Physical controls dominate stream metabolism and carbon cycling over nutrients and stoichiometry

Abstract

Streams process and transport vast carbon quantities along the aquatic continuum. The balance between processing and downstream export is governed by metabolism, which in turn is regulated by distinct physicochemical controls that vary through space and time. However, macronutrient stoichiometry is rarely considered over absolute concentrations. Here, we examined physical, chemical, and stoichiometric controls on stream metabolism, organic carbon mineralization, and carbon spiraling over 2 years. Stream discharge, temperature, and light were the primary drivers across metabolic and carbon cycling parameters. In generalized additive models, physical variables accounted for up to four times more deviance than stoichiometry, with consistently more important associations in partial least squares regressions. Increased discharge and temperature promoted heterotrophy and strongly controlled metabolic balance, while dissolved C : P and C : N provided minor moderating effects. Our study highlights varied physical controls on stream metabolic processes and suggests nutrient concentrations and stoichiometry are secondary regulators, especially when C : N : P are non-limiting—an important reflection when evaluating future effects of nutrient pollution on the global carbon cycle.