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Pelagic Sargassum beaching events are a global environmental issue that have caused increasing detrimental effects to coastal ecosystems since the 1970s. The 2015 pelagic Sargassum bloom in the Atlantic and the Caribbean caused significant impacts to coastal ecological communities in these areas. In enclosed systems, such as bays and lagoons, the Sargassum impacts could be particularly problematic due to the longer water residence times. An example of such an ecosystem is Laguna Grande bioluminescent lagoon, Puerto Rico. This lagoon slowly exchanges water with the ocean through a narrow channel (average about 5 m across), which results in the accumulation of the bioluminescent dinoflagellate Pyrodinium bahamense and other planktonic organisms. In this study we describe the 2015 pelagic Sargassum accumulation event in Laguna Grande, and report associated changes in the phytoplanktonic composition. Before this event, other studies conducted between 2003 and 2013 reported mean densities of P. bahamense ranging from 14,492 to 28,202 cells/L. However, the P. bahamense population was absent from the lagoon during the 2015 Sargassum influx. After the entrance of large quantities of pelagic Sargassum into the lagoon, accompanied with other marine algae and sea grasses, the macrophytes remained inside and began to decompose. Thirteen phytoplankton taxa not previously observed inside the lagoon were detected after the entrance of the macrophytes. We hypothesize that the changes in the phytoplanktonic composition in the lagoon were caused by altered water quality conditions following the entrance of the pelagic Sargassum and other macrophytes. [ABSTRACT FROM AUTHOR]