Effects of Multiple Drivers of Environmental Change on Native and Invasive Macroalgae in Nearshore Groundwater Dependent Ecosystems

Abstract

Groundwater dependent ecosystems (GDE) are increasingly recognized as critical components of sustainable groundwater management, but are threatened by multiple drivers of environmental change. Despite this importance, data that link drivers of hydrologic change to GDEs are scarce. This study adapts a land-sea modeling framework by calibrating marine models with macroalgal experiments to quantitatively assess impacts of climate and land use change on submarine groundwater discharge (SGD) and subsequent habitat suitability for a native (Ulva lactuca) and an invasive (Hypnea musciformis) macroalgae in nearshore GDEs in Kona, Hawai'i. Lab analyses demonstrate that while U. lactuca grows optimally in low-salinity, high-nutrient waters, H. musciformis appears constrained to a salinity threshold and exhibits low growth in low salinity despite high nutrient concentrations. Land-sea model results predict that while a dry future climate (Representative Concentration Pathway 8.5 mid-century) coupled with increased urban development will likely reduce SGD, protecting native forests may prevent further loss of SGD quantity. This prevention thus partially mitigates the decline in habitat suitability of U. lactuca due to the combined effects of climate and land use change. Findings also suggest that, in contrast to the native U. lactuca, reductions in SGD may favor H. musciformis growth if introduced to Kona. Collectively, this study demonstrates the importance of considering multiple drivers of environmental change on GDEs. This study bridges experiments with models to spatially map changes in species abundance beyond their current habitat conditions, and thus informs management actions that can explicitly incorporate future human and climate-related impacts.