Provenance of nutrients in submarine fresh groundwater discharge on Tahiti and Moorea, French Polynesia

Applied Geochemistry 100 (2019) 181–189

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Provenance of nutrients in submarine fresh groundwater discharge on Tahiti and Moorea, French Polynesia

T

Kathrin Haßlera,∗, Kirstin Dähnkeb, Martin Köllingc, Lydie Sichoixd, Anna-Leah Nickla,e, Nils Moosdorfa a

Leibniz Centre for Tropical Marine Research (ZMT), Fahrenheitstr. 6, 28359, Bremen, Germany Institute for Coastal Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Str. 1, 21502, Geesthacht, Germany c Zentrum für Marine Umweltwissenschaften, Universität Bremen, Bremen, Germany e Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Weßling, Germany d University of French Polynesia, Faaa, Tahiti, French Polynesia b

A R T I C LE I N FO

A B S T R A C T

Editorial handling by Dr N Otero

Submarine fresh groundwater discharge (SFGD) provides a pathway for dissolved nutrients and other solutes from land to the ocean. It connects pollution from anthropogenic land use with coastal marine waters. In case of the oligotrophic central South Pacific Ocean around Tahiti and Moorea, French Polynesia, nutrient concentrations are particularly low. Both islands are surrounded by tropical coral reefs, which are highly sensitive to nutrient concentrations in the ambient water so that a surplus of nutrients, e.g. from SFGD, could lead to the degradation of coral reef ecosystems. We examined nutrient contributions from different land cover classes to nutrient fluxes through SFGD by combining nutrient concentration data, spatial data, oxygen and hydrogen isotope ratios of water (δ18OH2O and δ2HH2O, respectively) and nitrogen and oxygen isotope ratios of nitrate (δ15NNO3- and δ18ONO3-). Undeveloped land provides measurable quantities of phosphate while nitrate concentrations are often below the detection limit. The bulk of the nutrient load in nutrient enriched groundwater is of anthropogenic origin. It enters the aquifer system at low altitudes, where catchments are characterized by anthropogenic land use. Elevated nitrate concentrations are mainly associated with septic waste/manure inputs in fresh water. This study elucidates sources of nutrients in the groundwater of two volcanic islands, highlighting the impact that even a small populated area along the coast of an island can have, as well as the differences in nutrient transport between these seemingly similar locations.

Keywords: Submarine groundwater discharge Nutrients French Polynesia Tahiti Moorea Volcanic islands Nitrogen and oxygen isotope ratios of nitrate

1. Introduction Submarine groundwater discharge (SGD) typically contains two components: submarine fresh groundwater discharge (SFGD) and recirculated saline groundwater discharge (RSGD) (Burnett et al., 2003; Moore, 2010; Taniguchi, 2002). SFGD has recently received increasing attention as a source of land-derived natural and anthropogenic nutrients and other solutes to coastal waters (Adyasari et al., 2018; Bishop et al., 2017; Burnett, 1999; Burnett et al., 2001; Moore, 1996), because groundwater often contains elevated nutrient concentrations (Garrison et al., 2003; Kim et al., 2005; Swarzenski and Kindinger, 2004). Nutrient inputs from SGD can locally be substantial (Garrison et al., 2003; Kim et al., 2005; Swarzenski and Kindinger, 2004) and exceed riverine nutrient fluxes (Burnett et al., 2003; Cable et al., 1996). On Hawaii,

∗

Street et al. (2008) found nutrient additions to the ocean to be the largest at SGD locations with a substantial fresh water component. Also on Moorea, SFGD has been suspected of acting as significant nutrient source to the coastal area (Knee et al., 2016). This study focuses on the fresh groundwater component of SGD and its solute concentrations. Dissolved silica (DSi) in fresh water is typically a weathering product, hence concentrations depend more on aquifer properties than anthropogenic inputs (Jansen et al., 2010). Nitrate (NO3−) and phosphate (PO43−) concentrations in groundwater depend on a range of parameters: nutrient input, soil and aquifer type, and climate (cf. Wick et al., 2012). Fertilized agricultural lands, livestock pastures and areas with high septic tank density (urban areas) can contribute nutrients to coastal waters around islands via SFGD. As a consequence, a growing population, ongoing agricultural development

Corresponding author. E-mail address: [email protected] (K. Haßler).

https://doi.org/10.1016/j.apgeochem.2018.11.020 Received 23 April 2018; Received in revised form 22 November 2018; Accepted 23 November 2018 Available online 24 November 2018 0883-2927/ © 2018 Elsevier Ltd. All rights reserved.

