Coastal ecosystems on oceanic islands provide critical ecological services to geographically isolated populations. Local residents are heavily dependent on nearshore farms for plant crops and adjacent reefs for protein. Increasing urbanization has made these ecosystems vulnerable to land-based sources of pollution arising from excessive sediment and nutrient delivery that stresses corals and drives persistent and harmful algal blooms [1
]. Terrigenous nitrogen (N) loading, linked to variability in anthropogenic land use, provides a strong control on phytoplankton, turf algae and macroalgae growth in coastal environments [2
]. Excessive algal growth is an economic and environmental concern, as the health of coastal areas is directly linked to tourism, a primary economic driver in tropical island locales [4
]. For example, an economic assessment of the persistent algal bloom in waters adjacent to the town of Kihei, Maui demonstrated that algal blooms caused by anthropogenic N input, led to $
20 million/yr of revenue loss from fewer vacation rentals, decreased tax base property values, and beach cleanup costs [5
]. Harmful macroalgal blooms in coastal environments are now common in many urban reefs [2
] and in the extreme, have been tied with the onset of fibropapillomatosis, a disease in herbivorous green sea turtles [9
Elemental analysis of macroalgal tissue from tropical coastal regions is a reliable indicator of anthropogenic eutrophication derived from coastal wastewater inputs [7
]. Although macroalgae have specific N and P requirements for growth [14
], in oligotrophic marine environments one or more of these nutrients are often limited. As a survival mechanism, tropical marine algae store available excess N and P [16
], increasing their resilience to temporary nutrient limited conditions [17
]. This adaptation allows macroalgal tissue composition to be used as a time-integrated record of recent nutrient inputs and sources, based on the isotopic composition of stored nutrients. Numerous studies have applied algal tissue analysis to complement more common water quality sampling efforts to better understand nutrient loading patterns across multiple time scales in oligotrophic coastal waters [7
While the focus of most coastal nutrient management is typically on point source and surface water discharges, [20
], the importance of submarine groundwater discharge (SGD) as a nutrient vector is becoming more recognized. On oceanic islands, SGD has potential to deliver nutrient loads 10 to 100 times higher than riverine inputs, in both pristine and human impacted tropical watersheds [11
]. Although quantifying SGD and associated nutrient loading is inherently more difficult than quantifying surface water inputs to coastal areas, well established methods using dissolved radon-222 (222
Rn) as a tracer of groundwater discharge [28
] have been applied successfully in tropical island settings [30
]. This naturally occurring radiogenic noble gas is an ideal groundwater tracer as 222
Rn has a short half-life of 3.8 days, is non-reactive through the full salinity range, and is typically found in very low concentrations in surface waters.
Predictable source dependent fractionation of the dual-isotopes of dissolved aqueous nitrogen and oxygen (δ15
O) of nitrate have been applied globally as tracers for fingerprinting N sources in terrestrial groundwater [26
] and in coastal surface waters [30
]. Similarly, algal tissue characteristics such as %N and δ15
N have also been used to effectively differentiate between nutrient sources. In ecosystems with excess N-loading from a wastewater source, macroalgae often have elevated N mass-fractions as well as enriched isotopic tissue-N compositions (δ15
N ≤ 7‰), whereas algae living in less impacted areas typically have N-isotope compositions matching the composition (δ15
N ≥ 6‰) of oceanic-N sources [8
This study examines water quality, macroalgal tissue N composition, and nutrient fluxes in SGD and baseflow across four watersheds on Tutuila, the main island in the U.S. Territory of American Samoa. These watersheds were selected because they span a gradient of human use impacts suggested by population density and land use. In American Samoa, land use factors have been linked to degradation of surface water quality and reef health [38
], but the role of SGD as a delivery mechanism for contaminants in the territory’s coastal waters has so far only been speculated [40
]. Prevalent onsite wastewater disposal systems (OSDS) (i.e., cesspools and septic tanks), numerous small scale pig rearing operations, and widespread agriculture have all been implicated as nutrient sources to Tutuila’s fresh and coastal waters [41
]. However, only limited source tracking efforts have so far been applied in American Samoa [35
The primary objective of this study is to develop a better understanding of how different land uses interact with local hydrogeology to deliver nutrients or other contaminants to the nearshore environment. To accomplish this, two “snapshot”-style measurement campaigns were conducted throughout the four study locations in August 2015 and 2016. During the first campaign, SGD derived nutrient fluxes were calculated using 222Rn as a groundwater tracer, and baseflow stage surface water nutrient fluxes were estimated through water sampling and using existing streamflow data. During both 2015 and 2016, water samples were collected from nearshore waters, coastal springs, streams at baseflow stage, and groundwater wells. In situ macroalgae were collected both years for analysis of tissue %N and δ15N, and, in 2016, specimens of experimentally managed macroalgae were deployed at fixed locations throughout the coastal zone to control for variability affecting in situ macroalgae. Ultimately, this work provides insight into the magnitudes and sources of coastal nutrient discharge in a tropical island setting, and clarifies the need to support integrated terrestrial and coastal resource management in American Samoa.
