Temporal dynamics of seston: A recurring nighttime peak and seasonal shifts in composition in a stream ecosystem

Abstract

We measured the baseflow concentration and composition of seston and suspended particulate organic carbon (POC) over a 1-yr period in White Clay Creek, a third-order stream in the southeastern Pennsylvania Piedmont, to assess temporal variability in seston concentration and quality at seasonal and diel timescales. Each month, we sampled stream water under baseflow conditions every 1.5 h over a 24-h period and measured seston and POC concentrations, carbon composition, pigment content, and 13C isotopic ratios. Seston and POC concentrations exhibited a strong diel pattern; nighttime concentrations exceeded daytime concentrations by 80% and 43%, respectively. Suspended chlorophyll a concentrations did not exhibit a diel pattern. We attribute the diel pattern of seston concentration to bioturbation by the nocturnal stream community, including crayfish, amphibians, eels, and macroinvertebrates. Seasonally, carbon content of seston increased from 9% throughout most of the year to 15% during November, December, and March, while seston d13C was depleted in the late fall and enriched in early spring months relative to the rest of the year. Chlorophyll a and pheophytin a concentrations in seston peaked during the early spring. Seasonal patterns in seston and POC composition reflect cycles of autumnal leaf litter inputs and vernal algal production. Bioturbation and shifts in organic carbon inputs mediate changes in POC quality and fluxes, which affect the bioavailability of POC and ultimately influence rates of heterotrophic respiration. Particulate organic carbon (POC) provides energy for microorganisms and macroinvertebrates in headwater streams and, when transported in the suspended load (seston), supports heterotrophic metabolism in downstream reaches and rivers (Wipfli and Gregovich 2002; Mayorga et al. 2005). POC transport represents a longitudinal linkage between upstream autochthonous and allochthonous production and downstream ecosystems, where the magnitude of connection is controlled by the quality, quantity, and timing of POC delivery (Mayorga et al. 2005).