Seasonal Cycling of Dissolved, Soluble and Colloidal Iron in the Celtic Sea

Shelf sea sediments are an important source of dissolved iron (Fe) to the open ocean, yet at present this source is poorly understood. In particular, very little is known about the relative contribution of soluble and colloidal iron to dissolved Fe in shelf regions. Here, we report the first seasonal study of dissolved, soluble and colloidal Fe cycling in the Celtic Sea.

Strong seasonal cycling of all forms of Fe within the Celtic Sea was observed. Winter mixing results in concentrations of dissolved Fe that are homogenously distributed throughout the water column in early spring. The dissolved Fe is typically comprised of 50-60 % cFe. During April the onset of stratification results in the initiation of the spring bloom and during this transitional period the coincident draw down of both soluble Fe and colloidal Fe and nitrate in surface waters were observed. This suggests a biological role for soluble Fe and colloidal Fe and/or a scavenging loss. The observed drawdown of dissolved Fe compared to published values of cellular Fe:N in coastal phytoplankton suggests that Fe must be either efficiently recycled or resupplied to surface waters during this period. During summer and autumn surface water dissolved Fe concentrations were 0.1 nM and alongside low nitrate values limit productivity in this region.

After the onset of stratification in mid-April, concentrations of dissolved Fe increase below the thermocline throughout the summer and autumn. This continual build-up of dissolved Fe in deeper waters results from tidal advection and sedimentary release of dissolved Fe after the spring bloom crashes. During this time period colloidal Fe contributed 60-80% of the dissolved Fe indicating the sedimentary release. Understanding the seasonal cycling of Fe in shelf systems is crucial in determining this source to the open ocean.