RAPID: Unprecedented Hypoxia in Cape Cod Bay

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

This is rapid response study of oxygen dynamics in southern Cape Cod Bay (CCB) in order to better understand the physical processes that are contributing to an unprecedented second consecutive summer of extensive bottom hypoxia. Comprehensive measurements will be made during the critical months of September and October, in order to resolve the important spatial and temporal scales needed to better understand the frontal dynamics of this region, which are hypothesized to play a key role in controlling bottom oxygen concentration. Hypoxia in coastal waters is typically associated with either large anthropogenic nutrient inputs or strong and persistent upwelling. Southern CCB has neither, yet extensive hypoxic bottom waters have developed for the second consecutive summer. This developing hypoxia event provides a unique opportunity to examine the physical and biogeochemical processes responsible for this novel hypoxia regime. Concentration of organic carbon in a localized area that also receives less oxygen input because of reduced mixing is expected to be the causal mechanism. Results from this research will be communicated to both the Massachusetts Department of Marine Fisheries and the Massachusetts Lobstermen’s Association. This research will help foster collaborations with these groups with the goal of assessing whether hypoxia is likely to persist in the coming year and become more severe as regional surface waters warm. Results from this research will help develop mitigation strategies to minimize economic losses and harm to the lobster fishery. It is hypothesized that in CCB, and other systems without persistent strong upwelling or large anthropogenic nutrient inputs, the production of organic carbon is insufficient to drive widespread hypoxia during the summer, but localized hypoxia can occur when convergent transport concentrates organic carbon in sufficient quantities to deplete bottom oxygen levels. In CCB, hypoxia is expected to develop in association with a strong bottom density front that occurs where the seasonal pycnocline intersects the seafloor. At this location, strong thermal stratification immediately adjacent to the seabed suppresses turbulence, preventing ventilation via vertical mixing and the front accumulates organic material that enhances respiration at this location. In order to maintain this bottom front during summer conditions, frequent relaxation/downwelling winds are needed. The mechanism of oxygen depleting associated with a bottom front is fundamentally different from what drives hypoxia in more commonly studied upwelling regions. However, it may be important in other environments with strong thermal stratification and variable wind forcing. This interdisciplinary research will resolve the mechanisms that drive hypoxia in CCB and provide insight into why low oxygen conditions appear to be occurring more frequently in this economically important region. The team will 1) conduct ship-based sampling roughly every 5 days to resolve the full three-dimensional distribution of temperature, salinity, dissolved oxygen, chlorophyll fluorescence and turbidity and how these variables respond to wind forcing; 2) deploy 3 bottom landers that span the hypoxic region on a N-S transect to continuously record bottom temperature, salinity and DO and water column circulation; and 3) conduct numerical simulations of the region using a coupled hydrodynamic-biogeochemical model to better understand the basic processes that result in low bottom oxygen. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.