Study Location Descriptions
Located near 14° S and 170° W, the island of Tutuila (142 km2
in area) hosts nearly 56,000 permanent residents and serves as the main population center of American Samoa [44
]. Because of its position within the South Pacific Convergence Zone, the climate is hot and humid, has prevalent year-round rainfall, up to 6000 mm/year, and is subject to a wetter season from October to May. Tutuila’s south shore is more exposed than the northern shore to the prevailing swell and southeasterly trade winds, and these forces often drive strong localized rip currents inside of the reef crest along the southern coast. Beyond the reef, currents are mainly driven by tides and most offshore currents move relatively slowly, at 10 cm/s or less, except in areas where currents are channeled, such as the far northern and eastern tips of the island [45
The four study locations selected for comparison each include a terrestrial watershed area and a coastal embayment. Listed from high to low population density, these locations are Pala Lagoon, Faga’alu Bay, Vatia Bay, and Oa Bay (Figure 1
). Each watershed drains a steep forested upper section that generally transitions to an alluvial-coastal plain of variable size. For the most part, development is concentrated in coastal areas and villages are located on these alluvial plains, except for the nearly pristine Oa location, which has no road access or residents. At all study locations, nearshore zones contain well developed fringing reefs, typically consisting of uniformly shallow (0.5–2 m deep) back reef flats that extend roughly 50 to 300 m from shore to fore reef crest; beyond which water depths rapidly increase. It is notable that soft sediments do not typically accumulate in nearshore areas except for in the interior portion of Pala Lagoon.
Geologically, Faga’alu, Vatia, and Oa are fairly typical examples of Tutuila’s radial watersheds, with headwaters composed of heavily eroded Pleistocene basalts, erupted 1.5 Ma [47
]. Since that time, alluvial transport of sediments and deposition of marine carbonates has created the small wedge-shaped coastal plains fronting the mouths of each bay. At least 30 other watersheds on Tutuila have a similar geologic structure, with Faga’alu and Vatia being two of the largest and Oa one of the smallest. In contrast, the hydrogeologic structure of the Pala Watershed, also referred to as the Tafuna Plain, differs significantly from the other study locations. The majority of the watershed is covered with a Holocene-age lava delta that has given the terrain a much lower slope and a much higher permeability to groundwater than the older Pleistocene rock that makes up the rest of the island [48
Pala Lagoon drains the largest and the most developed watershed of the four study locations. Numerous farms and residences are scattered over the 12.2 km2
lava plain, which currently has a population density of 480 people/km2
]. Only one small perennial stream runs along the northeastern margin of the Tafuna Plain. Faga’alu Bay has been previously identified as a priority watershed management area by the US Coral Reef Task Force due to concerns of declining reef health and stream water quality. A single main stream drains to an embayment above a steep, forested 2.5 km2
valley that has had a population density of 404 people/km2
since the 1990s [44
]. A sublocation consisting of a shallow reef flat fronting a rocky headland wrapping around and out the northern margin of Faga’alu Bay, termed Outer Faga’alu, was also delineated as an algae sampling location for in this study. This location is adjacent to the Utulei Wastewater Treatment Plant, which discharges municipal wastewater effluent from a submerged outfall in the harbor channel. Vatia Bay drains three radially oriented perennial streams in a lightly impacted 3.6 km2
watershed with a population density of roughly 132 people/km2
since the 1990s. Although anthropogenic impact in Vatia has been categorized as minimal [50
], reports of reef decline and increased algal growth, combined with a lack of wastewater infrastructure suggest more information is critically needed for natural resources management in Vatia [51
]. Oa bay is located on the northern coast, drains a small 0.6 km2
watershed, and is the least impacted of the study locations. The watershed has remained uninhabited and has only a single stream that, at baseflow stage, infiltrates completely once it reaches the alluvial wedge. A large coastal spring is located near the dry stream mouth during low tide.