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1998). Moorea has 16,000 inhabitants, mostly settling along its coast. As the older of the two islands, Moorea is more eroded but still the inland is in parts steep and impossible to cultivate. Agriculture on Moorea is more abundant than on Tahiti: pineapple fields dominate the gentle slopes within the eastern caldera. In the south-western part, an agricultural college cultivates a variety of tropical fruits and operates a pig farm. Cattle pastures are concentrated in the lower western caldera, but are also found inbetween settlements around the island. In flat areas of river valleys crops are cultivated on small scales. Tahiti is surrounded by a barrier reef (Rougerie et al., 1997), segmented by a series of channels which correspond to the main river outlets. Conditions in the shallow lagoon are mostly calm, except for some places in the north east of the main island where the barrier reef does not reach the sea surface (Williams, 1933). Moorea is surrounded by a barrier reef and near shore fringing reefs, disrupted by twelve deep channels resulting from freshwater outlets (Cooley, 1997). The climate in French Polynesia is wet tropical, marked by high amounts of seasonal rainfall. Temperature ranges from 27 °C during wet and 24 °C during dry season at sea level (Laurent et al., 2004). Rain occurs mostly during the wet season which extends from November until April (Laurent et al., 2004). On Tahiti mean annual precipitation varies from 1500 mm at sea level (on the leeward side, west coast) up to 8500 mm at high altitudes (Pasturel, 1993). High amounts of precipitation during the rainy season lead to episodic flooding of the rivers (Bouvier and Denat, 2006; Hildenbrand et al., 2005; Pheulpin et al., 2016; Williams, 1933; Wotling et al., 2000). On Moorea, the amount of precipitation also varies with altitudes from 1600 mm at sea level to between 5000 mm along the rim of the caldera (Maury et al., 2000). The heavy precipitation in the upland regions of the islands is a primary source of groundwater recharge (Hildenbrand et al., 2005). The hydrological and hydrogeological conditions on Tahiti are complex and only well studied in the Papeete area (Hildenbrand et al., 2005). Tahiti Nui consists of basaltic units and overlying porphyric accumulations that are productive aquifers, interlayered with thick vesicular autobrecciated impermeable material that forms confining layers. Most of the springs discharging either in the inner part of some valleys or at the ocean shore level in the northern half of Tahiti Nui are fed by a perched aquifer within the second shield. Dikes scattered through the volcanoes

and subsequent nutrient enrichment of SFGD makes SFGD highly interesting to reef management. This is especially important in light of uniformly low nutrient concentrations in offshore waters (Street et al., 2008), because high groundwater nutrient fluxes can lead to the degradation of reef ecosystems (Kim et al., 2011) and loss of species diversity (LaPointe, 1997). Oxygen and hydrogen isotope ratios in water (δ18OH2O and δ2HH2O) can be used to e.g. distinguish between different water bodies, to assess mixing between them, define the elevation of recharge, or magnitude of evaporation (e.g. Clark and Fritz, 1997). Nitrogen and oxygen isotope ratios of nitrate in turn indicate nitrate sources (e.g. Kendall et al., 2007; Wankel et al., 2006): Ideally, the combination of nitrogen and oxygen isotope ratios of nitrate (δ15NNO3- and δ18ONO3-, respectively) differs between individual NO3− sources and thus allows the identification of NO3− sources to water using their isotopic fingerprint in dissolved nitrate. This study uses isotopic tracers to infer the sources contributing nutrients to the coastal sea of Tahiti and Moorea via SFGD. 2. Study area Tahiti is the largest of the volcanic French Polynesian islands with a total area of 1042 km2 and a maximum elevation of 2241 m (Fig. 1). The island is comprised of two basaltic shield volcanoes linked by a narrow isthmus: the larger north-western cone, Tahiti Nui, and the smaller cone, Tahiti Iti, in the south-east (Fig. 1). Owing to the steep relief and rainforest in the inner part of the island, human activities are restricted to some stretches of coast and larger valleys. Tahiti has 187,000 inhabitants, most of whom live on the north-west coast of Tahiti Nui in and around French Polynesia's capital, Papeete (ISPFINSEE, 2012). Apart from that, long coastal stretches are only sparsely inhabited, or not at all (e.g. the south coast of Tahiti Iti). Most agriculture, particularly crop cultivation and larger cattle pastures, can be found along coastal stretches in the southeast of Tahiti Nui and in the north west of Tahiti Iti, predominantly on the Plateau de Taravao (Dupon and Yon-Cassat, 1993). Moorea is located approximately 16 km north-west of Tahiti. The island is 16 km long and 12 km wide (Fig. 1). It originates from a shield volcano, whose caldera remnants culminate at 1207 m (Le Dez et al.,

Fig. 1. Sampling points on Tahiti and Moorea. Also visible are the barrier reefs surrounding the islands. The location of the islands in the middle of the central South Pacific Ocean is indicated by the red dot in the small world map. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 182

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Chloride (Cl−) measurements were performed via ion chromatography on a Metrohm Advanced Compact IC 861. Samples with nitrate concentrations of at least 2 μmol L−1 were analyzed for the nitrogen and oxygen isotopic composition of dissolved nitrate (δ15NNO3- and δ18ONO3-) using the denitrifier method (Casciotti et al., 2002; Sigman et al., 2001). In samples with a nitrite concentration in excess of 2% of the respective nitrate concentration, nitrite was removed using sulfamic acid (Granger and Sigman, 2009) prior to N and O isotope analysis. The isotopic composition was determined on a GasBench II coupled to a Delta V Advantage mass spectrometer (ThermoFinnigan). We performed replicate measurements and included two international standards (IAEA-N3, δ15N = +4.7‰, δ18O = +25.6‰ and USGS 34 δ15N = −1.8‰, δ18O = −27.9‰; Böhlke et al. (2003)) with each batch of samples. The standard deviation of samples and standards (n = 5) was < 0.2‰ for δ15N and < 0.4‰ for δ18O. To correct for exchange with oxygen atoms from water, a bracketing correction was applied (Sigman et al., 2009). Throughout this study, δ15NNO3- and δ18ONO3- values are reported in per mill (‰) relative to AIR and VSMOW, respectively. The different sample types and their trends were compared using median values, because the presented data are not normally distributed.

