Beam Attenuation Coefficient, Light Scattering, Fluorescence protocols Wilford Gardner, Jan Gundersen, Mary Jo Richardson. Texas A&M University
Data Reduction Scheme The primary purpose for measuring the beam attenuation in JGOFS programs is to determine the concentration and distribution of particulate matter (PM) or particulate organic carbon (POC) in the water with continuous profiling rather than with limited discrete samples. Towards this end, a 25 cm Sea Tech Transmissometer was interfaced with the University of Washington's SeaBird CTD for all Arabian Sea cruises. Transmissometer data were analyzed for the five process cruises (TN043, TN045, TN049, TN050 and TN054) that occupied a standard set of stations. Data from the raw CTD files were binned at 2 db intervals through SeaBird's SEASOFT program, which has a spike removal subroutine which we have tested and found to remove transmissometer data spikes properly. The data were corrected for factory and field air calibrations. Beam transmission was converted to beam attenuation coefficients using c=-(1/r)*ln(%Tr/100) where c=beam attenuation coefficient (m^-1), r=beam path length (m), and Tr=% beam transmission. The Arabian Sea data set presented some challenges because 1-4 different transmissometers were used on any given cruise, complicating the data calibration. It is impractical to do a proper bench or air calibration prior for each CTD cast since the deck of the ship is not always a clean environment and atmospheric conditions can change rapidly and affect the air readings. One calibration method is to compare the beam attenuation at depth where the particle concentration is relatively invariant. The primary concern is ensuring that the optical windows are uniformly clean, which is best determined by comparing adjacent profiles. Unfortunately, many of the CTD casts extended only to 150 m or less, which was usually shallower than the particle minimum. Furthermore, the stations covered a wide geographic area, so it is more likely that the particle minimum at depth could vary. The primary method for comparing the beam attenuation signal to particulate matter (PM) concentration or particulate organic carbon (POC) concentration is to filter water samples and determine the dry weight using stable filters (0.4 um pore size Poretics filters in this case), or the amount of organic carbon on a glass fiber filter (0.7 um nominal pore size). The beam c data for those bottle depths (chosen as the cp value of the 2 db bin within which the sample depth fell) are then regressed against PM or POC using a Model II regression to determine the intercept where the concentration of particles in the water equals zero. Theoretically this value should be 0.364 since the transmissometers are set at the factory to read 0.364 in particle-free water. PM was filtered on four of the five cruises where beam c was analyzed. POC was measured on the one cruise for which no PM measurements were made (TN049) as well as most of the other cruises. In order to determine the attenuation specific to particulate matter, the attenuation due to water must be subtracted from the beam c values ( cp = c - cw). Practically, cw is determined as the minimum attenuation measured during each cruise. It must be noted that this minimum attenuation value is the "cleanest" water observed and is not particle free. Thus, the regressions of the cp data versus particle concentrations must be adjusted. A prediction of the PM concentration can be obtained from the resulting equations for each cruise: TN043 -> PM = 602 * cp (r^2 = 0.86) TN045 -> PM = 483 * cp (r^2 = 0.87) TN050 -> PM = 687 * cp (r^2 = 0.92) TN054 -> PM = 615 * cp (r^2 = 0.86) PM is in ug/Kg, and cp is attenuation per meter. Note that these are Model II regressions so the equations are the same if PM is regressed versus cp or vice versa. For comparison, the relationships between particle concentration and attenuation in surface waters of previous JGOFS programs were: PM = 1022*cp North Atlantic Bloom Exp. PM = 451*cp EqPac Spring Time Series PM = 647*cp EqPac Fall Time Series Chlorophyll Chlorophyll-a fluorescence distribution in the Arabian Sea was determined, in-situ, with a SeaTech Fluorometer. The fluorometer was interfaced with the Sea-Bird CTD, and the data were acquired in the same format as the transmissometer data. The Fluorometer is a standard irradiation/emission system. When chlorophyll a is excited by blue light (425 nm), it will fluoresce at a peak wavelength of 685 nm (red light). The