Organic processes in a coastal plain stream of the southeastern United States

Hydraulic characteristics, stream metabolism, and organic matter dynamics were studied in Payne Creek, a low-gradient Coastal Plain stream. Solute injection experiments and subsequent analyses with OTIS revealed that Payne Creek had a small transient storage zone with little exchange between the channel surface and the hyporheic water. Discharge showed positive relationships with the channel cross-sectional area (A) and dispersion coefficient (D), but did not affect the storage zone size (A_S) or the relative storage zone size (A_S/A). Accumulated organic matter in the stream functioned as a diffusive debris dam during fall, increasing both channel and storage zone areas. Lack of hydrologic interaction between stream surface and subsurface implies that organic processes are mainly controlled by activities in channel surface in Payne Creek. Mean daily gross primary productivity was 0.006 g O₂ m−2 d−1 ranging from 0 to 0.022 g O₂ m−2 d−1, which was much lower than previously reported values. Community respiration varied between 0.5 and 2.45 g O₂ m−2 d −1 with a mean daily respiration of 1.13 g O₂ m −2 d−1, and it was also in the low end of previously reported values. These low metabolic rates can be attributed to the low nutrient concentrations, low light intensity, and continuously shifting sandy substrates. The mean annual litterfall was 678 g AFDM in Payne Creek. Annual mean mass of particulate organic matter (POM) stored in the top 40 cm of stream sediment was 3.45 kg AFDM m−2, ranging from 2.52 kg to 4.32 kg AFDM m−2, and approximately equally divided between coarse particulate organic matter (CPOM) (48.6%) and fine particulate organic matter (FPOM) (51.4%). In the surface sediment, CPOM mass was highest in November following deciduous litter input, while the FPOM was highest in October. In deeper sediments, CPOM and FPOM amounts were similar on all sampling dates, and seasonal changes were not pronounced. Annual total export of CPOM from Payne Creek was 23.3 kg during the year 2000. An organic matter budget indicated that Payne Creek was a highly heterotrophic stream receiving more than 99% of energy from allochthonous sources, although 64% of organic matter inputs were not identified.