Residence Time in a Lagoon System

Cigu Lagoon is a lagoon located in southern-west costal area of Taiwan. It is not only an important site for fish farming but also provides tidal flats where migration birds come and stay. Cigu Lagoon is characterized by two inlets that connect the lagoon and the ocean, and the total width of the two inlets is only 1/20 of the lagoon length. Therefore, exchange of sea water is relatively limited compared to an open bay. This implies that nutrients and pollutants from inflow rivers are likely to be accumulated in the lagoon. Therefore, Cigu Lagoon is considered to be one of the most ideal lagoons for investigating ecological system of tidal flat in Taiwan. We conducted field observations to measure spatial distribution of salinity and water temperature by using an observation boat and the discharge of the river that flows into Cigu Lagoon in July of 2018. We applied a three-dimensional environmental model, i.e., Fantom, to analyze the variation in seawater exchange from spring to neap tide in Cigu Lagoon. Results suggest that the larger tidal amplitude occurred, the less the seawater exchange observed.