Nutrient inputs from submarine groundwater discharge on the Santiago reef flat, Bolinao, Northwestern Philippines

Marine Pollution Bulletin 63 (2011) 195–200

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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul

Nutrient inputs from submarine groundwater discharge on the Santiago reef flat, Bolinao, Northwestern Philippines Maria Isabel S. Senal a,⇑, Gil S. Jacinto a, Maria Lourdes San Diego-McGlone a, Fernando Siringan a, Peter Zamora a, Lea Soria a, M. Bayani Cardenas b, Cesar Villanoy a, Olivia Cabrera a a b

Marine Science Institute, University of the Philippines, Diliman, Quezon City 1101, Philippines Geological Sciences, University of Texas at Austin, Austin, TX 78712, USA

a r t i c l e

i n f o

Keywords: Submarine groundwater discharge Eutrophication Nutrient flux Electrical resistivity 222 Rn

a b s t r a c t Submarine groundwater discharge (SGD) on the reef flat of Bolinao, Pangasinan (Philippines) was mapped using electrical resistivity, 222Rn, and nutrient concentration measurements. Nitrate levels as high as 126 lM, or 1–2 orders of magnitude higher than ambient concentrations, were measured in some areas of the reef flat. Nutrient fluxes were higher during the wet season (May–October) than the dry season (November–April). Dissolved inorganic nitrogen (DIN = NO3 + NO2 + NH4) and soluble reactive phosphorus (SRP) fluxes during the wet season were 4.4 and 0.2 mmoles m 2 d 1, respectively. With the increase population size and anthropogenic activities in Bolinao, an enhancement of SGD-derived nitrogen levels is likely. This could lead to eutrophic conditions in the otherwise oligotrophic waters surrounding the Santiago reef flat. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Submarine groundwater discharge (SGD) refers to the water that comes out of the pores of sediments into the ocean–land boundary (Mulligan and Charette, 2009). Several studies showing that SGD makes a significant contribution to the nutrient budget of coastal waters have led to growing research interest in this phenomenon (Burnett et al., 2001; Johnson et al., 2008; Lee et al., 2009; Niencheski et al., 2007). This contribution to the nutrient budget is particularly important in areas where no major rivers are present, such as on the Santiago reef flat in Bolinao, Pangasinan, Philippines. Groundwater often has a high nutrient content due to recharge of waters containing land-derived nutrients and other pollutants into aquifers (Nolan et al., 2002). This process is of particular concern in highly urbanized and populated areas, where groundwater is more susceptible to contamination (Umezawa et al., 2008). Because SGD can introduce large amounts of nutrients into the water column, it has been linked with the occurrence of harmful algal blooms (Hwang et al., 2005; Lee et al., 2010), eutrophication, and species zonation on reefs (Johannes, 1980). Coral reefs, which naturally thrive in low-nutrient waters, are at risk when increasingly exposed to high-nutrient and low-salinity

⇑ Corresponding author. Address: Marine Science Institute, Velasquez Street, University of the Philippines, Diliman, Quezon City 1101, Philippines. Tel.: +63 2 3939924; fax: +63 2 9223944. E-mail addresses: [email protected], [email protected] (M.I.S. Senal). 0025-326X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2011.05.037

conditions. The goal of this study was to identify the sources and characteristics of the SGD on the Santiago reef flat, which is a site with one of the most extensive seagrass meadows in the Philippines and where major coral restoration efforts are underway. 2. Methodology 2.1. Study site Santiago Island is located in northwestern Philippines at approximately 16.411°N and 119.907°E (Fig. 1). It is surrounded by a reef flat with an area of approximately 32 km2 and is adjacent to the South China Sea (Nañola, 2002). The reef flat itself consists mostly of limestone and carbonate sediments. 2.2. Nutrient sampling Nutrient sampling was conducted in December 2008, April 2009, and August 2009. Sampling in December 2008 was spatially extensive in order to identify possible point sources of SGD on the reef (Fig. 1). Seawater samples were pumped and collected from <1 m above the seabed from 31 random sampling points. Samples were also obtained from the pump wells on Santiago Island (Fig. 1). Time-series sampling was conducted during the dry season (April 2009) and wet season (August 2009) off Silaki Island to observe the behavior of the SGD during a 24-h period. Water samples were obtained every 3 h from six 15  57 cm Lee type seepage drums (Lee, 1977). The drums were placed at different sampling points

