The tropical island of Puerto Rico contains 39 water reservoirs purposed towards raw water storage, treatment, and distribution. The modification of the river continuum via dam construction affects hydrological connectivity, water physiochemistry, transport of organic matter and sediments, migration of native amphidromous and catadromous species, and trophic structure. Reservoirs formed by dams without spillway discharge cannot support the migration of native shrimp or fish resulting in a nearly complete removal from upstream reaches. Out of the 46 fish species present in the island’s streams, only 9 are native. Historically, Centrarchidae, Cichlidae, and Poecillidae fish have been introduced as a food and recreational resources, while flood events have led to the accidental spread of aquaculture grown crayfish. However, it’s also recognized that people deliberately release crayfish for greater fishing opportunity. Non-native organisms can create niche competition with native species, introduce pathogens, diseases, withstand changes in water quality, and increase sediment movement. Reservoir sedimentation and potential pollution sources, combined with an increasing need for potable water storage have resulted in proposals for dredging systems like the water regulating reservoir Lago Regulador de Isabela (LRI). Located in the Northwest Karst region, LRI receives raw water from Lago Guajataca reservoir and irrigation canals, but besides outputs to water treatment facilities, the reservoir lacks migration pathways for aquatic organisms. The purpose of this study is to conduct a biological and limnological characterization of LRI via specimen collection and identification and in-situ physiochemical measurements. Introduced fish and crayfish are expected to dominate assemblages, while decreased oxygen in benthic waters will lead to the presence of species tolerant to decreased water quality. A lack of documented information about LRI presents a novel opportunity to produce necessary information for future management decisions geared towards water quality and biodiversity conservation of systems on which local communities depend on.