Life in the dark depths below the photic zone is given over to a food chain based on the consumption of detritus, the nutrient-rich rain of dead plants and animals that constantly falls from the sunlit waters above. This rain of detritus continually removes that third requirement for plant life – nutrients – from the photic zone. Marine life can flourish only where these nutrients are recycled to the surface by upwelling currents or where they are replaced by nutrients in the outflow of rivers.
These conditions are met along many coastlines of the world. More than half of the plant material (biomass) in the oceans consists of seaweeds anchored to intertidal (that region of the shoreline between the high and low tides) and subtidal (habitat below the level of the lowest tide) rocks. Most of the world's fisheries are concentrated in small areas of rich coastal upwellings; beyond the continental shelves, nutrient levels are usually so low that the open oceans are biological deserts.