Limnological Response of Las Curias Reservoir, San Juan, Puerto Rico: Successful Management of the Invasive Aquatic Fern, Salvinia molesta

Abstract

The anthropogenic deterioration of aquatic ecosystems affects water resources due to agricultural malpractices, pollution from domestic septic tanks, recreational activities, and poor watershed management, among other factors. This study examines the management of Las Curias Reservoir, San Juan, Puerto Rico, after the 2016 arrival of the invasive aquatic fern Salvinia molesta. In September 2019, a community-led initiative introduced the Cyrtobagous salviniae weevil, an effective biological control agent for S. molesta, and commenced a mechanical removal campaign using an aquatic harvester. Limnological sampling (September 2019 to September 2022) and drone flights were employed to measure physicochemical and floating plant cover changes, respectively, in the reservoir. Monitoring of weevils in the reservoir demonstrated rapid establishment and dispersal, which resulted in visible damage including browning of plants and eventually sinking of entire mats. From 23 July 2019, the reservoir surface was predominantly covered by salvinia, occupying an area of 17.7 ha (100% coverage). This coverage decreased to 12.6 ha (71%) by 29 January 2021. By 12 August 2022, the coverage had been substantially reduced to just 1.1 ha, representing only 6% of the reservoir surface. In 2022, the reservoir recorded an average dissolved oxygen concentration of 2.4 mg L−1 (±0.0, n = 144), the highest in the study period and indicative of ecosystem recovery. After three years of control efforts, dissolved oxygen, pH, and specific conductance returned to levels recorded prior to Salvinia molesta introduction. This ecosystem recovery, a first in Puerto Rico, could be attributed to early use of mechanical control and the long-term impact of biological control.