Long-term trends and temporal heterogeneity of water quality in tidally mixed estuarine waters

Dynamics of water quality in the Keonggi Bay, a shallow macrotidal temperate estuary of Yellow Sea, Korea were identified using the major water quality parameters such as dissolved oxygen (DO), ammonia, nitrate, phosphate, and chemical oxygen demand (COD). The study area during the last 18 years was in eutrophic and mesotrophic water in terms of the nutrient eutrophication index even with a slight decrease in DIN and COD concentrations during recent years. Monthly values of nitrate and ammonia significantly correlated with SS and salinity, respectively, indicating that re-suspension of sediment by vertical mixing and freshwater input are critical factors of monthly fluctuation in water quality. The lack of significant autocorrelation in water quality parameters suggested a significant tidal effect on temporal water quality fluctuation in the tidally mixed estuarine system. Principal component analysis (PCA) revealed a clear pattern of long-term trends of water quality. The early 1980s were the periods of best water quality, with worst conditions during the late 1980s and early 1990s. These long-term trends of water quality were well discriminated by PCA which can be further applied for the whole ecosystem interpretation with biological variables.