COS 272-3 - Wetland ecohydrology in the Anthropocene: the use of stable isotopes (δD and δ18O) to determine plants’ spatio-temporal water dynamics in a coastal urban wetland in Puerto Rico

Abstract: Coastal wetland ecohydrology derives from a spatiotemporal synergy among local/catchment weather conditions, historical anthropic modifications, freshwater inputs, and seawater inflow. We used the isotopic technique for determination of natural abundance of δD and δ¹⁸O as it differentially discriminates from evaporation and/or salinity. The study objectives were to determine climate variability effects on the water sources, marine-terrestrial connectivity, spatio-temporal mixing dynamics and plant water uptake in a tropical wetland. Historical hydrological modifications in the Ciénaga las Cucharillas in northeastern Puerto Rico, modified its marine connectivity. Monthly samples were collected from March to November 2021. Water was collected from in-situ precipitation, superficial subsurface inflow from the microbasin, sea water from the San Juan Bay, 0-10 cm surface soil, as well as from phreatic level and 2.5m depth in 10 monitoring wells in the study area. Laguncularia racemosa (mangrove), Achrosticum danaeifolium (fern) and Urochloa mutica (grass) were sampled for xylem water determinations. Precipitation events rather than seasonal effects determined water sources' stable isotopes dynamics where evaporation and salinity determined isotopic concentrations. Marine-terrestrial connectivity was shown in the deeper sub-subsurface samples and in phreatic water during dry events. There was a spatio-temporal mosaic rather than a linear gradient inland, where distance to the channel bringing water from the microbasin and deep-surface inputs from underground natural channels determine salinity in the wetland. Phreatic water (δD: -5.3 to 5.8; δ¹⁸O: -1.6 to 0.5) is the main water source of L. racemosa (δD: -24 to 3.6; δ¹⁸O: -2.7 to 7.7), while A. danaeifolium (δD: -28.8 to 23.3; δ¹⁸O: -3.1 to 10.6), and U. mutica (δD: -25.5 to 11.3; δ¹⁸O: -3.8 to 2.8), takeup water from the surface soil (δD: -20.4 to 17.6; δ¹⁸O: -3.4 to 2.4). Expected drier conditions in the Caribbean will modify species composition due to freshwater water source competition and salinity effects.