Abstract
The regulation of aquatic carbon cycles by temperature is a significant uncertainty in our understanding of how watersheds will respond to climate change. Aquatic ecosystems transport substantial quantities of carbon to the atmosphere and ocean, yet we have limited understanding of how temperature modifies aquatic ecosystem metabolic processes and contributions to carbon cycles at watershed to global scales. We propose that geomorphology controls the distribution and quality of organic material that forms the metabolic base of aquatic ecosystems, thereby controlling the response of aquatic ecosystem metabolism to temperature across landscapes. Across 23 streams and four years during summer baseflow, we estimated variation in the temperature sensitivity of ecosystem respiration (R) among streams draining watersheds with different geomorphic characteristics across a boreal river basin. We found that geomorphic features imposed strong controls on temperature sensitivity; R in streams draining flat watersheds was up to six times more temperature sensitive than streams draining steeper watersheds. Further, our results show that this association between watershed geomorphology and temperature sensitivity of R was linked to the carbon quality of substrates that changed systematically across the geomorphic gradient. This suggests that geomorphology will control how carbon is transported, stored, and incorporated into river food webs as the climate warms.
Highlights
The regulation of aquatic carbon cycles by temperature is a significant uncertainty in our understanding of how watersheds will respond to climate change
Aquatic ecosystems play an important role in watershed carbon (C) cycles and process a substantial amount of C from the terrestrial environment[1,2,3,4]; there are critical uncertainties in our understanding of how aquatic ecosystem metabolism will respond to climate induced shifts in temperature and hydrology
Our analyses show that a model allowing for a variable, stream-specific response of R to temperature performed substantially better in most cases than a model that set the temperature sensitivity at a single theoretical value (Supplementary Table 7)
Summary
The regulation of aquatic carbon cycles by temperature is a significant uncertainty in our understanding of how watersheds will respond to climate change. Will climate-driven temperature and hydrological change influence thermal characteristics of aquatic systems, and the quantity and quality of organic matter that fuels ecosystem metabolism and supports recipient food webs. A recent study of the temperature sensitivity of metabolic rates in streams across the US showed that the greater sensitivity of R to temperature than gross primary production (GPP) could decrease net ecosystem production (NEP) in streams as global temperatures increase[22] This higher and more variable temperature response of R has been supported by several studies in aquatic ecosystems[23,24,25,26,27], but reasons for this variation are not fully understood
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