0 1989, by the American Society of Limnology and Oceanography, Inc. Estimation of a photon budget for the upper ocean in the Sargasso Sea

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 ma-terial 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 optica