TEMPORAL AND VERTICAL-DISTRIBUTION OF TOTAL AMMONIA NITROGEN AND UN-IONIZED AMMONIA NITROGEN IN SEDIMENT PORE-WATER FROM THE UPPER MISSISSIPPI-RIVER

We examined the temporal and vertical distribution of total ammonia ni trogen (TAN) and un-ionized ammonia nitrogen (NH3-N) in sediment pore water and compared the temporal patterns of TAN and NH3-N concentratio ns in overlying surface water with those in pore water. Pore water was obtained by core extraction and subsequent centrifugation. We measure d TAN concentrations and calculated NH3-N concentrations from February through October 1993 at four sites in Pool 8, upper Mississippi River , at depths of 0 to 4, 4 to 8, and 8 to 12 cm below the sediment-water interface. Total ammonia nitrogen and NH3-N concentrations were signi ficantly different among sampling dates (p = 0.0001) and sediment dept hs (p = 0.0001). Concentrations of TAN and NH3-N in surface water were significantly less than those in pore water from all sediment depths (p < 0.05). Concentrations in pore water ranged from 0.07 to 4.0 mg TA N/L and less than 1 to 20 mu g NH3-N/L in winter, and from 0.07 to 10. 0 mg TAN/L and 1 to 175 mu g NH3-N/L in summer; greatest concentration s were usually found in sediments 8 to 12 cm deep. Annual mean TAN con centrations were positively correlated with silt and volatile solids c ontent and were negatively correlated with sand content. Because of th e high variability of TAN and NH3-N concentrations in pore water, sedi ment toxicity studies should take into account the season and the dept h at which sediments are obtained. The annual mean NH3-N concentration in pore water at one site (55 mu g/L) exceeded the concentration (30 mu g/L) demonstrated to inhibit growth of fingernail clams in laborato ry studies. However, these concentrations apparently were not lethal, as evidenced by the presence of fingernail clams at this site.