NH₄⁺ Regeneration and Uptake in Mesotrophic Lakes; Diurnal and Seasonal Variations

Abstract

We conducted ¹⁵NH₄⁺ dilution experiments to examine temporal variations in NH₄⁺ regeneration and uptake in the pelagic environments of mesotrophic Lakes Kizaki and Biwa. In Lake Kizaki, both NH₄⁺ regeneration and uptake rates showed no apparent diurnal periodicity during summer (July and August), and the regeneration: uptake ratio was close to one over the course of the day, suggesting that regeneration and uptake were tightly coupled regardless of light conditions. Size fractionation experiments showed that bacterial NH₄⁺ regeneration was substrantial throughout the day, suggesting that a microbial loop was responsible for the rapid turnover of NH₄⁺. Seasonal measurements revealed that NH₄⁺ regeneration and uptake rates were highest in summer in both lakes. Correlation analyses indicated that temperature, nutrients and chlorophyll-α concentrations had little effect on the uptake rates. The regeneration : uptake ratio was close to one (0.95±0.31, n=18) despite large variations in nutrients and biological conditions, suggesting that phytoplankton usually take up regenerated NH₄⁺ as rapidly as they can. Our data provide direct evidence that biological regeneration in the euphotic zone is the dominant process in NH₄⁺ supply to phytoplankton in pelagic environments of mesotrophic lakes.