The characteristics of ammonium and nitrate uptake by phytoplankton in Lake Kinneret1

Abstract

Seasonal and diurnal patterns of NH 4 + and NO 3 ‒ uptake were determined for Lake Kinneret phytoplankton. Nanoplankton generally, but not always, had a higher uptake of NH 4 + and NO 3 ‒ than did net plankton. Ammonium was always taken up preferentially and the phytoplankton had lower apparent affinity ( K s ) for this ion than for NO 3 ‒ . However, during the annual dinoflagellate bloom of Peridinium (February–May), when ambient levels of NH 4 + were low and those of NO 3 ‒ were high, a considerable portion of the total N flux was derived from NO 3 ‒ . The observed daily fluctuations of specific uptake rates for both NH 4 + and NO 3 ‒ can be partially explained by changes in light intensity. The finding that NO 3 ‒ was utilized by the dinoflagellates implies that control of the amounts of this nutrient, which is predominantly supplied from watershed sources, could be important in limiting eutrophication in Lake Kinneret.