Diel bio-optical variability observed from moored sensors in the Arabian Sea

As part of the Forced Upper Ocean Dynamics Program, which ran concurrently with the US JGOFS Arabian Sea Expedition, five moorings were deployed in th e historical axis of the Findlater Jet. In addition to other variables, moo red sensors collected photosynthetically active radiation (PAR), particulat e beam attenuation (C-p), stimulated fluorescence (FLU), and dissolved oxyg en (O-2) data from October 1994 to October 1995. Diel bio-optical signals w ere recorded during two periods between the Northeast and Southwest Monsoon s at 10, 35, and 65 m, Spectral analysis shows significant diel cycles of C -p, FLU, and O-2, but the strength of these cycles was not constant over ti me. Daily periodicity was lowest for all bio-optical signals just after a s trong storm during the 1994 Fall Intermonsoon period. During a phytoplankto n bloom associated with a cool advective feature, the FLU and O-2 diel sign als were most pronounced. Although these signals are biological responses t o the daily cycle of irradiance, they are mediated by hydrographic conditio ns; strongest when phytoplankton are confined within the mixed layer or the rmocline, and thus exposed to light intensities long enough to display thes e responses to PAR. Fluorescence quenching at 10 m due to high irradiance ( similar to 1000 mu Einstein m(-2) s(-1)) forced the ratio of fluorescence t o particulate attenuation into a diel periodicity at 10 m, but not at 35 m (noontime irradiance similar to 200 mu Einstein m(-2) s(-1)), where the FLU and C-p increases were almost in phase. Diel changes in C-p, when scaled t o particulate organic carbon, suggest a net production of similar to 20 mg C m(-3) d(-1) at 10 and 35 m. We estimate a specific growth rate from a cal culated particle production rate balanced by a constant grazing over 24 h t o be 0.77 d(-1), and using a C-c* of 424 my C m(-2), estimate a carbon : ch l a ratio between 85 and 115 for a 10-d window during the 1994 Fall Intermo nsoon period. (C) 1999 Elsevier Science Ltd. All rights reserved.