, 2013). While our ligand model produces an excess of ligands, relative to iron, from with DOC excretion and organic matter remineralization (i.e. positive L⁎), as supported by available data ( Boyd et al., 2010 and Boyd and Tagliabue, submitted for publication), neither model has external sources of ligands. Presuming dust and sediments are not expected to be sources of ligands (though Gerringa et al. (2008.) find indications for a sedimentary source of

ligands), the negative L⁎ values we find implies that our models are able to sustain a too large fraction of uncomplexed dissolved iron ( Bowie et al., 2001). This is likely a legacy of the too low and invariant ligand concentrations typically used in click here the past. Because of this, models needed to assume low scavenging rates to maintain iron concentrations at observed levels. Thus by increasing ligand concentrations towards measured levels, with unchanged scavenging rates, our models tend to

overestimate iron. We would argue that the distribution of L⁎ is a powerful argument that iron biogeochemical models need a more dynamic iron cycle, with faster scavenging but also higher surface ligand concentrations. Looking towards refining the representation of iron–ligand dynamics in ocean models, some improvement can be made by revisiting the assumptions regarding colloidal species and their cycling. As mentioned previously, our models account for colloidally associated losses of iron and ligands, but assume a fixed colloidal Natural Product Library screening fraction of 0.5. If this is replaced by a dynamic colloidal fraction that is computed as a function of temperature, ionic strength and pH (Liu and Millero, 1999 and Liu and Millero, 2002) and a simple doubling of the scavenging rate, the widespread increase in dissolved Fe, illustrated by L⁎, associated with dynamic ligands

is removed ( Fig. 8c). While this indicates some improvement, it only serves to highlight that more attention should be placed on the modeling of colloidal species in future work. The dynamism of ligand concentrations and their sensitivity to environmental variables implies the potential for significant changes in Phloretin response to fluctuations in climate. For example, climate change induced changes in productivity, warming, or light intensity will affect the sources and sinks of ligands, which may then feedback onto ocean productivity via iron concentrations. At first order, we speculate that a warmer, more stratified and less productive future ocean (Bopp et al., 2013) should drive enhanced photochemical and bacterial losses of ligands, as well as reduced production rates. The reduced ligand concentrations that result may lower iron concentrations and enhance the degree of iron limitation. The relative importance of these effects remains to be tested by climate models.