Diel variations in dissolved organic nitrogen in a coastal pond: Relationships with carbon and nitrogen metabolism

Abstract

Changes in dissolved inorganic and organic nitrogen compounds were followed over three light-dark periods in a 2000 m 3 shallow clay pond located on the Atlantic coast in the spring of 1988. Very strong diel variations in concentrations of dissolved organic nitrogen (DON) were observed with maxima (about 6–7 μM) during the day and minima (undetectable) at night. Nitrate concentrations also exhibited diel patterns with evidence of nitrification during the day and uptake at night. Ribulose bisphosphate carboxylase activity was related to nitrate concentrations only and β-carboxylases to ammonium concentrations only. Other nutrients did not show any clear variations with time. An inverse relationship between changes in DON and changes in nitrate was significant (74% of the variation in DON changes could be explained by variations in nitrate concentrations) and suggested that most of the nitrate taken up was transformed into DON, a phenomenon consistent with the lack of accumulation of 15N coming from labelled nitrate into the particulate matter. The lack of relationship between Rubisco and DON suggests that autotrophs were not directly involved in DON cycling or that the phytoplankton was partly heterotrophic. Concerning N budgets, for periods shorter than 24 h, DON cannot be ignored because its variations can be as great as those of dissolved inorganic nitrogen.