Water temperature in estuaries: river and ocean influences, effects of surface heat flux, and dynamical role of temperature stratification

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Estuaries are biologically productive areas of the ocean that support important fisheries and coastal ecosystems. However, they are under stress from changing river flows and weather patterns, nutrient loading, hypoxia, and harmful algal blooms. Water temperature is a major influence on algal growth rates and affects nutrient cycling rates. Increasing temperature results in lower dissolved oxygen saturation values, and higher respiration rates of organisms, increasing community respiration; therefore, warmer water can directly lead to worsening hypoxia, which is already a nationally and globally increasing problem. At higher temperatures, oysters are more susceptible to disease and less able to tolerate freshening of the water column. Fish and crustaceans have thermal tolerance limits, and some species of fish already experience thermal stress during current warm water anomalies. The processes that control water temperature in estuaries have received little attention in comparison to salinity, which is the main control on the density field and estuarine circulation. This project will determine how estuarine water temperature is dynamically controlled and understand which processes, including estuarine basin geometry, are more important. The study will use existing data and semi-idealized numerical simulations from seven mid latitude estuaries. These are ecosystems likely to experience increased thermal stress in coming decades, when precipitation, river flow, wind, air temperature, and cloudiness patterns, all of which impact estuarine temperatures, are forecast to change. This project will support a graduate student.Although temperature is as important as salinity in its effects on the ecology of estuaries, the existing physical oceanographic literature provides little to no framework for understanding the temperature structure, variability, and dynamics of estuaries. The major influences on water temperature in estuaries are (1) conservative mixing of ocean and river waters with different temperature as water parcels are advected through the estuary and (2) air-sea heat exchange. In rapidly flushed estuaries, the water temperature is determined by conservative mixing, so temperature could be predicted from the salinity field and the river and ocean temperatures. In slowly flushed estuaries, however, these river and ocean end member temperatures vary on time scales shorter than the transit time of water parcels passing through the estuary, making conservative mixing less useful as a basis for predicting temperature. Further, in slowly flushed estuaries, air-sea heat fluxes cause the water temperature to depart from the conservative mixing prediction. This project will determine what controls the structure and evolution of water temperature in seven estuaries across a broad range of parameter space and provide a framework for understanding other estuarine systems. The investigators will develop an analytical framework for understanding the dynamics that control estuarine water temperature and identify non-dimensional parameters that govern the temperature structure. The new framework will be tested by synthesizing existing long time series observations of seven estuaries (the Delaware Bay, Chesapeake Bay, Hudson River, Merrimack River, Columbia River, Alsea River, and Guadalquivir River) that span a range of parameter space. As part of this research, the range of natural variability in estuarine temperatures and forcing (i.e., air-sea fluxes and time-varying river and ocean temperatures) will be determined and its relation to estuary depth and transit time will be examined using idealized numerical model experiments. The main hypothesis to be tested is that vertical temperature stratification has substantial dynamical effects by modifying vertical mixing in regions of marginal stability, changing the estuarine circulation and length of the salt intrusion.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.