The four watersheds selected for this study span a gradient of human impact and physical hydrogeologic properties that were defined through assessment of population density, and land use analysis. Population data from the U.S. Census has been previously used to assign human impact classifications to Tutuila’s watersheds [50
], and a recently released high-resolution wildlife habitat map [52
] allows for geospatial assessment of land use in each watershed. Land use and population density metrics suggest Pala Lagoon should be subject to the highest anthropogenic impacts followed by Faga’alu then Vatia, and then Oa as its watershed is nearly 100 % forested with no human residents (Table 1
Land use and hydrogeology both act as important controls on coastal water quality and terrestrial nutrient delivery in tropical oceanic island settings. At all of the studied locations, SGD is a major pathway for anthropogenic as well as naturally derived nutrients discharging to coastal waters. This is not surprising as studies on other Pacific Islands and even on a global scale have found SGD rates to be comparable to anywhere between 10% and 1600% of riverine water fluxes [13
]. In all four locations studied on Tutuila, SGD rates were significantly higher than baseflow rates, as indicated by streamflow estimates by Wong [62
]. Trends in island wide nutrient loading followed levels of expected human impact in each watershed, whereas fluxes of N and P were highest in the Pala watershed and lowest in Oa, with Faga’alu and Vatia in between. In the three inhabited study watersheds, elevated δ15
N values in coastal groundwater, and onshore-offshore trends in algal and coastal water samples indicated wastewater or manure is likely to be a major source of coastal N in these areas. Additionally, high intra-location variation in water quality and algal parameters indicated the spatial distribution of nutrient loading is affected by heterogeneity in N-source locations and in subsurface flow paths, at remarkably fine scales.
4.1. Nutrient Levels in Coastal Management Context
While absolute magnitudes of nutrient concentrations found in Tutuila’s coastal waters were relatively low when compared to other islands where waste water injection wells or commercial scale agricultural applications are present [11
], observed N and P concentrations frequently exceeded local and federal water quality regulatory standards set for these environments. The American Samoa Water Quality Standards (ASWQS) were established by an AS-EPA [72
] administrative ruling, which specifies coastal waters with median concentrations of TDN and total dissolved phosphorus (TDP) exceeding 10.7 and 0.65 µmol/L, respectively, are in violation. Similarly, the National Coastal Assessment (NCA) Program of the U.S. EPA has established nutrient level “cutpoints” for assessing the condition of U.S. coastal resources, whereas tropical coastal surface waters with DIN and PO43−
concentrations greater than 3.6 and 0.32 µmol/L, respectively are considered to be in “poor” condition [73
If all coastal water samples from this study are pooled, they exceeded NCA “poor condition” cutpoints for DIN and PO43−
17% and 55% of the time, respectively, and they exceeded the ASWQS for TDN and TDP 13% and at least 25% of the time, respectively (Table 6
). Although the highest proportion of exceedances occurred in Pala Lagoon, it is also interesting to note that the NCA cutpoints for DIN and PO43−
were also exceeded 6% and 18% of the time respectively, in Oa Bay. While there is potential for sample location bias as sampling for this study was not spatially randomized, these data reveal that none of the study locations had water quality that always conformed to accepted standards, suggesting there are specific areas, or hotspots, within each location where nutrient input is concentrated to a level that is worth management attention and continued study. On the other hand, observed NCA cutpoint exceedances in the pristine Oa location, may imply these particular standards simply need to be reviewed and revised.
