README.ADATPAYS_BBOP # 2 September 98 # compiled by Margaret O'Brien from older READMEs. march/april 97 By Norm Nelson (BBSR) FILES are ASCII TEXT Each file contains one of the following, depending on the extension: .atp == absorption by total particulates .ad == absorption by detritus (depigmented particulates, Kishino method) .ays == absorption by cdom (yellow substances) Filename indicates date and time of sample (yyyymmddhhmm) Each file contains MEAN MEASURED spectra from one cruise. Header is in SEABASS-approved format. Addtional pertinent data (niskin bottle id numbers, number of scans averaged, etc.) are in the comments section. Samples are from niskin bottle casts, but columns are spectra (step=1nm), rather than casts. Field names reflect depths of samples: /fields=wvln,atp001m,atp020m,atp040m,atp060m, etc. NOTES ON THE ANALYSIS METHODS -- *.ATP and *.AD files: These absorption spectra were made from seawater samples collected on BATS cruises in the Sargasso Sea. Samples were filtered using GF/F filters. Measurements prior to Jan 1997 (BATS 100) were made using a Perkin-Elmer Lambda 2 spectrophotometer. Subsequent measurements have been made using a Perkin-Elmer Lambda 18 spetrophotometer. All measurements are made using the Quantitative Filter Technique (Mitchell, 1990, Cleveland and Weidemann, 1993, Nelson et al. 1998 {appendix}). Spectra are reported as means of multiple scans of the same filter. For complete information on measurement and data processing see the BBOP IOP Sample Method Manual (in progress). QFT correction coefficients were applied to the data before the absorption coefficient was computed. The coefficients used and the methods used to estimate them are described in the appendix of Nelson et al. (1998). bubble_correction: In some filters prepared with the in-line filtration rig a bubble can appear on the filter. The bubble is not placed in the spectrophotometer beam. Correction for the increased density of the filters away from the filter is applied to each spectrum where a bubble appears. Correction factor is: 1-(area of bubble/clearance area of filter) where the area of the bubble (usually elliptical in shape) is found by measuring the semimajor and semiminor axes to within 1 mm. 1998 and subsequent data will have bubble_correction == 1 since the inline filtration rig has been replaced with a bubble-free filtration system. .ad files: To estimate the contribution of nonphytoplankton absorbing materials to atp, we extract loaded GF/F filters in 100% methanol, in a method similar to Kishino et al. (1985). This method does not completely extract phycobilins, and in most .ad spectra an artifact of residual phycoerythrin is clearly visible between 500 and 525 nm. This peak can not be considered quantitative, and cannot be considered to have the correct shape, as the methanol extraction process does change the magnitude and location of the phycoerythrin absorption (J. Patch and N. Nelson unpubl. data). We have not found a satisfactory alternative to methanol extraction. Discussion of possible alternatives to methanol extraction will be found in an upcoming SeaWiFS TM (B.G. Mitchell, ed.). NOTES ON THE ANALYSIS METHODS -- *.AYS files: Complete methods are found in Nelson et al. (1998). All Nuclepore filters used have been preconditioned by filtering 2l of Milli-Q water and 250 ml of sample prior to filtering and conserving the sample. 1997 and 1998 spectra do not have the Bricaud et al. (1981) wavelength-dependent scattering correction applied as baseline offsets were consistent and small. Spectra have been adjusted to zero at 650 nm. REFERENCES Bricaud, A, A. Morel, and L. Prieur (1981). Absorption of light by dissolved organic matter in the sea (yellow substance) in the UV and visible domains. Limnol. Oceanogr. 26: 43-53. Cleveland, J.S., and A.D. Weidemann (1993). Quantifying absorption by marine particles: a multiple scattering correction for glass-fiber filters. Limnol. Oceanogr. 40: 428-433. Kishino, M., N. Takahashi, N. Okami, and S. Ichimura (1985). Estimation of the spectral absorption coefficients of phytoplankton in the sea. Bull. Mar. Sci. 37: 634-642. Mitchell, B.G. Algorithms for determining the absorption coefficient of aquatic particles using the quantitative filter technique. Ocean Optics X, Proceedings of SPIE, 1302: 137-142. Nelson, N.B., D.A. Siegel, and A.F. Michaels (1998). Seasonal dynamics of colored dissolved material in the Sargasso Sea. Deep-Sea Res. I 45: 931-957.