Ofu Island, part of the Manuʻa Islands in American Samoa, is in the South Pacific Ocean and is surrounded by a fringing reef ranging from 80–180 m in width. Our study sites were situated on the southern reef, within the Ofu island unit of NPSA ( Figure 1 ). Previous research on the south shore of Ofu Island (Oliver and Palumbi, 2009) has described the reef flat as having a series of semi-independent, shallow, sand-filled depressions or pools, wherein each pool exhibits greater water exchange with the offshore ocean via depressions in the reef flat than with adjacent pools. The reef flat pools of Ofu Island support high biodiversity, including nearly 300 fish species and 80 coral species, several of which are listed under the U.S. Endangered Species Act (Craig et al., 2001) and contains one of the best preserved Indo-Pacific coral-reef ecosystems in the U.S. National Park system (National Park Service, 2017). The reef also provides essential coastal protection by buffering wave energy and reducing beach erosion (Storlazzi et al., 2019). This study focused on two pools within the NPSA’ Ofu island unit: Papaloloa Point, located near the island’s airstrip (previously designated as Pool 200), and Fattuana Point, situated approximately 800 m northeast of Papaloloa Point (previously designated as Pool 300). Ofu Island consists predominantly of thin-bedded basaltic lava with high permeability (Davis, 1963) and represents the erosional remnant of a larger volcanic island that was bisected into Ofu and Olosega Islands by fluvial and marine erosion. The geology of the beach-ridge deposits where this study took place is characterized by homogeneous unconsolidated calcareous sand that was deposited during the seaward progradation during the past 2–3 kyr B.P. (Kirch and Hunt 1993).

Although a few spring-fed streams exist on Ofu Island, they percolate rapidly into the underlying volcanic bedrock. At sea level, the highly permeable basaltic substrate hosts a substantial volume of fresh groundwater, forming a thin freshwater lens within a high-recharge aquifer (Davis, 1963). The lens is likely thickest in the island’s interior of Ofu Island and thinnest at the coast, where abundant groundwater discharge may occur. The absence of a significant coastal plain, and steep slopes along the coast facilitate the direct discharge of groundwater from the freshwater lens into the Pacific Ocean. Coastal springs are readily identifiable along the present shoreline during spring low tides within the study area (Vaeoso, 2022). Early hydrological investigations of Ofu Island (Davis, 1963) revealed that during periods when upland springs—traditionally the primary source of drinking water—failed to meet demand, residents were compelled to rely on abundant SGD springs, locally known as vai puna o le mata o le vasa, underscoring their critical cultural and historical significance. This historical reliance is further exemplified by the Toʻaga archaeological site, which spans nearly three millennia of Samoan history and features a deep sequence of ceramics dating back over 3000 yr, when the beach shoreline was positioned further inland (Kirch et al., 1990; Anderson, 1995). The proximity of the Toʻaga site (<1 km to the east) to present SGD springs at Fattuana Point indicates that these springs may have been the only readily accessible water source for its former occupants. Similar cultural valuations of freshwater sources have been documented in other Pacific regions, such as Molokaʻi, Hawaii (Oberle et al., 2023), where SGD springs were also used for drinking water during times of water shortages.

Hydrodynamic processes within the reef flat pools are influenced by a combination of tidally modulated wave-forced flows and longer-period (>100 s) infragravity waves propagating across the reef flat (Cheriton et al., 2024). Ofu Island has a microtidal regime; at low tide, flow is focused from reef crest into the pools, whereas at high tide, flow is predominantly seaward from the pools (Maticka et al., 2022; Cheriton et al., 2024). Ofu Island is sparsely populated, with the main village located on the western shore, approximately 2.2 km from Papaloloa Point pool study site. Key infrastructure at Papaloloa Point includes a small airport with associated government buildings, Va’oto Lodge and its existing cesspool, and the NPSA Ofu Ranger Station ( Figure 1 ). Ofu Island has one active well, managed by the American Samoa Power Authority (ASPA) Well No. 202. Additionally, a small wetland, Va’oto Marsh, is situated adjacent to the airport and several government facilities.

