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

Abstract

As part of the Forced Upper Ocean Dynamics Program, which ran concurrently with the US JGOFS Arabian Sea Expedition, five moorings were deployed in the historical axis of the Findlater Jet. In addition to other variables, moored sensors collected photosynthetically active radiation (PAR), particulate beam attenuation ( C p ), stimulated fluorescence (FLU), and dissolved oxygen (O 2) data from October 1994 to October 1995. Diel bio-optical signals were recorded during two periods between the Northeast and Southwest Monsoons 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 time. Daily periodicity was lowest for all bio-optical signals just after a strong storm during the 1994 Fall Intermonsoon period. During a phytoplankton bloom associated with a cool advective feature, the FLU and O 2 diel signals were most pronounced. 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. Fluorescence quenching at 10 m due to high irradiance (∼1000 μEinstein m −2 s −1) forced the ratio of fluorescence to particulate attenuation into a diel periodicity at 10 m, but not at 35 m (noontime irradiance ∼200 μEinstein m −2 s −1), where the FLU and C p increases were almost in phase. Diel changes in C p , when scaled to particulate organic carbon, suggest a net production of ∼20 mg C m −3 d −1 at 10 and 35 m. We estimate a specific growth rate from a calculated particle production rate balanced by a constant grazing over 24 h to be 0.77 d −1, and using a C ∗ c of 424 mg C m −2, estimate a carbon : chl a ratio between 85 and 115 for a 10-d window during the 1994 Fall Intermonsoon period.