Salinity structure of a tidal freshwater ecosystem under multiple tidal conditions, Mission River, TX, USA

The ecological health and integrity of coastal estuaries critically depends on the balance between the quantity, quality, and timing of freshwater inflow. This balance may be upset by subtle changes in numerous hydrologic conditions, including precipitation rates and frequencies, runoff conditions, and tides. Certain hydrologic conditions will create an abnormally long freshwater residence time in a lower river reach--on the order of months between episodic storms--which will drastically alter the quantity, quality, and timing of estuarine freshwater inflow. We term this fresh, tidal, lentic river reach the 'tidal freshwater ecosystem' (TFE) and find that it remains largely overlooked by hydrologic and estuarine sciences. We hypothesize that TFEs occur in coastal rivers with small bed slope and riverine discharge, enabling denser saltwater intruding inland via tidal motion to impede freshwater discharge to the estuary. However, the balance of forces governing the relative rates and volumes of freshwater discharge, saltwater intrusion, and freshwater-saltwater mixing are not well understood in TFEs, especially with regard to the influence of vertical salinity structure (whether stratified, well-mixed, or a combination) on the retardation of freshwater discharge. In this study we sought to empirically characterize the salinity structure of a river known to contain a tidal freshwater reach, the Mission River of southern Texas. During high and low spring and neap tides, we surveyed a ~ 22 km-long tidal section of the river by towing two instruments: a multi-parameter probe measuring temperature, electrical conductivity (EC), and dissolved oxygen (DO) at mid-channel depth; and, at the water surface, an electrical resistivity geophysical cable measuring water and channel bed sediment electrical resistivity. We also profiled the water column every 0.25 km using a second multi-parameter probe. The data successfully resolved longitudinal and vertical salinity variations within the tidal river channel, enabling objective identification of the tidal freshwater reach and quantitative interpretation of the role of mixing in the TFE. For example, under neap tidal conditions we observed a linear decrease in EC with distance from the estuary at both the surface and mid-water column depth, consistent with a well-mixed longitudinal profile. However, DO concentrations did not follow the EC trend, despite relatively constant temperature and pH. The contrasting pattern in DO may suggest biogeochemical influences on TFE water chemistry in addition to influences from the river's density structure. The data encourage further field study and numerical modeling to more completely characterize the physical and biogeochemical function of TFEs, especially whether they may support a unique freshwater ecology while acting as a detaining reservoir for nutrients and freshwater vital to estuary health.