In February 2020, the U.S. Geological Survey (USGS) conducted a field campaign at the Papaloloa Point and FatTuana Point reef flat pools within the Ofu island unit of NPSA ( Figures 1 , 3 , 4 ). The study aimed to quantify SGD and its role in nutrient delivery and possible impact on the reef health.

Spatial ERT measurements from our study represent the first application of this technique on Ofu. Given the relative uniformity of the shoreline and coastal plain geology (Kirch and Hunt, 1993), variations in ERT are assumed to be predominantly driven by changes in salinity (Oberle et al., 2017; Buquet et al., 2016; Johnson et al., 2015). The ERT profiles delineate a distinct nearshore freshwater layer at Papaloloa Point, a feature that is nearly absent at Fattuana Point ( Figure 2 ). Additionally, the profiles reveal a depth-dependent increase in salinity, with the saltwater interface predominantly occurring at mean sea level. The observed freshwater saturation at mean sea level is consistent with the presence of tidally driven SGD springs. In particular, the shore-perpendicular ERT profile at Papaloloa Point Center ( Figure 2A ) documents a thin freshwater lens (~1 m thick) extending toward the main SGD spring as reported below.

UAS-TIR imagery identified cooler sea-surface temperatures in the Papaloloa Point and Fattuana Point pools, confirming active SGD ( Figures 3 , 4 ). Near the SGD point sources captured in the UAS-TIR surveys, YSI transects recorded a 3–4 unit decrease in salinity ( Figures 3B , 4B ). Based on UAS-TIR imagery, the estimated SGD plume areas were 750 m², 200 m², and 400 m² at Papaloloa Point Center, Papaloloa Point East, and Fattuana Point, respectively, but the imagery also identified spatial heterogeneity at the meter scale. The difference between the average temperature in the SGD plumes and surrounding waters ranged from 2°C to 4°C.

The radon buoy data at Papaloloa Point exhibited a classic inverse relationship between tidal stage and ²²²Rn concentrations, indicating increased groundwater discharge during low tide when hydraulic gradients between coastal groundwater and seawater were greatest ( Figure 5 ). At Fattuana Point, however, there was a 2–3 hr lag between the minimum water level at low tide and maximum ²²²Rn concentrations. The highest mean SGD rates were observed at Papaloloa Point Center, followed by Fattuana Point, and Papaloloa Point East ( Table 1 ). Maximum SGD rates at the point sources ranged from 0.15 m d^-1 to 2.90 m d^-1. Within 40 m offshore, SGD rates decreased by 4% at Papaloloa Point Center and 54% at Fattuana Point, whereas SGD was undetectable offshore at Papaloloa Point East ( Table 1 ).

The SGD-influenced area delineated with the UAS-TIR ( Figures 3 , 4 ) was multiplied with radon-derived advection rates to calculate volumetric SGD rates ( Table 1 ). Following a similar pattern to SGD rates reported above, the highest SGD flux occurred at Papaloloa Point Center ( Table 1 ), with a maximum flux of 2,175 m³ d^-1, whereas fluxes at Papaloloa Point East and Fattuana Point were an order of magnitude lower ( Table 1 ). Although SGD fluxes diminished with distance from shore, corals within 40 m of point sources at Papaloloa Point Center and Fattuana Point nonetheless receive input from SGD as documented from the flux measurements made at the offshore buoy locations ( Table 1 ).

