Particulate organic carbon and inherent optical properties during 2008 North Atlantic Bloom Experiment

The co-variability of particulate backscattering (b(bp)) and attenuation (c(p)) coefficients and particulate organic carbon (POC) provides a basis for estimating POC on spatial and temporal scales that are impossible to obtain with traditional sampling and chemical analysis methods. However, the use of optical proxies for POC in the open ocean is complicated by variable relationships reported in the literature between POC and c(p) or b(bp). During the 2008 North Atlantic Bloom experiment, we accrued a large data set consisting of > 300 POC samples and simultaneously measured c(p) and b(bp). Attention to sampling detail, use of multiple types of POC blanks, cross-calibration of optical instruments, and parallel measurements of other biogeochemical parameters facilitated distinction between natural and methodological-based variability. The POC versus c(p) slope varied with plankton community composition but not depth; slopes were 11% lower for the diatom versus the recycling community. Analysis of literature POC versus c(p) slopes indicates that plankton composition is responsible for a large component of that variability. The POC versus b(bp) slope decreased below the pycnocline by 20%, likely due to changing particle composition associated with remineralization and fewer organic rich particles. The higher b(bp)/c(p) ratios below the mixed layer are also indicative of particles of lower organic density. We also observed a peculiar platform effect that resulted in similar to 27% higher values for downcast versus upcast b(bp) measurements. Reduction in uncertainties and improvement of accuracies of POC retrieved from optical measurements is important for autonomous sampling, and requires community consensus for standard protocols for optics and POC.