TIDE-SALINITY-CHEMICAL-ISOTOPIC RELATIONS IN TROPICAL ESTUARIES TO CHARACTERIZE THE SOURCES AND TRANSPORT OF WATER AND SOLUTES DURING TIDAL CYCLES (Invited Presentation)

Time series of tides and salinity were collected at minute intervals for 4 d (4 semidiurnal tides) in a tropical estuary. We used tide and salinity data to develop a mixing model that characterized the presence and timing of seawater, river, groundwater and rain and their mixture at the mouth of a river, a tidal creek at mid-estuary and a tidal creek at the estuary head. The tide expressed as normalized water levels vs. salinity (T-S) plots for the river mouth station revealed mixing with seawater during rising tides and freshwater-diluted seawater (brackish) drainage from the mangrove forest during ebb tides. In the mangrove creek station, the T-S relationships initially showed constant salinity, followed by sharp rises in salinity to peak high tide caused by seawater intrusion. The salinity decreased precipitously at the start of tidal ebbing due to the influx of freshwater (rain?) diluted brackish water from the mangrove forest. The T-S relationship for the tidal creek station at estuary head showed constant salinity which decreased only near peak rising tide because of river water dilution. During tidal ebbing, the salinity increased from groundwater influx to background salinity. We tested the T-S model to include chemical and isotopic data collected for 24 h (1 semidiurnal tide) in the tidal creek station at the head of the estuary. We observed clearly defined periods of fresh river water, mixed fresh-saline water and groundwater, and their chemical and isotopic signatures. Establishing T-S and T-chemical/isotopic patterns for multiple locations in mangrove estuaries over sub-tidal to tidal scales define location-specific water and solute sources and their cycling during tides. Our results can inform short to long-term investigations for scientific studies and in estuarine ecosystem management.