UC San Diego

Freshwater ecosystems have less surface area than the oceans but play an outsized role in the global carbon cycle. Organic matter produced in situ or entering as terrestrial detritus may accumulate in sediments, be respired and released as CO2 to the atmosphere, or exported to the ocean. Aquatic decomposition of terrestrial detritus, periphyton primary productivity, benthic invertebrates (decomposers and grazers), and invertebrates that emerge from rivers contribute to the transport of carbon both downstream and between the terrestrial and aquatic ecosystems. I asked how decomposition rate, primary productivity, benthic macroinvertebrate biomass, and emergent macroinvertebrate biomass change with stream order in Yosemite Valley and El Portal, California, USA. In Summer 2019, I collected data at 19 sampling sites in the Merced River and its tributaries (Tenaya Creek and Yosemite Creek). Along with my main variables – decomposition rate, periphyton growth rate, benthic invertebrate biomass, and emergent invertebrate biomass – I considered environmental factors – elevation, temperature, and waterflow velocity. I found faster leaf decomposition at lower elevation and higher temperatures. Also, decomposition declined marginally with increasing invertebrate biomass. Other fluxes and stocks showed no relationship to stream order. Low elevation, high order rivers may therefore become more heterotrophic and release more CO2 to the atmosphere as a result of faster mineralization of allochthonous organic material without apparent compensatory increases in photosynthetic uptake. How the balance of photosynthesis and respiration in ecosystems will respond to climate change is one of the major unknown feedbacks in forecasting the future course of carbon accumulation.