Bacterial dynamics at the sediment-water interface of a stratified, eutrophic reservoir.
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Christian, Bradley W.
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Sediment-water interfaces (SWIs) are loci of dynamic physical, chemical, and biological interactions in stratified, eutrophic reservoirs. Seasonal reservoir mixing and stratification affects SWI physicochemical processes as well as bacterial abundance, diversity, biomass, and metabolism. Because SWI bacteria transform chemicals and release nutrients that affect water quality and eutrophication, seasonal changes in these bacterial dynamics help define reservoir carbon and nutrient cycles and trophic interactions. Four studies were conducted to assess SWI bacterial dynamics in Belton Reservoir, a eutrophic, monomictic impoundment. The first utilized [3H]-L-serine to measure SWI bacterial activity and biomass production. Highest activity and production occurred during summer stratification under anoxic conditions. Lowest activity and production occurred under oxic conditions during autumnal overturn and winter mixing. The second study consisted of two parts, both utilizing Biolog EcoPlates to measure SWI carbon substrate utilization rates (CSURs). The first part tested the effectiveness and interpretability of EcoPlates. Optimal use was dependent upon inoculum density, incubation temperature, and aerobic/anaerobic incubation techniques. The second part concluded that CSURs for carbohydrates were highest during onset of stratification and winter mixing, CSURs for amino acids were highest during winter mixing, and CSURs for carboxylic acids were highest during late season stratification. The third study analyzed quantities and sources of SWI carbon, nitrogen, and bulk organic matter (OM). OM concentration did not differ among seasons. Inorganic carbon and nitrogen differed seasonally. OM C/N ratios and stable isotopes (13C and 15N) were significantly different at the SWI of the shallowest depths, indicating that OM at this site was of allochthonous origin. The last study utilized automated ribosomal intergenic spacer analysis (ARISA) and denaturing gradient gel electrophoresis (DGGE) to elucidate total and sulfate-reducing (SRB) SWI bacterial diversity and similarity. Total SWI bacterial diversity did not significantly differ. During stratification, high similarity occurred among sites on individual dates. During mixing, high similarity occurred through time. Although SRB are functionally strict anaerobes, they exhibited higher richness during oxic rather than anoxic conditions.