Regional environmental degradation after mid-2010s, possible links with flow pattern change in a semi-enclosed sea (Ise Bay, Japan)

Worldwide, oligotrophication has been revealed to be occurring under the reduction of the anthropogenic nutrient load in coastal waters. Recent environmental changes such as oligotrophication and/or climate change have been reported in previous studies, however outcomes differ depending on the situation. This study demonstrated the environmental degradation during the 2000s–2010s based on in-situ data in Ise Bay, a semi-enclosed bay in Japan, and examines about the contribution of a change in the flow pattern (offshore water intrusion) using a diagnostic hydrodynamic model. Oligotrophication, increased DIN/DIP ratio and increased thickness of the low DO water mass occurred in the mid-2010s when the fisheries yield decreased. Results of a diagnostic model suggest that the flow pattern in spring and autumn alternated to a horizontal circulation from an estuarine vertical circulation. The flow pattern change contributed to the environmental degradation (oligotrophication and hypoxia) in the western area which is hardly influenced by the offshore water intrusion.