Anthropogenic nutrient loading is well recognized as a stressor to coastal ecosystem health. However, resource managers are often focused on addressing point source or surface water discharge, whereas the impact of submarine groundwater discharge (SGD) as a nutrient vector is often unappreciated. This study examines connections between land use and nutrient loading through comparison of four watersheds and embayments spanning a gradient of human use impact on Tutuila, a high tropical oceanic island in American Samoa. In each study location, coastal radon-222 measurements, dissolved nutrient concentrations, and nitrogen isotope values (δ¹⁵N) in water and in situ macroalgal tissue were used to explore SGD and baseflow derived nutrient impacts, and to determine probable nutrient sources. In addition to sampling in situ macroalgae, pre-treated macroalgal specimens were deployed throughout each embayment to uptake ambient nutrients and provide a standardized assessment of differences between locations. Results show SGD-derived nutrient flux was more significant than baseflow nutrient flux in all watersheds, and δ¹⁵N values in water and algae suggested wastewater or manure are likely sources of elevated nutrient levels. While nutrient loading correlated well with expected anthropogenic impact, other factors such as differences in hydrogeology, distribution of development, and wastewater infrastructure also likely play a role in the visibility of impacts in each watershed.

Submarine groundwater discharge (SGD) is an important phenomenon that governs hydrological cycles at the land‐to‐ocean transition zone. SGD manifests as cold and buoyant freshwaters influx from the seafloor to the water column that contains carbon, nutrients, metals, and greenhouse gases, altering coastal areas' oceanographical and biochemical properties. Here, we present electromagnetic imaging of large‐scale freshwater plumes in high‐resolution, offshore west Hawai'i. Electrical resistivity models detect multiple vertical freshwater plumes extending from the seafloor to the ocean surface. Additionally, our models image extensive spatially distributed surface freshwater. The resistivity of these plumes and surface freshwater ranges from ∼1 to 30 Ωm. Resistivity‐to‐salinity calculation indicates a plume‐scale salinity range of ∼0.3–9.9, containing up to 87% of freshwater. Our results imply that substantial volumes of freshwater occupy water column plumes in Hawai'i. This study offers a new and effective method to elucidate hydrogeologic and ocean processes affecting biogeochemical cycles in coastal waters worldwide.

To understand community impacts and needs after the August 2023 Maui wildfires, we conducted a rapid survey-based field investigation two weeks after the incident.