Net Ecosystem Metabolism is Independent of Elevation in Mountain Lakes of the Northern Rocky Mountains, USA

Gross primary production (GPP) and ecosystem respiration (ER) are fundamental metabolic processes in ecosystems, and their difference represents net ecosystem production (NEP) or the overall balance of carbon within an ecosystem. Widespread application of diel oxygen techniques has furthered understanding of the drivers of GPP and ER within aquatic ecosystems. However, in mountain lakes, which lie on a natural gradient of terrestrial inputs across elevation, metabolism is rarely estimated using diel oxygen techniques due to logistical difficulties. We collected high-frequency oxygen, light, and temperature data from 14 mountain lakes across an elevational gradient to estimate GPP, ER, and NEP and tested the prediction that lakes at higher elevations tend to be autotrophic (GPP > ER) due to atmospheric nutrient deposition, whereas lakes at lower elevations tend to be heterotrophic (GPP < ER) due to higher terrestrial carbon inputs. However, we found no apparent shift in NEP in response to elevation. Instead, lake NEP was positively correlated with the nitrogen:phosphorus ratio of in-lake nutrients. GPP and ER were both positively correlated with the ratio of watershed to lake area, a proxy for hydrologic processes in the lake. Across all lakes, estimates of metabolism, nutrient concentrations, and dissolved organic carbon concentrations were low relative to temperate and boreal lakes. In addition, allochthonous carbon is likely recalcitrant in the lakes we examined. We suggest that nutrient limitation and hydrology, rather than allochthonous carbon loading, may control metabolism in mountain lakes. Future work in mountain lakes should consider elevation-dependent properties lakes such as UV radiation intensity, lake morphometry, lability of allochthonous carbon, and nutrient stoichiometry as potential drivers of lake metabolism.