Seston and Dissolved Organic Carbon Dynamics in a Southern Appalachian Stream

Abstract

Suspended paniculate matter (seston) and dissolved organic carbon (DOC) were stud- ied along a 6.4-km section of a southern Appalachian headwater stream from July 1978 to August 1979. Samples were collected at six sites encompassing stream orders 1-4 and a 635 m elevation change. Nonstorm concentration of particulate organic matter (POM) measured as ash-free dry mass (AFDM) ranged from 0.53-3.68 mg/L during the study. No significant changes in seston concentration occurred from upstream to downstream sites, and only one site had concentrations significantly different from the others. Significant seasonal trends in seston concentrations existed over all stations, and these were associated with corresponding hydrograph changes. During the first storm following leaf-fall both seston concentration (AFDM 22.5 mg/L) and particle size in transport increased. Non- storm DOC/particulate organic carbon (0.45 x POM) ratios average =1. DOC concentrations (mg/L) increased over the first 2 km of stream but remained relatively constant farther downstream. The increase was largely due to high molecular mass fractions. Seven size classes of seston were measured, and discriminant analysis was used to assess longitudinal changes in particle size distribution. The largest and two smallest size classes increased in relative abundance downstream while the four intermediate classes decreased. Although the mean particle size decreased downstream, the ratio of coarse particulate organic matter (CPOM)/fine particulate organic matter (FPOM) increased down- stream. We suggest that CPOM/FPOM ratios do not necessarily reflect downstream reductions in mean particle size. CPOM/FPOM ratios and downstream changes in mean particle size should be viewed as separate phenomena. The ratio of benthic CPOM/suspended CPOM decreases downstream, which is apparently attributable to the lack of retention devices in higher order streams. Food quality available to collector organisms, as indicated by adenosine triphosphate (ATP) measurements, was highest in the two smallest (0.45-43 /^m) seston size classes. No apparent relationship existed between ATP and percent organic matter in the various seston size classes.