Diel variations of bio-optical properties in the Sargasso Sea

Abstract

Concurrent time series measurements including: percent transmission of a collimated beam of 660 nm light (converted to beam attenuation coefficient, C), photosynthetically available radiation (PAR), and chlorophyll (chl) fluorescence were obtained at 7-8 depths within the upper 160m of the Sargasso Sea (34N 70W) from Mar-Oct 1987 using a moored array of instruments (see Dickey et al., this volume). A subset of these data, the spring deployment (the first of three) from March to mid May, has been analyzed with respect to the diel phase variations in the bio-optical properties. Among the features noted, the relationship between PAR and chl-fl changed from cM-fl lagging the PAR signal by ca 90 degrees, through 180 degrees of shift to leading by -90 degrees. The transition period corresponding to this change was marked by inconsistent behavior in thephase relationships between other bio-optical variables, but the changes were short-lived and returned to their previous offsets from the daily PAR. These changes are thought to be the result of a succession of species caused by a combination of warm outbreaks of Gulf Stream waters importing a foreign particle assemblage into the Sargasso Sea, contemporaneous with wind events causing deep mixing. This hypothesis is consistent with recent observations at sea and in laboratory studies.