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.

Coastal groundwater-dependent ecosystems and associated species, including native macroalgae, are highly adapted to the lowered salinity and nutrient subsidies of natural flows of submarine groundwater discharge (SGD). However, with climate and land-use change-induced shifts in SGD, invasive macroalgae are outcompeting native species, with drastic implications for these important ecosystems. This article contributes to a synthetic understanding of these processes by providing a salient case study from Oʻahu, Hawaiʻi, and by reviewing evidence on the links between SGD and the composition of macroalgal communities. Results point to the critical importance of sound land, water, and wastewater management policies to reduce impacts on SGD and native coastal groundwater-dependent ecosystems, particularly in the context of climate change.

Untreated and minimally treated wastewater discharged into the environment have the potential to adversely affect groundwater dependent ecosystems and nearshore marine health. Addressing this issue requires a systems approach that links land use and wastewater management decisions to potential impacts on the nearshore marine environment via changes in water quality and quantity. To that end, a framework was developed to assess decisions that have cascading effects across multiple elements of the ridge-to-reef system. In an application to Kona (Hawai‘i, USA), eight land use and wastewater management scenarios were evaluated in terms of wastewater system upgrade costs and wastewater related nutrient loads in groundwater, which eventually discharge to nearshore waters, resulting in potential impacts to marine habitat quality. Without any upgrades of cesspools or the existing wastewater treatment plant (WWTP), discharges of nutrients are expected to increase substantially with permitted development, with potential detrimental impacts to the marine environment. Results suggest that converting all of the existing cesspools to aerobic treatment units (ATU) and upgrading the existing WWTP to R-1 quality provide the highest protection to nearshore marine habitat at a cost of $569 million in present value terms. Other wastewater management options were less effective but also less costly. For example, targeted cesspool conversion (a combination of septic and ATU installation) in conjunction with the WWTP upgrade still provided a substantial reduction in nutrients and potential impacts to marine habitat quality relative to the present situation at a price point roughly $100 million less than the entirely ATU option. Of note, results were more sensitive to the inclusion of the WWTP upgrade option than they were to assumptions regarding the efficiency of the cesspool conversion technologies. The model outputs also suggest that the spatial distribution of potential impacts should be carefully considered when comparing different wastewater management scenarios. When evaluated separately, the WWTP option reduced total nutrients by more than the targeted cesspool conversion option at a fraction of the cost. However, potential improvements in marine habitat quality only occurred in the immediate vicinity of the WWTP, whereas the benefits under targeted cesspool conversion were more evenly distributed along the coast.

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.

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