MECHANICAL AQUATIC WEED MANAGEMENT IN THE LOWER VALLEY OF THE RIO-NEGRO, ARGENTINA

A major irrigation system in the Lower Valley of the Rio Negro, Argent ina, has been invaded by aquatic plants, with Potamogeton illinoensis Morong dominant in irrigation channels and Potamogeton pectinatus L. d ominant in drainage channels. Although several other macrophytes are p resent, problems are largely caused by the dominant species. Results a re presented for plant biomass response to weed control treatments usi ng a chain-cutting method in the principal irrigation channel of the s ystem. Peak above-ground biomass of Potamogeton illinoensis was reduce d by about 38% by this physical control regime. The treated population s regrew rapidly after spring clearance, but did not regrow after subs equent mid- and late-season clearance operations, even though untreate d population biomass remained high during this period. The highest den sity of Potamogeton illinoensis ramets was found in treated areas. Cha in-cutting produced no discernible effect on dissolved oxygen, water t emperature, water conductivity, pH or light extinction coefficient com pared with untreated check sectors of the channel.