Organic Carbon Cycling and Ecosystem Metabolism

Organic matter (OM) regulates biogeochemical cycling and provides a fundamental thermodynamic stability to freshwater ecosystems. In this chapter we highlight key knowledge of freshwater OM composition, sources, and cycling. OM consists of a heterogeneous mixture of plant, microbial, and animal products, either alive or as fragments in various stages of decomposition (i.e., detritus). Freshwater OM is either autochthonous, meaning produced within the ecosystem, or allochthonous, meaning of an external, often terrestrial, source from the catchment. OM includes carbon (C) as well as varying quantities of other elements and can be in dissolved or particulate form (DOM, POM; DOC, POC). Gross primary production (GPP) and ecosystem respiration (ER) govern OC fixation and breakdown and can thus be used to characterize organic carbon (OC) cycling at the whole-ecosystem scale. Most ecosystems are heterotrophic (ER > GPP), meaning metabolism and food webs are subsidized by allochthonous OC. Once OC enters ecosystems from internal GPP or allochthonous sources, it can be buried, emitted to the atmosphere after respiration, or exported. OC fluxes, composition, and cycling are changing in response to anthropogenic alterations of land cover, hydrology, chemistry, and climate—how multiple stressors alter OM fluxes and metabolism remains a critical knowledge gap in our understanding of carbon cycling in inland waters.