marine ecosystems, the largest of all ecosystems, cover approximately 71% of the earth's surface and contain approximately 97% of the planet's water. they generate 32% of the world's net primary production. they are distinguished from freshwater ecosystems by the presence of dissolved compounds, especially salts, in the water. approximately 85% of the dissolved materials in seawater are sodium and chlorine. seawater has an average salinity of 35 parts per thousand of water. actual salinity varies among different marine ecosystems.