Abstract
Inorganic carbon and NH4+ uptake by phytoplankton was studied during a period of nitrogen depletion in early fall of 1985 in Lake Suwa to investigate a possible effect of NH4+ supply from the bottom sediments on phytoplankton production. Phytoplankton samples from the experimental enclosures were, after being size-fractionated using 40 μm netting, incubated with the simultaneous addition of 13C inorganic carbon at tracer level (6.515.6% in excess of ambient DIC) and 15NH4+ in substrate saturated concentration (541330% in excess of ambient NH4+) under saturation light intensity for 34 hr. The <40 μm fraction, consisting of small-sized green algae, monads and diatoms, showed considerably higher specific nitrogen uptake rates than the >40 μm fraction, exclusively composed of large colonial blue-green algae Microcystis spp. There was little difference in specific carbon uptake rates and no difference in photosynthetic activity between the two size fractions. It was suggested that during a period of nitrogen depletion in Lake Suwa a large supply of NH4+ from the bottom sediments could more favorably affect the small-sized phytoplankton than the large colonial Microcystis.
