Estimation of a photon budget for the upper ocean in the Sargasso Sea

Abstract

A photon budget based on the quanta absorbed and subsequent biological utilization is computed for a site in the northwestern Sargasso Sea in spring. The budget includes sources, transitions, and sinks for photosynthetically available radiation (PAR). The attentuation of PAR is partitioned into separate components and the loss caused by each component is estimated as a function of depth in the upper 100 m. The major sources of attenuation to PAR include seawater (35-76%), total particulate material (20-65%), and dissolved organic material (O-l 3%). The total particulate material attentuation could be partitioned into attenuation of phytoplankton (12-39%) and of detrital material (g-27%). Although the greatest fraction of photons is absorbed by water, all the variability within the budget is caused by biological processes. Source terms include bioluminescence and chlorophyll a fluorescence, which are orders of magnitude less than radiant energy attenuation, but they are detectable and biologically significant. This photon budget is an estimate, on a quantum basis, of how radiant energy is absorbed and how various fractions are converted to chemical energy or lost to nonphotosynthetic processes. The budget serves to focus on sources, transitions, and sinks of photons and elucidate the mechanisms linking biological processes and ocean optical properties; to identify the major absorbers in the ocean and quantify their relative influence; to demonstrate an accounting with respect to the penetration of radiant energy into natural waters and point to major assumptions and uncertainties in our current estimate; and to establish a perspective that can serve as a basis for the predictive linkage of optical variability and bioluminescence. Acknowledgments

Determination of the cross‐section absorption coeffcient of individual phytoplankton cells by analytical flow cytometry

“…For modeling light-limited rates of photosynthesis, the error in estimating total photon absorption will decrease with depth as a consequence of the spectral narrowing of E,,, due to differential spectral attenuation. The error associated with estimating photon absorption and the subsequent modeling of light-limited photosynthesis will, therefore, decrease for deepliving cells as E,,, converges toward pure blue-green light in the open ocean (Smith et al 1989 The advantage of using the FCM to determine g is that it is uniquely capable of rapidly analyzing a large number of cells and, as a consequence, can improve the statistical interpretation of variability in a&t within and among water masses. The value of this approach is exemplified by Figs.…”

The response of photosynthetic absorption coefficients to irradiance in culture and in tidally mixed estuarine waters

“…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,…”

Effect of pigment composition on absorption properties of phytoplankton

“…Knowledge of these inherent optical properties of phytoplankton is therefore a necessary requirement for models of light penetration , of light utilisation by phytoplankton (Dubinsky et al 1986, Sakshaug et al 1989, Smith et al 1989, and even of mixed-layer dynamics (Lewis et al 1983, Platt et al 1984, Kamykowski 1987, Sathyendranath et al 1991. Bio-optical models for remote sensing of phytoplankton biomass and primary productivity of the oceans are also based on the optical properties of phytoplankton (Gordon & Morel 1983, Sathyendranath & Morel 1983, Platt 1986, Sathyendranath & Platt 1989.…”