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Fig. 1. Sampling stations in December 2008 and location of Lee type seepage meters during the April and August 2009 sampling (inset).

north of Silaki Island (Fig. 1, inset). Water samples from areas outside the range of the seepage meters (i.e., ambient samples) were obtained using Niskin bottles. No night time sampling was conducted in April 2009. Samples from the dug wells on Silaki Island were obtained in August 2009 using Niskin bottles. Nutrient samples were filtered using 0.45 lm cellulose acetate syringe filters, and the filtrates then were frozen at 8 °C until analysis for NO3 + NO2, NH3, and PO4. Silicate samples were stored at 5 °C. The samples were analyzed using methods adapted for the Skalar San + analyzer (Skalar Analytical B.V., Breda, The Netherlands) and wet chemical techniques described by Strickland and Parsons (1972). 222 Rn concentrations were measured using a Durridge RAD 7 analyzer (Durridge Company Inc., MA, USA). The average integrated seepage flux was calculated from the 222Rn readings. Nutrient fluxes were calculated as a product of the average integrated seepage flux and the average nutrient concentration from the seepage meters (Taniguchi et al., 2008). Electrical resistivity (ER) measurements were continuously profiled using a towed SuperSting R8 marine ER system (Advanced Geosciences, Inc., TX, USA) similar to that used in Breier et al. (2005) as described by Cardenas et al. (2010).

3. Results

of magnitude higher than those of the rest of the reef (the highest of which was 0.60 lM). For example, in Malilnep Channel, the highest measured value for NO3 + NO2 was 125 lM, which is four orders of magnitude higher than that found in most areas of the reef. This value was measured in the channel at a site <1 m deep. A plot of the nutrient concentrations also showed that the highest NO3 + NO2 and NH3 concentrations (Fig. 2a and b) were near Malilnep Channel, but this was not the case for PO4 and SiO3 (Fig. 2c and d). Electrical resistivity (ER) profiles measured along Malilnep Channel showed the presence of interspersed relatively fresher water below the seafloor (Cardenas et al., 2010). An ER profile between Santiago and Silaki Islands also showed the presence of a saltwater wedge beneath the seafloor, which is a possible indication of tidal or wave pumping in the area. Most of the reef had ER values >0.5 X m. 222Rn readings over the entire reef flat ranged from 0.4 to 3.9 dpm/L. Areas with high 222Rn activities had correspondingly high ER values (Cardenas et al., 2010). On Santiago Island, NO3 + NO2 concentrations in the pump wells were highly variable and ranged from 0.1 to 293 lM ± 120. Well samples with high NO3 + NO2 concentrations (11–292 lM) were accompanied by relatively low NH3 concentrations of 2–3 lM, whereas high NH3 concentrations (19–76 lM) were accompanied by low NO3 + NO2 concentrations (<0.1–1 lM). Generally, high concentrations of NO3 + NO2 were found with low concentrations of NH3 (Fig. 3).

3.1. Spatial sampling (December 2008) 3.2. Temporal sampling (April and August 2009) The NO3 + NO2 concentrations in most areas of the reef were 0.01 lM (Fig. 2a). However, a number of sampling points north of Silaki Island had NO3 + NO2 concentrations that were an order

Nutrient concentrations measured from the seepage meters were generally higher than ambient values, even during the dry

M.I.S. Senal et al. / Marine Pollution Bulletin 63 (2011) 195–200

197

Fig. 2. Nutrient concentrations in the Santiago reef flat, Bolinao, northwestern Philippines.

