Synthesis of iron fertilization experiments: From the Iron Age in the Age of Enlightenment

Abstract

Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx‐2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo‐nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (>60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth‐integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

Southern Ocean phytoplankton physiology in a changing climate

“…While the SO has some of the highest macronutrients (N, P, Si) concentrations of any marine province across the globe, Si and Fe are spatially and temporally variable (Martin, 1990;Coale et al, 2003Coale et al, , 2004Hiscock et al, 2003;De Baar et al, 2005;Maldonado et al, 2005). This irregularity in nutrient availability greatly influences the spatial dynamics of biological communities and more specifically, the regulation of marine primary production (Hiscock et al, 2003).…”

“…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).…”

“…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). Enrichment of Fe can drive shifts in the community structure from a nanoplankton (<10 m) to a microplankton (>10 m) dominated assemblage (De Baar et al, 2005), increasing the sinking of particulate organic carbon (POC) and thus export to the deep ocean (Boyd et al, 2000). As such, it is the bioavailable pool that shapes SO phytoplankton communities and caps the efficiency of the biological pump.…”

“…Microorganisms can also modulate their Fe biological requirement using enzyme replacement and modification of the photosynthetic antenna (Behrenfeld and Milligan, 2013;Petrou et al, 2014). Diatoms exhibit the highest sensitivity to Fe limitation (Miller et al, 1991;Morel et al, 1991) with shifts to larger sizes (>10 m) in response to Fe fertilisation (De Baar et al, 2005). Experiments carried out with cultured diatoms in the laboratory showed a relationship between Fe, the surface/volume ratio (S/V) and the iron biological requirement for growth (De Baar et al, 2005;Timmermans et al, 2004;Sarthou et al, 2005), with larger diatoms being associated with greater iron requirement.…”

“…Diatoms exhibit the highest sensitivity to Fe limitation (Miller et al, 1991;Morel et al, 1991) with shifts to larger sizes (>10 m) in response to Fe fertilisation (De Baar et al, 2005). Experiments carried out with cultured diatoms in the laboratory showed a relationship between Fe, the surface/volume ratio (S/V) and the iron biological requirement for growth (De Baar et al, 2005;Timmermans et al, 2004;Sarthou et al, 2005), with larger diatoms being associated with greater iron requirement. Therefore, Fe bioavailability is not only influenced by its chemical forms, but also by the different uptake strategies, biological requirements and interactions of the phyto-and bacterio-plankton communities (e.g., Barbeau et al, 1996;Hutchins et al, 1999).…”