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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. [ABSTRACT FROM AUTHOR]

Giant salvinia (Salvinia molesta Mitchell, Salviniaceae) is a free-floating fern native to Brazil that alters natural processes in aquatic ecosystems, causing ecological problems for water resource managers. Due to its ease of dispersal through fragmentation and rapid growth, giant salvinia can quickly become problematic, requiring persistent management efforts. If left unmanaged, giant salvinia creates a thick mat of vegetation, which limits sunlight penetration, reducing aquatic macrophytes, lowering dissolved oxygen and stressing aquatic organisms. Giant salvinia was first reported in Lago Las Curias, Cupey, San Juan, Puerto Rico, USA, in 2016 and covered the entire 19 ha waterbody in 2018. In 2019, we conducted surveys to examine the extent of the invasion and quantify ecological damage to the aquatic environment. We found that the lake is lacking in dissolved oxygen, with a mean concentration of 0.38 mg L-1, and mean concentrations of nitrate, ammonium, and phosphate were < 1 mg L-1. Mean pH in 2019 was 6.4, down from 7.6 in years prior to giant salvinia. We estimated total fresh giant salvinia biomass to be 3,449 metric tons and dry biomass to be 51 metric tons. Other floating vegetation associated with the giant salvinia mat included Pistia stratiotes, Pontederia crassipes, Typha sp., Oxycaryum cubense, Cyperus esculentus and Mikania sp. Uncontrolled growth of giant salvinia impeded access to the waterbody and degraded resource quality in the aquatic ecosystem. The lack of dissolved oxygen and decreasing pH creates conditions for sedimentary nutrients release, creating a potential internal loading of phosphorus. To control an infestation of this magnitude we suggest biological control with the salvinia weevil (Cyrtobagous salviniae) which has proven successful in controlling giant salvinia, and at a fraction of the cost of other control methods. Ultimately, an integrated management plan, including chemical and mechanical control, needs to be established to control giant salvinia and other invasive freefloating plants, and monitor the restoration of ecological services at Lago Las Curias. As a long term goal, nutrient fluxes into the lake must be reduced by eliminating non-point sources of pollution throughout the watershed. [ABSTRACT FROM AUTHOR]