Intellectual Merit: Sustained phytoplankton blooms along the outer South Atlantic Bight (SAB) continental shelf off Long Bay are observed in winter in multi-year satellite chlorophyll imagery. This section of the shelf lies north of the "Charleston Bump" (between 32.5-33.5°N), where the Gulf Stream is often strongly deflected offshore. Due to this offshore deflection, this is not an area where nutrient input to the shelf would be enhanced by upwelling associated with Gulf Stream frontal eddies, a major mechanism of nutrient input in other parts of the SAB shelf (Lee et al., 1991). Yet prior in situ observations suggest that there is recurring input of nutrients from the upper slope to the outer shelf off Long Bay from winter to early spring. This project will investigate a fundamental aspect of physical-biological coupling in the outer shelf to upper slope region. The PIs will test the hypotheses that: 1) the persistence of winter blooms on the outer shelf off Long Bay results from repeated episodes of nutrient input and mixing which maintains nutrient-sufficient conditions for extended periods; 2) several physical mechanisms are involved, including enhanced mixing energy from the internal tide along this section of the upper slope/shelf break; 3) the relatively high nutrient, intermittently turbulent environment will favor larger bloom-forming phytoplankton. The latter could have important implications for higher trophic levels, including early life history strategies of fish that spawn along the shelf margin off Long Bay in winter to early spring.
This project will combine several maturing observational technologies to address the following:
1. What is the frequency and magnitude on on-shelf transport of nitrate from the upper slope?
2. What are the mechanisms of nutrient delivery from the upper slope to the outer continental shelf zone that are operating off Long Bay under the range of hydrographic and forcing conditions encountered in winter?
3. What is the 3-D structure of outer shelf hydrography and associated winter bloom features and how do these evolve through multiple nutrient input/mixing events?
4. What are the rates of nitrate utilization and primary production associated with the winter blooms?
5. Does the winter regime consistently favor a bloom assemblage dominated by larger diatom forms?
Near-continuous cross-shelf and upper slope observations will be obtained with two autonomous gliders, time-series measurements on the outer shelf and slope from a set of moored instruments (including a moored profiling system at the shelf break), and repeated cross- and along-shelf ship surveys using a towed, undulating package. Ship station work will include measurements of primary production and on-board analyses of key functional characteristics of the phytoplankton assemblage (cell forms, abundance, size and bio-volume distributions) using a microfluidics/imaging system. In combination, these systems will provide a level of spatial and temporal resolution of physical, nutrient and biological fields that could not be achieved in earlier, station-based field studies and the basis for improved understanding of physical mechanisms of recurring nutrient input to the shelf, and how the nutrient, mixing, and circulation regime in winter structures the phytoplankton community.
Broader Impacts: This project will provide a deeper understanding of shelf/slope exchange processes and how these influence shelf ecosystems, generating information that will contribute to implementation of ecosystem-based management in the region. Graduate students in Marine Sciences and in Electrical and Computing Engineering will have important roles in the study. A grade 6-12 education component will build on prior interactions with the COSEE SE program activities and will emphasize: 1) the inherently interdisciplinary nature of oceanography, focusing on physical-biological coupling in a regional context, and 2) the interface of science and engineering, illustrated by observational technologies employed. Coastal naturalists will be engaged through a seabird survey component of the field program that will augment existing information on pelagic seabirds in winter and define their association with oceanographic features on the central South Atlantic Bight shelf and slope.
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 with the project included two students at the University of North Carolina, Chapel Hill and three students at the Georgia Institute of Technology. (Although presently within the University of Georgia, the Skidaway Institute was not part of a degree-granting institution at the start of the project). The Georgia Tech students played a key role in glider operations, in particular for field trials and evaluation of an improved environmental robotics system for glider mission control. Volunteer seabird observers were also invited to participate in the winter cruises to Long Bay and have reported their observations in a regional ornithological society journal.
