US JGOFS Antarctic Environments Southern Ocean Process Study (AESOPS) Beam Attenuation Coefficient, Fluorescence, Light Scattering, PM filtration protocols Wilford Gardner, Mary Jo Richardson Texas A&M University Data Reduction Scheme Beam attenuation via transmissometers The primary purpose for measuring beam attenuation in JGOFS programs is to determine the concentration and distribution of particulate organic carbon (POC) or particulate matter (PM) in the water with continuous profiling rather than with limited discrete samples. Towards this end, a 25-cm path-length Sea Tech transmissometer was interfaced with the R/V Palmer's SeaBird CTD for all Ross Sea cruises. During the Polar Front cruises, a new model of SeaTech transmissometer (pathlength=20 cm) was interfaced with the R/V Revelle's SeaBird CTD. In addition, during the final process cruise (Process II, RR Kiwi-09) D. Stramski and G. Mitchell (SIO) provided two WetLabs transmissometers (660 and 480 nm) that were also interfaced with the SeaBird CTD for casts from station 9-30 where the cast did not exceed 1000 m. Beam attenuation data posted here for stations 9-30 (<<1000 m) of RR Kiwi-09 were from the 660 nm WetLabs transmissometer because it was more stable in the surface waters. For the casts deeper than 1000 m, we provided an earlier model SeaTech transmissometer with 5000 m capability, and those SeaTech data are posted in the data base. Transmissometer data were analyzed for the seven Palmer cruises (NBP9604, NBP9604A, NBP9605, NBP9701, NBP9703, NBP9708 and NBP9802), and the four Revelle cruises (Kiwi 06-09). Data from the raw CTD files were binned at 2 db intervals through SeaBird's SEASOFT program, which has a spike removal subroutine that we have tested and found to remove transmissometer data spikes properly. 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. In order to determine the attenuation specific to particulate matter (beam cp), the attenuation due to water is subtracted from the beam c values (cp = c - cw). In principal, cw should be 0.364 for the SeaTech transmissometers since they are set at the factory to read 0.364 in particle-free water. 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. The WetLabs transmissometers are designed to yield cp directly with final calibrations provided by R. Reynolds (SIO). To match the cp values of the SeaTech and WetLabs profiles on simultaneous casts, an additional correction of 0.0151 per m was required. The extreme cold conditions of the Southern Ocean and Ross Sea presented some challenges because it is not practical to do a proper bench or air calibration prior to each CTD cast, thus requiring other calibration methods. Relative calibrations between casts can be made by comparing the beam attenuation at a depth where the particle concentration is relatively invariant - usually deeper than 300 m. The primary concern is ensuring that the optical windows are uniformly clean. Linear offsets in beam cp can be made to the entire profile when clear-water values differ significantly. Occasionally the CTD casts extended only to 150 m or less, which was usually shallower than the particle minimum. The stations during some cruises covered a wide geographic area, especially on the Polar Front cruises, so it was possible that the particle minimum at depth could vary. Washing the transmissometer windows just before launching the CTD apparently caused ice to form in the subzero air of the Ross Sea, and the ice sometimes took many meters to thaw, making some surface data unusable. The primary method for correlating beam cp with the concentration of particulate organic carbon (POC) or particulate matter (PM) was by linear regression of the data. Bottle POC concentrations The concentration of particulate organic carbon was determined by Walker Smith's group after filtration of water samples through a GFF filter (0.7 µm nominal pore size). (Please see Chapter 15 of the JGOFS protocols (1994) for POC methods.) POC was filtered on the four process cruises in the Ross Sea and on the four survey and process cruises in the Polar Front. Bottle samples were obtained primarily in the upper 200-250 m, so extrapolation to deeper depths is less certain. In the Ross Sea there were often bottom nepheloid layers of resuspended sediment and the POC equations cannot be used to estimate POC concentrations in those areas. Bottle PM concentrations Water samples were vacuum filtered (0.5 atm) in-line from the Niskin bottles through pre-weighed 0.4 um pore size Poretics filters and washed with three aliquots of either distilled water or 1M ammonium formate, dried and returned to the lab for reweighing. Extensive experiments during NBP9701 showed no difference between washing with distilled water versus ammonium formate. Therefore all PM regression data are from the distilled water washes, as in all other JGOFS PM measurements. PM was filtered on one cruise in the Ross Sea (NBP9701) and on the two process cruises in the polar front. Regressions against beam cp The beam c data for those bottle depths (chosen as the beam c value of the 2 db bin within which the sample depth fell) are then regressed against POC or PM using a Model II regression to determine the slope of the regression. (Model II regressions minimize the deviation of data points from the regression line in both the x and y direction; Model I regressions minimize deviations in only one direction.) POC Concentrations A prediction of the POC concentration (µg/l) can be obtained from the resulting equations for each cruise: Ross Sea NBP9604a -> POC = 593 * cp + 7.56 (r^2 = 0.710) NBP9701 -> POC = 635 * cp + 63.1 (r^2 = 0.818) NBP9703 -> POC = 752 * cp + 0.782 (r^2 = 0.065) (Use of the NBP9604A equation would probably be better for NBP9703) NBP9708 - > POC = 519 * cp + 24.7 (r^2 = 0.878) Polar Front RR_KIWI06, Survey 1 -> POC = 645 * cp + 10.3 (r^2 = 0.822) RR_KIWI07, Process 1 -> POC = 412 * cp + 8.69 (r^2 = 0.899) RR_KIWI08, Survey 2 -> POC = 367 * cp + 36.5 (r^2 = 0.839) RR_KIWI09, Process 2 -> POC = 510 * cp + 40.7 (r^2 = 0.749) POC is in ug/liter, and cp is attenuation per meter. Note that these are Model II regressions . PM Concentrations Ross Sea A prediction of the PM concentration was determined from filtration during only one cruise, and is the only estimate for determinations of PM concentration during other Ross Sea cruises: NBP9701 -> PM = 890 * cp - 5.02 (r^2 = 0.926) Distilled water rinse Polar Front RR_KIWI07, Process 1 -> PM = 1395 * cp - 21.5 (r^2 = 0.912) RR_KIWI09, Process 2 So -> PM = 1771 * cp - 23.2 (r^2 = 0.956) RR_KIWI09, Process 2 No -> PM = 666 * cp + 5.93 (r^2 = 0.896) RR_KIWI09, Process 2 Sta 15-> PM = 867 * cp + 3.82 (r^2 = 0.980) PM is in ug/liter, and cp is attenuation per meter. Note that data for Process Cruise 2 were separated into the Southern transect (So) and Northern transect (No) because of very different relationships. Station 15 (part of the northern transect) was given its own correlation and was not used in the Northern regression. These differences were not seen in the POC or Fluorescence data. Chlorophyll Chlorophyll-a fluorescence distribution in the Ross 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. 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 scientists from the Smith/Marra/Barber primary productivity team and analyzed onboard the ship using a Turner Fluorometer). Linear regressions between fluorometer-determined chlorophyll-a fluorescence, and the chlorophyll-a concentrations determined using a Turner fluorometer were very good when concentrations were high (r^2 = 0.88), but were sometimes very poor during periods of low concentrations (r^2 = 0.06). Regressions were made for each cruise individually. Regressions between chl a concentrations and fluorescence (Fl) are: Ross Sea NBP9604a -> Chl a = 2.10 *Fl + 0.0395 (r^2 = 0.873) NBP9701 -> Chl a = 0.665*Fl + 0.363 (r^2 = 0.876) NBP9703 -> Chl a = 2.27*Fl - 0.0125 (r^2 = 0.061) (Use of equation for NBP9604A would probably be better for NBP9703) NBP9708 -> Chl a = 0.908*Fl + 0.397 (r^2 = 0.88) Polar Front During the Polar Front cruises in-situ fluorescence was measured with a Chelsea fluorometer interfaced with the CTD. The Chl/Fl slopes are much greater with the Chelsea instrument. Comparison of the bottle chl a determined with a Turner fluorometer and the HPLC bottle data showed a trend of decreasing Turner fluorescence during the sequence of cruises. No such trend is apparent in the Turner fluorescence versus Chelsea in-situ fluorescence (i.e. no temporal trend in the slopes listed below). The Model II regressions are as follows: RR_KIWI06, Survey 1 -> Chl a = 11.7 *Fl - 0.160 (r^2 = 0.884) RR_KIWI07, Process 1 -> Chl a = 12.9 *Fl - 0.180 (r^2 = 0.807) RR_KIWI08, Survey 2 -> Chl a = 8.67 *Fl - 0.119 (r^2 = 0.849) RR_KIWI09, Process 2 -> Chl a = 11.7 *Fl - 0.217 (r^2 = 0.707) LSS - SeaTech Light Scattering Sensor Light back-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 the volume is roughly the shape of a stretched balloon. Back-scattered light from the particulate matter is measured by a detector next to the LED. 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 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. The LSS malfunctioned occasionally during the later Ross Sea cruises and those profiles were eliminated. For the Revelle cruises in the Polar Front, an LSS was mounted inside the rosette pointing upward at the lanyards used to close the bottles. The lanyards were in the LSS sensing volume, so the absolute values of the data are not directly comparable with the Ross Sea values, but the values among Polar Front cruises are intercomparable. These LSS data can be regressed against particle concentration as with POC or PM to make comparisons between Ross Sea values and Polar Front values and to obtain continuous estimates of POC or PM.