Optical backscattering in the Arabian Sea - continuous underway measurements of particulate inorganic and organic carbon

Continuous surface measurements of temperature, salinity, fluorescence and optical backscattering were made during R/V Thompson cruise no. TN053 in th e northern Arabian Sea ("Bio-Optical cruise"; October-November, 1995). The cruise covered the early NE monsoon period. Optical measurements involved a lternate estimates of total backscattering and acidified backscattering app roximately every 1.5-2min (measured after addition of a weak acid to dissol ve calcium carbonate). The difference between total and acidified backscatt ering equals "acid-labile backscattering". Total and acid-labile backscatte ring were converted to the concentration of particulate organic carbon (POC ) and particulate inorganic carbon (PIC; calcium carbonate), respectively, and discrete samples taken along the cruise track were used for calibration . Backscattering data were frequently coherent with temperature, salinity, and density variability. Acid-labile backscattering values revealed that ca lcium carbonate accounted for 10-40% of the total optical backscattering in the region, and the semi-continuous records demonstrated distinct patches of coccolith-rich water. The northern Arabian Sea had the highest acid-labi le backscattering. Results suggest that PIC: POC ratios can vary over about four orders of magnitude. Highest surface values of PIC: POC approached on e in several places. We also report qualitative observations of phytoplankt on community structure made aboard ship, on fresh samples. (C) 2001 Elsevie r Science Ltd. All rights reserved.