Bioavailable colloidal iron in river water originated from the forest

In samples of water from five sites on the Shukunohe, Ohno and Kunebetsu rivers, Japan, most of the filterable iron (< 0.45 μm) was colloidal iron (0.45-0.025 μm). There was a correlation between dissolved iron and colloidal iron; both are assumed to be associated with humic substances, since the carbon isotope ratio of colloidal organic substances on glass-fibre filters corresponded to the ratio of mountain humic soil. In culture experiments, the freshwater phytoplankton Melosira granulata var. angustissima f. spiralis grew rapidly in a medium containing colloidal iron, to a maximum of &SIM;250 000 cells mL(-1). However, there was little growth with amorphous colloidal iron and no growth in control medium containing &SIM;0.1 μM dissolved Fe. In a multispecies phytoplankton culture, colloidal iron also promoted growth, with a maximum of &SIM;60 μg Chl-a L-1. These results indicated the bioavailability of colloidal iron.