The total amount of energy acquired by primary producers in an ecosystem is called gross primary production (GPP). However, of this energy, producers use some for metabolic processes, and some is lost as heat, decreasing the amount of energy available to the next trophic level. The remaining usable amount of energy is called the net primary productivity (NPP). In terrestrial ecosystems, NPP is driven by climate, while light penetration and nutrient availability drive NPP in aquatic ecosystems. Gross Primary Production Energy can be acquired by organisms in three ways: photosynthesis, chemosynthesis and by the consumption of other organisms. Autotrophs, or producers, synthesize their food. Chemoautotrophs occur in ecosystems where sunlight is unavailable and use chemicals as an energy source-like hydrogen sulfide, H2S, from deep ocean hydrothermal vents-while photoautotrophs transform energy from sunlight into usable energy for the rest of the organisms in an ecosystem. The rate at which these producers obtain and transform this energy is known as the ecosystem's gross primary production (GPP), which is also a measure of the total amount of energy accumulated by primary producers in an ecosystem. Net Primary Production However, not all of the energy obtained by producers is available for use by other organisms in the ecosystem. During both chemosynthesis and photosynthesis, energy is used by primary producers to fuel their cellular respiration, and some is lost as heat as a byproduct of metabolic processes. The energy left over after respiration and metabolism by primary producers is known as net primary production (NPP), which is then available to primary consumers on the next trophic level. NPP of Ecosystems Ecosystems with the highest NPP are tropical wet rainforests and estuaries influenced by warm temperatures, high humidity and an influx of nutrients. Low productivity ecosystems include deserts and the Arctic, which are dry and either too hot or too cold for high rates of plant growth. In aquatic ecosystems, quantities of both light and nutrients control primary production. The depth of light penetration drives high primary productivity in both shallow coastal waters, and at the surface of deep-ocean and lakes. Phytoplankton, which produces almost 40% of the earth's oxygen, thrives at the surface of deep marine and fresh waters, while in shallow waters, vastly diverse coral reefs and aquatic plants thrive. Areas of nutrient inflow have exceptionally high levels of primary production. Examples are estuaries where nitrogen-rich freshwater mixes with saltwater, or ocean upwellings where the organic matter of the deep ocean circulates to the surface. The inflow of macronutrients, such as phosphorus and nitrogen, increases primary production as these are otherwise limiting factors in the growth of photosynthesizing organisms. An excessive influx of these nutrients from agricultural runoff can cause exponential growth in algal and phytoplankton populations, depleting the water of oxygen, and negatively affecting the aquatic flora and fauna-a process known as eutrophication.