can store groundwater and act as hydraulic connections (Hildenbrand et al., 2005). Moorea mainly consists of a shield volcano made up of five units (Le Dez et al., 1998). The northern part of the shield collapsed immediately at the end of the building stage, leaving an isolated remnant. This collapse was possibly triggered by the intrusion of numerous sills and dikes below the future caldera floor. No detailed study of Moorea's hydrogeology is available. On the islands of Tahiti and Moorea point source SGD occurrences (focused discharge of mainly fresh water) are locally known. For example, the SGD site “La Source” in the north west of Tahiti is a famous dive site (Moosdorf and Oehler, 2017) and on a beach section in the north east of Moorea locals capture SGD within a concrete pipe and use it for washing. 3. Methods We relied on local knowledge to direct us to SGD sites, which are known to the inhabitants. Characteristic concentric circles on the sea surface mark the locations of shallow, near shore point sources. Springs further offshore and in deeper waters, which are only accessible by diving, can be recognized under water by blurry patches which result from the mixing of fresh water with salt water. Fieldwork was conducted during January and February 2016, the second half of the wet season. We sampled all point source SGD sites we could locate along the coasts of Tahiti and Moorea, as well as streams or rivers closest to SGD sites or whose catchments are clearly dominated by one of the land use categories. In total, we collected water samples from 39 streams and rivers, 20 public water supply wells and coastal springs, 13 SFGD sites and 7 samples from the coastal sea surface (∼0.5–1 m depth, Fig. 1). Lacking a land-use dataset for French Polynesia, we qualitatively defined land use for the individual sites based on field observations of their surroundings reinforced by visual inspection of satellite images (Google Earth and ESRI Digital Globe). A list of the land use information per station is provided in table S1 (electronic supplement). The rainwater sample was collected on Moorea directly at the coast. As this sample is drip water from a rooftop it might contain dissolved nutrients of non-atmospheric origin, e.g. from decomposition of plant litter on the rooftop, solutes could be affected and the sample is not used for quantitative analyses in this study. Where possible physicochemical parameters (temperature (T), electrical conductivity (EC), oxygen saturation (O2 sat.) and pH) were measured directly on site in the field using handheld multi parameter probes (Hach Lange HQ 40d multi). Physicochemical parameters of samples collected from submarine springs by diving were determined right after bringing the sample up to the surface. For nutrient analyses, 60 ml of water were filtered through a 0.2 μm disposable CA syringe filter into a sample-rinsed high-density polyethylene bottle, immediately cooled and kept cool until analysis. For δ18OH2O and δ2HH2O and for Cl− analyses 2.5 ml of water were filtered through a 0.2 μm disposable CA syringe filter into a sample-rinsed Eppendorf tube, immediately cooled and kept cool until analysis. For δ15NNO3- and δ18ONO3- 40 ml of water were filtered through a 0.45 μm disposable GF syringe filter into a sample-rinsed high-density polyethylene bottle, immediately cooled, frozen within 8 h of sampling and kept frozen until analysis. We used a Tecan plate reader or a Skalar continuous flow analyzer for photometric nutrient analyses. Oxygen and hydrogen isotope ratios in water (δ18OH2O and δ2HH2O) were analyzed with a Picarro L2130i Cavity Ringdown Spectrometer. Oxygen and hydrogen isotopic compositions were measured using a calibration with internal lab reference waters which are calibrated against the VSMOW2/SLAP2 scale. The water isotope data are expressed in δ-notation in per mil (‰) relative to VSMOW. Following Coplen (2011), we present positive δ-values accompanied by a “+” for clarity.

3.1. Extrapolating fresh groundwater endmembers from SGD samples In order to compare nutrient concentrations and δ18OH2O and δ2HH2O values of individual samples we used the average chloride concentration of the groundwater samples (235 μmol L−1 for Tahiti and 234 μmol L−1 for Moorea) and the average chloride concentration of seawater samples (560,000 μmol L−1) to extrapolate SGD samples to fresh groundwater endmember (SFGD) compositions. All values discussed in the following are extrapolated SFGD compositions, with the exception of Cl− concentrations and physicochemical parameters. Even though water with chloride concentrations up to ∼2790 μmol L−1 at 25 °C (and more at higher temperatures) is considered “fresh”, groundwater samples with chloride concentrations above 400 μmol L−1 were excluded from the calculation of the average fresh water chloride concentration as they represent “outliers” in the groundwater data. On the SGD samples, equation (1) after Hunt and Rosa (2009) was applied:

cfr = csw + (cmix − csw ) ×

sfr − ssw smix − ssw

(1)

