Using a Mountain Landscape to Understand Drivers of Spatial Variation in Aquatic Ecosystem Metabolism

Abstract:

Lake metabolism studies have focused on dynamics and drivers within the surface mixed layer, despite demonstrated vertical and lateral variation in metabolic rates in many systems. This spatial bias hinders our ability to predict lake responses to stressors, because gross primary productivity (GPP) and ecosystem respiration (ER) may have habitat-specific constraints or drivers. Here we make use of compressed environmental gradients and topographic heterogeneity in a mountain landscape in the Sierra Nevada, California, to characterize spatial variation in aquatic ecosystem metabolism within and among lakes. Over the course of three summers (2017-2019), we performed in-situ benthic and pelagic incubations, diel oxygen profiling, and isotopic sampling in 15 lakes that spanned a 2000-m elevation gradient and substantial size and morphometry gradients (surface area 2-60 hectare; max depth 3 – 80 m). Despite large differences in terrestrial carbon inputs and mean water temperature among lakes, we found consistent differences in ecosystem rates between pelagic and littoral habitats, as well as consistent increases in GPP with depth, often peaking within the hypolimnion. Depth-integrated whole lake metabolism showed no elevational trends, and in all lakes ER was closely linked to GPP (R² = 0.94). Littoral zones were generally autotrophic, with rates of GPP and ER exceeding those in the open water, though rates varied among substrate type. Our results suggest that basin morphometry should exert strong controls on whole-lake productivity by dictating the relative contribution of different habitat types.