Seasonal variability in sediment porewater Fe and potential for water column release in the Celtic Sea
Abstract:
The intra-annual cycling and release of Fe from shelf sediments to the overlying water column has been studied over three seasons, before, during and after the phytoplankton bloom in the Celtic Sea, UK. Porewater dissolved Fe (dFe, < 0.15 μm) was predominantly in the reduced Fe(II) form (70 to 100 %), most of which appeared to be present in the soluble size fraction (<0.02 μm; 75 to 100 %). This indicates that iron reduction is an important mechanism of dFe formation in this region. In cohesive surface sediments (< 1 cm) dFe concentrations changed from ~ 0.1 μM during pre-bloom conditions, to 5 – 15 μM during bloom conditions, and to ~ 1 μM during the post-bloom. Higher concentrations observed during the bloom coincide with the deposition of large amounts of bloom originated organic material to the seafloor, which is decomposed and leads to the release of Fe into porewaters. These data indicate that the pool of porewater dissolved Fe that could be released into overlying waters is at a maximum over a short period of the annual cycle. We also measured up to 20 nM dFe in oxic seawater in contact with sediment, and up to 70 % was in the reduced Fe(II) form. The measured oxidation rates of Fe(II) in waters overlying sediments were also substantially slower than empirical predictions. We conclude, the largest inventory of porewater dFe with potential to be released to the water column occurs during the spring bloom, it comprises mostly Fe(II), and is likely to be stabilized by complexation to organic ligands in the overlying water.