NOVELTY - Reduction of algal biomass in a body of eutrophic water (1) comprises providing a hydrocarbon production installation or its part; populating a body of eutrophic water that is proximate to the hydrocarbon production installation with organisms that feed on algae; and culturing the organisms that feed on algae in the eutrophic water, reducing algal biomass.    USE - The method is for reduction of algal biomass in a body of eutrophic water (claimed) or offshore waters near an oil and gas production platform. It is useful in many of the coastal waters in the United States and worldwide where hydrocarbon production installations are installed. The method is used to prevent or reduce the loss of seagrasses and perennial macroalgae by reducing the phytoplankton biomass; to prevent the formation of harmful algal bloom or to reduce the algal biomass in a harmful algal bloom; and to prevent hypoxia in eutrophic zone.    ADVANTAGE - The diversity of algae in an algal bloom is modified; and the productivity of oligotrophic water is increased. The method avoids the development of harmful algal bloom. It prevents the dominance within a population of algae by certain algal species that produce toxins in the body of water, e.g. dinoflagellates. Fishes that are fed on algae can be used as food fish, aquaculture feed, and as biomass for making biofuels or industrial feedstocks. The method can be used to remove heavy metal, such as mercury, from contaminated water proximate to a hydrocarbon producing installation. Bacteria and algae take up heavy metal in the water as they grow. The heavy metal accumulates in the fishes that consume the algae. By harvesting fishes that are fed on algae in the contaminated water, the heavy metal is removed from the water and remains in the fishes, which can be processed appropriately at low cost.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) production of biofuel and/or fish meal comprising providing hydrocarbon production installation or its part; populating a body of eutrophic water that is proximate to the hydrocarbon production installation with fishes that feed on algae; culturing the fishes in the eutrophic water where the fishes feed on algae in the eutrophic water; harvesting the fishes; and processing the harvested fish to produce biofuel and/or fish meal;    (2) modification of the diversity of algal species in a body of water comprising more than 105 algal cells/ml or increasing algal productivity of body of oligotrophic water comprising providing a hydrocarbon production installation or its part; transporting a stream of water comprising macronutrient or micronutrient from a distance or a different water depth relative to the body of water by the hydrocarbon production installation; and discharging the stream of water into the body of water, where concentration of the macronutrient or the micronutrient in the stream of water and in the body of water is different;    (3) a system comprising a hydrocarbon production installation or its part; a body of eutrophic water containing algae proximate to the hydrocarbon production installation; and organisms that feed on the algae in the body of eutrophic water; and    (4) hydrocarbon production installation-based system for regulating algal biomass in a body of water, comprising a nutrient management module, an algae module, and an aquaculture module, where modules are operably associated with a hydrocarbon production installation.    DESCRIPTION OF DRAWING(S) - The drawing is a flowchart for the application of planktivorous-trophic level bioremediation technology.    Reduction of algal biomass in a body of eutrophic water (1)    Reduction of algal biomass in inland waters (2)    Reduction of algal biomass in waste water discharged e.g. from industrial plants, animal husbandry operations, or fish farms (3)