DIEL VARIATIONS IN DISSOLVED ORGANIC NITROGEN IN A COASTAL POND - RELATIONSHIPS WITH CARBON AND NITROGEN-METABOLISM

Changes in dissolved inorganic and organic nitrogen compounds were fol lowed over three light-dark periods in a 2000 m(3) shallow clay pond l ocated on the Atlantic coast in the spring of 1988. Very strong diel v ariations in concentrations of dissolved organic nitrogen (DON) were o bserved with maxima (about 6-7 mu M) during the day and minima (undete ctable) at night. Nitrate concentrations also exhibited diel patterns with evidence of nitrification during the day and uptake at night. Rib ulose bisphosphate carboxylase activity was related to nitrate concent rations only and beta-carboxylases to ammonium concentrations only. Ot her nutrients did not how any clear variations with time. An inverse r elationship between changes in DON and changes in nitrate was signific ant (74% of the variation in DON changes could be explained by variati ons in nitrate concentrations) and suggested that most of the nitrate taken up was transformed into DON, a phenomenon consistent with the la ck of accumulation of N-15 coming from labelled nitrate into the parti culate matter. The lack of relationship between Rubisco and DON sugges ts that autotrophs were not directly involved in DON cycling or that t he phytoplankton was partly heterotrophic. Concerning N budgets, for p eriods shorter than 24 h, DON cannot be ignored because its variations can be as great as those of dissolved inorganic nitrogen.