Limnological responses to active management of the invasive aquatic fern <em>Salvinia molesta</em> in Las Curias Reservoir, San Juan, Puerto Rico.

Abstract

The anthropogenic deterioration of aquatic ecosystems affects water resources due to agricultural malpractices, pollution due to domestic septic tanks, recreational activities, poor management of watersheds, and others. For instance, introducing invasive aquatic weeds has been recognized as a significant problem in watersheds worldwide due to their ability to create anoxic conditions. This study focuses on the management of the Las Curias Reservoir in San Juan, Puerto Rico since the arrival of the aquatic fern <em>Salvinia molesta</em> around 2016. Since December 2019, a community driven initiative involving state, federal and academic institutions led to the introduction, in December 2019, of the <em>Cyrtobagous salviniae</em> to the reservoir. This weevil is considered an effective biological control agent for <em>S. molesta</em>. Simultaneously, community members initiated a mechanical removal campaign using an aquatic harvester. Limnological samplings since September 2019 to March 2022 was conducted to measure physicochemical responses in the reservoir in response to the reduction of <em>S. molesta</em> cover. In addition, monthly drone flights were conducted to measure changes in aerial plant coverage. Propagation of <em>S. molesta</em> was aggravated due to eutrophication after an increase in nutrient-rich sewage discharges from septic tanks after hurricane Maria in 2017. By 2019, the reservoir was completely covered by the fern. It is not until August 2020 that we noticed a considerable change in the reduction of plant cover of 43% (100,462 m²). Upon the reduction of plant coverage, we notably found increases for the period of (2019-2022) in the mean water temperature along the epilimnion of 0.96 ºCِ, 1.37 ºC in the thermocline, and 1.05 ºC in the hypolimnion. Mean dissolved oxygen increased 4.17 mg L-1 along the epilimnion, 0.43 mg L-1 in the thermocline, and 0.04 mg L -1 in the hypolimnion. Crucial nutrients concentrations have also increased during this period. The recovery of the Las Curias ecosystem has been the result of combined biological and mechanical controls, applied effectively for the first time in Puerto Rico.