Ecological indicators are defined as quantifiable metrics that can be used to monitor the state of ecosystems and their response to environmental perturbations. In the global oceans, commonly used indicators are typically based on the presence and distribution of phytoplankton (as indexed by the concentration of chlorophyll-a [Chl-a]), which form the base of oceanic food webs. Phytoplankton phenology (the timing of phytoplankton growth) and phytoplankton size structure are particularly important ecological indicators that can be derived via ocean colour remote sensing. Phytoplankton phenology has a direct control on food availability, which subsequently impacts the survival of higher trophic levels and the structure of marine ecosystems. Meanwhile, phytoplankton size structure can be used to define the major functional groups that ultimately influence marine food web structure, biogeochemical cycling and carbon export. The Red Sea is a relatively unexplored tropical marine ecosystem, particularly in relation to its large-scale biological dynamics. In light of recent evidence of rapid regional warming, the need to monitor the response of the Red Sea to potential future ecosystem modifications is becoming more imminent. Using a combination of contemporary oceanographic tools, with an emphasis on ocean colour remote sensing, this PhD thesis attempts to validate the retrieval of phytoplankton ecological indicators in the Red Sea - specifically phytoplankton abundance, phenology and size structure. The interannual variability of both indicators and their linkages with the regional physical environment are also explored.
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