Linking Transpiration to Reef Nitrogen Supply on a Tropical Coral Island

Coral reef islands are biodiversity hotspots with high conservation value, but we have a poor understanding of how island vegetation, through transpiration, influences groundwater nutrient supply to adjacent reef systems. Here we combine stable isotope tracing, geophysical surveys, and satellite analysis to unravel the links between transpiration and the discharge of groundwater nitrate to the waters surrounding a coral reef island (Lady Elliot Island in the Great Barrier Reef). Over a 2-year study period (2020-2021) there was a net loss of freshwater from the island, that is, evapotranspiration (ET) exceeded rainfall, largely caused by high rates of transpiration (73% of ET). Transpiration was higher in forested areas, and groundwater salinity appeared to be higher there. By tracing the nitrogen stable isotope signature of groundwater nitrate into reef organism tissue we were able to map groundwater nitrogen discharge spatially and temporally. Groundwater nitrogen discharge was focussed on one side of the island and did not vary seasonally as expected, despite reduced rainfall and seabird guano inputs over the austral winter/spring. Based on our results, we propose that transpiration by island vegetation slows groundwater flow and concentrates nitrate in the groundwater being released to the surrounding reef system. High concentrations of nitrate in groundwater (up to 27 mmol L-) were observed on Lady Elliot Island and these seem to have increased since 2014, but further work is required to understand if this is a normal scenario for tropical coral islands or the result of the islands' revegetation program. Coral reef islands are biodiversity hotspots that are considered to be biological refuges against climate change. As such there is a concerted effort to fully understand and adaptively manage these ecosystems. However, we lack fundamental information on how the presence of vegetation, and specifically transpiration, influences the nutrient biogeochemistry of tropical islands. This is important as coral reefs are considered to be susceptible to nutrient availability. Here we combine a range of biogeochemical techniques to unravel the links between transpiration and the discharge of groundwater nitrogen to the waters surrounding a coral reef island (Lady Elliot Island in the Great Barrier Reef). We find that high rates of transpiration across seasons alters the nutrient dynamics of the aquifer. In particular we propose that transpiration slows groundwater flow and concentrates nitrate in groundwater being released to surrounding reefs. It is not clear whether this process is part of a natural cycle or if it is influenced by the active revegetation program on the island. Future work should try to address this knowledge gap as this has important implications for the continued management of these critical ecosystems. Transpiration by island vegetation appears to slow groundwater flow and concentrate nitrate in groundwater It is not clear whether transpiration drives long term increases in groundwater nitrate concentrations or discharge to the local reef The relationships between revegetation, transpiration, and groundwater nitrogen dynamics need to be addressed further