Besides energy, all organisms require water and various nutrients. The elements of primary importance in these nutrients are carbon, nitrogen, oxygen, and phosphorus. Nutrients, unlike energy, are recycled through ecosystems in nutrient (biogeochemical) cycles. For each element, the cycle may involve 1) a reservoir, in which the nutrient is present but is temporarily unavailable, 2) an exchange pool, which is the primary source of nutrients, and 3) the biotic community, which consists of the organisms through which nutrients pass. Human activities have altered nutrient cycles.