emission detector is filtered to a peak response in order to make the measurement insensitive to the excitation source. The amount of fluoresced light detected is converted to a voltage range of 0 to 5 volts. A signal gain of 10x was used, setting sensitivity to 3mg chl-a m^-3. The fluorometer is set to sample with a three second time constant to smooth the data. A baffle has been placed in front of the emission detector in an attempt to make it insensitive to ambient light (SeaTech Fluorometer Manual). The SEASOFT software converts the measured voltage into a relative chlorophyll-a value using the equation: [volts * signal gain/5] + offset = mg chl-a m^-3 These relative values were calibrated using discreet chlorophyll samples (taken by various JGOFS scientists and analyzed onboard the ship using a Turner Fluorometer). There is a good (r^2 = 0.90) linear correlation between fluorometer-determined chlorophyll-a fluorescence, and the chlorophyll-a concentrations determined using a Turner fluorometer. Regressions were made for each cruise individually, but the correlations (based on the standard deviation of the slope and intercept) were improved when data from cruises TN049, TN050, and TN054 were combined. Prior to TN049, chlorophyll samples were taken from the Trace-Metal rosette, which contained no CTD or fluorometer for accurate depth or fluorescence measurements. We attempted a comparison between standard CTD/fluorometer profiles made close in time to the Trace-Metal casts on which chlorophyll measurements were made, but the lack of accurate depths or water density for the discreet samples plus the temporal variability between casts introduced too much scatter for a useful correlation. There were too few chlorophyll a measurements made on the standard CTD casts during TN043 and TN045 to independently calibrate the fluorometer. This added to the appeal of a general calibration for the fluorescence signal for all cruises, though we recognize that data for two cruises were not included. We emphasize for future work that it is necessary to have a fluorometer and CTD on the rosette at the time chlorophyll samples are being taken in order to accurately calibrate the fluorescence signal. Furthermore continuous profiles from a fluorometer provide higher resolution than discreet samples alone. Slightly different slopes and intercepts were observed in the fluorescence/chlorophyll correlations for samples above and below the chlorophyll maximum. Therefore the depth of the chlorophyll maximum was determined by visual inspection of each profile (to avoid confusion with individual spikes) and the samples were divided into two categories, separated at a depth 10 m beneath the maximum fluorescence value. The assumption (substantiated by inspection of the data) is that chlorophyll-containing particles within the subsurface chlorophyll maximum are more similar to those above the maximum than below. A model II linear regression on each group of data indicated a very slight difference in slopes between the two groups, but a substantial offset in the intercepts. This results in a difference in the concentration of predicted chlorophyll based on the fluorescence above and below the chlorophyll maximum. Similar differences in chlorophyll fluorescence above and below the chlorophyll maximum were noticed by Pak et al.(1988). Equations are provided here for both regions in the Arabian Sea. Above the depth of the chlorophyll maximum: Chl a = 0.357*Fl + 0.078 (r^2 = 0.86) Below the depth of the chlorophyll maximum: Chl a = 0.389*Fl - 0.05 (r^2 = 0.93) LSS - SeaTech Light Scattering Sensor Light scattering due to particles was monitored using a SeaTech Light Scattering Sensor (LSS). The LSS projects light from two 880 nm (infrared) LEDs into a sampling volume that varies depending upon the concentration of particulate matter, but that is roughly the shape of a stretched balloon. Back-scattered light from the particulate matter is measured by a detector. The range on the LSS was set to 0 - 33 mg/l. The amount of light detected is scaled to a 0-5 volt output, but in the Arabian Sea most values were less than 0.5 volts. The LSS output depends upon the nature of the particulate matter and will vary with changes in particle size distribution, shape, index of refraction, organic/inorganic content etc. Therefore the LSS requires site-specific calibration. The LSS was interfaced with the SeaBird CTD and the data were handled in the same format as the transmissometer and fluorometer data.