Nutrient concentrations were lowest in fore-reef water samples compared to all other sample types ( Table 2 ). Elevated SiO₄ ^4- levels in the SGD, Va’oto Lodge cesspool, and ASPA Well No. 202 samples indicated freshwater movement through the basaltic aquifer (Stefánsson and Gíslason, 2001), whereas lower SiO₄ ^4- concentrations in the Va’oto Marsh indicated contributions from other freshwater sources (e.g., rainwater). The cesspool sample contained the highest PO₄ ^3- and NH₄ ⁺ concentrations; however, NH₄ ⁺ was minimal in the SGD samples. PO₄ ^3- concentrations in the SGD and marsh samples were lower than in the cesspool. In contrast, N+N concentrations were elevated in the cesspool, SGD, and ASPA Well No. 202 samples ( Figure 6A ). Excluding NH₄ ⁺, nutrient concentrations correlated significantly (Pearson-product correlation coefficient (r) at p ≤ 0.05) with temperature and salinity (Prouty, 2025), with correlation coefficients ranging from -0.65 to -0.95 and -0.72 to -0.98 ( Supplementary Figure 1 ), respectively supporting the hypothesis that SGD is the primary pathway for nutrient delivery to the pools. Nutrient concentrations at higher salinities are close to oceanic levels, reflecting the effectiveness of physical mixing (e.g., dilution) on nearshore nutrient concentrations (Bienfang et al., 2009). The δ¹⁵N_NONO3 values from select SGD samples (N+N > 4 μmol L^-1) ranged from 5.7‰ to 9.8‰, overlapping with ASPA Well No. 202-water δ¹⁵N_NONO3 values ( Table 3 ). In contrast, the cesspool sample was ¹⁵N-depleted (δ¹⁵N_NONO3 = -9.2 ± 0.7‰), indicating nitrification (microbial NH₄ ⁺ oxidation to NO₃ ^-) ( Figure 6B ). Mean δ¹⁵N (± SD) values in macroalgae were 5.9 ± 0.6‰ and 3.7 ± 1.3‰ at Fattuana Point and Papaloloa Point, respectively ( Table 3 ). Water sample δ¹⁸O_NONO3 values ranged from -1.8 to 4.8‰, with the highest values in the cesspool ( Table 3 ). The slope between δ¹⁵N_NO and δ¹⁸O_NONO3 at both pools was less than 0.5, indicating minimal, if any fractionation due to denitrification (Kendall et al., 2007).

Nutrient fluxes were estimated using mean SGD fluxes and SGD nutrient concentrations normalized to the salinity of groundwater to obtain an unmixed SGD endmember. This approach was necessary because SGD is a mixture of recirculated seawater and freshwater (e.g., Peterson et al., 2009), as confirmed by the high variability in salinity within the SGD samples ( Table 2 ). Despite higher SiO₄ ^4- and N+N concentrations in the Fattuana Point SGD point-source samples, nutrient fluxes at Papaloloa Point Center were nearly 10 times greater due to higher SGD rates ( Table 4 ).

The field measurements revealed that the SGD plumes exhibit variable spatial extents across each pool. Notably, the largest plume was detected by infrared imaging (i.e., UAS-TIR) at Papaloloa Point Center, whereas comparatively smaller plumes were observed at Papaloloa Point East and Fattuana Point. Differences in the spatial extent of these SGD plumes is supported by a range of SGD fluxes at each point source, and the ERT observations that demonstrate different inland freshwater heads. These differences indicate that subsurface hydrological dynamics strongly influence plume development.

Fresh, cooler water discharging from coastal springs was detected in both Papaloloa Point and Fattuana Point, with a higher flux of SGD and nutrients at Papaloloa Point, particularly at Papaloloa Point Center as discussed above. However, the higher nutrient concentrations at Fattuana Point may indicate variations in subsurface filtration prior to discharge, potentially influenced by differences in sediment composition and hydraulic conductivity (Burnett et al., 2006; Moore, 2010). The permeable calcareous sand and gravel at Papaloloa Point likely function both as a nutrient sink and a freshwater reservoir, facilitating greater groundwater storage and nutrient attenuation through adsorption and microbial processes (Bokuniewicz, 1995). In contrast, the absence of a similar coastal plain at Fattuana Point would result in more limited filtration capacity and reduced groundwater residence times, leading to higher nutrient concentrations in SGD. The observed differences in response times between tidal fluctuations and SGD variations reinforce the conclusion that the hydrogeologic settings at the two sites are distinct. The 2–3-hr time lag at Fattuana Point indicates that SGD originates from an aquifer that has lower diffusivity, indicating a lower permeability and a smaller local storage (Li and Jiao, 2003; Robinson et al., 2006).

