Chemical Composition and Microbial Activity of Seston in a Southern Appalachian Headwater Stream

Chemical composition and microbial activity of seston (i.e., fine particulate organic matter and associated inorganic material in transport) in a southern Appalachian headwater stream were evaluated to determine whether changes in microbial activity associated with decreasing particle size were related to changes in seston surface area and/or chemical composition. As seston particle size decreased from 500 to 10 μm, organic content measured as ash free dry mass decreased from 72.5% to 36.7%. Simultaneously, the nutritional quality of the organic fraction declined, as evidenced by increases in lignin and cellulose content and corresponding reductions in acid-detergent-soluble materials (simple carbohydrates, proteins, and lipids). Microbial activity, measured as mass-specific 14C-glucose mineralization and 3H-thymidine incorporation rates, increased significantly as particle size decreased, despite the reduced nutritional quality of smaller particles. Microbial biomass (ATP) and activity (glucose mineralization) per unit seston surface area were proportional to particle size over the 10-250-μm particle size range. Smaller particles of lower nutritional quality supported lower area-specific microbial biomass and activity. Production: biomass ratios of microorganisms associated with seston were low (1.21 to 6.08 × 10-9/hr), suggesting that these microorganisms may have become inactive in response to the gradual decline in quality of detritus as it decomposed.