This document stipulates protocols for measuring bio-optical and radiometric data for the SIMBIOS Project. It supersedes the earlier version, and is organized into four parts: Introductory Background, Instrument Characteristics, Field Measurements and Data Analysis, Data Reporting and Archival. Changes in this revision include the addition of three new chapters: (1) Fundamental Definitions, Relationships and Conventions; (2) MOBY, A Radiometric Buoy for Performance Monitoring and Vicarious Calibration of Satellite Ocean Color Sensors: Measurement and Data Analysis Protocols; and (3) Normalized Water-Leaving Radiance and Remote Sensing Reflectance: Bidirectional Reflectance and Other Factors. Although the present document represents another significant, incremental improvement in the ocean optics protocols, there are several protocols that have either been overtaken by recent technological progress, or have been otherwise identified as inadequate. Revision 4 is scheduled for completion sometime in 2003. This technical report is not meant as a substitute for scientific literature. Instead, it will provide a ready and responsive vehicle for the multitude of technical reports issued by an operational Project. The contributions are published as submitted, after only minor editing to correct obvious grammatical or clerical errors.

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Ocean Optics Protocols for Satellite Ocean Color Sensor Validation

Little is known about the combined impacts of future CO2 and temperature increases on the growth and physiology of marine picocyanobacteria. We incubated Synechococcus and Prochlorococcus under present-day (380 ppm) or predicted year-2100 CO2 levels (750 ppm), and under normal versus elevated temperatures (+4°C) in semicontinuous cultures. Increased temperature stimulated the cell division rates of Synechococcus but not Prochlorococcus. Doubled CO2 combined with elevated temperature increased maximum chl a–normalized photosynthetic rates of Synechococcus four times relative to controls. Temperature also altered other photosynthetic parameters (α, Φmax, Ek, and ) in Synechococcus, but these changes were not observed for Prochlorococcus. Both increased CO2 and temperature raised the phycobilin and chl a content of Synechococcus, while only elevated temperature increased divinyl chl a in Prochlorococcus. Cellular carbon (C) and nitrogen (N) quotas, but not phosphorus (P) quotas, increased with elevated CO2 in Synechococcus, leading to ∼20% higher C:P and N:P ratios. In contrast, Prochlorococcus elemental composition remained unaffected by CO2, but cell volume and elemental quotas doubled with increasing temperature while maintaining constant stoichiometry. Synechococcus showed a much greater response to CO2 and temperature increases for most parameters measured, compared with Prochlorococcus. Our results suggest that global change could influence the dominance of Synechococcus and Prochlorococcus ecotypes, with likely effects on oligotrophic food-web structure. However, individual picocyanobacteria strains may respond quite differently to future CO2 and temperature increases, and caution is needed when generalizing their responses to global change in the ocean.

Effects of increased temperature and co2 on photosynthesis, growth, and elemental ratios in marine synechococcus and prochlorococcus (cyanobacteria)1

Observations from the Sargasso Sea have shown that the light attenuation spectrum is a function of both phytoplankton pigments and a detrital-like component that varies independently. Here we examine the nature and dynamics of these detrital-like variations by analyzing a time-depth series of visible and ultraviolet light absorption spectra for colored (chromophoric) dissolved materials [CDOM; ag(λ)] and detrital particulates [ad(λ)] collected at the US JGOFS Bermuda Atlantic time-series study (BATS) site. At 440 nm, CDOM absorption, ag(440), made up of on an average more than one-half of the total non-water absorption coefficient, while detrital particulate absorption, ad(440), was generally a minor constituent. The vertically integrated stock of CDOM (0–140 m) increased from the beginning of spring until the end of summer in 1994 and 1995, although near-surface values (⩽40 m) were strongly depressed near the surface at the peak of summer. This summertime reduction of the mixed layer ag(λ) is likely due to photooxidation of CDOM to optically inactive forms. Further, values of CDOM absorption were unrelated directly to indices of total water column dissolved organic carbon (DOC) stocks measured at BATS. We hypothesize that CDOM is produced as a by-product of the microbial breakdown of DOC and is destroyed due to photooxidation. Summertime CDOM dynamics at this site can be quantified with these two processes, as mixing below the summertime mixed layer can be assumed to be small. Our results imply, for the first time in the blue ocean, a link between microbial community activity and CDOM dynamics and clearly provide an explanation for the nature of non-chlorophyll light attenuation observed in the Sargasso Sea.

