NOVELTY - Producing biogas involves providing a polyculture of aquatic plants to a growth system (10); continuously providing water, carbon dioxide, air and nutrients to the polyculture contained within growth system; growing the polyculture for a time sufficient to produce an aquatic plant-based biomass; withdrawing a portion of aquatic plant-based biomass contained within the growth system; and treating the withdrawn aquatic plant-based biomass to produce a biogas.    USE - For producing biogas and electricity (claimed), useful as a low-cost fuel for heating or energy production purposes.    ADVANTAGE - The method is efficient, sustainable, and results in a minimal amount of waste products. The cyanobacteria consume portions of the carbon dioxide and the nitrogen oxide, and then convert these gases into nitrates and nitrites. The nitrates and nitrates, in turn, are fertilizers used by the duckweed species as nutrients. Thus, no nitrogen oxide, which is a pollutant, is entrained in the gas and no disposal or regeneration of catalysts is necessary. The system for converting duckweed into biogas is thus a closed loop and sustainable system.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for producing electricity involving providing a polyculture of aquatic plants to a growth system; continuously providing water, carbon dioxide, air and nutrients to the polyculture contained within the growth system; growing the polyculture for a time sufficient to produce an aquatic plant-based biomass; withdrawing a portion of the aquatic plant-based biomass contained within the growth system; treating the withdrawn aquatic plant-based biomass to produce a biogas; and combusting the biogas in an internal combustion engine.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic block diagram of growth system for growing polyculture.    Growth system (10)    Growing tray (12)    Blower (14)    Air stream (16)    Injection point (18)    Air/carbon dioxide stream (20)    Water stream (22)