on the bacterioplankton and phytoplankton assemblages and on the microbial cycling of particulate and dissolved dimethylsulfoniopropionate (DMSP p and DMSP d ) and dimethylsulfide (DMS). Averaged concentrations of total dissolved Fe (TDFe) decreased linearly with increasing water density along the transect, from 3.4 nmol L 21 at the two inshore stations to 1.0 nmol L 21 at the offshore stations, as a result of the vertical and lateral mixing between the Fe-rich coastal water and the Fe-poor Alaska Current. The Fe-rich inshore stations were dominated by diatoms and characterized by low DMSP p : chlorophyll a (Chl a) ratios (ca. 26 nmol mg 21 ) and bacterial DMS yield (, 4%). In contrast, the Fe-poor offshore stations were dominated by prymnesiophytes and exhibited high DMSP p : Chl a ratios (ca. 84 nmol mg 21 ) and bacterial DMS yield (8%). Chl a, DMSP p , and the abundance of total bacteria and three bacterial clades (Gammaproteobacteria, Roseobacter, and Betaproteobacteria) were positively correlated with the TDFe gradient. At the Fe-poor offshore stations, the positive correlation found between TDFe and the DMSP p : Chl a ratios suggests that Fe supplied by mixing stimulated DMSP production in the prymnesiophyte-dominated assemblage, a response similar to that generally observed during the first days of most of the large-scale ocean iron fertilizations (OIFs). These results suggest that the stimulation of DMSP production takes place whatever the Fe supply mode: atmospheric dust deposition, as simulated by OIFs, or mixing, as reported in this study.

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Fe-poor water collected at Sta. P20 in the Gulf of Alaska in June 2011 was enriched with different concentrations of volcanic ash (0.12, 1.2, and 10 mg L 21 ) from two subduction zone volcanoes, Kasatochi and Chaiten, and incubated onboard under in situ conditions for 6 d. The experimental setup also included a control (no addition) and a positive control (addition of 0.6 nmol L 21 FeSO 4 ). Following a 4 d lag period, there were increases in carbon fixation rates (up to a factor of 10) and chlorophyll a concentrations (up to a factor of 3) in the positive control and in the ash-enriched (1.2 and 10 mg L 21 ) treatments. Diatoms dominated at the end of the incubations, but cyanobacteria, dinoflagellates, pelagophytes, and haptophytes were also stimulated by the presence of ash. Deposition of , 1 mg ash L 21 , which is in the lower range of those estimated to have caused the August 2008 bloom following the eruption of the Kasatochi volcano in the Aleutian Islands, would suffice to trigger a bloom in the Gulf of Alaska.