2023 Volume 27 Issue 2 Pages 1-13
The bottom layer of Ise Bay has experienced remarkably large areas of hypoxic water since 2006. In order to identify the mechanisms of the expansion of hypoxic water after 2006, we conducted a long-term (2003–2016) numerical simulation that was based on a hydrodynamic and lower trophic ecosystem model of the area. The model reproduces the density structures and variability of hypoxic water in the bottom layer of Ise Bay. Interestingly, the simulated hypoxic water areas on the date of monthly observation differs remarkably from the simulated monthly mean values. This implies that once-a-month observation cannot capture precise variation of hypoxic water due to a lack of sampling frequency. The simulation results indicate that the intrusion of outer sea water into the bottom layer (BLI: Bottom Layer Intrusion) plays a key role for driving the year-to-year variation of bottom layer hypoxic water. Our results show that the occurrence of BLI is principally driven by the fortnight fluctuation of tide as clarified in previous research. In addition, BLI is also related to (1) the vertical mixing of sea water due to strong wind, (2) the intensified estuarine circulation caused by river discharge and (3) the upwelling of cold and high-density waters from the outer sea. Among these three factors we found that wind speed and river discharge are related to the recent expansion of hypoxic water in Ise Bay. Since 2004 the mean wind speed in August-September has become weaker and the river discharge has decreased. This may have led to the decreased occurrence of BLI after August, resulting in the expansion of bottom layer hypoxic water in recent years.
