Table of Contents

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

CTD casts were conducted in Long Bay, South Atlantic Bight, SE United States from the middle continental shelf (~ 30 m depth) to upper slope (~260 m depth). The primary cross-shelf CTD survey line was off Myrtle Beach, SC between 33.17/-78.33 and 32.76/-77.91. On cruise SAV12-03, CTD profiles were also conducted at stations on parallel cross-shelf lines ~26 km to the SW and NE of the central line.

NOT FINAL - awaiting further calibration.

Instrument configuration of the R/V Savannah CTD/carousel water sampler:
Sea-Bird SBE 25 Sealogger CTD; Sea-Bird SBE 33 deck box; Sea-Bird SBE 32C carousel pylon; Sea-Bird SBE 43 oxygen sensor; WET Labs ECO chlorophyll fluorometer; WET Labs ECO CDOM fluorometer; WET Labs ECO BB optical backscatter sensor; LI-COR 193SA PAR sensor; Sea-Bird SBE 5P pump. The carousel was equipped with eight 8-L external-closure water sampling bottles (Ocean Test Equipment Model 110).

The instrument inventory for the ship’s CTD system (including spares for all components) was managed by the ship’s Marine Technician (John Bichy). Instrument calibrations were performed by manufacturers within suggested intervals. Calibration factors and calibration dates are listed in the file “ctd_con_report.txt” that accompanies the cast data files. The sampling rate for the CTD system was set at 1 Hz.

The deployment procedure included a near-surface “soak” (at least one minute) prior to profiling to allow air to be purged from tubing lines and sensors to equilibrate. The package was then lowered (descent rate of ~0.3-0.4 m/s) to a target depth, typically about 2 m above the seafloor. If desired, water samples were collected during the upcast using the carousel water sampling system. Water sample depths were selected based on the real-time profiles obtained during the downcast.

Methods Reference:

Fofonoff, N.P. and R.C. Millard, Jr, 1983.  Algorithms for computation of fundamental properties of seawater.  UNESCO Technical Papers in Marine Science #44.

Initial CTD data processing used the Sea-Bird data processing software package with input of the raw data file (*.hex) and associated configuration file (*.con) for each cast. Processing steps are described in the file “ctd_process.txt” that accompanies the data files. For bin-averaging, scans acquired during the initial surface “soak” period prior to profiling were removed and only the downcast was processed using 1-m bins for station depths < 100 m and 2-m bins for station depths > 100 m. The bin-averaged data was output in ASCII file format (*b.cnv). Parameter names and units for the output data files are listed above (by column).

Factory calibration factors for CTD package sensors were applied to calculate conductivity, pressure, and temperature (see “ctd_con_report.txt”). Depth was calculated from pressure using the “seawater” setting in the Sea-Bird software and density was calculated by the EOS80 equation of state for seawater (Fofonoff and Millard, 1983). Oxygen (kg/m3, % saturation), optical backscatter (1/m) and PAR (as quantum scalar irradiance, micro-mole photons / m2 s1) were calculated using the manufacturer-supplied calibration factors and algorithms included in the Sea-Bird data processing package. Chlorophyll fluorescence and CDOM fluorescence are reported here as the raw voltages (0-5 volts total signal range).

BCO-DMO Processing:

- added conventional header with dataset name, PI name, version date, reference information, replacing ctd header
- renamed some parameters to BCO-DMO standard
- created toplevel file
- added ctd metadata columns to cast data: cruise_id, cast, year, mon, day_gmt, time_gmt, lat_deg, lat_min, lon_deg, lon_min, depth, station, comment

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.