Tidal stirring and its impact on water column stability and property distributions in a semi-enclosed shelf sea (Seto Inland Sea, Japan)

The Seto Inland Sea is a partially enclosed shallow sea with a complex bathymetry involving several basins, separated by narrow straits, and numerous islands. The influence of tidal stirring on density stratification and on the distribution of nutrients and chlorophyll in this region has been investigated using a combination of a numerical model of the tidal flow and the results of two extensive surveys of property distributions in the summer regime. The Princeton Ocean Model (POM) has been used to determine the spatial structure of the tidal flow and the results have been verified by comparison with an extensive database of current and elevation measurements. The model indicates a large difference in total energy flux between the western entrance section, which is controlled by a progressive wave regime, and the eastern entrance section which is associated with a predominantly standing wave regime. The combined energy flux input matches the total energy dissipation for the M2 in the Inland Sea which is found to be ∼3.4×109 W. Two ship surveys in August 2002 and 2003 along a central section show a highly differentiated pattern of vertical structure. The potential energy anomaly ϕ  , an index of vertical stability based on density, exhibits a marked inverse correlation with tidal energy dissipation D¯ thus confirming the control of stratification by tidal stirring. In most straits, vertical differences of nutrients and chlorophyll are small or negligible while in the low-energy basins there are pronounced vertical differences associated with density stratification. These sluggish basins exhibit oligotrophic conditions in the surface layer but large nitrate concentrations below the pycnocline and significant chlorophyll maxima in subsurface layer. The physical pattern of varying stratification and fronts is reflected to a considerable degree in the distributions of biochemical properties.