Dataset: CTD-ML

   PI:              William Broenkow
   of:              Moss Landing Marine Laboratory (MLML)
   dataset:         High resolution CTD and beam attenuation data
   dates:           May 18, 1989 to June 07, 1989
   location:        N: 59.8117  S: 46.24  W: -20.7483  E: -17.68
   project/cruise:  North Atlantic Bloom Experiment/Atlantis II 119, leg 4
   ship:            R/V Atlantis II
 

Methodology:

CTD PROFILES - (Broenkow, MLML)

The MLML CTD/Rosette (Yarbrough et al., 1989) was used to make profiles of conductivity, temperature, dissolved oxygen, beam attenuation and in situ fluorescence. Conductivity was measured with a Sea-Bird conductivity cell and MLML pump, temperature with a platinum thermometer (tau = 0.3 sec) and pressure with a Digiquartz transducer. Data were digitized at 0.8 m intervals. Corrections were applied to temperature, salinity, and pressure using laboratory calibrations done before and after the cruise. Pressure corrections for the compressibility of the Sea-Bird cell were applied using the algorithm provided by Sea-Bird Electronics. Corrected data were compared with salinity and temperature field calibration data provided by the Scripps CTD group. Scripps corrected CTD data and ours show excellent agreement. Maximum salinity differences between the SIO and MLML profiles are about +/- 0.02 S.

Oxygen

The oxygen electrode data were obtained with a Beckman polarograph electrode modified at MLML to obtain near-membrane temperatures. The data have been corrected to oxygen concentrations by comparison with titrated calibration samples obtained during ATLANTIS II 119.5. Most of these calibration samples were analyzed by MLML personnel, and the RMS difference with Scripps titrations was 3 umole/kg. Oxygen concentrations were computed from oxygen reduction current via the WHOI algorithm (Owens and Millard, 1984) using near-membrane temperatures and in situ pressure. Corrections for membrane porosity changes may be large, and cynicism is advised when using these data.

Beam Attenuation

The MLML transmissometer is a modified Martek instrument based on the Scripps Visibility Laboratory design (Petzolf and Austin, 1968). Beam attenuation is measured through the folded 1 m path with a Wratten 45 (480 nm) filter and an IR blocking filter. Calibration is done in the laboratory by adjusting instrument gain to a transmission reading of 85.5% in dry air. Drift is estimated aboard ship before and after each cast by diligent cleaning of the windows using alcohol.

Fluorescence

The MLML profiling fluorometer uses Variosens electronics (Frungel and Koch, 1980) and produces log-scaled signals. Excitation is via a Xenon flash lamp and a broad band filter (350-550 nm half power). Fluorescence emission was detected by silicon diode through a 670 nm (half power) long pass filter. These raw data are converted to "rescaled fluorescence" units by comparison with extracted pigment analyses. We provided our own chlorophyll calibrations during ATLANTIS II 119.5 by fluorometric analysis of acetone extracts of water filtered through Whatman GF/F (0.7 micron) filters. The "rescaled fluorescence" units are numerically equivalent to chlorophyll-a concentrations in ug/liter. The term "rescaled fluorescence" is used to acknowledge the fact that fluorescence and chlorophyll concentrations may not covary because of variation in quantum yield. The RMS difference between "rescaled fluorescence" and extracted chlorophyll was 0.27 ug/liter.

Depth calculation for the CTD data files

The depth values in these CTD files have been calculated from pressure by the US JGOFS Data Management Office using the algorithm below. The latitude used in computation was the latitude recorded in the CTD data file. The CHECKVALUE was used to verify the accuracy of the computation. The stated accuracy of this algorithm is 0.1 meters. The calculated depths have been rounded to the nearest whole meter.

function DEPTH=depth(P,LAT); DEPTH Computes depth given the pressure at some latitude D=DEPTH(P,LAT) gives the depth D (m) for a pressure P (dbars) at some latitude LAT (degrees). Fofonoff and Millard (1982). UNESCO Tech Paper #44. Notes: (ETP3, MBARI) This algorithm was originally compiled by RP @ WHOI. It was copied from the UNESCO technical report. The algorithm was endorsed by SCOR Working Group 51. The equations were originally developed by Saunders and Fofonoff (1976). DSR 23: 109-111. The parameters were re-fit for the 1980 equation of state for seawater (EOS80). CHECKVALUE: D=9712.653 M FOR P=10000 DECIBARS, LAT=30 DEG CALCULATON ASSUMES STD OCEAN: T = 0 DEG C; S = 35 (IPSS-78) X = sin(LAT/57.29578); X' = X*X; GR = GRAVITY VARIATION WITH LAT: ANON (1970) BULLETIN GEODESIQUE GR = 9.780318*(1.0+(5.2788E-3+2.36E-5*X')*X') + 1.092E-6*P D = DEPTH BEFORE GRAVITY CORRECTION D = (((-1.82E-15*P+2.279E-10)*P-2.2512E-5)*P+9.72659)*P DEPTH = D/GR