In comparison, the coastal plain at Papaloloa Point may act as a highly permeable aquifer that can exhibit a more immediate response to tidal forcing along the coast (Bokuniewicz, 1992). This is supported by the near-synchronous fluctuations in SGD observed at Papaloloa Point, which align closely with tidal cycles ( Figure 5 ). Results from the ERT data are consistent with this interpretation, demonstrating high resistivity and a diffuse discharge zone near Papaloloa Point Center and a smooth boundary between the surface and saturated zone ( Figure 2A ). At Papaloloa Point, and especially at Papaloloa Point Center, a thicker and more extensive freshwater lens formed, likely because of the presence of a coastal beach platform composed of several meters of Pleistocene to Holocene calcareous sediment (Davis, 1963). This configuration promotes freshwater saturation at mean sea level and facilitates the formation of tidally driven SGD springs ( Figure 5A ). Additionally, the greater hydraulic tidal connectivity at Papaloloa Point may make it more vulnerable to saltwater intrusion, particularly under conditions of reduced freshwater input or sea-level rise (Michael et al., 2013). This intrusion could contribute to the dilution of nutrient concentrations and an overall increase in salinity, as indicated in the field data ( Table 2 ). In contrast, at Fattuana Point, in the absence of such a coastal beach platform, SGD is directly expelled from the underlying Pleistocene to Pliocene volcanic talus deposits, with only a thin veneer of beach sediment present. The localized high-conductivity (low-resistivity) anomaly and lack of a diffuse, coastal freshwater lens at Fattuana Point ( Figure 2D ) suggest that SGD here is derived from a more inland aquifer, and the increased time required for the tidal pressure to affect the aquifer (Robinson et al., 2006), results in the observed 2–3-hr lag ( Figures 5E, F ).

Recirculated volumetric fluxes from Papaloloa Point Center are within the lower range (168 to 6,115 m³ d^-1) reported in Shuler et al. (2019) from three watersheds on the adjacent island of Tutuila Island with similar geologic structure. The smaller plume at Papaloloa Point East delivered significantly less freshwater and nutrients, with a negligible offshore (~40 m) SGD signal. The observed variability between coastal springs located approximately 130 m apart within a single pool underscores the spatial heterogeneity of SGD along the shore, demonstrating that traditional box-model methods may misrepresent SGD-driven nutrient fluxes. The high local-scale spatial variability in SGD is further supported by UAS-TIR imagery, which captured a non-uniform distribution of cooler water from the springs ( Figures 3 , 4 ). SGD plume size and shape varied with distance from shore but were also influenced by external forces such as currents, waves, and internal discharge dynamics. For example, at Papaloloa Point Center, winds and tides predominantly controlled the SGD plume’s surface area. At Fattuana Point, an asymmetric SGD plume extended westward ( Figure 4 ), further demonstrating that the plume extent and distribution are affected by external hydrodynamic forces. Given these spatial heterogeneities, nutrient fluxes and hydrodynamics must be considered when evaluating the impact of SGD on coral reef health and macroalgal distribution.

Despite variations in nutrient concentration and fluxes, SGD remains the dominant nutrient delivery pathway in both pools, consistent with estimates that SGD contributes nutrient loads 10 to 100 times higher than riverine inputs to many tropical watersheds (Santos et al., 2021). Unlike riverine inputs, which fluctuate with precipitation, tidally driven SGD provides a near-continuous nutrient source to reefs (Nelson et al., 2015; Jiang et al., 2021, Camacho-Cruz et al., 2025).

Although we did not conduct a direct tracer study, nutrient concentrations and isotopic values indicate multiple possible nutrient sources to the pools. Elevated NH₄ ⁺ and PO₄ ^3- levels in the cesspool relative to the ASPA Well No. 202 demonstrate that these nutrients may serve as tracers for cesspool contributions versus pristine freshwater because they were negligible in the ASPA Well No. 202 sample ( Table 2 ). In contrast, SGD N+N could originate from both the cesspool and groundwater, given their elevated concentrations in both sources. Isotopic signatures can provide additional information on origin and transformation processes (e.g., Kendall et al., 2007). The ¹⁵N-depleted cesspool sample ( Table 3 ), combined with high NH₄ ⁺ concentrations, reveal incomplete nitrification (microbial oxidation of NH₄ ⁺ to NO₃ ^-), leading to significant δ¹⁵N fractionation. In some cases, high δ¹⁵N_NONO3 values in SGD can indicate denitrification, a process commonly associated with wastewater treatment and detected in tropical SGD samples (Prouty et al., 2017). However, the cesspool was untreated, and the slope between the SGD δ¹⁵N_NO and δ¹⁵O_NONO3 values does not indicate denitrification. Instead, the SGD δ¹⁵N_NONO3 values are within the range reported from coastal water samples collected from four watershed on Tutuila along a human land-use gradient (Shuler et al., 2019; 7.4 to 10.3‰). These results suggest that the coral-reef ecosystem in these Ofu pools may be more susceptible to elevated nutrient levels than in other parts of American Samoa.