4.2. Comparison of Watersheds Along an Expected Land-Use Gradient
With increasing urbanization, coastal ecosystems on oceanic islands have become vulnerable to land-based nutrient loading, which causes stress to and ultimately can change benthic community composition [1
]. Even in this relatively isolated Pacific island archipelago, the complex issues of island geology, hydrology and vulnerabilities from land-based sources of nutrients have led to significant gradients in impact.
4.2.1. Highly Impacted
Pala Lagoon was expected to be the most impacted study location. Not surprisingly, the watershed’s combination of high population density, large area, and extremely permeable underlying bedrock contributed to the highest SGD flux and SGD-derived nutrient loading rates found in this study. Nutrient concentrations in Pala Lagoon’s CGW were high, likely due to both upgradient land use and limited nutrient attenuation through the conductive bedrock. This nutrient rich SGD, discharging from prevalent coastal springs along the western shoreline, appeared to drive a strong onshore-offshore nutrient gradient. Higher N and P concentrations and δ15
N values in water and algal tissues near the coastline were observed to decrease with distance towards the lagoon outlet where mixing with offshore waters, biological uptake by the Lagoon’s well populated benthic-macroalgal community, or both served to attenuate nutrients. This onshore-offshore gradient in δ15
N and N concentration is indicative of terrigenous-N inputs and is commonly found at locations where wastewater is discharged via SGD or surface waters to coastal ecosystems [8
Almost all spring and stream samples in Pala Lagoon showed some enriched δ15
N values, ranging from 7.2 to 9.2‰, with little co-enrichment in δ18
O values, which indicates this enrichment is likely not the result of denitrification. This, in combination with Pala Lagoon’s high DIN flux indicates wastewater or manure sources within the watershed are the main sources of DIN, a conclusion that is also supported by land-use data. Although a municipal wastewater collection system exists in the Tafuna area, over half of the households are not connected to it and thus still rely on OSDS units [43
]. This in combination with the region’s highly permeable geologic substrate allows effluent from OSDS or piggeries to move rapidly through the subsurface, with limited time and surface area for nutrient attenuation, resulting in high-N loading along the coastline.
4.2.2. Moderately Impacted
In contrast to the expected onshore-offshore gradient in δ15N values, Faga’alu bay displayed an opposite and enigmatic trend. Measurements from in situ algal tissues collected near the northern-outside point of Faga’alu Bay in 2015 lead to suspicions that the Utulei Wastewater Treatment Plant (WWTP) may be a source of high-δ15N nitrogen to Pago Pago Harbor and the bay. This WWTP discharges primary-treated effluent directly into the harbor via an ocean outfall located about 0.5 km to the north of Faga’alu. In 2016, additional in situ algal tissues, three deployed Hypnea samples, and a water sample were collected at the outer Faga’alu location to investigate this hypothesis further. Algal tissue δ15N values from this location were consistently higher than δ15N values of algal tissues and water collected from the stream, well, and coastal groundwater in the Faga’alu study location, indicating detectable levels of N from WWTP effluent do affect algae and water in the vicinity of the ocean outfall. Although this point-source nutrient discharge is monitored and regulated by AS-EPA, coastal resource managers have not fully considered its potential effects on Faga’alu Bay.
In Vatia, co-enrichment in DIN, δ15N and δ18O values from coastal water and coastal springs suggests partially-denitrified N is present, likely from an OSDS source. In the bay, coastal spring samples on the north side had 2–3 times the DIN, elevated NH4+ and generally higher δ15N values when compared to spring samples from the southern side of the bay; a trend that was consistent over the duration of this study. This geochemical signature is likely indicative of mixing with OSDS effluent, as there are number of homes served by OSDS within about 50 m of the northern coastline. In contrast, there are few homes upgradient of the more southerly Vatia springs, and these residences are all at least 150 m away from the coast. Coastal water samples in Vatia also showed high δ15N values in samples taken proximal to the northern spring group, although these values were not observed along the central part of the bay front, which also faced many homes with OSDS units. The highest algal and water sample δ15N values observed in Vatia were found repeatedly in a single area of SGD discharge, and adjacent samples did not appear to be geochemically similar. This illustrates the potential for small-scale heterogeneity in subsurface flow paths or variability in source proximity to affect the geochemistry of SGD.
