Abstract

This report contains results from the fifth cruise of the Marine Optical Characterization Experiment. A variety of spectroradiometric observations of the upper water column and atmosphere were made by investigators from the University of Miami, NOAA, CHORS, CICESE, Oregon State University, University of South Florida, UABC, NERSC and MLML. Data presented here were obtained by oceanographic CTD profiler: salinity, temperature, dissolved oxygen, beam attenuation and chlorophyll-a fluorescence; and by water samplers: total suspended matter and suspended organic carbon and nitrogen, salinity, and dissolved oxygen.

Introduction

The purpose of the Marine Optical Characterization Experiment (MOCE) is to obtain in situ ocean data characterizing the upper ocean bio-optical properties. The purposes of these data are twofold: 1) providing surface truth for the SeaWiFS ocean color satellite (launched on 18 July 1997, and made operational on 18 September) and 2) to develop bio-optical algorithms relating water-leaving radiance to dissolved and suspended particulate material concentrations in surface waters.

Data contained in this report were obtained using a Sea-Bird SBE911plus CTD profiler. Values of electrical conductivity, temperature, pressure, dissolved oxygen, beam attenuation at 660 nm, chlorophyll-a fluorescence at 680 nm and 670 nm were obtained at 24 Hz. The CTD descent speed is 30 m/min. Following data correction and calibration we preserve corrected values including derived salinity, density, and potential temperature in 1 dbar bins. Details of the shipboard procedures are given in Broenkow et al. (1994) and some details of the Sea-Bird CTD data processing procedure are given in Broenkow et al. (1995). With the exception of beam attenuation and fluorescence, we follow Sea-Bird (Sea-Bird, 1994) data processing procedures.

Two classes of data are reported here. Vertical CTD profiles of water column properties and water analyses for particulate materials. In addition to making particulate analyses from CTD rosette bottles, we also report those data from samples taken from the ships sea chest during long-track horizontal profiles and while on station.

Methods

Water samples were collected by 10 custom-made 17-liter water samplers attached to the Sea-Bird Carousel. Water samples were obtained on the upcast at inflection points or other significant depths in the water column based on observation during the down cast. The primary purpose of the water samples is to obtain large volume samples for analyses of total suspended material (TSM, MLML), phytoplankton pigment analyses by high performance liquid chromatography (HPLC, CHORS), particulate organic carbon and nitrogen (POC, PON, MLML), particle size analyses (NOAA/NESDIS) and calibration samples for CTD oxygen.

Long-track water samples were taken in coordination with other investigators during tracks between stations and from high to low fluorescence. Water samples were provided by the ship sea chest with an intake at 5 meters. Additionally water samples were collected from the ships sea chest and by bucket during each station. The purpose was to determine the variability of TSM/POC/PON and pigments during the station.

Near local apparent noon, Secchi depth measurements were made with a 30 cm, all white, Secchi disk, carefully avoiding surface glint. The depth estimates were made both by lowering the disk until it faded from view and raising it again until it returned to view. The reported Secchi depths are the mean of the two readings. Ocean color as sensed by the human eye was estimated by Munsell color chips (Munsell Color Company, Baltimore Md.) selected by R.W. Austin (Scripps Visibility Laboratory). Two or more observers compared the color of the Secchi disk suspended at half its disappearance depth.

Total suspended particulates were determined by filtering 0.5 to 9 liters of water through 47 mm diameter, 0.45 µm pore-size Millepore HP/EP mixed-ester cellulose filters. These filters were desiccated and tared to a constant (±20 µg) weight and stored in separate Petri dishes. Water was vacuum filtered aboard ship using a pressure differential of 0.5 to 0.7 atmospheres. Sea salts were removed by two 10 ml rinses of deionized (Mille-Q) water. These filters have a 6 mm hydrophobic edge which eliminates the need to rinse sea salts from the filter rim. After sample collection the filters were returned to the Petri dish, dried at 60C, and stored until analysis ashore. Suspended sediment weights were determined by weighing each filter on a Mettler H54-AR balance. Weighing was repeated three times or more until the difference between weights was less than 40 µg.

Separate samples were filtered for particulate organic carbon and nitrogen analyses. Approximately 1 to 4 l of water was pressure filtered through 25 mm Whatman glass fiber GF/F filters having a nominal pore size of 0.7 µm. These filters were pretreated by ashing in a muffle furnace at 500 C for two hours. Each filter was stored in an ashed aluminum-lined Petri dish. Following filtration, the filters were returned to the Petri dish, folded, gently creased, dried at 60C, and stored until analysis ashore. Organic carbon and nitrogen were determined by combustion analysis with a Leeman Labs Model 440 Element Analyzer. Acetanilide standards were analyzed every 15th sample, and the maximum deviation of these standards never exceeded the 5% limits, which is the accepted precision of the method (University of Maryland, 1992). The limits of detection are 1 µg C mg^-1 sample for carbon and 0.1 µg N mg^-1 for nitrogen.

Data Management

SeaBird CTD/Carousel data are collected using SeaBird software on a DOS laptop computer. Data acquisition and processing procedures are explained in detail by Feinholz and Broenkow (1994) and processing steps are illustrated in a tutorial (Broenkow, et al., 1994). Data from all instruments are kept in an MLML_DBASE format which can be displayed, edited and processed with a single suite of programs (Broenkow and Reaves, 1994). CTD data files are named by instrument (SBE) and the a sequential file number. After the data have been processed and corrected the finished data set were written to a CD-ROM. The MOCE-5 CTD data are comma delimited ASCII files. Each ASCII file has a file header, a description of each variable followed by the data. Missing data are indicated with a blank. The file naming convention will be maintained, but ASCII files will have the file extension .TXT rather than .MLD used for the MLML_DBASE binary files.

