In object to measure the physico-chemical speciation of iron, water samples were taken at 4 m depth during a spring bloom in Ekhagen Bay, Baltic Sea. Sampling was performed at 6 dates between March 18th and June 2nd year 2003. Cross flow ultrafiltration and DGT (Diffusive Gradients in Thin films) were the methods used. One of the most important objectives of the study was to determine the Fe concentration in the ”truly” dissolved <1 kda fraction. this was done by a modification of a previously described method using nh4 to co-precipitate fe together with mg(oh)2. thereafter, the fe content was determined by isotope dilution together with icp-ms. by using this method, the concentration of fe in the <1 kda fraction was quantified. the concentration of iron ranged between 5.2 and 17 nm in this fraction. the lowest amount of “truly” dissolved fe (<1 kda) was detected at may 13th when the chlorophyll were at its second peak, 12.6 µg/l for chlorophyll a. this indicates a correlation between phytoplankton production and bioavailable iron. iron measurements by ultrafiltration and dgt-technique showed fairly the same patterns, but did not display exactly the same concentrations. fe concentrations from dgt measurements varied between 7.9 and 28.5 nm, with a minimum at april 20th. clearly, the bioavailable pool of iron is decreasing during the spring bloom in ekhagen, although the reasons for this are not fully understood. possible explanations are active uptake by phytoplankton, scavenging onto mn-oxyhydroxides or attachment to tep (transparent exopolymeric particles).