Timing is everything: Understanding short- and long-term variability in light and temperature on inter-biome freshwater ecosystem production

Background/Question/Methods Light and temperature are dominant drivers of gross primary production (GPP) and ecosystem respiration (ER) that often covary seasonally. As most aquatic ecosystems are net heterotrophic and donor controlled, metabolic demands are constrained by light limiting carbon (C) production and temperature limiting ecosystem metabolic demands of organic C. The LTER Network consists of freshwater ecosystems in (sub)tropical (FCE, LUQ), temperate (AND, CWT, KNZ, NTL), tundra (BNZ), and arctic biomes (ARC), offering a unique opportunity to assess the relative temperature-dependence of GPP and ER across a range of temperature and light (photosynthetically active radiation; PAR). Synthetic comparisons of ecosystem production constrained by predictable patterns in temperature and PAR are valuable for understanding network and continental scale C processes as well as the contribution of inland fresh waters to the global C cycle. Understanding how the temperature dependence of GPP and ER vary broadly among freshwater ecosystems is important as ecosystems worldwide are experiencing increases in air and water temperatures; whereas changes in light availability are often site-specific.

Results/Conclusions

Long-term measures of ecosystem productivity at individual sites coupled with inter-biome (e.g., Lotic Inter-site Nitrogen Experiment, LINX) and network-scale comparisons (Global Lake Ecological Observatory Network, GLEON) illustrate that variance in GPP and ER is coupled but explained by ecosystem trophic state. Most freshwater ecosystems have positive ER-temperature relationships supported by metabolic theory, but ecosystems with pulsed light (arctic springs) or allochothonous organic C (temperate deciduous-forested systems) have negative ER-temperature anomalies explained by temporal dynamics in organic C production or terrestrial inputs. Low-level PAR increases variation in GPP in open-canopy streams and lakes, which is likely due to differential time-lag responses of GPP and ER to changes in light and temperature. Ecosystem heterotrophy in subtropical wetlands is driven by dry seasonal water depth, emergent macrophytes, and detrital organic C that collectively enhance water column heterotrophy. Tropical forested stream productivity is driven by storm-induced loading of terrestrial organic C (enhanced ER) and sedimentation (reduced GPP). Development of methods and modeling coupled with long-term data of ecosystem productivity throughout freshwater continental and global networks are continuing to advance our knowledge of the capacity and role of these vulnerable and changing ecosystems to retain organic C.