Cfr stands for the concentration/isotopic composition in the pure SFGD sample (i.e. the fresh groundwater endmember), Csw for the concentration/isotopic composition in the seawater endmember, Cmix for the concentration/isotopic composition in the SGD sample, Sfr for the chloride concentration of the freshwater endmember, Ssw for the chloride concentration of the seawater endmember and Smix for the chloride concentration of the SGD sample. 4. Results 4.1. Physicochemical parameters Surface sea water from the lagoons surrounding Tahiti and Moorea has a median EC of 53,800 μS cm−1, the median EC of stream water is lower than that of groundwater (Table 1). The rain water sample from Moorea exhibits the lowest EC in our data set (30 μS cm−1). Median stream water and groundwater EC values from Tahiti (126 μS cm−1 and 175 μS cm−1, respectively) are lower than those from Moorea (191 μS cm−1 and 366 μS cm−1, respectively). Sea water is well oxygenated to slightly oversaturated with oxygen, fresh water samples from Tahiti are generally more oxygenated than samples from Moorea (Table 1): stream water, groundwater and SGD samples from Tahiti are generally well oxygenated, whereas the same sample types from Moorea are less saturated in oxygen. 183

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Table 1 Median values (and ranges) of EC, T, pH, and O2 sat. of the different water types.

sea water stream water

all Tahiti Moorea Tahiti Moorea Tahiti Moorea Moorea

groundwater SGD rain water

EC [μS cm−1]

T [°C]

pH

53,800 (48,800–56,600) 126 (60–400) 191 (90–29,800) 175 (100–630) 366 (140–1,200) 31,900 (1,350–45,500) 7,660 (420–37,200) 30

29 25 27 26 27 26 26 24

8.2 8.0 7.9 7.6 7.8 8.0 7.5 5.5

(27–30) (24–31) (23–31) (23–27) (25–28) (24–29) (25–27)

(8.2–8.3) (6.2–8.9) (6.6–8.5) (7.0–8.0) (6.3–8.2) (7.4–8.4) (6.4–8.2)

O2 sat. [%]

Number of samples

108 (80–114) 102 (26–133) 87 (66–113) 95 (58–106) 87 (41–107) 98 (94–104) 68 (48–82) 97

7 27 12 13 7 9 5 1

samples from both islands are generally enriched in nutrients (NO3−, NH4+, PO43− and DSi) as compared to stream water samples (Table 2, Fig. 3). Considering the 25th to 75th quartile of the data NO3− concentrations in groundwater and SFGD from Tahiti show a higher variability and reach higher values than in samples from Moorea (Table 2, Fig. 3a). The same is true for stream water samples. NH4+ concentrations display the exact opposite trend: concentrations in all fresh water samples show a higher variability and reach higher values in samples from Moorea than in samples from Tahiti (Table 2, Fig. 3b). PO43− concentration ranges in the same fresh water type (groundwater plus SFGD and stream water, respectively) are similar on both islands whilst the median for samples from Moorea is higher than for samples from Tahiti (Table 2, Fig. 3c). Furthermore, minimum PO43− concentrations found in SFGD are almost three times the minimum concentrations found in groundwater and the median of the PO43− concentration is also noticeably higher in SFGD than in groundwater (Table 2, Fig. 3c). DSi shows the same general behavior as PO43− with more pronounced differences between the same sample types from the two different islands (Table 2, Fig. 3d).

Figure 2. δ2HH2O vs. δ18OH2O of seawater, rain water, stream water, groundwater and fresh submarine groundwater (SFGD) samples from Tahiti and Moorea. Groundwater samples define a local meteoric water line (LMWL, black line). For comparison, the global meteoric water line (GMWL, dashed blue line, Craig (1961)) is also shown.

4.3.1. N and O isotopic composition of nitrate δ15NNO3- and δ18ONO3- of all samples with NO3− concentrations ≥2 μmol L−1 range from −0.06‰ to +20.62‰ for N and −3.90‰ to +18.82‰ for O isotope ratios (Table 2, Fig. 4). In a δ18ONO3- vs. δ15NNO3- plot (Fig. 4), all but three samples plot in the overlapping fields of the signatures of NH4+ in fertilizer and precipitation, soil NH4+ and NO3− in manure and septic waste (Kendall et al., 2007). In stream water, sample KH-15 approaches values found in synthetic fertilizer (Fig. 4) whereas the isotopic signature of nitrate in groundwater sample KH-67 falls into a range typical for septic waste and/or manure.

4.2. Isotopic composition of water Seawater at the study site is slightly enriched in δ18OH2O as compared to the global mean ocean value of ∼0‰, in line with other findings from the Pacific (LeGrande and Schmidt (2006); Fig. 2, Table 2). Values of fresh water samples are more depleted in δ18OH2O than the seawater samples (Fig. 2, Table 2). With the exception of the rain water sample and three stream water samples from Moorea, fresh water samples from Tahiti and Moorea exhibit a linear correlation between their oxygen and hydrogen isotopic composition (Fig. 2, LMWL, R = 0.96, p = 3.8 × 10−20) which is parallel to the global meteoric water line (Fig. 2, GMWL, slope of 8, y-intercept 10; Craig (1961)).

