Abstract
The Zostera beds around the estuary of the Monzen River in Iwakuni City, Yamaguchi Prefecture, were divided into a recovery period (2009-2014) and a decline period (2015-2020) based on annual changes in bed area over time. During this period, precipitation showed an increasing trend. Water temperature and the amount of fine sediment deposition on the leaves were higher during the decline period than during the recovery period, while turbidity was lower. The thermal tolerance limits for Zostera marina and Z. japonica are estimated to be 28°C and 29°C, respectively, so the cumulative time above these temperatures was used as an indicator of heat stress. After 2015, a negative correlation was observed between the cumulative time and the area of Zostera beds. Fine sediment deposition showed a positive correlation with cumulative precipitation (January to September) and a negative correlation with Zostera bed area. These results suggest a combined effect of heat stress and fine sediment deposition on Zostera beds. The increase in precipitation likely led to greater sediment deposition, making the estuarine area more susceptible to the effects of global warming.
