Late-Holocene paleoenvironmental history of bioluminescent Laguna Grande, Puerto Rico

Coastal lagoons are valuable natural resources and ecotourism hotspots that are at great risk due to human development and global climate change. In order to protect these resources it is important that we fully understand the paleoenvironmental history of these unique ecosystems and their sensitivity to environmental change. Here we present a multi-proxy (pollen, charcoal, grain-size, dinoflagellate cyst, stable isotope geochemistry, and foraminifera) 3600 cal yr B.P. record of paleoenvironmental change for Laguna Grande, Puerto Rico, which is a particularly unique coastal lagoon as it contains high abundances of the bioluminescent dinoflagellate Pyrodinium bahamense. Sedimentary evidence indicates that the Laguna Grande basin began accumulating sediment ~ 3600 cal yr B.P. coincident with regional sea level rise, but the modern lagoon itself is geologically quite young and did not become fully enclosed until ~ 575 cal yr B.P. due to longshore depositional processes and mangrove swamp development. Foraminifera assemblages suggest a potential breach of a protective barrier ~ 1550 cal yr B.P. perhaps as a result of a tropical storm or tsunami and is coincident with an increase in fire activity. Only two high-energy depositional units (tempestites) could be identified in the lagoon sediments over the last 575 cal yr B.P. indicating minimal impact by tropical storms or tsunamis on the lagoon since isolation from the open ocean. Fossil dinoflagellate cyst concentrations and influxes do not approach modern levels until ~ 200 cal yr B.P. and are currently at the highest levels in the history of the lagoon, indicating the lagoon may have become bioluminescent only recently, perhaps as a result of anthropogenic nutrient additions. Sedimentation rates support this hypothesis indicating a recent tripling of sedimentation rates when compared to millennial-scale sedimentation rates. This study highlights the utility of paleolimnological data in the management and preservation of at-risk coastal lagoon ecosystems.