TEMPORAL VARIATION OF SALINITY AND DISSOLVED OXIGEN IN A SALTMARSH FORMED AFTER THE GREAT EAST JAPAN EARTHQUAKE

A new saltmarsh was created due to land subsidence by the Great East Japan Earthquake, 2011 and erosion by the following tsunami attack. Water level, salinity and dissolved oxygen (DO) were measured from 2012 to 2019 with the time interval of 10 or 20 minutes in this saltmarsh, and variation of salinity and DO were discussed. Although maximum salinity level reached 32 and it does not change every year, lower level of salinity decreased gradually. Since the subsided land rises by 5 cm every year, seawater was difficult to intrude into the estuary during rainy season and storm discharge flow into the saltmarsh increased every year. DO increased rapidly in the morning (2.8 – 3.6 mg/L/h) and it decreased rapidly in the afternoon (3.4 – 4.3 mg/L/h), then is decreased gradually from night to the next morning (0.4 – 1.1 mg/L/h). DO in the surface water became supersaturated during daytime. This is caused by production and consumption by aquatic plant. DO in the bottom water became anoxic at night and for several days after rainfall. The reason is that the vertical mixing of bottom salt water was regulated due to wrapping by freshwater in addition to the consumption by aquatic plant.