Spatial and temporal variations of estuarine stratification and flushing time across the continental U.S

Estuarine circulation attributes such as stratification and flushing time significantly influence estuarine ecological processes. Stratification reflects how much vertical mixing occurs in an estuary, while flushing time can describe the exchange rate of pollutants between the estuary and ocean. A recently developed estuarine characterization framework used estuarine geophysical attributes and water exchange datasets to characterize estuarine circulation for 360 estuaries in the continental U.S. between 1950 and 2015. The estuaries were grouped into nine ecoregions according to the Marine Ecoregions of the World. In the Gulf of Mexico and along the East Coast, most estuaries were well-mixed (63-93%), with 3-5% strongly-stratified estuaries. Along the West Coast, strongly-stratified estuaries dominated (46-63%), with the exception of the Puget Trough basin and the southern CA ecoregion with 83% and 75% well-mixed estuaries. The stratification type of some estuaries varied seasonally. Generally, they were more stratified winter through spring, then mixed during the summer, with the exception of southern FL, which had a reverse pattern due to the positive correlation between the stratification parameter and freshwater inflow (97% estuaries with R ² > 0.9). The flushing times of the 300 well-mixed and partially-stratified estuaries were estimated using Tidal Prism (TPM) and Freshwater Fraction Methods (FFM). Flushing time seasonal variation exhibited a negative correlation with freshwater inflow (R ² > 0.8 for 50% of estuaries using TPM). Generally, estuarine flushing times were short in winter and long in summer (reversed in FL and a portion of the Gulf of Mexico). On the West Coast, estuaries tended to flush quickly compared with estuaries in other regions, even though they usually had low freshwater inflows, since other factors, e.g., the estuarine volume, affected the flushing time as well. To ensure appropriate interpretation of responses to change in nutrient loading, the significant intra- and interannual variations in stratification and flushing time need to be incorporated into management and assessment of estuaries.