Local land-use and global climate change are rapidly degrading oligotrophic lakes. Robust water quality indicators are needed to determine whether oligotrophic lakes are maintaining their low-nutrient state or transitioning towards eutrophic conditions. Lake metabolism, the balance of carbon production and respiration, is an integrated ecosystem metric that may indicate impending ecosystem shifts to a meso- or eutrophic state. Ecosystem metabolism modeled from one deep site within a lake is often assumed to be representative of the entire lake; little is known about the relationship between lake metabolism estimates at near-shore sites and nutrient loading from the nearest inflow tributary (hereafter ‘localized nutrient loading’). In this study, we asked: How do summer lake metabolism estimates at near-shore sites (4-7 m deep) compare to a single deep-site (13 m deep) estimate? We addressed this question in oligotrophic Lake Sunapee, New Hampshire (USA), a lake with multiple inflow tributaries that vary three-fold in their nutrient load to the lake. We used high-frequency measurements of dissolved oxygen, water temperature, and light to estimate metabolism at three near-shore sites and one deep-site. We also measured stream discharge and nutrient concentrations in the closest inflow streams to each near-shore lake site to estimate localized nutrient loading.
Results/Conclusions
Net ecosystem metabolism at Lake Sunapee’s deep-site was consistently positive (range = 0.01 – 0.5 mg-O2/L/day) whereas all three near-shore sites averaged near zero net ecosystem metabolism (range = -0.4 - 0.3 mg-O2/L/day). On average, the magnitude of variability in diel dissolved oxygen concentrations was greater at the near-shore sites (~0.5 mg/L/day) than at the deep-site (~0.15 mg/L/day). Localized stream nutrient loading was positively correlated with daily maximum GPP and R rates, where the highest localized nutrient loading and maximum gross primary production (2.7 mg-O2/L/day) and respiration rates (2.5 mg-O2/L/day) occurred at the same site. Our work suggests that near-shore metabolism can be used as an indicator of ecosystem change as a result of localized nutrient loading. Developing a relationship between more easily measured ecosystem metabolism metrics compared to time intensive nutrient sampling may help lake managers target high priority areas of the watershed to reduce nutrient loading to lakes. Linking spatially variable lake metabolism to heterogeneous nutrient loading provides insight into how lakes integrate catchment land use, providing a sensitive indicator of impending shifts in lake trophic state.