Temporal variation in organic carbon spiraling in Midwestern agricultural streams

Inland freshwaters transform and retain up to half of the carbon that enters from the terrestrial environment and have recently been recognized as important components of regional and global carbon budgets. However, the importance of small streams to these carbon budgets is not well understood due to the lack of globally-distributed data, especially from streams draining agricultural landscapes. We quantified organic carbon pools and heterotrophic metabolism seasonally in 6 low-order streams draining row-crop fields in northwestern Indiana, USA, and used these data to examine patterns in organic carbon spiraling lengths (SOC; km), downstream velocities (VOC; m/d), and turnover rates (KOC; day−1). There were seasonal differences in SOC, with the longest spiraling lengths in winter (range: 7.7–54.4 km) and the shortest in early and late summer (range: 0.2–9.0 km). This seasonal pattern in SOC was primarily driven by differences in discharge, suggesting that hydrology tightly controls the fate of organic carbon in these streams. KOC did not differ seasonally, and variability (range: 0.0007–0.0193 day−1) was controlled by differences in stream water soluble reactive phosphorus concentrations. Compared to previous studies conducted primarily in forested streams, agricultural streams tended to be less retentive of organic carbon. These systems function predominantly as conduits transporting organic carbon to downstream ecosystems, except during low, stable-flow periods (i.e., late summer) when agricultural streams can be as retentive of organic carbon as forested headwaters. High organic carbon retention in the late summer has implications for coupled carbon and nitrogen cycling (i.e., denitrification), which may play an important role in removing nitrate from stream water during periods of low flow.