Environmental DNA Reveals the Impact of Submarine Groundwater Discharge on the Spatial Variability of Coastal Fish Diversity

Simple Summary In environments characterized by small-scale islands, terrestrial inputs, like freshwater discharges, can significantly impact fish communities compared to ocean currents. While the influence of river discharge on marine biodiversity is well documented, the effects of submarine groundwater discharge on nearshore fish communities are less understood. Here, our results provide evidence that submarine groundwater discharge crucially influences coastal fish diversity by altering salinity and nutrient supply. This study also showed that fish community composition varies considerably over a small spatial scale, reflecting habitat partitioning on the target island.Abstract Submarine groundwater discharge (SGD) has recently been recognized as an influential factor in coastal ecosystems; however, little research has been conducted on its effects on coastal fish diversity. To investigate the relationship between SGD and fish diversity, we conducted a survey at the coastal island scale using the environmental DNA (eDNA) method. Our findings indicate that fish species richness and functional richness peak at stations with high SGD. Environmental variables, such as salinity, dissolved inorganic nitrogen (DIN) concentration, and SGD, significantly influence fish diversity. Carnivore fish richness was negatively correlated with salinity, while planktivore fish richness was positively correlated. Additionally, SGD and DIN concentrations were found to be crucial in shaping omnivorous and pelagic communities, respectively. This study highlights the role of SGD in enhancing nutrient conditions favorable for diverse fish communities and demonstrates the effectiveness of eDNA metabarcoding for rapid marine biodiversity assessment. These findings provide valuable insights for coastal ecosystem monitoring and management.