Physical and Biological Controls on Diel Dissolved Oxygen and Water Quality Dynamics along the Potomac River Continuum
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Fundamental physical controls on dissolved oxygen – such as salinity, light availability, and water temperature – have distinct longitudinal gradients within river ecosystems. These gradients are particularly important in major tributaries of the Chesapeake Bay, which transition from non-tidal streams and rivers to tidal estuaries. Spatial and temporal changes in dissolved oxygen (DO) and other water quality parameters can be tightly linked with varying physical and biological controls along river flowpaths. However, studies analyzing longitudinal patterns in dissolved oxygen have been limited, largely due to the intense diel cycling and short-term variability of dissolved oxygen. Lack of intensive research into sub-daily dissolved oxygen dynamics, as well as historical disconnects between stream ecology and estuarine ecology, continue to impede holistic environmental monitoring, modeling, and management from flowing freshwater to estuaries. This research leverages continuous and discrete water quality data collected from the Chesapeake Bay's Potomac and Anacostia watersheds, covering more than 160 km along the non-tidal and tidal mainstem of the Potomac River. We explore drivers of water quality and dissolved oxygen along the watershed-estuary continuum using a combination of longitudinal stream synoptic monitoring and high-frequency sensor data. We explored physical and biological controls on dissolved oxygen through longitudinal and seasonal patterns in turbidity, chlorophyll-a, pH, salinity, and water temperature. For example, we found that mean daily dissolved oxygen and the daily range of pH both significantly decreased along the flowpath of the tidal Potomac River estuary in the Summer and Fall (R2 > 0.48, p < 0.001; R2 > 0.70, p < 0.001). We present a simple mechanistic model of dissolved oxygen, which also highlights promising remote sensing applications for sub-daily dissolved oxygen estimation. We combine sensor data, routine sampling, and longitudinal stream synoptic monitoring to holistically understand drivers and longitudinal patterns in dissolved oxygen, salinity, and organic matter, as well as the interplay of extreme climatic events and anthropogenic inputs in regulating biogeochemical transformations and water quality impacts from rivers to coastal zones.