Estimates of nutrient loading to Faga’alu and Vatia Bays were similar with Faga’alu having about twice the nutrient loading as Vatia. This might be expected as Faga’alu has about twice population and developed land as Vatia. However, the population density of Faga’alu is over three times that of Vatia, and the natural land use proportions of each watershed are fairly similar (Table 1
). Therefore, it is likely that other factors also influence nutrient loading in these watersheds. An interesting difference between the two watersheds is the presence of a wastewater collection system that serves a portion of the residents in Faga’alu. According to self-reported 2010 U.S. Census information [44
], of the 169 reported households in Faga’alu Village, 117 reported being connected to a public sewer and only 52 reported using an OSDS. Vatia in contrast, has no public sewer infrastructure, therefore all residents likely use some type of OSDS. This suggests that a number of the residents in Faga’alu are not contributing wastewater effluent to the watershed and therefore their impact is significantly reduced, which may help to explain why nutrient loading in Faga’alu is not larger than observed.
Another factor that may increase the impact of OSDS in Vatia village is the distribution of development. Many homes in Faga’alu are located up the valley whereas most of the residences in Vatia are located close to the shore. To quantify this idea, building location data were obtained from the American Samoa Department of Commerce [75
], and the distribution of all buildings within Pala, Faga’alu, and Vatia Watersheds were calculated as a function of their straight-line distance to the coastline (Figure 7
). The median distance from the coast for structures in Vatia is 87 m, whereas in Faga’alu and Pala Watershed median values for building distance from the coast are 152 and 1300 m, respectively. Because longer subsurface travel distance may provide increased time for attenuation reactions for nutrients (e.g., denitrification and sorption) the overall distance CGW travels between N-sources and the coast likely plays a role in final SGD nutrient compositions.
4.2.3. Least Impacted
Geochemical parameters observed in Oa Watershed were interpreted as reference values, reflecting the nutrient signature for pristine, unimpacted land-use. Spring and stream samples from, Oa Watershed, had δ15
N values within 4.4 to 6.9‰, indicating the expected range of source water δ15
N lacking any anthropogenic impacts. Coastal waters in Oa generally reflected this δ15
N signature, with the three of the four coastal samples that had enough N + N for analysis showing values of 6.2 to 6.9‰, and with one enigmatic sample located on the eastern edge of the bay that had a δ15
N value of 8.5‰. The δ15
N values of C. fastigata
collected from Oa were generally within the upper range of values reported for this species located in other lightly impacted/pristine regions of American Samoa [35
]. Calculated nutrient loads to Oa Bay were significantly lower than in other watersheds, which is a factor of both low concentrations of N in CGW and the watershed’s small size (Table 4
Although Oa Bay typically had lower DIN and δ15
N values, area scaled PO43−
loading in Oa was found to be the second highest of the four studied watersheds. This relatively high PO43−
loading in a pristine location, as well as a general lack of correlation between PO43−
and expected land-use impact in the other watersheds, suggests that coastal P loading on Tutuila is likely to be controlled by factors other than land use. Across the four study locations, levels of phosphorus in most sample types appeared to be fairly consistent, yet were often high in comparison to national and local standards and cutpoints. Cho [71
] approximates the global average P concentration in SGD to be around 0.75 µmol/L, whereas the average P concentrations observed in unmixed coastal spring samples from this study was 2.3 µmol/L. This apparent P surplus in Tutuila’s waters may be attributed in part to natural weathering of volcanic rock. The oceanic basalts from which Tutuila is constructed contain amounts of phosphorus that are up to three-times higher than in continental rocks [76
], and as this rock is weathered and dissolved by groundwater, high concentrations of TDP are able to leach out [73
]. Chadwick et al. [77
] suggests that higher P concentrations are found in younger Hawaiʻian soils, and these can be mobilized into groundwater and baseflow by erosion and weathering [78
]. Another potential non-anthropogenic P-source prevalent in Pacific Islands, and particularly in less-impacted locations such as Oa Bay, is biological addition from seabird guano. Indeed, an island-wide survey of nesting seabird colonies on Tutuila conducted in 2004 [79
] documented two small coastal-seabird colonies in Oa bay of four and nine individuals each, and numerous colonies in the steep coastal areas outside of Vatia Bay. While P input from coastal seabird guano likely only impacts coastal waters, and not inland streams or wells, biogenic nutrient sources also have the potential to influence coastal nutrient budgets and should be considered when assessing nutrient-mass balance.
