The Valsequillo reservoir, located near the city of Puebla, Mexico, is a hard-water eutrophic subtropical system, with minimum temperatures in winter (November-December), and marked dry-rain seasons with fluctuating depth. The reservoir has been infested with water hyacinth for over three decades. A management program involving the use of triturating machines was applied from December 1996 to February 1997. After trituration, remains were allowed to settle to the bottom. The purpose of this study was to monitor the changes in the water quality and the biological communities before and after physical control of weeds. A monthly sampling of surface water was performed at four stations one year before the treatment. After trituration, one year sampling was also carried out. Variables measured included temperature, Secchi disk transparency, depth, pH, dissolved oxygen, oxygen saturation, hardness, nitrate content, nitrite content, ammonia, orthophosphates, and numerical abundance of phytoplankton, zooplankton, and nekton.Weed control affected changes in all variables measured, as a result of residual decomposition of triturated matter. Secchi transparency and oxygen levels decreased and pH became slightly more alkaline. More important changes occurred for nutrients. Orthophosphate concentration increased, for nitrate and nitrite, increase was about 320% and 450% respectively. Ammonia reached lethal values for at least four months after trituration. As a result, phytoplankton decreased initially, and when it flourished again, the Bacillariophyta were replaced by Cyanophyta. Euglenophyta were important in both years. Of zooplankton, calanoids decreased, but cyclopoids and cladocerans maintained similar numbers, although the latter group changed in composition in that Ceriodaphnia was replaced by Moina. Fish disappeared from the system after weed trituration. In the second year a small recovery of water quality occurred, but water hyacinth also started to develop again. At present, Valsequillo is again covered by water hyacinth.
