Award: OCE-1032285

This collaborative interdisciplinary research project investigated physical exchange and biological responses during winter along the margin of the broad continental shelf off the Southeastern United States. The study area was Long Bay off Myrtle Beach, South Carolina (Figure 1) where satellite imagery and prior field work had shown blooms of phytoplankton often develop and persist on the outer shelf through the winter (Figure 2). The persistence of blooms indicated recurring nutrient input through winter. Major objectives of the study were to investigate the role of multiple physical mechanisms of nutrient input from the upper slope to outer shelf under varied winter conditions, characterize the response of the phytoplankton community, and assess export of fresh organic matter from the outer shelf to the deep ocean. The production and export of organic matter along the shelf margin are topics of long-standing interest for the SE US continental shelf and for shelf systems globally. An intensive field campaign was conducted in the winter of 2012 that included deployments of three sets of moored instruments (mid-shelf, shelf break, and upper slope), operation of two autonomous gliders through the winter, and ship-based surveys and sampling during research cruises conducted between December, 2011 and early April, 2012. A follow-up cruise was conducted in February of 2013. The project also included collaborative work in environmental robotics focused on development and field trials of an automated glider mission control system. Winter 2012 was unusually mild along the US eastern seaboard. Analysis of buoy records and a 10-year compilation of satellite sea surface temperature imagery for Long Bay indicated that heat loss from shelf waters was less than average and the position of the winter density front was considerably inshore from the anticipated outer shelf location (Figure 3). Circulation on the upper slope and outer shelf was strongly influenced by several large filaments of Gulf Stream water that flowed southwestward along the shelf margin (Figure 4). On the shelf, glider records showed rapid shifts between a vertically uniform and stratified water column. Shelf circulation was correlated with wind stress but the response varied with the strength of stratification. The major phytoplankton bloom encountered during the study developed in mid-January on the middle shelf. The bloom included gelatinous colonies of the prymnesiophyte, Phaeocystis globosa, which resulted in pronounced spiking in fluorescence profiles. To better estimate the bloom chlorophyll biomass, the total chlorophyll fluorescence signal was partitioned into colonies (spikes) and small cells (the baseline signal) and separate calibration factors estimated for the two components were employed. About 40-60% of the total chlorophyll in this feature was estimated to be in the colonies (up to several mm in size). The aggregation of the small cells (7 um) into large colonies was significant for organic carbon export from the shelf. In late January, the mid-shelf bloom was transported to the shelf break as dense shelf water slumped seaward under warmer, lighter water derived from a southwestward flowing Gulf Stream filament (Figure 5). A moored fluorometer/turbidity sensor at 170 m depth on the upper slope provided evidence for downslope transport of fresh bloom material. Very high chlorophyll concentrations were observed at the mooring site during two events of 12-24 hr duration (Figure 6). Glider and ship profiles to within 2 m of the seafloor at this time did not detect elevated chlorophyll at depth. It thus appears that these events resulted from downslope transport of accumulated bloom material in the benthic boundary layer (within 2 m of the seafloor). Aggregation of the gelatinous colonies could have contributed to rapid settling of bloom material at the shelf break and export to depth on the slope. Graduate training associated ...