Nutrient dynamics in submarine groundwater discharge through a coral reef (western Lombok, Indonesia)

Submarine groundwater discharge (SGD) from tropical volcanic islands locally transports significant amounts of terrestrial nutrients into the coastal ocean, which can affect sensitive coastal ecosystems such as coral reefs. In northwestern Lombok, terrestrial submarine groundwater discharges through several submarine springs at the seafloor into a coral reef. In order to understand the transport mechanisms of nutrients from land into the reef, we investigated nutrient fluxes via SGD within the area using a combination of different methods, including recharge estimates in the coastal catchment area, echosounder profiling of the seafloor, discharge measurements, and hydrochemical analyses (stable isotopes of water, radon, and nutrients). Different types of springs can be distinguished based on their morphology. One spring showed a crater-shaped structure while other springs consisted of fissures. Recharge rates in the coastal catchment amounted to 5 L s(-1), while discharge rates from the largest spring were 5 L s(-1) for total SGD and 2.5 L s(-1) for fresh SGD, which is similar to discharge rates from smaller onshore springs at the slopes of a volcano. SGD was in general a source of nutrients such as dissolved silicon (similar to 226 mol d(-1)), phosphate (0.61 mol d(-1)), or total dissolved nitrogen (13.4 mol d(-1)). Nitrate concentrations varied among the different springs, probably due to different sources in the hinterland, transformations (e.g., denitrification), or biological uptake during transport from land into the ocean. As nitrate can considerably affect coral reef health, our findings suggest that especially nitrate should be monitored around submarine springs in Indonesia.