Carbon dioxide availability sometimes limits productivity. An aquatic ecosystem's productivity is usually determined by light intensity, water temperature, and availability of inorganic nutrients. Productivity is greatest in shallow waters near continents and along coral reefs due to abundant nutrients and sunlight. Inorganic nutrients are limiting at the surface of open ocean waters with nitrogen and phosphorus in especially short supply. This is a primary reason for the relatively low productivity of open oceans.
Marine phytoplankton is most productive where upwellings bring nutrient-rich waters to the surface.
Thermal vent communities are also very productive though they are not very widespread and contribute little to marine productivity. Freshwater ecosystem productivity also varies from the surface to the depths in relation to light intensity. Availability of inorganic nutrients is sometimes limiting, but biannual turnovers bring nutrients to the surface waters. The amount of energy available to each trophic level is determined by the efficiencies with which food energy is converted to biomass in each link of the food chain. Secondary productivity is the rate at which consumers convert the chemical energy in the food they eat into their own biomass.