Production and resuspension of planktonic foraminifers at the shelf break of the Southern Middle Atlantic Bight

Mesoscale hydrographic events that characterize circulation at the shelf break of the US East Coast enhance foraminifer production to levels comparable to that in regions of oceanic upwelling, including subpolar waters, equatorial zones, and coastal upwelling areas. In this work, we quantify the magnitude of shelf break production events and attempt to constrain the response of foraminifers to high frequency, hydrographic forcing. Specifically, we examined production and resuspension of planktonic foraminifers on the shelf break and upper slope of the Middle Atlantic Bight using trap samples, temperature data, and ancillary data collected as part of the SEEP II experiment offshore from the Delmarva Peninsula. We examined a time series of 30 sediment trap samples with an average trapping period of 13 days from February 1988 to May 1989 and cross-shelf transects of trap samples taken during three selected trapping periods (snapshots). Hydrography during the time series and snapshots was interpreted from 395 daily temperature sections. onic foraminifer fluxes exceeded 6000 tests/m2/d in nine trapping periods of the time series. The largest flux event exceeded 40,000 tests/m2/d and resulted from storm-driven resuspension and transport of particles from the shelf to the upper slope. Two other events were clearly associated with resuspension. In contrast, other peaks in foraminifer flux were due to production that was 1.5–7 times higher than that in gyre-center ecosystems and comparable to production in oceanic upwelling locales. Peaks in foraminifer production were associated with specific hydrographic events that lifted nutrient-rich, deep water into the euphotic zone near the shelf break. These events stimulated phytoplankton blooms, which nourished rapid growth and reproduction of planktonic foraminifers. The trophic events lasted a minimum of ∼6 days and occurred with greater frequency than those of typical open-ocean ecosystems. The shelf break, in general, may be a region of significant planktonic foraminifer production despite its proximity to unfavorable conditions that limit the range of planktonic foraminifers on the shelf.