Biochemical implications of the Bottom Boundary Layer Detachment at the Mid-Atlantic Bight shelf-break front

Abstract

The Mid-Atlantic Bight frontal system along the U.S. northeast shelf is rich in biodiversity. In this region, primary production is influenced by a variety of upwelling processes, including internal instabilities of the front, off-shore forcing from Gulf Stream rings, and wind-driven flows. It is noteworthy that the concentrations of chlorophyll-a (Chl-a) in the shelf-break region are not consistently enhanced throughout the year, although local increases of phytoplankton biomass have been observed in some circumstances. In this work, we investigate the frontal dynamics of one of the possible mechanisms affecting primary production: upwelling via detachment of the Bottom Boundary Layer (BBL). The annual variability of the surface Chl-a in the shelf-break region reveals a 5- to 20-day period, which is potentially consistent with nutrient upwelling associated with the BBL detachment. Details of the process are examined using in situ data by quantifying along-isopycnal changes in properties. As frontal isopycnals rise in the water column, nitrate tends to decrease and Chl-a tends to increase, suggesting utilization of upwelled nutrients by phytoplankton. However, significant fluctuations can be attributed to sample size, intrinsic data variability, and the assumption of homogeneity in the along-shelf dimension.