Risk to native marine macroalgae from land-use and climate change-related modifications to groundwater discharge in Hawai'i

Coastal groundwater-dependent ecosystems benefit from lowered salinity, nutrient-rich submarine groundwater discharge (SGD). Across Pacific islands marine macroalgae appear to have been challenged by and adapted to the stress of lowered salinity with a trade-off of nutrient subsidies delivered by SGD. Human alterations of groundwater resources and climate change-driven shifts brought modifications to the magnitude and composition of SGD. This paper discusses how native macroalgae have adapted to SGD nutrient and salinity gradients, but that invasive algae are outcompeting the natives near SGD with nutrient pollution. It is important to re-evaluate land and water use practices by modifying groundwater sustainable yields and improving wastewater infrastructure to keep SGD reductions minimal and nitrogen inputs in optimal ranges. This task may be particularly challenging amidst global sea level rise and reductions in groundwater recharge, which threaten coastal groundwater systems and ecosystems dependent on them.