Seasonal dynamics of colored dissolved material in the Sargasso Sea

We made modifications to the procedure used for absorption measurements of aquatic particles retained on glass-fiber filters that extend the procedure’s application range to “case 2” waters with high suspended sediment content and allow the determination of phytoplankton pigment absorption in situations where the standard solvent extraction method is not effective. The first result was achieved by combining light-transmission and light-reflection measurements (the latter made possible by the use of a commercially available integrating-sphere attachment for the dual-beam spectrophotometer) so as to remove the spurious contribution to the measured absorption caused by sample backscattering. The second result was obtained by bleaching the sample with a NaClO solution-a method that proved satisfactory even with water-soluble pigments (e.g. the important class of the phycobilins) and solvent-resistant Chlorophyceae. This process also allows for depigmentation of the particle suspension, and thus it was used to evaluate the empirical expression for converting the filter-retained sample absorbance to the equivalent particle suspension value. Both features of the modified procedure have been positively tested through measurements carried out on solvent-resistant phytoplankton species and on samples of inorganic suspended sediment. Light-transmission measurements on particles retained in glass-fiber filters (Yentsch 1962) are convenient for determining the light absorption spectrum of particle suspensions consisting mainly of natural phytoplankton and organic and inorganic detritus (Gordon and Morel 1983). The procedure has two basic advantages: particles can be concentrated so that instrumental accuracy requirements can be met regardless of the high dilution occurring in situ, and information is provided on the absorption of the cells in vivo. The main problem in data analysis arises from the large modification of light transmission due to multiple scattering by the filter, which results in an overestimate of particle absorption relative to suspension state (Butler 1962). Several empirical expressions have been derived for converting the absorption of filter-retained particles (for convenience also referred to as “sample” in the following) to the equivalent absorption of particle suspension. A review on this subject is given by Cleveland and Weidemann ( 199 3). Kishino et al. (1984, 1985) proposed a procedure for discriminating absorption by phytoplankton pigments from absorption by detritus based on measurements performed before and after pigment extraction by methanol. Although some doubts remain about the effectiveness of the procedure (Bricaud and Stramski 1990) and its validity range shows some limitations (little or no effect on some algal species), solvent extraction is the method generally used to identify pigment absorption in natural phytoplankton populations. It is current practice to minimize the loss of forward

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An alternative approach to absorption measurements of aquatic particles retained on filters

Determination of spectral absorption coefficients of particles, dissolved material and phytoplankton for discrete water samples

: Absorption spectra measured for aquatic particles concentrated onto glass-fiber filters require a correction for the increase in pathlength caused by multiple scattering in the glass-fiber filter. A multiple scattering correction was calculated from optical density spectral for 48 phytoplankton cultures of seven species representing a variety of cell sizes, pigment groups, and call-wall types. The relationship between optical density in suspensions and on filters was not wavelength-dependent. Differences between blank filters were always spectrally neutral. Small differences between relationships for single species were inconclusive. Given the absence of wavelength-dependent effects, we report a single general quadratic relationship, OD(susp)(lambda) = 0.378 OD(filt)(lambda) + 0.523 OD(filt)(lambda)2 (r2 = 0.988), for correcting glass- fiber filter spectra. For independent samples, the average error in predicting OD(susp)(lambda) with this algorithm at any wavelength was 2%. Greatest errors were in spectral regions of low absorption. Absorption spectra for particles concentrated onto glass-fiber filters can be quantitatively corrected for multiple scattering within this limit. Applicability of the algorithm to field samples of varied composition was enhanced by using a large number of spectra and a range of cell types in algorithm development. Optical oceanography, Physical oceanography, Absorption, Scattering.

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J. S. Cleveland

Papers

Quantifying absorption by aquatic particles: A multiple scattering correction for glass-fiber filters