汽水湖・中海における窒素代謝 Ｉ 植物プランクトンによる無機態窒素の取り込み

Abstract

The uptake rates of ammonium and nitrate by the natural phytoplankton community in the shallow brackish lake Nakanoumi were measured thirteen times from August 1983 to July 1984 using the 15N tracer technique. Ammonium was uptaked in preference to nitrate, and nitrate use began when ammonium was depleted to about 3 μg N·liter-1. The result of the multiple regression analysis implied that the uptake rate of ammonium was modified primarily by water temperature, phytoplankton standing stock (POC concentration) and ambient concentration of ammonium. Nitrogen appears to be the most important limiting element for the phytoplankton growth in the euphotic layer at the central part of Lake Nakanoumi from May to December, because the halocline acts as an effective barrier for the vertical transport of enriched nitrogen nutrient in the overlying water. The seasonal pattern of nitrate concentration in the euphotic layer at the central part of Lake Nakanoumi is also discussed relative to nitrate uptake rate by the phytoplankton standing stocks.