Mechanism studies of seasonal variability of dissolved oxygen in Mass Bay: A multi-scale FVCOM/UG-RCA application

- A coupled physical-biogeochemical model was developed to simulate the DO field in Mass Bay.
- DO in Mass Bay exhibits a well-defined seasonal cycle, highest in March-April and lowest in October.
- An EOF analysis indicates that DO in Mass Bay features both seasonal and spatial modes.
- Horizontal advection plays a dominant role in the DO variability in the northern bay.
- The DO variation in the southern bay is controlled predominantly by local biogeochemical processes.

Long-term (1992-2010) water quality monitoring records reveal that the dissolved oxygen (DO) concentration in Mass Bay exhibits a well-defined seasonal cycle, highest in March-April and lowest in October. This pattern persists in all years with insignificant interannual variability. A multi-domain-nested coupled physical-biogeochemical model was developed and applied to simulate the DO field over the 16-year period 1995-2010. The model-computed DO and nitrogen concentrations were in good agreement with observations. An EOF analysis of the modeled DO field indicates that DO in Mass Bay features both well-defined seasonal and spatial modes. The magnitude and phase of the DO seasonal cycle vary more significantly in the southern bay than in the northern bay. Horizontal advection, which is connected to the western Gulf of Maine coastal currents, plays a dominant role in the DO variability in the northern bay. The southern bay features a well-defined local retention mechanism with a longer residence time. In this region, the DO variation is controlled predominantly by local biogeochemical processes. Since the photosynthetic minus respiration production of DO is always balanced to a large degree by the oxidation of organic matters, reaeration becomes a major driver for the seasonal cycle of DO.