Shelf break frontal dynamics and its ecosystem implications at the Mid-Atlantic Bight

Shelf break fronts represent areas of high productivity found at the edges of continental shelves. The Mid-Atlantic Bight frontal system along the U.S. northeast shelf is one of those. In this region, the enhanced primary production is maintained by upwelling mechanisms. Nevertheless, it is known that here the concentrations of chlorophyll-a (Chl-a) do not consistently exhibit enhancement throughout the year even if local increases of phytoplankton biomass have been observed, hence the whole process is not completely clear. This thesis aims to investigate the frontal dynamics in order to clarify some of the mechanisms controlling primary production. A comprehensive inter-annual analysis (2003-2020), conducted using satellite images of surface Chl-a concentration, revealed periodic variability aligned with Spring and Autumn blooms, showing coherence with surface wind variability. This has not been verified for short-lived Chl-a enhancements suggesting that the observed variability is driven by a combination of upwelling mechanisms not directly connected to winds. Furthermore, the connection between upwelling mechanisms, involving the detachment of the bottom boundary layer (BBL), and the observed increase in Chl-a concentration at the shelf break front is investigated. In situ data are used to examine BBL detachment, employing two methods: accumulated properties change algorithm and along isopycnal change in properties, which revealed a significant BBL variability influenced by Gulf Stream features and seasonal stratification conditions. Moreover, we investigated the correlation between the detaching BBL, nutrient, and Chl-a abundance along isopycnals. Results indicate that the relationships between nitrate decline and phytoplankton biomass increase are not as straightforward as expected, which might be related to the limited statistical support, attributed to sample size, intrinsic data variability, and the assumption of bidimensionality.