Riverine input of bioavailable iron supporting phytoplankton growth in Kesennuma Bay (Japan)

The effects of riverain iron and nutrient inputs on phytoplankton growth in Kesennuma Bay were studied. The effects of iron and fulvic acid-iron complex additions on phytoplankton growth were studied in iron-enriched and -limited culture experiments of coastal marine diatom Chaetoceros sp. (the dominant species inside and outside of the bay) using media prepared from bay and outer waters. Bay water is not iron-limited. The addition of Fe(III) to bay water or autoclaved bay water gave no increase in cell yield. However, when bay water was autoclaved after UV-irradiation, there was little growth. This suggests that the UV irradiation destroyed organic compounds that affected iron bioavailability. Outer water is iron-limited. The addition of Fe(III) to outer water increased cell yield and iron-enriched outer water prepared by autoclaving after adding fulvic acid-Fe increased also cell yield. When outer water after adding Fe(III) was autoclaved, there was little growth. This suggests that fulvic acid made the iron bioavailable. The riverain inputs of organically bound iron, such as fulvic acid-Fe, and nutrients probably play an important role for supporting phytoplankton growth in the bay.