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 degreesN, 61.5 degreesE) deployed from October 1994 to October 1995, included fluorometers, PAR irradiance sensors, Lu-638 sensors, and a spectral radiometer. An annual cycle of phytoplankton biomass was determined by transforming signals from the optical sensors into chlorophyll alpha (chl alpha). 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 alpha rose from 15 to 25mgm(-2), while during the Southwest Monsoon, chl alpha increased from 15 to a maximum > 40mgm(-2) We present an empirical relationship between the ratio of downwelling E(d)443/E(d)550 (blue to green wavelength ratio) and integral euphotic zone chi a determined by moored fluorometers (r(2) = 0.73). There is a more significant relationship between E-d 443/E-d 550 measured at one depth in the water column (65 m) and the average vertical attenuation coefficient for PAR (K-par) between 0 and 65 m (r(2) = 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. (C) 2001 Elsevier Science Ltd. All rights reserved.