An annual cycle of phytoplankton biomass in the Arabian Sea, 1994–1995, as determined by moored optical sensors

A surface-to-bottom mooring in the central Arabian Sea (15.5°N, 61.5°E) deployed from October 1994 to October 1995, included fluorometers, PAR irradiance sensors, Lu683 sensors, and a spectral radiometer. An annual cycle of phytoplankton biomass was determined by transforming signals from the optical sensors into chlorophyll a (chl a). Half-yearly phytoplankton blooms with water-column stratification were observed near the end of each monsoon, as well as biomass increases in response to mesoscale flow features. During the Northeast Monsoon, the integrate water-column chl a rose from 15 to 25 mg m−2, while during the Southwest Monsoon, chl a increased from 15 to a maximum >40 mg m−2. We present an empirical relationship between the ratio of downwelling Ed443/Ed550 (blue to green wavelength ratio) and integral euphotic zone chl a determined by moored fluorometers (r2=0.73). There is a more significant relationship between Ed443/Ed550 measured at one depth in the water column (65 m) and the average vertical attenuation coefficient for PAR (KPAR) between 0 and 65 m (r2=0.845). Because biofouling was a significant problem at times, data return from any one sensor was incomplete. However, optical sensor/data intercomparison helped fill gaps while permitting investigation of the temporal variability in observed phytoplankton biomass.