5. Discussion

4.3. Nutrient concentrations

5.1. Groundwater recharge areas

Sea water is generally depleted in NO3−, contains no to little NH4+ and DSi, but does contain PO43− (Table 2). Groundwater and SFGD

The linear correlation between δ2HH2O and δ18OH2O (Fig. 2, LMWL: r = 0.96, p = 3.8 × 10−20) indicates that the groundwater samples

Table 2 Medians (and ranges) of δ18OH2O and δ2HH2O, and NO3−, NH4+, PO43−, DSi and Cl− concentrations of the different sampled water types.

sea water stream water groundwater SFGD rain water a b

all Tahiti Moorea Tahiti Moorea Tahiti Moorea Moorea

δ18OH2O [‰]

δ2HH2O [‰]

NO3− [μmol L−1]

NH4+ [μmol L−1]

PO43[μmol L−1]

DSi [μmol L-1]

Cl− [mmol L-1]

0.8 (+0.4 to +0.9) (-5.5 to −2.5) (-4.4 to −1.3) (-5.4 to −3.2) (−4.7 to −3.2) (−6.5 to −2.8) (−4.5 to −3.8) −1.4

5.3 (+4.1 to +6.1) (-28.8 to −7.8) (-20.4 to −5.0) (-28.3 to −11.9) (−22.2 to −11.7) (−38.4 to −7.2) (−22.3 to −18.3) −0.2

0 (0–0) 1 (0–15) 2 (0–7)a 19 (0–60) 6 (0–14) 26 (0–44) 9 (0–13) 1

0.09 (0.00–0.18) 0.05 (0.00–7.63) 0.2 (0–17.90) 0.08 (0.00–0.82) 0 (0.00–31.75) 0.01 (0.00–0.71) 0.05 (0.00–1.12) 0.96

0.26 0.82 1.06 1.99 1.31 2.65 3.34 0.61

15 (0–35) 288 (120–1078) 509 (294–644) 294 (264–1161) 561 (403–1015) 497 (283–661) 644 (426–1168) 2

552 (505–566) 0.13 (0.05–91.5) 0.21 (0.13–291) 0.18 (0.14–3.81) 0.37 (0.13–6.36) 31.9 (9.51–471) 6.34 (1.59–350) 0.13

An outlier of 21 μmol L−1 NO3−was excluded from the statistics. An outlier of 10.22 μmol L−1 PO43− was excluded from the statistics. 184

(0.19–0.39) (0.06–2.63) (0.37–2.43) (0.38–3.36) (0.68–3.82) (1.14–3.73)b (1.94–3.79)

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Fig. 3. (a) NO3−, (b) NH4+ (c) PO43− and (d) DSi concentrations in groundwater, SFGD and stream water samples from Tahiti and Moorea. Black rimmed boxes enclose the 25th to 75th percentile of the data, the horizontal lines mark the median (for groundwater and SFGD samples combined) and error bars indicate the complete range of values. The legend in (a) also applies to (b), (c) and (d).

by other isotope fractionating processes. Generally, strong changes in δ18OH2O can be observed on volcanic tropical islands, e.g. Hawaii (Scholl et al., 1996). Evaporation from surface waters results in the enrichment of the heavy isotopic species in the remnant water body along an evaporation line and usually leads to a slope less than 8 in δ2HH2O vs. δ18OH2O space (Gat, 1996). The same will apply to mixing with seawater, as seawater is enriched in the heavy isotopes as compared to meteoric water. We thus attribute the deviation of some samples in the dataset from the LMWL (e.g. KH-62, KH-85 influenced by mixing with sea water (high EC) and KH-77 influenced by evaporation (low EC); Fig. 2) to one or a combination of these two processes. In the rainwater sample the “amount effect” (Dansgaard, 1964) could have led to fractionation: a very high amount of rainout during the precipitation event could have resulted in more positive δ values. The altitude and distance from the coast effects on δ18OH2O (and 2 δ HH2O) are extensively used in hydrological studies to determine groundwater recharge areas (e.g. Bishop et al., 2017; Gat, 1996; Gonfiantini et al., 2001; Rogers et al., 2012; Siebert et al., 2012; Wassenaar et al., 2011). In accordance with this and given the measured median sea water δ18OH2O of +0.8‰, samples with more negative δ18OH2O values originate from precipitation at higher altitudes farther inland. This is further supported by more negative isotopic compositions in freshwater samples from Tahiti (highest peak 2241 m) than in samples from Moorea (highest peak 1207 m). Given overall negative δ18OH2O values in freshwater samples from Tahiti and Moorea, major recharge of freshwater reservoirs seems to occur preferentially inland at high altitudes. This is supported by rainfall patterns of the

Fig. 4. δ18O vs δ15N ratios of nitrate in SFGD, groundwater and stream water samples from Tahiti and Moorea, as well as isotopic composition of potential nitrate sources to the samples (modified after Kendall et al. (2007)). The shaded field represents the possible range of δ18O-NO3 values produced by nitrification in equilibrium with fresh water samples. The two indicated clusters are discussed in the text.

