Award: OCE-1657803

The continental shelfbreak 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 shelfbreak 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 synoptic measurements, such a feature is curiously absent from time-averaged measurements, both remotely sensed and in-water. Why would there not be a mean enhancement in phytoplankton biomass as a result of the upwelling? One hypothesis is that grazing by zooplankton prevents accumulation of biomass on seasonal and longer time scales, transferring the excess production to higher trophic levels and thereby contributing to the overall productivity of the ecosystem. However, another possibility is that the net impact of these highly intermittent processes is not adequately represented in long-term means of the observations, because of the relatively low resolution of the in-water data and the fact that the frontal enhancement can take place below the depth observable by satellite. A unique opportunity to test these hypotheses arose with deployment of the Ocean Observatories Initiative (OOI) Pioneer Array south of New England. The combination of moored instrumentation and mobile assets (gliders, autonomous underwater vehicles) facilitated observations of the frontal system with unprecedented spatial and temporal resolution. This provided a four-dimensional (space-time) context in which to conduct a detailed study of frontal dynamics and plankton communities needed to test the aforementioned hypotheses. We carried out a set of three research expeditions to obtain cross-shelf sections of physical, chemical, and biological properties within the Pioneer Array. Nutrient distributions were assayed together with hydrography to detect the signature of frontal upwelling and associated nutrient supply. We expected that enhanced nutrient supply would lead to changes in the phytoplankton assemblage, which were quantified with conventional flow cytometry, imaging flow cytometry (Imaging FlowCytobot, IFCB), optical imaging (Video Plankton Recorder, VPR), traditional microscopic methods, and phytoplankton pigments. Zooplankton were measured in size classes ranging from micro- to mesozooplankton with the IFCB and VPR, respectively, and also with microscopic analysis. Biological responses to upwelling were assessed by measuring rates of primary productivity, zooplankton grazing, and net community production. Intellectual merit: We tested the hypothesis that enhanced productivity at the shelfbreak front is not adequately represented in long-term means of the observations because of the relatively low resolution of the in-water measurements and the fact that the frontal enhancement can take place below the depth observable by satellite. Despite high resolution sampling throughout the water column, we did not find systematic enhancement of phytoplankton, primary production, zooplankton, or grazing at the frontthe enhancements we observed were ephemeral. This challenges our fundamental conceptual model of how this system works. Surprisingly, the highest phytoplankton biomass and productivity we measured were in intense subsurface diatom hotspots in the slope sea that were likely of Gulf Stream origin. This may have significant consequences for the regional ecosystem. As the climate has warmed, the changing large-scale circulation of the northwest Atlantic has resulted in increasing western boundary current instability. As a consequence, onshore intrusions of Gulf Stream (GS) water into the Northeast U.S. continental shelf have become increasingly frequent. The impacts of this shift on marine ecosystems have yet to be resolved. While these intrusions of low-nutrient GS water have been thought to potentially diminish biological productivity, our results suggest that subsurface diatom blooms enhance it. These results suggest that changing large-scale circulation has consequences for regional productivity that are not detectable by satellites by virtue of their occurrence well below the surface. Broader impacts: The project provided training for 3 postdoctoral scholars, 16 graduate students (3 from Brazil), and 16 undergraduates. Journalists aboard two cruises produced two video mini-documentaries and broadly disseminated the findings. Last Modified: 12/18/2023 Submitted by: DennisJMcgillicuddy