The effects of enhanced phytoplankton production on iron speciation and removal in mesocosm experiments in a landlocked basin of Hopavågen, Norway

This study examined the concentration, transformation and removal of various operationally defined iron species, including one control and one experimental mesocosm bag (40 m3 each) in the landlocked basin of Hopavågen, Norway. The experimental mesocosm bag was spiked with nitrate, ammonia, phosphate and silicate as part of the MesoNor-97 project. During the mesocosm experiments concentrations of total filter-passing iron (TFFe<0.4 μm) decreased from about 30 to about 5 nmol l−1, while Chl a increased to 38 μg l−1 on day 9 in the nutrient-enriched mesocosms (bag 7). Colloidal iron (ColFe) and chelex-labile iron (ClxLFe) appeared have the greatest bioavailability and/or to be most susceptible to biologically mediated removal. In contrast, on day 9, anionic organic Fe complexes (AnOFe) was still present and hydrophobic–lyphophlic–non polar organic Fe complexes (HpOFe) increased considerably in the surface water of the enriched mesocosm. Sediment trap data show that the iron flux reached its highest level while primary production was at a maximum. A rather high level of atmospheric iron flux was calculated (33.6±16 μmol m−2 day−1) based on sediment trap data during the mesocosm experiment.