(including SFGD) are of meteoric origin and fractionated by altitude effects (Craig, 1961; Dansgaard, 1964): With increasing altitude, precipitation becomes increasingly depleted in the heavy isotopes. Water samples plotting further away from the LMWL are additionally affected 185

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stations has been documented from field observations (Table S1, electronic supplement). However, this very local land-use information, which does not represent catchments of the sampling stations, shows no significant correlations with nitrate concentrations or isotopic composition. More generally, anthropogenic septic waste is mostly generated in the west of the island and often treated in decentral water treatment facilities (Centre d’Hygiène et de Salubrité Publique, 2013), many of which discharge into the soil. The Moorea school of Agriculture estimates that farming practices usually apply 350 kg/hectare of fertilizer (personal communication), of which 1360 and 1910 tons were imported to French Polynesia in 2013 and 2015, respectively (Service du Développement Rural, 2016). In addition, most of the livestock farms do not treat their effluents (Centre d’Hygiène et de Salubrité Publique, 2013) and can be point sources of nitrate pollution in addition to the manure used as organic fertilizer. This suggests an anthropogenic origin for the elevated NO3− concentrations observed in some locations on Tahiti. Other studies on volcanic islands have made similar observations: On the North Shore of Kauai (Hawaii, USA), Knee et al. (2008) interpreted site-specific differences in groundwater nitrate and nitrite concentrations to reflect a possible agricultural nitrogen source. On Maui (Hawaii, USA) Bishop et al. (2017) found that sugarcane and pineapple fields contributed the largest amount of nitrogen in SFGD. Septic systems, cesspools, and near coast wastewater injection wells also contributed nitrogen to groundwater, although in much smaller quantities (Bishop et al., 2017). In the Flic en Flac lagoon on Mauritius, major inputs of nitrate and phosphate in SFGD during periods of heavy rain arose from agricultural and domestic sources (Ramessur et al., 2012). The δ15NNO3- and δ18ONO3- data strengthen the hypothesis of human caused nutrient enrichment in fresh water: Comparing the presented data set with δ15NNO3- and δ18ONO3- composition ranges representing specific sources (compiled by Kendall et al., 2007) (Fig. 4) leads to setting the following endmembers: 1) The synthetic fertilizer endmember represents synthetic fertilizer as a direct N source. It has a composition of intermediate δ15NNO3- and high δ18ONO3-. This endmember is represented by sample KH-15, which was taken in a river running through an area of greenhouses. 2) The endmember representing septic waste/manure, sample KH-67, was sampled from a spring in a churchyard. It shows high δ15NNO3- and intermediate δ18ONO3- values (Fig. 4). δ15NNO3- and δ18ONO3- in groundwater are positively correlated (Fig. 4, linear regression, R = 0.66, p = 0.03). At first glance this positive correlation seems to hint towards nitrate consumption, e.g. by denitrification. During denitrification, N and O isotope effects are coupled, and increase along a defined slope (∼1 or less, depending on the denitrifying species, Granger et al., 2008; Granger et al., 2004), thus resulting in heavier isotopic composition of NO3− at lower concentrations. However, the data presented in this study showed the opposite behavior: in groundwater samples NO3− concentrations increase together with δ15NNO3- (Fig. 6, linear regression, R = 0.93, p = 3.23 × 10−5). Thus, microbial denitrification seems an unlikely cause for the observed pattern, but cannot be ruled out based on the available data. The δ18O and δ15N isotopes in nitrate analyzed in this study form two clusters. The first cluster (δ18ONO3- > +4‰, δ15NNO3- > +6‰, Fig. 4) lies within the oxygen isotope range of nitrate produced by nitrification of soil NH4+ in equilibrium with δ18OH2O of the collected fresh water samples, assuming that nitrifying bacteria incorporate two oxygen from soil water and one from atmospheric O2 during nitrification (Mayer et al., 2001). The second cluster exhibits lower δ18ONO3and δ15NNO3- values (Fig. 4) that seem to be explainable only by nitrification of NH4+ from fertilizer and soil NH4+, in this case increasingly incorporating oxygen from water during nitrification under increasingly anoxic conditions. Thus, δ18ONO3-- values can be lowered to a minimum of −5‰ (the minimal observed δ18OH2O values). Because NH4+ is immobile in soils due to adsorption and ion exchange on clay

