This uppermost ocean layer (0-200m depth) is the most influenced by the atmosphere, and is defined and structured by light availability. Photosynthesis in these ecosystems accounts for half of all global carbon fixation. That productivity supports diverse marine life, including many visual predators, like tuna, that rely on the high light environment. Migration is a common life history trait across all groups: either vertical – rising from the depths to feed at the surface at night to evade daytime predators; or horizontal – between breeding and feeding grounds. Detritus from this zone is an important nutrient source for lower oceanic layers.