No abstract

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.

In terrestrial ecosystems, transitional areas between different plant communities (ecotones) are formed by steep environmental gradients and are commonly characterized by high species diversity and primary productivity, which in turn influences the foodweb structure of these regions. Whether comparable zones of elevated diversity and productivity characterize ecotones in the oceans remains poorly understood. Here we describe a previously hidden hotspot of phytoplankton diversity and productivity in a narrow but seasonally persistent transition zone at the intersection of iron-poor, nitrate-rich offshore waters and iron-rich, nitrate-poor coastal waters of the Northeast Pacific Ocean. Novel continuous measurements of phytoplankton cell abundance and composition identified a complex succession of blooms of five distinct size classes of phytoplankton populations within a 100-km-wide transition zone. The blooms appear to be fueled by natural iron enrichment of offshore communities as they are transported toward the coast. The observed succession of phytoplankton populations is likely driven by spatial gradients in iron availability or time since iron enrichment. Regardless of the underlying mechanism, the resulting communities have a strong impact on the regional biogeochemistry as evidenced by the low partial pressure of CO 2 and the nearly complete depletion of nutrients. Enhanced phytoplankton productivity and diversity associated with steep environmental gradients are expected wherever water masses with complementary nutrient compositions mix to create a region more favorable for phytoplankton growth. The ability to detect and track these important but poorly characterized marine ecotones is critical for understanding their impact on productivity and ecosystem structure in the oceans.ecotone | transition zone | iron | high-nitrate, low-chlorophyll | flow cytometry

Significant changes in the abundance and composition of phytoplankton were observed along Line P in the northeast subarctic Pacific as a result of a rapid warming of surface waters in 2014–2015. This feature, labeled “the blob,” reached ~ 4°C above normal and restricted winter ocean–surface nutrient renewal due to increased stratification. As a result, surface nutrients were the lowest observed and nitrate depletion in summer extended farther offshore than in the last three decades. Within this nitrate‐depleted region, there was unusually low phytoplankton biomass and a dramatic increase in the dominance of cyanobacteria, including Prochlorococcus, which had not been previously observed in this region. Farther offshore, in the iron‐limited region, phytoplankton biomass and the abundance of haptophytes and chlorophytes increased during “the blob.” By 2016, surface nutrient and phytoplankton concentrations were still low, but at most of the stations, phytoplankton composition was similar to that observed before the warming occurred, except for an increase in diatoms farther offshore. These changes at the base of the food web could have ecosystem‐wide implications.

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.