Diel Variations of Bio-optical Properties in the Sargasso Sea Michael Hamilton, T.e. Granata, T. D. Dickey, J.D. Wiggert Ocean Physics Group Department of Geological Sciences University of Southern California Los Angeles CA 90089-0740 D. A. Siegel Department of Geography University of California, Santa Barbara Santa Barbara CA 93106 J.Marra,C.Langdon Lamont-Doherty Geological Observatory Palisades NY 10964 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 (chI) 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-f1 changed from chl-f1lagging 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 the phase 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. l.INTRODUCTION The Biowau experiment was carried out over nine months encompassing the seasons of an oceanic year, from March through November 1987 at 34N, 70W in the Sargasso Sea. Meteorological data were collected from a package on the surface buoy, and moored instrument packages located at eight depths within the upper 160m measured the physical and bio-optical properties of the euphotic zone. Multi-variable moored systems (MVMS) were used for bio-optical measurements including transmission of a beam of collimated light at 660 nm (C660, m·!), stimulated fluorescence of cellular and dissolved chi-a and phaeopigments (chl-fl, equivalent ug chll-!), dissolved oxygen concentration (02, JlM), and photosynthetically available radiation (PAR, 10 17 quanta cm· 2 s+). In addition, MVMS units were used for physical measurements including temperature and horizontal components of current. Data from the MVMS were sampled every 4 min. Meteorological measurements were made from the surface buoy at an interval of 7.5 min. Shortwave radiation and atmospheric parameters including wind direction and speed, air temperature, barometric pressure, and relative humidity were determined. An overview of the MVMS and meteorological measurements is presented in Dickey ct. al. 5 . Several scales of variability are apparent within the Biowatt measurements, including daily, synoptic, mesoscale, and seasonal. One of the more pronounced forcing scales is at the daily or dicl frequency, related to solar insolation, which facilitates meaningful comparison of the diel cycles. The relationship of the dicl cycles of the bio-optical properties of the upper ocean as compared to the daily cycles of the sun must be related to local biologieal processes. Changes in physiology, quantity, community, or biological processes should be reflected as periods of departure from the background signal of daily, sun-forced cycles of increase and decrease of the bio-optical variables. The utility of quantifying the regular relationships of the diel cycles of the bio-optical properties of the ocean, then, includes the separation of locally produced and advected variations in particle, chlorophyll, and oxygen concentration. 214 / SPIE Vol. 1302 Ocean Optics X (1990)