Effect of natural iron fertilization on carbon sequestration in the Southern Ocean

Abstract

The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial-interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization--an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below--as invoked in some palaeoclimatic and future climate change scenarios--may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

Controls on mesopelagic particle fluxes in the Sub-Antarctic and Polar Frontal Zones in the Southern Ocean south of Australia in summer—Perspectives from free-drifting sediment traps

“…This is in agreement with the dominance of more or less heavily processed particles (faecalaggregates) during SAZ-Sense and also agrees with many other studies in Polar regions (e.g. Blain et al, 2007;Pollard et al, 2007;Wassmann et al, 2008).…”

“…This coincides with results from the Kerguelen Ocean and Plateau compared Study (KEOPS) that was carried out in PFZ waters (Blain et al, 2007), where most of the particle flux originated from faecal matter (Ebersbach and Trull, 2008). Similar to our dominating faecal-aggregates, KEOPS reported on aggregates of faecal matter that were responsible for downward POC export.…”

“…The elevated biomass observed at P3, which is present throughout the year as indicated by satellite remote sensing (Mongin et al, 2011), was not accompanied by elevated primary or new production, and did not translate into increased sinking flux, at least not during the short shipboard observational period reported here. This provides a cautionary note, as have other studies of natural iron fertilisation (Blain et al, 2007;Pollard et al, 2007), about the effectiveness of iron inputs (which were elevated at the P3 site; Bowie et al, 2009) to increase biological pump strength. The clear influence of zooplankton grazing in the control of particle production and export observed via the use of the polyacrylamide gel traps emphasises the influence of higher trophic levels on biological pump responses to stimulation of photosynthesis.…”

“…Increased iron supply is observed in this region of the SAZ (Bowie et al, 2009), as are surface biomass levels as observed by satellite remote sensing (Mongin et al, 2011). Thus, this site offers the opportunity to observe the response of export to natural iron inputs to the Subantarctic Zone, which can be compared to other Southern Ocean natural iron fertilisation studies such as CROZEX (Pollard et al, 2007) and KEOPS (Blain et al, 2007).…”

Southern Ocean phytoplankton physiology in a changing climate

“…Over 30 years of research has demonstrated that Fe limitation controls the efficiency of the biological pump and the structure of phytoplankton communities in the SO (e.g., Martin, 1990;Coale et al, 2003;De Baar et al, 2005;Maldonado et al, 2005;Blain et al, 2007;Boyd et al, 2007;Smetacek et al, 2012). However, only the biologically active pool (defined here as bioavailable) is the fraction that can be effectively taken up by microorganisms to support their nutrient-dependent metabolic processes and growth (Hassler et al, 2012).…”