Iron distribution and speciation in oxic and anoxic waters of the Baltic Sea

Abstract

A survey to determine the distribution of iron in the Baltic Sea has been carried out to investigate the role of iron speciation at surface waters in the fertilization and initiation of summer diazotrophic cyanobacterial blooms. Levels of total dissolvable iron (II) (Fe(II)), total dissolved iron (Fediss) and iron associated with suspended particulate material (FeSPM) were measured in surface waters from all areas of the Baltic. Additionally, depth profiles were made at three characteristic stations for vertical process studies.The highest surface values of Fe(II) (~39nmolL−1), coinciding with maximum Fediss concentrations (~600nmolkg−1) and maximum FeSPM values (~250nmolL−1), were found in the Bothnian Bay, the region of the Baltic Sea with the lowest salinity. Fe(II) concentrations in the Baltic Proper ranged from between 0.6 and 4nmolL−1, with the minimum values in the transition area between the Baltic and the North Sea. In the depth profiles, we found Fe(II) maxima in anoxic layers (≈30nmolL−1 at 160m in the Gotland Basin), which coincided with maximum Fediss (~1200nmolkg−1 at 160m) and decreasing concentrations of FeSPM, revealing a pool of possibly bioavailable Fe(II) in deep waters. Net diffusion fluxes of 10.1μmolm−2day−1 for Fediss and 0.28μmolm−2day−1 for Fe(II) through the redoxcline were calculated. For Fe(II) a half-time of 162h was calculated for the oxygen minimum zone, while a half-time of 2min was calculated for the ~70m depth horizon, where oxygen is already present.This result indicated that the Fe(II) pool in the Gotland Basin deep waters can be neglected as a major Fe(II) source for the surface waters of the Baltic Proper.To our knowledge, this is the first comprehensive investigation of Fe speciation in oxic and anoxic Baltic Sea waters.