This diagram shows the relationships between several elements of the Caspian Sea ecosystem. Dissolved nutrients such as nitrogen (N) and phosphorus (P) flow into the Caspian Sea from tributary rivers and are consumed by plankton. Dead plankton that sink to the seafloor support the bottom feeding sturgeon and return nutrients to the water column. Schools of kilka feed on the plankton and in turn become prey to seals, sturgeon, birds, and fishermen. In a pristine system, sturgeon would return to the rivers to spawn, supplying the precious eggs to the lucrative caviar industry.