islands that show highest rainfall at high altitudes (Pasturel, 1993). A previous study in the north – western corner of Tahiti, greater Papeete area, rules out main infiltration at low (< 500 m) or intermediate (500–1000 m) altitudes based on isotopic composition of rain (Hildenbrand et al., 2005). The therein published relationship between δ18OH2O in rain and altitude suggests recharge area elevations of around 1500 m for the sampled fresh water. An exact recharge area cannot be determined using the previously published isotopic data because significant seasonal and yearly variations in the isotopic compositions have been reported for e.g. Hawaii (Scholl et al., 1996) and Jamaica (Ellins, 1992). Rain would infiltrate the aquifer at or above the geological contact between the impermeable base of the second shield and the overlying fractured volcanic flows (Hildenbrand et al., 2005). Based on the δ18OH2O data in groundwater samples presented in this study, the main groundwater recharge on Tahiti and Moorea occurs mostly above 400 m. However, at such high altitudes no significant natural nutrient sources are expected on these islands. Accordingly, the locally elevated nutrient concentrations in some groundwater samples, including SFGD, indicate that some recharge also occurs at lower altitudes where human activities provide high concentrations of nutrients. These enter the aquifer in small volumes of low altitude recharge and mix with the high altitude recharge resulting in locally elevated NO3− and PO43− concentrations. The low mineralization of fresh water on Tahiti and Moorea (minimal EC = 97 μS cm−1, Table 1) most likely results from high mean annual precipitation and short groundwater residence times in the aquifer. Groundwater residence times in Tahiti were estimated to below 50 years (Hildenbrand et al., 2005). Less dilution of fresh water from Moorea than from Tahiti could be owed to several factors: First, annual precipitation is lower on Moorea than on Tahiti. Moreover, Moorea is older than Tahiti, and therefore weathering is more advanced, resulting in porous aquifers and a more gentle topography. If the hydraulic head resembles the topography, it is also less steep, hence the flux of water through aquifers may be slower. In addition to less precipitation on Moorea, this should result in longer residence times in the aquifer and longer reaction times of groundwater with aquifer minerals. Generally, higher DSi concentrations in samples from Moorea (Fig. 5d) also suggest longer residence time of groundwater in the aquifer. 5.2. Sources of nutrients On Tahiti and Moorea, excluding obvious outliers, groundwater (including SFGD) shows higher values of nutrient concentrations with larger variability (nitrate, phosphate and dissolved silica) as compared to stream water samples (cf. Fig. 3). Stream water is a mixture of groundwater and surface runoff in variable proportions. Consequently, nutrient concentration in stream water depends on the concentration in the contributing groundwater and in the fraction of surface water runoff at the time of sampling. Surface water usually contains lower DSi and PO43− concentrations than groundwater. This is because soils on Tahiti and Moorea can be expected to be depleted of primary minerals due to fast weathering, as has been observed on Hawaii (Vitousek et al., 1997). Contrarily, groundwater may equilibrate with a much larger reservoir of still “fresh” minerals and for a longer time (Schopka and Derry, 2012). Thus nutrients from other sources (such as fertilizer, plant litter, etc.) also have more time to accumulate in groundwater as compared to surface water. Nutrient concentrations in the rain water sample were 1 μmol L−1 NO3− (insufficient for isotope analyses), 1 μmol L−1 NH4+ and 0.6 μmol L−1 PO43−. Accordingly, rain water should not contribute significantly to the nutrient budget in fresh water bodies. 5.2.1. Nitrate On Tahiti, high NO3− concentrations coincide with areas of high anthropogenic activities (see study site description, Fig. 5a). While no formal land use map of Tahiti is available, land use around the sampling 186

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Fig. 5. Nitrate and phosphate concentrations and δ15NNO3- for stream water, groundwater and SFGD samples from Tahiti (a, c and e, respectively) and for Moorea (b, d and f, respectively). Note the different scaling for Tahiti and Moorea. (For interpretation of the references to colour in this figure legends, the reader is referred to the web version of this article.)

minerals (Alshameri et al., 2018) and is also lost to the atmosphere by volatilization (Paramasivam and Alva, 1997), soil NH4+ is an unlikely source for both clusters, especially for samples with high NO3− concentrations. A more likely explanation for the observed isotopic compositions is immobilization of NO3− as organic N, subsequent remineralization to NH4+, and finally nitrification back to NO3−. (“mineralization-immobilization turnover concept”, Jansson and Persson, 1982; Mengis et al., 2001). During this process δ15NNO3- values remain unchanged in case of quantitative (i.e. using all available NO3−) MIT, or are lowered compared to the source NO3− if the MIT process is incomplete. The most sensitive step in this case will be the final nitrification step – ammonia oxidation is tied to very high isotope effects (Casciotti et al., 2003), and thus, especially incomplete ammonia oxidation will have a notable effect on the resulting bulk isotope values in nitrate. If the MIT concept applies to our situation, the main N source to both clusters of samples is NO3− fertilizer with an increasing contribution from manure/septic waste with increasing NO3− concentrations and increasing δ18ONO3- and δ15NNO3- values that are observable in the first cluster. Manure and septic waste are usually characterized by δ15NNO3- > 4‰, which applies to the first cluster (Widory et al.,

Fig. 6. δ15N ratios versus NO3− concentrations groundwater and SFGD from Tahiti showing increasing δ15N ratios with increasing concentrations.

