The decline and fate of an iron-induced subarctic phytoplankton bloom

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.

Ocean Iron Fertilization Experiments: Past–Present–Future with Introduction to Korean Iron Fertilization Experiment in the Southern Ocean (KIFES) Project

“…That being said, EIFEX was the exception. Significant changes in export production were not found in any of the 25 other OIF experiments, suggesting that the effect of iron addition on this component of the biological pump remains a question that needs to be resolved possibly by future OIF experimentation (Bidigare et al, 1999;Nodder et al, 2001;Boyd et al, 2004;Buesseler et al, 2004;Coale et al, 2004;Aono et al, 2005;Tsuda et al, 2007;Smetacek et al, 2012;Martin et al, 2013). …”

“…Generally, chlorophyll-a concentrations increased substantially 2-to 20-fold with max values of ~0.1 mg m -3 (FeeP) to 22 mg m -3 (SEEDS-1) ( Fig. 7c and Table 3) when nitrate concentrations sharply decreased from 3-5 days after iron addition (Tsuda et al, 2003;Coale et al, 2004;Boyd et al, 2004;Tsuda et al, 2007;Peloquin et al, 2011;Smetacek et al, 2012). SEEDS-1 and SEEDS-2, performed under similar conditions, presented similar initial 20 chlorophyll-a concentrations (0.8 mg m -3 ), but their responses to iron addition were different.…”

“…However, there was little information about differences in phytoplankton biomass and communities along the longitudinal dust gradient (Duce and Tindale, 1991;Moore et al, 2002). To investigate the relationship between phytoplankton biomass/community and this dust 5 gradient, the Subarctic Pacific iron Experiment for Ecosystem Dynamics Study-1 (SEEDS-1) was conducted in July-August 2001 (13 days) in the western subarctic gyre using the RV Kaiyo-Maru (Tsuda et al, 2003, and the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES) was performed in July-August 2002 (25 days) in the in the Gulf of Alaska using the RV John P. Tully, El Puma, and Kaiyo-Maru (Boyd et al, 2004(Boyd et al, , 2005. The main objective of SERIES was to investigate the duration of phytoplankton blooming (i.e., start to finish) after iron addition.…”

“…To determine whether the second step of biological pump, export of carbon to the deep sea (i.e., increased export production), is enhanced after iron addition, sediment trap and chemical tracers such as natural radiotracer thorium-234 ( 234 Th; half-life = 24.1 days) have been used together and/or individually to estimate the export flux of 15 particulate organic carbon (Bidigare et al, 1999;Nodder et al, 2001;Boyd et al, 2004;Buesseler et al, 2004;Coale et al, 2004;Aono et al, 2005;Tsuda et al, 2007;Smetacek et al, 2012;Martin et al, 2013). 234 Th has a strong affinity for suspended particles, and the extent of 234 Th removal in the water column is indicative of the export production below the euphotic zone associated with surface primary productivity (Buesseler, 1998).…”

Temporal Variation in Iron Flux Deposition onto the Northern North Pacific Reconstructed from an Ice Core Drilled at Mount Wrangell, Alaska

“…Iron is an essential micronutrient controlling phytoplankton growth in the ocean, and several iron-enrichment experiments conducted in the subarctic Pacific Ocean have demonstrated this important role of iron (e.g., Tsuda et al 2003;Boyd et al 2004). Possible sources of the iron supplied to the surface water of the northern North Pacific are deposition of mineral dust from the atmosphere (e.g., Duce and Tindale 1991) and mesoscale oceanic intrinsic processes, such as vertical winter mixing and horizontal Fe-rich intermediate water transport (Nishioka et al 2011).…”