The role of effective discharge in the ocean delivery of particulate organic carbon by small, mountainous river systems

Recent research has shown that small, mountainous river systems (SMRS) account for a significant fraction of the global flux of sediment and particulate organic carbon (POC) to the ocean. The enormous number of SMRS precludes intensive studies of the sort conducted on large systems, necessitating development of a conceptual framework that permits cross‐system comparison and scaling up. Herein, we introduce the geomorphic concept of effective discharge to the problem of source‐to‐sink POC transport. This idea recognizes that transport effectiveness is the product of discharge frequency and magnitude, wherein the latter is quantified as a power‐law relationship between discharge and load (the ‚rating curve’). An analytical solution for effective discharge (Q_e) identifies two key variables: the standard deviation of the natural logarithm of discharge (Σq), and the rating exponent of constituent i (b_i). Data from selected SMRS are used to show that for a given river Q_e‐POC , Q_e‐sediment, Q_e for different POC constituents (e.g., POC_fossil vs. POC_modern) differs in predictable ways, and Q_e for a particular constituent can vary seasonally. When coupled with the idea that discharge peaks of small rivers may be coincident with specific oceanic conditions (e.g., large waves, wind from a certain direction) that determine dispersal and burial, these findings have potentially important implications for POC fate on continental margins. Future studies of POC transport in SMRS should exploit the conceptual framework provided herein and seek to identify how constituent‐specific effective discharges vary between rivers and respond to perturbations.