SEASONAL-VARIATION OF DISSOLVED AND ADSORBED AMINO-ACIDS AND AMMONIUMIN A NEAR-SHORE MARINE SEDIMENT

Dissolved free amino acids;(DFAA) and ammonium in the pore water of a coastal sediment at a water depth of 40 m were investigated monthly fo r more than a year, to improve knowledge on how the concentrations and distributions of these dissolved compounds varied during an annual cy cle. Seasonal changes in adsorbed amino acids and ammonium were also s tudied and adsorption coefficients were calculated. Ammonium distribut ion in the pore water showed clear seasonal trends. In the warmer peri od (August and September) there were high concentrations in the pore w ater, and in winter the lowest concentrations were measured. Pore wate r concentrations of DFAA were in general low, but showed seasonal tren ds during the year. At the end of summer when the bottom water tempera ture reached its maximum (about 14 degrees C), the concentrations of D FAA were also at their maxima, However, a response to the input of org anic matter to the sediment was also observed as increased pore water DFAA concentrations. The seasonal response was not strong, which proba bly was due to several processes (e.g. adsorption, degradation and bac terial assimilation) removing DFAA and thereby preventing large pools of DFAA from being built up in the pore water. Adsorption of ammonium followed the concentration of dissolved ammonium in the pore water and there was neither a relation with input of organic matter nor with te mperature in the bottom water. The adsorption coefficient (the dimensi onless K) for ammonium was 1.07 +/- 0.11 and did not vary during the y ear. The adsorption coefficients for amino acids, which were all highe r than K for ammonium, did show seasonal trends and the periods with h igh concentrations of adsorbed amino acids were related not only to hi gh concentrations of DFAA, but also to recent input of organic matter and probably also to the stimulation of bioturbation as a result of hi gh temperature in the bottom water at the end of summer. The results d emonstrated a seasonal variation in K of amino acids, but not ammonium , indicating that the processes controlling dissolved concentrations w ere substantially different for these compounds.