Continental seas have the richest life of the oceans, more species and greater numbers than anywhere else. This is because it has both sunlight and nutrients for photosynthesis. This causes microscopic algae and cyanobacteria to bloom, and then animals eat the phytoplankton. Most of the nutrients in the oceans are washed down from the continents by rivers. In particular, the continents are almost the only source for some key elements such as iron. The lack of nutrients away from continents explains why so much of the Pacific Ocean is almost barren of life, and why the shelves are so rich in life.