Diel and seasonal patterns in water quality continuously monitored at a fixed site on the tidal freshwater Potomac River

Abstract

Recent advances in water quality monitoring have facilitated the acquisition of temporally rich datasets that allow comprehensive analysis of patterns and underlying processes and drivers at multiple scales. We analyzed data from a continuous water quality monitor on the tidal Occoquan River, a tributary of the tidal Potomac River and Chesapeake Bay. Temperature, conductivity, dissolved oxygen (DO), and pH were collected at 15 minute intervals from April through early November of 2010. Results of time series analysis indicate that, on a short-term basis, conductivity manifested an underlying semidiel pattern, presumably driven by tidal excursion. In comparison, DO, pH, and temperature exhibited a diel pattern correlated with the daily light and temperature cycle. Longer-term patterns were related to longer-term climatic factors such as a dry summer with low freshwater inputs, seasonal progressions of light and temperature, and a river discharge spike in early October. Examination of multiday patterns in DO and pH using 15 minute data during a climatically stable period illustrated both diel and semidiel patterns. Patterns in a period of strong hydrological forcing revealed a disruption of diel and semidiel patterns for several days with a general restoration of patterns thereafter. Both diel and seasonal data suggest that abundant submerged aquatic vegetation in the study area was the main primary producer component driving diel and seasonal DO and pH patterns.