Optical Signatures of Plankton in the Open Ocean: From Individual Cells to Global Patterns

Marine plankton ecosystems play a major role on Earth, having implications for the global carbon cycle and the food-web structures. Ocean color satellites and networks of autonomous platforms equipped with optical sensors are the primary tools used to study phytoplankton dynamics. They provide long term records while offering a synoptic view of our oceans, enabling to study impact of climate variability on planktonic ecosystems. Interpretation of these observations rely heavily on optical theory and how light propagating through the water is affected by particles who absorb and scatter light (e.g. phytoplankton, sediments). However, the complexity of the optical properties of natural seawater often obscures their interpretation. I address some of the current challenges in optical theory by analyzing measurements of inherent optical properties and phytoplankton size distribution (PSD). The PSD built spans four seasons across regions of the western North Atlantic, including large variability which highlight the dynamic annual cycle of phytoplankton of this area. Previously established algorithms used to estimate phytoplankton size algorithms based of optical properties are assessed as to date they have not been validated with actual size measurements. Additionally, the contribution of phytoplankton to particulate attenuation and backscattering and its efficiency to absorb light are computed for the upper ocean. The PSDs revealed that phytoplankton dominate attenuation and backscattering signals ( 75 %) reinforcing the idea that these properties are good predictors of phytoplankton biomass. Additionally, spectral slopes of attenuation and backscattering also correlate well with the PSD. This suggests that ocean color algorithms should focus on improved retrieval of backscattering spectra. A data logger was developed to improve current recording of optical data during long term deployment on research vessels. It was successfully deployed \u3e650 days at sea. Finally, I proposed a novel method to detect a subset of diel migrating organisms (SDMO) responsible for anomalies in particulate backscattering and ultra-violet fluorescent profiles from autonomous platforms. This method demonstrates the seasonality of SDMO in the world’s ocean in regions where such data has been lacking and provides the distribution of SDMO which play an important role in the biological pump, extending observations beyond “classical” methodology