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The accuracy of models for primary production and light propagation depends on correct assignment of absorption to photosynthetic pigments. The phytoplankton absorption coefficient is comprised of two components: photosynthetic and photoprotective absorption coefficients. A method based on the fluorescent excitation of chlorophyll a is used to quantify the photosynthetic absorption coefficient for phytoplankton grown in culture and sampled from Puget Sound, Washington. The difference spectrum between total phytoplankton and photosynthetic absorption should be equivalent to photoprotective absorption. For cultures, the difference spectra exhibit peaks near 460 and 490 nm and broad-band absorption between 400 and 450 nm. However, for field samples an additional pronounced peak is observed around 440 nm, similar in shape to the chlorophyll a Soret peak. If the 440-nm peak were associated with photosystem I chlorophyll a, the photosynthetic absorption coefficient will be underestimated by Ͻ15% for these samples.Variability in both coefficients is predictable as a function of irradiance. The photosynthetic coefficient varies inversely with growth irradiance, and the photoprotective coefficient varies directly with irradiance. This direct relationship with irradiance accounts for much of the variability in the spectral shape of the total phytoplankton absorption coefficient. The ratio of the photosynthetic absorption coefficient to the total phytoplankton absorption coefficient increases as a function of decreasing irradiance for cultures and for field samples collected from stratified regions of the water column. This ratio is a photoadaptive parameter that can serve to integrate physiological response to irradiance and has the potential to provide estimates of mixed layer dynamics.Modeling light propagation through the water column and determining a photon budget for the euphotic zone requires an estimate of the phytoplankton absorption coefficient. The phytoplankton absorption coefficient often is the most variable optical component of the water column. The composition and quantity of the phytoplankton assemblage affect both the spectrum and the magnitude of the absorption coefficient. Phytoplankton adapt the composition and concentration of photosynthetic and nonphotosynthetic pigments in response to variations in irradiance intensity and spectral composition (Yentsch and Yentsch 1979;SooHoo et al. 1986;Mitchell and Kiefer 1988;Johnsen and Sakshaug 1993). The accuracy of photon budgets and of bio-optical models for ocean productivity is improved by the incorporation of spectral information into the irradiance and absorption coefficient measurements (e.g., Bidigare et al. 1987Bidigare et al. , 1992Smith et al. 1989). The accuracy also is improved by 1 Present address: Technology, Planning and Management Corporation,

The effects of environmental variables, particularly irradiance, on the sinking rates of phytoplankton were investigated using cultures of Chaetoceros gracilis Schütt and C. flexuosum Mangin in laboratory experiments; these data were compared with results from assemblages in the open ocean and marginal ice zone of the Greenland Sea. In culture experiments both the irradiance under which the diatom was grown and culture growth rate were positively correlated with sinking rates. Sinking rates (ψ) in the Greenland Sea were smallest when determined from chlorophyll (mean ψchl= 0.14 m · d−1) and biogenic silica (ψsi= 0.14 m · d−1) and greatest when determined from particulate carbon (ψc= 0.55 m · d−1) and nitrogen (ψN= 0.64 m · d−1). Field measurements indicated that variations in sinking may be associated with changes in irradiance and nitrate concentrations. Because these factors do not directly affect water density, they must be inducing physiological changes in the cell which affect buoyancy. Although a direct response to a single environmental variable was not always evident, sinking rates were positively correlated with growth rates in the marginal ice zone, further indicating a connection to physiological processes. Estimats of carbon flux at stations with vertically mixed euphotic zones indicated that approximately 30% of the daily primary production sank from the euphotic zone in the form of small particulates. Calculated carbon flux tended to increase with primary productivity.