the pelagic water column is the major part of the living space at sea. this is the water between the surface and the top of the benthic zone, where living organisms swim, float or drift, and the food chain starts with phytoplankton, the mostly microscopic photosynthetic organisms that convert the energy of sunlight into organic material which feeds nearly everything else, directly or indirectly. in temperate seas there are distinct seasonal cycles of phytoplankton growth, based on the available nutrients and the available sunlight. either can be a limiting factor. phytoplankton tend to thrive where there is plenty of light, and they themselves are a major factor in restricting light penetration to greater depths, so the photosynthetic zone tends to be shallower in areas of high productivity. zooplankton feed on the phytoplankton, and are in turn eaten by larger animals. the larger pelagic animals are generally faster moving and more mobile, giving them the option of changing depth to feed or to avoid predation, and to move to other places in search of a better food supply.