These productive regions are often associated with eastern-boundary current systems on the transition between marine shelves and the open ocean, forming where divergence of surface water causes upwelling of cold, nutrient-rich water. Bursts of primary productivity are associated with naturally variable, often wind-driven upwelling events and support a very high biomass of plankton, fish, and marine birds and mammals. Small species like sardine and anchovy that operate at low trophic levels dominate fish communities, may vary greatly in abundance through time, and play important roles in food webs.