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

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 (r(2) = 0.68) was explained by phytoplankton between 2 and 20 mu 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.