Results

CTD profiling results are shown in a standard format in Appendix 1 and sea chest data for total suspended material (TSM) and particulate organic carbon and nitrogen (POC, PON) are given in Appendix 2. In this section we will describe the oceanographic profiling results with regard to oceanography of the Baja California. However, since station sampling was done primarily with the intent of making midday bio-optical observations, traditional oceanographic gridded station sampling was not possible. Thus the oceanographic results presented here cannot describe the complex nature of circulation and mixing around the Baja California Peninsula.

Table 2 presents some layer depths useful in the context of characterizing the hydrography of the photic zone. The mixed layer depth varied from nil to 30 m. The depth of the fluorescence maximum lay between 3 and 67 m, and the oxygen saturation maximum was found between 2 and 50 m.

During MOCE-5 field check samples for dissolved oxygen were taken during each cast. These samples were taken at interesting depths conforming to the requirements for characterizing the near-surface photic zone of most interest to the MODIS program. Deeper samples were taken to provide calibration data points through the oxygen minimum.

Because of the known difficulty in using membrane oxygen electrodes, considerable work is involved in making field calibration measurements. The agreement between Winkler titration results with membrane electrode measurements shows considerable scatter (up to 0.2 ml/l) between the Winkler and CTD samples (Fig. 2). This situation accurately reflects the reality of electrode oxygen measurements; their accuracy is relatively poor, but the profiles reveal structure that is difficult to determine without a large number of bottle samples. Each oxygen electrode has a finite life of a few hundred hours of use, and the sensor degrades throughout its lifetime such that calibrations must be done on a cruise-by-cruise (or cast-by-cast) basis. Oxygen electrode data are corrected using a linear regression of Winkler and electrode percent oxygen saturation (Fig. 2). The equation to correct the oxygen is

%O_2cor = [%O_2elect + 0.68123]/0.92387

where %O_2cor is the corrected percent saturation, %O_2elect is the uncorrected electrode percent saturation, 0.68123 is the offset, and 0.92387 the slope. The corrected electrode oxygen saturation is used to calculate the oxygen in ml/l.

The modified Marek transmissometer used on previous MOCE cruises was replaced with a 25 cm 660 nm C-Star transmissometer. Air calibrations were performed prior to each CTD cast by noting the voltage when the transmissometer is clean and dry. Beam attenuation was calculated using a modification to Wet Lab equations. Wet Lab calculates beam attenuation as

c = -1/x (ln(V_sig - V_d) / (V_ref - V_d))

where x is the path length (0.25 m), V_sig is the transmissometer voltage, V_d is the voltage with the path blocked (0.055) and V_ref is the voltage with clean water in the path (4.753). Note that air calibrations are not included in the equation. Figure 3 shows the history of air calibrations during the cruise. Because of the decline in air calibrations V_ref must be increased. The corrected reference voltage,V_refcor, is calculated as

V_refcor = V_ref /[V_air / AirCal]

where V_air is the C-star calibrated air calibration (4.835) and AirCal is the air calibration taken before each cast. This corrected reference voltage is then used in Wet Lab equation above.

The MLML Fluorometer is powered by an external battery which must be recharged frequently. During MOCE-5 a number of CTD casts were made with a low battery charge. These data were corrected empirically by adding 0.72 to all the affected data. Figure 4 shows the relationship between the corrected MLML fluorometer and Chelsea fluorometer. Note that the MLML fluorometer is more sensitive than the Chelsea fluorometer.

The TS plot in Figure 5 shows that the study area lies in the transition region between the cool 10C low salinity 33.5 California current water and the warm > 25C high salinity >35 psu Pacific Equatorial waters. Because of this, the data obtained during MOCE-5 were taken over an oceanographically varied area.

The transect plots of salinity, temperature, oxygen, MLML Fluoromoter, Chelsea Fluorometer, and beam attenuation in Figures 6-11 show the variation from station to station of each variable. The transect plot of Salinity and Temperature (Fig 6 and 7) show the transition from California Current Water (sbe0106-118) to Pacific Equatorial waters (sbe0119-125) in the Gulf of California and then back out in to the California Current Waters (sbe0126-129). Figure 8 shows the oxygen maximum at the surface in the Gulf of California and at deeper depths outside the gulf. Figures 9-11 show that all the highest fluorescence and beam attenuation peaks were in the Gulf, with the exception of sbe0108.

Acknowledgements

We appreciate the efforts of the captain and crew of R/V Melville. Craig Hunter performed the TSM, POC and PON analyses. Stephanie Flora reduced the CTD profile data. This work was supported by National Oceanic and Atmospheric Administration, National Environmental Satellite Data Information Service Grant No. NA77ECO252 to William Broenkow.

Cruise Participants

References

Broenkow, W.W., M.E. Feinholz, S.J. Floraand J.A. Gashler. 1994. Shipboard Techniques for Oceanographic Observations. Moss Landing Marine Laboratories Tech. Pub. 94-2. Moss Landing CA 95039.

Broenkow, W.W., M.E. Feinholz and J.A. Gashler. 1995. Data Reduction using the SBE 911/plus CTD system and SEASOFT Programs. Moss Landing Marine Laboratories Tech. Memorandum. 95-1. Moss Landing CA 95039