Understanding the macroscale response of lake water quality to measures of regional and global climate
Lakes have been called sentinels of climate change. In fact, lake water quality can be influenced both directly and indirectly via the terrestrial landscape by climate. Because water quality varies over time in a single lake and over space in a single year, both temporal and spatial variation should be considered when evaluating the effects of climate on water quality. However, temporal and spatial variability in the direct and indirect effects of climate on lakes have not been evaluated at broad (e.g., subcontinental) extents that include large numbers of lakes. In this study, we quantified the effect of several climate metrics on the water quality in 7500 lakes varying in hydrology, morphometry and landscape setting in 17 US states. Using association rule mining and hierarchical models, we compared how regional climate metrics and annual global climate indices were related to water quality metrics, including water transparency (Secchi), total phosphorus (TP), and chlorophyll a (Chl a) concentrations. Regional climate metrics included average, annual, and monthly precipitation and temperature, as well as Palmer Drought Severity Index; global climate indices included El Nino Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO).
Temporal anomalies in lake water quality were associated with climate anomalies for individual lakes in multi-year time series. For example, we found high TP conditions occurred more frequently when mean annual air temperatures were high. In addition, we found that the strength and direction of the response to global indices varied regionally. Although regional patterns in geology and land use can confound and/or mediate the potential causal effect of climate, the temporal responses of water quality to climate were generally consistent with the large-scale spatial patterns over climate gradients. Therefore, if we fail to account for these regional climate patterns and their indirect effects on lakes, we could greatly underestimate TP. These results demonstrate a strong effect of climate on water quality, and emphasize the need to consider local and regional landscape properties for understanding the response of individual lakes to global climate patterns. Studying such a large spatial extent with data from >7500 lakes allowed us to measure the climate signal in water quality and to determine the most relevant spatial and temporal scales for measuring climate.