Fig. 3. Dissolved inorganic nitrogen concentrations in groundwater samples from the pump wells in Santiago Island. The broken line divides the higher and lower range of values.

season, although the difference was not significant (Table 1). NH3 concentrations inside the seepage meter (1.9–22 lM) were an order of magnitude higher than ambient values (0.7–1.8 lM). The changes in PO4 and NO3 concentrations over the duration of the 24-h sampling period and with the tides were determined (Fig. 4). Nutrient concentrations increased during low tide. This behavior of nutrients supplied by SGD is expected: As hydrostatic pressure on the seafloor is relieved during low tide, SGD becomes stronger due to larger pressure gradients between the seawater column and the pore water. The difference between ambient nutrient concentrations and those measured from the seepage meter was more pronounced

in August 2009 than in April 2009 (Table 1). The nutrient concentrations measured from the seepage meters also were significantly different between April and August 2009, with concentrations in August 2009 one order of magnitude higher than those in April 2009, especially for PO4 and NH3 (the latter reached 90 lM in August). However, the increase in the SiO3 concentration between sampling dates was minimal. Significantly higher nutrients fluxes were measured during the dry season than the wet season. NH3 accounted for 98–99% of the total dissolved inorganic nitrogen (DIN) flux that was measured in both sampling periods (Table 2). Nutrient concentrations in water samples taken from dug wells on Silaki Island, north of the reef flat, were generally higher than those in the seepage meter samples, except for NH3. The NO3 + NO2 concentrations of samples from the groundwater dug wells were 2–5 orders of magnitude higher than the concentrations measured from the seepage meters, with values as high as 1644 lM for the wells (Table 1). NH3 concentrations of dug well samples were an order of magnitude lower than the samples from seepage meters. SiO3 concentrations were an order of magnitude higher in the dug well samples than in seepage meter samples.

4. Discussion Dug and pump wells with high NH3 concentrations (Fig. 3) could be indicative of high amounts of dissolved organic matter in the groundwater and/or could be parts of flow paths connected to areas of intense ammonification (i.e., plumes from septic wastes). In contrast, those with high NO3 + NO2 concentrations (Fig. 4) have been interpreted as belonging to long groundwater flow paths (Andersen et al., 2007; Bowen et al., 2007; Kroeger,

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Table 1 Nutrient concentrations of Silaki Island during the time-series sampling. Sampling date

Water type

April 2009

Seepage meter (n = 10) Range Mean ± s.d. Seawater (n = 10) Range Mean ± s.d.

August 2009

Seepage meter (n = 14) Range Mean ± s.d. Seawater (n = 14) Range Mean ± s.d. Wells in Silaki Island (n = 2) Range

PO4 (lM)

SiO3 (lM)

NH3 (lM)

NO3 + NO2 (lM)

0.3–0.8 0.5 ± 0.1

8.8–26.4 15.3 ± 5.1

1.9–22.0 5.5 ± 6.1

0.2–0.6 0.4 ± 0.1

0.2–0.7 0.4 ± 0.1

4.3–8.3 5.5 ± 1.4

0.7–1.8 1.2 ± 0.4

0.1–0.4 0.2 ± 0.1

0.3–4.6 2.7 ± 1.5

12.4–22.2 19.1 ± 3.4

11.3–89.9 60.4 ± 28.1

0.04–10.6 1.2 ± 2.7

0.08–0.4 0.2 ± 0.1

4.8–15.8 7.6 ± 2.7

1.4–5.7 2.7 ± 0.9

0.001–0.5 0.1 ± 0.2

3.9–9.5

141.1–178.8

3.1–3.9

28.0–1644.1

Fig. 4. Tidal height (smooth line) and the PO4 and NH3 concentrations of seepage meter samples from Silaki Island (marked line) during the time-series sampling on August 2009.

2003); as the groundwater passes through these flow paths, rapid nitrification occurs, which results in elevated NO3 + NO2 concentrations (Bowen et al., 2007; DeSimone and Howes, 1998). In several areas in the United States and the United Kingdom, the increase in DIN in groundwater has been attributed to the increase in wastewater disposal into septic tanks due to increasing population size (Kroeger et al., 2007; Wakida and Lerner, 2005). To date, this process does not appear to be happening on Santiago Island,

