Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

At selected stations and depths, Niskin water samples were collected using the ship’s CTD/carousel water sampler (Sea-Bird SBE 25 Sealogger CTD; Sea-Bird SBE 32C carousel pylon; 8-L Ocean Test Equipment Model 110 external-closure water sampling bottles).  Samples were transferred to 2 L polycarbonate bottles from the Niskin bottles via Tycon tubing and processed in the ship’s wet lab.  Triplicate samples (250 mL) were filtered onto GF/F glass fiber filters.  The filters were folded and inserted into screw-capped cyrovials labelled with a cyropen (date, station, depth, volume filtered, replicate number), then frozen in liquid nitrogen.  Sample information was recorded on a sample log sheet that was scanned for an electronic pdf record after each cruise.  

Samples were stored in liquid nitrogen until analyzed at the shore laboratory.  Analysis was by the fluorometric method, based on JGOFS protocols (1996).  Frozen filters were placed in a polyethylene tube to which 10 mL 90% acetone was added for extraction.  After several hours extraction in the freezer, the filters were disrupted using a probe-type sonicator, with care to keep samples cold and in dim light.  Filter debris was cleared by centrifugation and the cleared extracts were decanted into cylindrical glass cuvettes and read in a Turner Designs Model 10AU fluorometer, with readings recorded before and after addition of one drop 2 N HCl.  Chlorophyll and phaeopigment concentrations were calculated as described in the JGOFS protocols (1996).  The fluorometer scale factor (calibration factor) was determined using a chlorophyll reference solution (90% acetone extract of a filtered diatom culture) with the reference concentration determined spectrophotometrically (extinction at 664 nm) using a Cary Model 3 spectrophotometer.  The stability of the laboratory fluorometer over time was tracked by periodic measurements of a stable fluorescence reference solution (Basic Blue 3 in Milli-Q water).

Water sample concentrations of particulate chlorophyll and pheopigments (in chlorophyll equivalents) were calculated as described in the JGOFS Protocols (1996).

BCO-DMO Processing:

- added conventional header with dataset name, PI name, version date
- renamed parameters to BCO-DMO standard
- changed ~ to nr_ ('near')
- changed blank flag cells to '-'

NSF Project Title: Mechanisms of nutrient input at the shelf margin supporting persistent winter phytoplankton blooms downstream of the Charleston Bump

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. 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 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.

References:
Lee, T. N., J. A. Yoder, and L. P. Atkinson, 1991: Gulf Stream frontal eddy influence on productivity of the southeast U.S. continental shelf. J. Geophys. Res, 96, 22191-22205.