THE ROLE OF BACTERIA IN THE NUTRIENT EXCHANGE BETWEEN SEDIMENT AND WATER IN A FLOW-THROUGH SYSTEM

The contribution of bacteria to phosphorus (P) and nitrogen (N) releas e from, or retention in, sediment was studied in a flow-through system . ''Live'' and formaldehyde-''killed'' sediment communities were incub ated in 25-liter bottles with a continuous flow of P- or P + N-enriche d water. Sediment bacteria in the killed communities were inhibited by adding formaldehyde (final concentration 0.04% v/v) to the sediment b efore the start of the experiment. Bacterial activity in the live sedi ments measured with [H-3]thymidine and [C-14]leucine incorporation tec hniques did not change essentially during the experiment period (7-8 d ays). Chemical mechanisms were found to be of principal importance in PO4-P retention in the sediment. In the live samples, the net retentio n of PO4-P was lower than in the killed samples, which was likely due to the reduced O-2 conditions in the sediment as a consequence of bact erial mineralization. In total P exchange, however, bacteria increased the retention rate by recycling dissolved organic P in the sediment. In the live communities the retention of N was very efficient, and all the introduced NH4-N and NO3-N was immobilized by sediment bacteria. Nitrogen enrichment, however, did not alter the P exchange rates. The gradual emergence of bacterial activity (and grazing) in the killed co mmunities, subsequent to the dilution of formaldehyde concentration, e nhanced the release of PO4-P and NH4-N from sediment.