Transmissometer measurement of POC

The relationships between beam attenuation coefficient, c, at 660 nm, measured using a Sea Tech transmissometer, and particulate matter concentrations, size distributions, and chemistry determined by the Multiple Unit Large Volume in situ Filtration System (MULVFS) were investigated in the central equatorial Pacific during 1992 US-JGOFS cruises TT007 and TT011. Results from 24 pairs of simultaneous casts to 1000 m from the two 3000 km long transects showed highest correlation between beam attenuation coefficient and particulate organic carbon (POC) when POC was summed over all size classes sampled. Regression of c vs in situ POC data from TT007 and TT011 yields slopes of 6.34±0.03 and 6.09±0.03×10-5 m-1 (nmol C l-1)-1, intercepts of 0.366±0.002 and 0.368±0.003 m-1 and r2 values of 0.97 and 0.95. A reanalysis of 1982 data from the NW Atlantic and 1996 results from sub-polar NE Pacific waters indicate that the calibration specifics of c vs POC are largely independent of ocean environment, season, or depth sampled. Our results suggest that the transmissometer can provide a quantitatively useful estimate of POC over a diverse suite of oceanographic environments. This opens the opportunity for deploying transmissometers from platforms other than ships to characterize the high-frequency (diurnal and longer time scales) variability of particulate organic carbon within the upper kilometer of the ocean. A framework for interpreting the vast body of beam attenuation coefficient measurements made in the oceans over the past two decades is provided.