A 2 year time series of optical, biogeochemical, and physical parameters, taken near the island of Bermuda, is used to evaluate the sources of temporal variability in light availability and utilization in the Sargasso Sea. Integrated assessments of light availability are made by examining the depth of constant percent incident photosynthetically available radiation (%PAR) isolumes. To first order, changes in the depth of %PAR isolumes were caused by physical processes: deep convective mixing in the winter which led to the spring bloom and concurrent shallowing of %PAR depths and the occurrnce of anomalous thermohaline water masses during the summer and fall seasons. Spectral light availability variations are assessed using determinations of diffuse attenuation coefficient spectra which illustrate a significant seasonal cycle in colored detrital particulate and/or dissolved materials that is unrelated to changes in chlorophyll pigment concentrations. Temporal variations in the photosynthetic light utilization index &PSgr; are used to assess vertically integrated light utilization variations. Values of &PSgr; are highly variable and show no apparent seasonal pattern which indicates that &PSgr; is not simply a ''biogeochemical constant.'' Determinations of in situ primary production rates and daily mean PAR fluxes are used to diagnose the relative role of light limitation in determining vertically integrated rates of primary production ∫PP.

The mean depth of the light-saturated zone (the vertical region where the daily mean PAR flux was greater than or equal to the saturation irradiance Ik) is only ~40 m, although more than one half of ∫PP occurred within this zone. Production model results illustrate that accurate predictions of ∫PP are dependent upon rates of light-saturated production rather than upon indices of light limitation. It seems unlikely that significant improvements in simple primary production models will come from the partitioning of the Earth's seas into biogeochemical provinces. ¿ American Geophysical Union 1995