Contributions of phytoplankton and bacteria to the optical backscattering coefficient over the Mid-Atlantic Ridge

Abstract

Understanding the sources of the optical backscattering coefficient from particles (b bp) aids interpretation of ocean colour measured from satellites. The hypothesis tested is that phytoplankton make a significant contribution to the b bp in the open ocean. Using phytoplankton pigment, flow cytometry and microscopy data, measured concurrently with in situ b bp in the central North Atlantic Ocean, we separated the phytoplankton signal from that of bacteria. Three distinct pelagic communities were detected, all associated with the sub-polar front. A significant percentage of variance in the b bp signal (r2 = 0.68) was explained by phytoplankton between 2 and 20 -m in diameter (nanoeukaryotes). To further test the hypothesis, b bp was calculated using literature values of the backscattering properties and cell size of phytoplankton and flow-cytometric cell counts. In agreement with previous modelling studies, bacteria, due to their great abundance, dominate the biological b bp signal. However, the variations in b bp were related to changes in phytoplankton abundance (particularly of nanoeukaryotes), and the use of high backscattering efficiency factors per cell for nanoeukaryotes yielded computed values close to the observed b bp. We found no evidence that the detrital component played a significant role in b bp at this site. However, given the limited methods available to test this result, such lack of evidence is inconclusive. To be able to explain the remaining proportion of the variation in b bp, a significant methodological advance is required for the better quantification and characterisation of the organogenic detritus. © Inter-Research 2012.