Time series of chlorophyll and turbidity measured every second on three moorings located at Long Bay, S. Carolina in the South Atlantic Bight, located at mid-shelf at 30 m, shelf break at 74 m and upper slope at 171 m along a central shelf/slope survey line SE of Myrtle Beach, SC between 33.17/-78.33 and 32.76/-77.91.

Table of Contents

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

Mooring data records for optical sensors:   

• LB_2012_LB1 mooring (30 m depth):  Three ECO units were deployed, one on the bottom mooring frame (S/N 738) about 0.5 m above bottom, and two on a taut-line mooring attached to the frame, nominally at 15 m depth (S/N. 736) and 5 m depth (S/N 735).  Sampling for the three LB1 ECO units was three consecutive 1-second readings at 6-minute (360 second) intervals.
• LB_2012_LB2 mooring (76 m depth):  One ECO unit was deployed on the ADCP mooring frame about 0.5 m above bottom (S/N 739).  This unit had an apparent electronic failure during the deployment which resulted in a distinct positive baseline shift for turbidity and negative baseline shift for chlorophyll.  The data record was considered suspect at this point and only data from the initial deployment period (before the baseline shift) is reported.  Sampling for the LB2 unit was three consecutive 1 second readings at 6-minute (360 second) intervals. 
• LB_2012_LB3 mooring (171 m depth):  The ECO unit (S/N 740) was mounted about 0.5 m above the bottom on the lower portion of the Floatation Technologies AL-200 Trawl Resistant Bottom Mount frame which held a 150 kHz ADCP unit.  Due to the longer deployment period for the LB3 instrument and internal memory limitations, sampling was set for a single 1-second sample at 6-minute (360 second) intervals (instead of three 1-sec samples for the ECO units at LB1 and LB2). 

Instrument calibrations and inter-comparisons:  Manufacturer supplied calibration factors for chlorophyll concentration and turbidity were employed following pre- and post-deployment checks for consistency.  The five ECO FLNTUSB units were serviced at WET Labs and calibrated as a set in May, 2011. Instrument inter-calibrations and the factory chlorophyll calibration factors were checked by obtaining a concurrent set of measurements in the water column before deployment.  All ECO units were mounted on a piece of angle iron that was hung on a weighted line at the same depth as the ship’s CTD/carousel system (20 m below the surface at the shelf break station LB2 to avoid surface irradiance effects on the chlorophyll fluorescence signal).  The units were held at depth for 5 minutes (acquiring 4-5 readings).  Niskin bottle water samples were collected about mid-point of the sampling period for chlorophyll analyses (triplicate filtered samples frozen and stored in liquid nitrogen on the ship, analyzed in the shore lab by the fluorometric method).  After recovery of the moorings, the units were photographed to record bio-fouling.  The four functioning ECO units (except for S/N 739) were then mounted on the ship’s CTD/carousel frame for a post-deployment set of inter-comparison measurements at two depths in the water column (35 m and 10 m), again with Niskin samples collected for chlorophyll analyses.  For chlorophyll estimated from fluorescence, the pre- and post-deployment readings showed agreement to within 0.1-0.15 mg/m3 for readings at a measured concentration of 0.4 mg/mg3 (both pre- and post-deployment) and 0.2 mg/m3 at a measured concentration of 1.25 mg/m3.  For turbidity the pre-deployment check showed close agreement between all units (average 0.066 +/- 0.002 NTU at 20 m depth).  The post-deployment check appeared to be compromised by drying of a film on the optical windows after recovery prior to the post-deployment checks.  For the turbidity data records, baseline readings over the deployment periods (minimum values) only showed marked increase for S/N 738 deployed on the mooring frame at LB1 (increasing from about 0.4 to 1.2 NTU during the latter part of the deployment).  For the data reported here, there has been no correction for the apparent increase in baseline readings for this instrument. 

Wetlabs calibration sheets: See Supplemental Files

Internal Clock Drift:  Time was recorded as UTC with internal clocks set to a common GPS-based reference during the pre-deployment set-up.  Internal clock drift during deployments was checked against the reference after recovery.  Temporal offsets at recovery ranged from -9 seconds to +528 seconds with an average offset of +359 seconds (or about the sampling interval of 360 seconds).  Given that most events in the optical records occurred on time scales of hours to days, no clock drift correction was applied.  Time stamps in raw files in the format MM/DD/YY HH:MM:SS were converted to the Microsoft Excel epoch convention (01/01/1900 = 1) and to the Matlab serial date number format (01/01/0000 = 1) for Excel and Matlab files respectively.    

Chlorophyll was calculated from digital counts output data as: 

            Chl (ug/L) = Scale Factor * (Output Counts – Dark Counts)

Turbidity was calculated as: 

            NTU = Scale Factor * (Output Counts – Dark Counts)

Instrument-specific scale factors and dark counts for chlorophyll and turbidity provided on the WET Labs “FLNTU Characterization Sheet” were employed (see Related files and references).  Raw data files were initially imported into Excel for calculations, then formatted for import into Matlab (including conversion of time stamps to the Matlab serial date format).

Examination of data records for the unit deployed at the shelf break mooring LB_2012_LB2 (S/N 740) indicated an electronic malfunction occurred at 14-Feb-2012 03:10:20.  This resulted in a distinct positive shift in the output baseline for turbidity simultaneous with a distinct negative shift in the baseline for chlorophyll readings.  The subsequent chlorophyll record appeared only to capture peaks of higher concentration events.  The data subsequent to this point was considered to be bad and was not included in the record submitted to the BCO-DMO. 

BCO-DMO Processing:

-extracted data from MatLab .mat files
-added conventional header with dataset name, PI name, version date
-renamed some parameters to BCO-DMO standard
-added yrday_utc and ISO_DateTime_UTC to served view
 

Website: http://nccoos.org/projects/long-bay-wintertime-blooms/

Coverage: outer South Atlantic Bight (SAB) continental shelf off Long Bay

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.