Benthic control on the statistical distribution of transported sediment carbon in a low-gradient stream

Results from a numerical model that simulates particulate organic carbon source, fate and transport were used to generate the statistical distribution of transported sediment carbon in a low-gradient, agriculturally-impacted stream over a five-year model simulation. Results suggest that the statistical distribution of transported sediment carbon is Gamma distributed (RMSEA = 0.066) for the low-gradient stream. The distributional form of transported sediment carbon is governed by seasonal variability of temporarily stored benthic carbon and the relative contributions of benthic, bank and upland carbon sources. Results of the study suggest that shape and skew of the Gamma distribution are governed by biological activity (i.e., autochthonous production and decomposition) of the streambed. Analysis was performed to examine how field sampling factors, including flow conditions during sampling, sampling frequency, and the sampling temporal domain including event, seasonal and annual variability, capture the statistical distribution of transported sediment carbon. Contrary to conventional wisdom, sampling flow conditions and sampling frequency showed little impact on the sampled distribution of transported sediment carbon, which reflects the amalgamation of streambank and upland carbon sources on the stream bed in this low-gradient stream. Annual variability, i.e., wet and dry years, and seasonal variability were needed to adequately capture the statistical distribution of transported sediment carbon, which reflects the stochastic nature of the hydrologic regime annually and the seasonal variability of biological processes. The results provide a testable hypothesis, and a sampling design approach, for the statistical distribution of transported sediment carbon in low-gradient systems where benthic biological processes are prominent.