An integrated system for converting ocean thermal energy to electrical power for use in synthesis of anhydrous ammonia and a mariculture operation for raising a selected species of marine fauna. The mariculture operation is carried out in a lagoon located adjacent to a deep ocean region where the surface water temperature is at least 21.degree. C and the deep nutrient-rich water temperature is no more than about 10.degree. C. A separate working fluid, such as ammonia, operates in a Rankine cycle between the warm and cold seawater temperatures. The deep ocean water is used to condense the working fluid and is then mixed with warm surface water that has been used to evaporate the same working fluid in a closed power cycle. The resulting mixture is directed to the lagoon, where the nutrient-rich component contributes to the growth rate of a selected mariculture species. The warm surface water temperature may be increased by creating a monomolecular evaporation retarding layer in the ocean surface surrounding the power plant intake.