Unlike water samples, algal δ¹⁵N values integrate nutrient sources over time. At Fattuana Point, the mean (± SD) algal δ¹⁵N value is 5.9‰ (± 0.6) and 3.7‰ (± 1.3) at Fattuana Point ( Table 3 ). These values are either within or below the range reported from Tutuila (Shuler, 2019; 5.5 to 9.6‰). Previous studies in the Ofu–Olosega reef flat pools (Garrison et al., 2007) found that oceanic and atmospheric sources, rather than anthropogenic inputs, were the dominant nutrient contributors based on algal δ¹⁵N values. However, results from our current study yield N+N and PO₄ ^3- concentrations that exceed those established by the American Samoa Water Quality Standards for coastal waters, 10.7 and 0.65 μmol L^-1, and can be considered poor condition according to the National Coastal Condition Assessment (NCCA) program (NCCA, 2010). Despite the small population size on Ofu, the SGD point source N+N and PO₄ ^3- values were as high, if not higher, than those reported for moderately to highly impacted (i.e., land-use) watersheds on Tutuila Island (Shuler et al., 2019).

Regardless of nutrient origin, SGD supplies elevated nutrients to the Ofu pools, potentially influencing macroalgal distribution. Notably, V. fastigata was absent from areas of the pools where no SGD was observed, indicating that SGD-derived nutrients may be promoting algal growth. If SGD-derived nutrients were the primary driver of algal proliferation, higher V. fastigata densities would be expected at Papaloloa Point due to its greater SGD flux. However, surveys demonstrate that the algal outbreak was more widespread in Fattuana Point ( Figure 4 ), indicating a more complex relationship between SGD and algal growth. Several factors may help explain this paradox.

Macroalgae require hard substrate for anchoring. In the Ofu reef flat pools, available substrate includes basalt boulders and coral. At Papaloloa Point Center, the reef-flat region immediately surrounding the spring is primarily a large, sand area (>2,000 m²) with very little hard substrate or coral cover, as evident in the aerial imagery ( Figure 1 ). The absence of coral could be due to SGD input or other factors, but the lack of hard substrate likely limits algal establishment. In contrast, Fattuana Point provides more suitable substrate, with higher coral cover and basalt boulders near and to the west of the SGD spring. SGD impacts on coral reef health are also influenced by water motion, particularly residence time. Nutrient exposure depends on nutrient concentration and how long water remains in an area. Differences in residence time may contribute to the observed disparity in algal distributions. Due to lost instruments, precise residence time calculations were not possible. However, temperature differences suggest hydrodynamic variability between sites. Warmer waters at Fattuana Point imply less mixing and longer residence times, whereas cooler temperatures at Papaloloa Point indicate more dynamic conditions and, thus, lower residence times. This aligns with findings from Cheriton et al. (2024), who examined infragravity waves on the southwest Ofu reef flat and found that at Papaloloa Point, moderate infragravity waves (significant wave heights > 0.2 m) occurred nearly 25% of the time, compared to only 13% of the time at Fattuana Point. The higher wave energy at Papaloloa Point likely enhances water mixing and drives increased flow, leading to shorter residence times (Storlazzi et al., 2018). In contrast, the lower wave energy at Fattuana Point suggests longer residence times, thereby allowing SGD-derived nutrients to accumulate over time (Oberle et al., 2019). This would lead to sustained nutrient enrichment, which, when combined with suitable substrate availability, would create favorable conditions for the proliferation of V. fastigiata. In contrast, the more vigorous water mixing at Papaloloa Point likely dilutes nutrient concentrations despite higher SGD fluxes, limiting nutrient retention and the potential for algal overgrowth.