Semi-annual monsoons over the Arabian Sea drive reversing basin-scale circulation patterns, unique in the world’s oceans. The resulting surface pressure fields and baroclinic adjustments change mixing dynamics and the nutrient supply to the euphotic zone, forcing regular oscillations in primary productivity and biomass, without relatively large changes in surface radiation or seawater temperatures. This thesis explores the bio-optical response to the varying physical forcing and nutrient injection in the Arabian Sea from seasonal to hourly time scales. The data primarily come from sensors on two surface-to-bottom moorings, deployed consecutively, in the central Arabian Sea (15.5N, 61.5E) from October 1994 to October 1995. Temperature, salinity, orthogonal current, dissolved oxygen, irradiance sensors, transmissometers, and natural and stimulated fluorometers were moored at four depths in the upper 80 meters of the water column. Despite biofouling problems during relatively long (six month) deployments in productive tropical waters, the moorings led to the first high temporal resolution, year-long water-column record of phytoplankton biomass in the Arabian Sea. The analysis relates the observed biomass response to both seasonal (monsoon) as well as shorter term, mesoscale, hydrographic feature forcing. Shipboard diffuse attenuation coefficients and extracted chlorophyll a values were incorporated to attain an internally consistent record. Biannual phytoplankton blooms coinciding with water column stratification were observed near the end of each monsoon, as well as biomass increases in response to mesoscale features. During the fall Northeast Monsoon, the areal chlorophyll a rose from 15 to 25 mg m^2, while during the Southwest Monsoon, areal chlorophyll a increased from 15 to at least 50 mg m^2.

A prototype multi-spectral radiometer was moored from April to October, 1995. Data from this instrument documented the change in water column spectral irradiance with the onset of the Southwest Monsoon, as well as during the passage of a geostrophic flow feature. Empirical relationships are presented between the ratio of downwelling Ed443/Ed550 (blue to green wavelengths) and integral euphotic zone chlorophyll a as well as between Ed443/Ed550 measured at one depth in the water column (65m) and the average vertical attenuation coefficient for PAR. The irradiance ratio pigment biomass model compared well with the estimates of biomass from the fluorometers, and the observed biomass change is coincident with the PAR euphotic zone depth shoaling.

Diel bio-optical signals were observed during both intermonsoon periods. Spectral analysis showed significant diel cycles of particulate beam attenuation, stimulated fluorescence, and dissolved oxygen, but the strength of these cycles was not constant over time. Although these signals are biological responses to the daily cycle of irradiance, they are mediated by hydrographic conditions; strongest when phytoplankton are confined within the mixed layer or thermocline, and thus exposed to light intensities long enough to display these responses to PAR.