Iron Speciation in the Trondheim Fjord from the Perspective of Iron Limitation for Phytoplankton

Concentrations of different Fe species in the Trondheim fjord waters were determined in different seasons by serial columns. Colloidal Fe (ColFe), Anionic organic-Fe complexes (AnOFe), Hydrophobix-lyphophlic organic-Fe complexes (HphOFe) and Chelex labile iron (ClxLFe) were determined before, during and after large river discharges and phytoplankton blooms. Time series of Fe species were obtained at a single location accompanied by data for humic substances, major nutrients, and phytoplankton biomass. Marked seasonal changes in the concentration of iron in various species were evident, and were directly associated with river discharge, especially in the surface waters, but also due to phytoplankton blooms. ColFe dominated in surface waters when the river discharge was high, but decreased with increasing depth. ClxLFe concentrations were lower than that of ColFe formed during the blooms and high river discharge periods. A clear association between the concentration of ClxLFe and phytoplankton blooms implies that ClxLFe may constitute most of the easily available Fe forms. AnOFe was high only during periods of high river discharge. HphOFe increased with phytoplankton bloom. The close correspondence between the distribution of colloidal Fe and dissolved humic substances with fluorophores (fHS), and between Fe (II), particulate Fe (PFe), and fHS in surface waters, made apparent the role of HS on iron speciation, the formation of colloidal Fe and Fe removal. Iron removal seems to be more effective than the riverine inputs and replenishment of Fe to control the dissolved Fe distribution in the fjord. Therefore, phytoplankton growth rate can be sporadically limited by iron in the Trondheim fjord and adjacent coastal waters especially if there will be enhancement of major nutrients.