Globally, phytoplankton blooms in lakes are increasing as a result of climate and land use changes that promote nutrient loading. However, the relative contributions of these drivers for increasing phytoplankton biomass in differing situations is not well established. For example, the relative importance of the two drivers may vary temporally in part because peak nutrient loads in north temperate lakes tend to occur in the spring, several months before summer phytoplankton blooms. Here, we investigated the relative contribution of land use and climate to sub-annual metrics of phytoplankton biomass in oligotrophic Lake Sunapee, New Hampshire, USA. This lake has been experiencing increasing phytoplankton blooms in recent years against a backdrop of warmer air temperatures and ongoing land use change in the catchment. We calibrated and validated a one-dimensional hydrodynamic model coupled with an aquatic ecosystem model to simulate lake dynamics over three decades. First we analyzed the effects of land use and climate in the 30-year baseline simulations. Then, we generated climate and land use scenarios based on the predictor variables identified in baseline simulation data analysis.
Results/Conclusions
Over the 30-year simulation, temperature and phosphorus loading were important predictors of chlorophyll-a. However, the relative importance varied with season: mean summer temperature was the primary predictor of summer chlorophyll-a, whereas spring phosphorus loading was the primary predictor of September chlorophyll-a. This finding suggests that sub-annual dynamics may be critical in understanding within-year ecosystem function and responses to changing stressors. Scenario testing highlights potential non-linear effects: September chlorophyll-a increased by 27% with higher spring phosphorus loading, whereas September temperature resulted in a slight decrease (3%) in chlorophyll-a relative to baseline conditions. In contrast, summer chlorophyll-a increased with both warmer summer temperature (6%) and higher spring phosphorus loading (23%). While previous studies have observed synergistic effects of warmer temperatures and higher nutrient concentrations on phytoplankton blooms in eutrophic lakes, our study suggests that land use and climate may have both positive and negative effects on phytoplankton biomass that vary at different times of year. Detailed process-based ecosystem modeling over long time scales coupled with robust observational data provides a powerful tool for understanding the interacting effects of land use and climate on lake ecosystems.