SEASONAL VARIABILITY OF LIGHT AVAILABILITY AND UTILIZATION IN THE SARGASSO SEA

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 precesses: deep convective mixing in the winter which led to the spring bloom and concurrent shadowing of %PAR depths and the occurrence of anomalous thermohaline and later 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 Psi are used to assess vertically integrated light utilization variations. Values of Psi are highly variable and show no apparent seasonal pattern which indicates that Psi is not simply a ''biogeochemical constant.'' Determinations of in situ primary production rates and daily mean PAR fluxes ate used to diagnose the relative role of light limitation in determining vertically integrated rates of primary production integral 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 I-k) is only similar to 40 m, although mole than one half of integral PP occurred within this zone. Production model results illustrate that accurate predictions of integral 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.