Award: OCE-1657855

The continental shelf-break of the Middle Atlantic Bight supports a productive and diverse ecosystem. Current paradigms suggest that this productivity is driven by several upwelling mechanisms at the shelf-break front. This upwelling supplies nutrients that stimulate primary production by phytoplankton, which in turn leads to enhanced production at higher trophic levels. Although local enhancement of phytoplankton biomass has been observed in some measurements, it is absent from time-averaged measurements. Why would there not be a mean enhancement in phytoplankton biomass as a result of the upwelling? A number of hypotheses have been proposed to explain this conundrum, but the entire program was designed to assess the importance of each. We conducted three cruises to obtain cross-shelf sections of physical, chemical, and biological properties of the Mid-Atlantic Bight. Nutrient distributions were measured together with oceanographic conditions to detect the signature of upwelling and associated nutrient supply. We expected that enhanced nutrient supply will lead to changes in the phytoplankton assemblage, which will be quantified with using multiple techniques. Increased nutrient supply should also result in increased primary productivity, which was assessed by both direct measurements and a vertically resolved biological model. The cruises were conducted in spring, late spring and summer to provide a seasonal composite of potential impacts on primary productivity. The spring cruise was noteworthy in that a large accumulation of the haptophyte alga Phaeocystis pouchetti was found. This alga is important because it has negative consequences to benthic fauna throughout the coastal region. Indeed, anecdotal reports of aquaculture disruptions during the bloom were reported, and it is likely that this bloom resulted in economic losses in coastal New England. We found that the bloom seemed to be initiated near Nantucket Shoals and occupied substantial areas (ca. 8,000 km2 at its maximum). The alga sank to the bottom of the water column, but there was still enough light to allow growth. It depleted nutrients and contributed substantially to overall productivity. The bloom dissipated by early May, as determined by satellite images. Based on continuous measurements of phytoplankton composition in Martha’s Vineyard, we found that these blooms occur approximately every three years. Hence they are an important component of both interannual variability and coastal impacts. The spring cruise found a very unusual feature: a narrow band of an unusual alga - Thalassiosira diporocyclus, a diatom. The growth of the alga was confined to a very narrow band ca. 50 m below the surface. They were stimulated by the interaction of a warm core ring with the continental shelf, which resulted in the vertical movement of nutrients into the euphotic zone. Both direct measurements and a detailed bio-optical model showed the importance of these "hotspots", in that they contributed significantly to integrated primary production. They likely are enhanced bands of increased trophic transformations that might play a significant role in food-web dynamics of the MAB. The summer cruise was characterized by the interaction of a "streamer" – a thin filament generated by a warm-core ring interacting with the continental shelf. Streamers have distinctive temperatures and phytoplankton biomass and represent a transfer of material onto the continental shelf. As the region in summer has a relatively low primary productivity, we followed the evolution of the streamer through time to detect its impacts. The analysis showed that the streamer represented a major form of cross-shelf mixing and caused a substantial onshore transport of heat and salt, as well as a substantial offshore transport of organic carbon and oxygen. In summary, while the three cruises were significantly different from each other, each had its own special impacts that were documented. Shelf-break interactions with deep water features were shown to be very important, and undocumented blooms on the shelf may become significantly more important in the future. Our study provided data that were highly resolved in space and time, and will serve as a benchmark for future MAB investigations. Last Modified: 10/31/2023 Submitted by: Walker O Smith