A statistical framework for testing impacts of multiple drivers of surface water quality in nearshore regions of large lakes

Abstract

In nearshore regions of large freshwater ecosystems, complex biophysical processes across large geographic regions, combined with the common logistical challenges of data collection by multiple research agencies and shifting monitoring survey designs over time, present challenges for detecting and managing the influence of multiple sources of nutrients and pollution. We present a statistical framework using linear mixed models (LMMs) to test impact of multiple drivers on nearshore water quality of large lakes. Under this framework, we analyzed a 12-year dataset of water quality variables that were measured from a nearshore region along the Canadian shoreline of Lake Ontario (~86 km2), near Pickering and Ajax. Spatial interpolation showed that almost all water quality parameters decreased in magnitude from the shoreline to the offshore. Two exceptions to this nearshore-offshore gradient occurred in a region that extended ~8 km southwest from the outfall of the Duffin Creek Water Pollution Control Plant (DCWPCP) where ammonia + ammonium (NH3+4) increased and pH decreased slightly. Other LMMs combined explanatory factors into major groups (geographic proximities to shoreline tributary mouths and stormwater drains [inflows], tributary discharges, discharge or loading from the DCWPCP, and climate factors). These models showed that geographic proximity to shoreline inflows and/or tributary discharges were the most important drivers for most water quality parameters including concentrations of phosphorus, a key variable for regulating proliferation of harmful algae blooms and nuisance benthic algae in the Great Lakes. Air temperature was correlated with decreased phosphorus concentrations and nitrate + nitrite, whereas total precipitation and snow were correlated with increased concentrations of most nutrients except NH3+4, which was negatively correlated with duration of lake ice cover in winter. Our framework highlights how influence of individual nutrient sources can be distinguished from climate factors within a dominant nearshore-offshore gradient in water quality within nearshore regions of large lakes.