Showing papers in "Virginia journal of science in 1991"

Sediment Denitrification Potential in the Elizabeth River, Virginia

Abstract: Sediment denitrification potential from two sites in the Elizabeth River estuary was studied over a nine-month period using the acetylene blockage method. Rates of microbial processes in this environment are of interest because of the high concentration of toxics present in some parts of the system. Highest rates were found in the highly polluted Southern Branch of the Elizabeth River with nitrate amended sediment ranging from 2-262 nmol N20/h per 20ml of sediment and exhibiting maximal rates during spring and fall. Rates in the Main Stem of the Elizabeth River were lower, with less than 1-85 nmol N20/h/20ml in nitrate amended sediment, and maxima in late fall. Unamended sediment from the Southern Branch denitrified in spring (2-131 nmol N20/h/20ml) and fall (1-124 nmol N20/h/20ml) only. Main Stem unamended sediment denitrified only minimally in the spring. Sediment denitrification potential was independent of temperature and dissolved oxygen in the water column. Comparison of phytoplankton abundance values and potential denitrification rates suggest that denitrification potential may be stimulated by phytoplankton bloom senescence. Comparison to other published studies shows sediment denitrification potential in the ;Elizabeth River to be within the range of values reported for other environments. INTRODUCTION Denitrification may serve as a mechanism for removal of excess nitrate in eutrophic aquatic environments. Generally denitrification rates are seen to be nitrate limited (Gordon et al.1986; King and Nedwell, 1985; Oremland et al.1984), so that in a eutrophic environment denitrification would be expected to increase. However, the presence of toxics could inhibit microbial processes in the sediment, including denitrification. Pseudomonas andAlcaligenes species are considered the major contributors to denitrification in aquatic sediments. In addition, strains of Bacillus, Corynebacterium, Micrococcus, Achromobacter, and Nitrosomonas denitrify, indicating a wide diversity in bacterial denitrifiers (Knowles, 1982; Payne, 1973). Since denitrification is carried out by many sediment bacteria, it may be viewed as an indicator of the status of the sediment microbial population. The Elizabeth River is an interesting environment for the study of microbial processes in a heavily industrialized region. Contamination from heavy metals and other toxic compounds such as polynuclear aromatics poses a serious problem to this estuary, and it is considered to be a system under stress (V.S.W.C.B. Gen. Inf. 114 VIRGINIA JOURNAL OF SCIENCE Bull #557, 1984). The multitude of industries surrounding the river include shipbuilding, naval operations, waste treatment plants, coal facilities, chemical facilities, and power generating plants. The effects of such contamination and industrial activities on microbial processes in this system have not been assessed. In this study we determined denitrification potentials at two sites in the Elizabeth River over a nine-month time period using the acetylene blockage method. To our knowledge this is the first such study in the Elizabeth River estuary. In order to determine factors controlling sediment denitrification potential, rates were compared to temperature, and dissolved oxygen in the water column. In addition, denitrification potentials at one site were compared to phytoplankton abundance in the water column. METHODS Sites and Sampling Estuarine sediment was obtained from two sites in the Elizabeth River (Figure 1). Sediment was collected using a Ponar Grab (Wildco Instruments) and placed in sterile glass jars for transportation back to the laboratory. Water depth was between 0.9-1.8 m. One site was in the upper reaches of the Elizabeth River Southern Branch, adjacent to a nitrate fertilizer plant. This was an organic rich sediment. In a previous study, water column nitrate at the site was measured at 6.1 μM (Alden et al. 1988). Sediment from the second site was taken from the lower reaches of the Elizabeth River in the Main Stem, behind the docks of Norfolk International Terminal. The sediment from this site had patchy areas with relatively high sand content. Water column nitrate at this site was 0.5 μM (Alden et al. 1988). In the laboratory, the sediment was homogenized and diluted with surface water samples ( 4:1, sediment:water, vol:vol) from each respective site. The slurry was then dispensed in 20 ml portions (graduated cylinder) into sterile 125 ml Erlenmeyer flasks which were then sealed with rubber stoppers and gassed with N2 for 5 min to obtain anaerobic conditions. Duplicate flasks were prepared for each condition. Acetylene (Union Carbide) was added through the rubber stoppers. ( which had wells cored out of the top 2/3 portion) by injection to the heads pace gas using a 20 ml syringe (Stylex) for a final concentration of 10% (Taylor, 1983). In the last two experiments acetylene was freshly generated in a separate flask by the reaction of calcium carbide and water and added as above. Nitrate additions in the form ofKN03 (10 mM solution) were made by injection into the slurry to obtain a 100 μ M concentration in each flask. Sediments were incubated within 3 hours of collection in an incubator-shaker set at 100 rpm and U>°C. Incubation time (time= 0) began with the addition of potassium nitrate ( executed immediately after addition of acetylene) in nitrate amended flasks, or, in flasks with ambient nitrate concentrations immediately after addition of acetylene. For phytoplankton determination, two composite water samples of 15 liters each were taken above and below the pycnocline, using an intake hose and shipboard pump, at a mid-channel station in the Southern Branch, monthly from February through December 1989 (Figure 1). A 500 ml water sample was then taken from each composite sample and preserved with Lugols solution for phytoplankton analysis. A settling and siphoning procedure followed to obtain a SEDIMENT DENITRIFICATION POTENTIAL 115