Seasonal Variation in Intrusion Frequency of Oceanic Water from Middle and Lower Layers and Its Significance in Bottom Water Hypoxia in Ise Bay, Japan

Abstract:

The intrusion of oceanic water is an important event to raise the exchange rate of water in the enclosed bay. In this study, an attempt was made to quantify seasonal variation in the intrusion frequencies of oceanic water from the mouth of Ise Bay, which is a semi-enclosed water body having common characteristics of summer season hypoxia that is typically observed in eutrophic enclosed bays. Real-time, long-term water quality and hydrodynamic data of Ise Bay have been utilized in this research. The frequency of oceanic water intrusion from the middle and lower layer of Ise Bay was calculated by two methods, first, by comparing the density structure of bay mouth water column layers with the density structure of inner bay water column layers. In the second method, the classical tidal harmonic analysis was conducted to separate the tidal currents from the observed layered hydrodynamic data and subsequently, intrusion frequencies were calculated by using residual flows. Residual flow indicated the existence of normal estuarine circulation with bottom inflow and surface outflow in winter season while in the summer season this bottom intrusion was restricted, this phenomenon was more prominent in 2013 and 2014 as compared to 2012. From the analysis results of both methods, it was found that winter season has more intrusion from the deeper layers having an average monthly frequency of 87% and 82% respectively, whereas summer season has lesser bottom water intrusion with the average monthly frequency of 66% and 64% respectively. Intrusion frequencies gave evidence of more supply of oxygen-rich bottom water in the winter season while lesser supply in the summer season. This shift of oxygen-rich ocean water intrusion from the lower layer to the upper layers plays a vital role in the development of lower dissolved oxygen water mass at the bottom of the inner bay.