Bioavailable colloidal iron in river water originated from the forest

Abstract

In samples of water from five sites on the Shukunohe, Ohno and Kunebetsu rivers, Japan, most of the filterable iron (<0.45 m) was colloidal iron (0.45–0.025 m). 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 ~250 000 cells mL^–1. However, there was little growth with amorphous colloidal iron and no growth in control medium containing ~0.1 M dissolved Fe. In a multispecies phytoplankton culture, colloidal iron also promoted growth, with a maximum of ~60 g Chl-a L–^1. These results indicated the bioavailability of colloidal iron.