A Multi-scale Coupled Physical-Biogeochemical FVCOM System for Massachusetts Bay: Application for Mechanism Studies of Seasonal Variability of Dissolved Oxygen

A long-term (1992-present) water quality monitoring program reveals that the dissolved oxygen (DO) in Massachusetts Bay (MB) exhibits a clear seasonal cycle with little interannual variability. A multi-domain nested coupled physical-biogeochemical FVCOM model system was developed with the aim at assessing water quality conditions and identifying the key mechanisms controlling the temporal/spatial variability of DO in MB. Built on good agreement in model-computed and observed DO, nitrogen and other water quality variables, results of a 14-year (1995-2008) simulation indicate that DO in MB is dominated by two modes: 1) seasonal cycle and 2) spatial variation. A well defined DO seasonal cycle, with highest DO in March-April and lowest in October, persists in all years. The magnitude and phase of the DO season cycle varies spatially, with less variation in the northern part of MB than to the south. . Horizontal advection plays an essential role in the DO variation in the northern MB, where it is connected to the western Gulf of Maine coastal processes and southwest mean circulation. The southern region, particularly within Cape Cod Bay, features a well-defined local retention mechanism with a longer residence time. In this region, DO variation is mainly driven by local processes associated with reaeration, oxidation, sediment oxygen demand (SOD) and photosynthesis-respiration. Although biogeochemical, advection and mixing processes vary year to year, the DO in MB is characterized by a "self-regulation" process, which keeps their net contributions to DO relatively constant each year, and thus leads to a persistent DO seasonal cycle.