TECHNOLOGY FOCUS - BIOLOGY - Preferred Method: The organisms are confined to an enclosure or a substrate operably associated with the hydrocarbon production installation. The body of eutrophic water comprises more than 1x 105 algal cells/ml, algal bloom, harmful algal bloom, or hypoxic zone; and the algal cells are cells of Cyanobacteria, diatoms, dinoflagellates, raphidophytes, and/or haptophytes. The dimensions of at least one dominant species of algae range from 200 to 2000, preferably 0.2-2 mu m. At least one dominant species in the eutrophic water is a species of Anabaena, Aphanizomenon, Microcystis, Noctiluca, Alexandrium, Prorocentrum, Gymnodinium, Ceratium, Pfiesteria, Dinophysis, Gyrodinium, Heterosigma, Chattonella, Skeletonema, Synedropsis, Pseudo-nitzschia, Leptocylindrus, Chaetoceros, Phaeocystis, Nannochloris, Stichococcus, Aureococcus, Miraltia, Guinardia, Spermatozopsis, Urosolenia, Nitzschia, Cyclotella, Cryptomonas, or Pedinophora. The organisms that feed on algae comprise a population of fishes, a population of shellfishes, or populations of fishes and shellfishes. They comprise planktivores and/or omnivores. At least one of the major species of planktivores is pelagic phytoplanktivore. At least one major species of the organisms that feed on algae is a species of fish in the order Clupeiformes, Siluriformes, or Cypriniformes. The organisms comprise menhaden, anchovies, shiners, shads, sardines, pilchards, herring, smelts, catfish, milkfish, carps, mullets, hilsas, oysters, mussels, scallops, and/or clams. They comprise Brevoortia patronus or Brevoortia tyrannus, which is confined to a net cage. The organisms are fishes introduced at a density of 300-600 or 600-900 individuals/m2. The step of culturing the organisms comprises replenishing the eutrophic water in the enclosure with algae; and does not comprise adding fish meal, fish oil, nitrogenous fertilizer, and/or phosphorus fertilizer into the eutrophic water. It comprises maintaining the level of dissolved oxygen in the eutrophic water above 3 mg/L. The method further comprises harvesting at least one of the organisms after culturing. The method of increasing algal productivity of body of oligotrophic water comprises discharging a stream of fluid comprising iron, such that the concentration of iron in the body of oligotrophic water increases to a level greater than 0.1, 0.5, 1, 5, or 10 nM. The nutrient management module comprises pipelines and/or risers that are fluidically connected with hydrocarbon production installations that are horizontally spaced in a geographic area, or vertically disposed from below the seabed up to the height of a deck. The algae module comprises imaging subsystems for measuring algae density, counting algae, and/or classifying algae taxonomically, and data subsystems for analysis, storage, and display of algal biomass data. The aquaculture module comprises enclosure(s), and operating subsystems. The enclosure is a floating cage, a submersible cage, or a submerged cage. The structural components of the hydrocarbon production installation form parts of a frame of a cage. The operating subsystems comprise an environmental monitoring subsystem, a feeding subsystem, an aeration subsystem, a transfer subsystem, and a fish processing facility.