Seasonal variability in the concentration and flux of organic matter and inorganic nitrogen in a semiarid catchment, San Pedro River, Arizona

Abstract

Streams and rivers in semiarid environments are characterized by highly variable discharge associated with seasonal precipitation patterns. To characterize how this variability in discharge affects the concentration and flux of organic matter and inorganic nitrogen, we measured discharge and collected water samples in the San Pedro River in southeastern Arizona from April 2001 through September 2002. Mean daily discharge during the study ranged from 2.5 × 103 m3 day−1 to 9.4 × 105 m3 day−1, with low discharge in late June followed shortly thereafter by the highest discharge in July or early August after the onset of monsoon storms. Dissolved Organic Matter (DOM) dominated carbon and nitrogen flux during the nonmonsoon season, while Particulate Organic Matter (POM) dominated flux during the monsoon. The concentrations of Dissolved Organic Carbon and Nitrogen (DOC, DON), and NO3‐N typically increased 2‐ to 10‐fold, while POM increased by 2 to 3 orders of magnitude during the monsoon season. Concurrent increases in concentration and discharge resulted in 96% of the annual organic carbon (DOC + POC) and 97% annual nitrogen (DIN + DON + PON) flux occurring during the monsoon season with the majority of both organic carbon (92%) and total nitrogen (83%) being transported as particulates. These data indicate that POM transported during the monsoon season may be a major source of carbon and nitrogen to the stream ecosystem, potentially fueling heterotrophic activity in sediments and the hyporheic zone after the monsoon season and associated floods have ended.