Effects of Vegetation Control by Grass Carp on Selected Water-Quality Variables in Four Florida Lakes

Abstract

Grass carp Ctenopharyngodon idella (50/hectare) were introduced into three central-Florida lakes with dense infestations of the aquatic macrophyte hydrilla Hydrilla verticillata; aquatic vegetation and water quality were monitored for 43 months. They also were introduced (61/hectare) into a north-Florida reservoir with dense populations of Illinois pondweed Potamogeton illinoensis and Eurasian watermilfoil Myriophyllum spicatum; aquatic vegetation and water quality were monitored for 48 months. In the three central-Florida lakes, the hydrilla has remained absent for 6 years and is possibly eradicated. The only remaining plants are either in very shallow water, woody plants, or unpalatable species such as spatterdock Nuphar luteum. In the north-Florida reservoir, the Illinois pondweed was eliminated 2 years after the initial stocking and the Eurasian watermilfoil was greatly reduced 3 years after stocking. The reduction of Eurasian watermilfoil lasted 3 years and the Illinois pondweed remained absent 8 years after grass carp were stocked. Turbidity increased in all four lakes; chlorophyll α decreased significantly in three lakes and exhibited no significant changes in the fourth. Three lakes exhibited long-term increases in nutrient-related variables: Kjeldahl nitrogen in two central-Florida lakes and the north-Florida reservoir, orthophosphorus in two central-Florida lakes and total phosphorus in one central-Florida lake. The least-developed (for human habitation) and deepest central-Florida lake experienced initial increases in Kjeldahl nitrogen, orthophosphorus, and nitrate-nitrogen, but these variables returned to baseline levels in subsequent years. Nitrate nitrogen decreased in the north-Florida reservoir and one central-Florida lake. The degree of lake enrichment by grass carp feeding activities probably is related to nutrient loading, water depth, and the reduction of the nutrient buffering capacities of the submersed and marginal plant communities. Received February 5, 1982 Accepted August 4, 1983