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to low background levels. These additions stem from the populated areas along the coast, while most of the island's interior is uninhabited. While a fertilizer signal in δ15NNO3- and δ18ONO3- values can be seen in many samples, elevated concentrations of NO3− in waters correlate with isotopic signatures characteristic for septic waste/manure. While NO3− and PO43− concentrations are positively correlated, the total PO43− concentrations are too low to robustly invoke non-natural sources. In addition, some PO43− is mobilized within the aquifer at the SGD sites since PO43− concentrations are higher in SFGD than in groundwater. On Moorea PO43− is also present in all fresh water samples and concentrations in SFGD are higher than in groundwater. Nutrients in waters on Moorea originate from the same anthropogenic sources as on Tahiti, which is supported by the similar nitrate N and O isotope data. The processes governing nutrient reactions in the aquifer seem different between the islands, however. On Moorea, nitrification of surface NH4+ in the unsaturated soil profile is a likely source of nitrate, while on Tahiti, the MIT process transfers NO3− from fertilizer into NO3- with a different isotopic signature. Although the two studied islands are geochemically and geologically similar, different processes dominate the coastal nutrient fluxes. Thus, studies of SFGD nutrient fluxes from volcanic islands need to account for regional differences between study locations despite seemingly similar settings.

2004, 2005). On Moorea, high anthropogenic activities coincide with NH4+ enriched water samples at some sites, which suggests that NH4+ (and thus also nitrified NO3−) is of anthropogenic origin. Furthermore, on Moorea NO3− concentrations are inversely correlated with NH4+ concentrations (Spearman Rank Order Correlation, R = −0.62, P = 0.02), suggesting nitrification of surface NH4+ in the unsaturated soil profile as a likely source of nitrate. NO3− concentrations in the study area correlate positively with 15 δ NNO3- values. Also, some high nutrient concentration samples clearly coincide with intense anthropogenic land use in the inferred catchment (Fig. 5). In natural parts of the study area, the NO3− concentrations are low compared to concentrations measured in other regions (e.g. temperate forests: Spoelstra et al., 2010). This points towards anthropogenic inputs of NO3− adding to naturally low background concentrations. 5.2.2. Phosphate On Tahiti, a positive correlation of PO43− concentrations with NO3− concentrations (linear regression; R = 0.4; p = 0.004) could hint at an anthropogenic origin of PO43−, but the PO43− concentrations are too low to clearly point toward that. A natural contribution to the PO43− load on both islands is evident in the presence of a minimum PO43− concentration of 0.37 μmol L− in all samples. This source is probably due to weathering of the basaltic basement and aquifer rocks, because basaltic and related rocks are known to contain appreciable amounts of phosphate minerals (Hartmann et al., 2012). Lavas from Tahiti Nui contain between 0.3 and 1.2 wt.% P2O5 (Hildenbrand et al., 2004). SFGD systematically contains higher PO43− concentration than groundwater samples. This could be attributed to 1) chemical weathering of rocks adding PO43− to the groundwater until it discharges in the ocean as the FSGD likely had a longer residence time than upstream groundwater (cf. Lasaga, 1984); or 2) less anthropogenic PO43− input due to changed regulations in the past decades (cf. Köhler, 2006), or 3) desorption processes in the subterranean freshwater/saltwater mixing zone (cf. Flower et al., 2017). Unfortunately, at this point it seems impossible to disentangle the individual processes leading to the elevated PO43− concentrations.

Acknowledgements We are grateful to Pierre Sasal and Herehia Helme for sharing their local insight and for support in the field. We would also like to thank M. Birkicht, J. Mawick and T. Sanders for their help with the chemical analyses. The comments of Michael Kersten, the associate editor, and two anonymous reviewers helped to improve this manuscript. This research was funded through the Federal Ministry of Education and Research (BMBF), Germany (Grant No #01LN1307A to N. Moosdorf). The reported data set can be accessed at https://doi.pangaea.de/10. 1594/PANGAEA.885682. Appendix A. Supplementary data

5.2.3. Implications for coral reef conservation Elsewhere, local studies report examples of human activities that are responsible for high nutrient concentrations in fresh water samples. SFGD transports these nutrients to the coastal sea and its ecosystems. High groundwater nutrient fluxes were related to the degradation of reef ecosystems (Kim et al., 2011) and loss of species diversity (LaPointe, 1997). Depending on the respective flux, the observed anthropogenic nutrient enrichment of groundwater and hence SFGD poses a potential threat to these sensitive ecosystems. This becomes of growing concern in light of a growing population and ongoing agricultural development within SFGD associated catchments and the resulting increase in nutrient addition, which stands in stark contrast to the uniformly low nutrient concentrations in sea water samples. On Tahiti and Moorea, coral reef degradation could result in economic problems, e.g. a decline in dive tourism and a reduced catch for local fishermen. Consequently, reef management must involve sensible land management, such as establishing groundwater conservation areas in order to ensure coastal water quality. In this regard quantifying SFGD fluxes and groundwater abstraction, as well as identifying groundwater flow paths and ages should provide a basis for qualified decisions.

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6. Conclusions We deduce the sources of nutrients delivered to the coast via fresh submarine groundwater discharge on Tahiti and Moorea, French Polynesia based on coupled N and O isotope ratio analyses in dissolved NO3−. On Tahiti, fertilizer and septic waste/manure locally add nutrients 188

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