In terms of coastal ecosystem health, naturally high phosphate levels in themselves may not be concerning; however, this does suggest that Tutuila’s coastal waters are likely to be N, rather than P limited. Therefore, even small additions of N from anthropogenic sources into these environments could start to change species composition; excess N-loading could stimulate excessive algal growth and possible eutrophication more easily than if the system was P limited.
4.3. Management Considerations and Future Directions
Groundwater inputs can strongly influence watershed nutrient loading and cause impacts to coastal ecosystem health. In all four of this study’s watersheds, calculated daily nutrient loads from SGD during the 2015 study period were significantly greater than loads from baseflow-stage stream inputs, underscoring the need to consider coastal groundwater quality in addition to surface water quality when undertaking management actions. Additionally, it is likely that there is significant interaction between coastal groundwater and stream baseflow in these watersheds, again showing the need for management of groundwater quality, as it directly affects surface-water quality. Future approaches to coastal land management and development would benefit from considering how changes in land use impact the quality of coastal groundwater, surface water, and therefore, nearshore reef health.
In Pala Lagoon and Vatia Bay, elevated N concentrations and δ15
N values in both water and algal tissue suggest discharging N in these areas is primarily derived from a wastewater or manure source. This conclusion is also supported by the work of Shuler et al. [43
] where it was found that OSDS sourced wastewater was the predominant N source to the aquifer underlying the Tafuna Plain. Detectable impact of wastewater on the coastal environment in these embayments is a strong motivation for the development of new wastewater collection systems or expansion of existing systems. On the other hand, elevated δ15
N values observed within Faga’alu bay and adjacent to the Utulei WWTP warrant further investigation of this facility as a source of N to surrounding coastal and harbor areas.
While it was found that SGD nutrient loading estimates from this study correlate well with current population and land-use magnitudes, this conclusion does inherently rely on the assumption that nutrient inputs and transport are in a steady-state with nutrient discharge. For the purpose of predicting present-day nutrient loading on Tutuila this assumption is probably valid, considering that land use and human population in the locations studied have remained fairly stable over at least the last 30 years [49
]. However, subsurface water and solute transport may be complicated in heterogeneous basaltic aquifers. These aquifers can be characterized as dual-porosity systems where fractures within the rocks transmit water quickly and zones with only primary porosity transmit water much more slowly [80
]. For example, Bertrand et al. [82
] used an artificial tracer to examine water and contaminant transport in heterogeneous basalt flows that exhibited dual-porosity behavior. They found that while almost all of the water and tracer was quickly advected through fractures, a significant portion of the tracer was lost and was hypothesized to have been captured and stored in slow, primary-porosity zones. This behavior of solute storage within zones of lower-permeability matrix has been well documented in other dual-porosity systems [83
]. This process might be seen as a temporary benefit that provides natural-subsurface nutrient removal. However, natural resource managers should be aware that if land use is modified with the intention of mitigating nutrient inputs, dissolved nutrients could be slowly released from the aquifer for a period of time after inputs have been reduced. Therefore, it may take years or even decades for benefits from future land-use management to be realized as aquifers slowly re-equilibrate.
Of equal concern to nutrient perturbation of coastal ecosystems is the potential risk of illness due to wastewater effluent from both WWTPs and OSDS units in American Samoa. The American Samoa EPA performs a limited amount of recreational water sampling for fecal indicator bacteria (FIB) and posts public warning signs about the risk of illness from swimming at local beaches [41
]. However, the sources of FIB and the relationship of bacteria concentrations to physical conditions remain largely unknown. Future studies using a combination of the methods used in this study and DNA-derived source tracking could provide clarity on major waste/bacterial sources across small spatial scales. Although a handful of recent SGD investigations included a biotic component, there is a relative lack of knowledge regarding the effects of SGD on marine ecosystems and humans that use them. As concluded in a recent review by Lecher and Mackey [86
], there are currently few studies that focus on the impact of groundwater at an ecosystem level, yet these were found to be the most insightful.