but the results of this study will provide a baseline if the population continues to grow and this process begins to occur. Cardenas et al. (2010) suggested that the SGD flow paths from the island into the reef flat are associated with the geologic features (e.g., faults or structures) that cut across Santiago and Silaki Islands and through the reef flat. Therefore, land-derived nutrients can actually be introduced into the reef flat through SGD. The NO3 + NO2 concentrations that were measured north of Silaki Island, north of Santiago Island, and in Malilnep Channel were significantly higher than the concentrations on the rest of the reef (Fig. 2a). The extremely high values of 126 lM and 13 lM in Malilnep Channel cannot be attributed to rainfall or remineralization processes. These NO3 + NO2 concentrations were of the same magnitude as the NO3 + NO2 concentrations of the pump well samples from Santiago Island. This finding suggests that SGD is the likely source of NO3 and probably of other nutrients as well. Swarzenski et al. (2001) observed the same phenomenon in Crescent Beach Spring, Florida, where the SGD was geochemically similar to the artesian groundwater along the coast of Crescent Beach. The 222 Rn readings and high resistivity values in Malilnep Channel, northwest of Santiago Island, and north of Silaki Island, where the NO3 + NO2 concentrations were high, further support this interpretation. SiO3 concentrations from the pump wells on Silaki Island (Table 1) were within the range of the SiO3 concentration in groundwater for the major sandstone and limestone aquifers in England (Lloyd and Heathcote, 1985). The comparable nutrient values of ambient samples and those from the seepage meters indicate that most of the SGD is probably recirculated into the pore sediments (Table 1). The salinity difference between water inside and outside the seepage meters is about 0.05 ppt and appears to be modulated by the tides. The increase in the nutrient concentrations during the wet season (Table 1) despite the lower integrated seepage flux (Table 3) indicates that

Table 2 Nutrient concentrations on reefs where SGD was also found. Reef, Location

NO3 + NO2 (lM)

Discovery Bay, Jamaica 4.3–9.8 Southern Florida, USA 0.4–1.2 Guajaruba Reef, Brazil Dry season 6.1 Wet season 8.2 Shiraho Reef, Japan Summer seasona 3.4–136.1 North of Silaki Island, Santiago reef flat, Philippines Dry season 0.2–0.6 Wet season 0.04–10.6 a

Values were converted into lM from mg/L in the original text.

NH3 (lM) 0.3–0.5 0.2–2.4 10.7 4.8 1.3–2.6 1.2–22.0 11.3–89.9

DIN (lM)

PO4 (lM)

Reference

0.1–0.2 0.8–3.4

Lapointe (1997) Lapointe (1997)

0.4 1.4

Costa et al. (2000)

4.7–138.7

0.09–0.3

Blanco et al. (2011)

1.4–22.6 11.3–100.5

0.03–0.8 0.3–4.6

This study

4.6–10.3 0.6–3.6 16.8 13.0

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M.I.S. Senal et al. / Marine Pollution Bulletin 63 (2011) 195–200 Table 3 SGD, DIN and DIP/SRP fluxes at various coastal sites. Location

SGD flux (m3 m

Kahana Bay, Oahu 0.11a Bangdu Bay, Korea 0.44 Eilat, Israel 0.06–0.26 North Inlet, S. Carolina 0.03 Spencer Beach, Hawaii 0.12–0.17 Kamiloloa, Hawaii 0.18–0.33 Bolinao, Pangasinan, Philippines April 2009 (Dry) 0.16 August 2009 (Wet) 0.07 a

2

d

1

)

DIN flux (mmol N m

2

d

1

)

DIP or SRP fluxes (mmol P m

2

d

1

)

Reference

3.4 (TDN) 21.4 2.9–10 2.42 3.3–4.4 0.7–1.3

0.17 (TDP) 0.16 0.02–2.0 0.91 0.11–0.15 0.16–0.30

Garrison et al. (2003) Hwang et al. (2005) Shellenbarger et al. (2006) Krest et al. (2000) Street et al., 2008 Street et al. (2008)

0.9 4.4

0.08 0.2

This study This study

After Street et al. (2008). Total discharge to Kahana Bay (90,000 m3 d

1

) assumed to be distributed over the entire study area (0.8 km2).

there is an increase of non-recirculated SGD at this time of year and that rainfall influences the transport of groundwater offshore (Blanco et al., 2010; Costa et al., 2006). The significant increase in NH4 and PO4 concentrations in the seepage meter samples in August 2009 compared to the values in April 2009 points to a more direct release of NH4- and PO4-rich sources into the groundwater during the wet season, which is in agreement with Costa et al. (2006). SiO3 concentrations for the wet season were only slightly higher than those for the dry season (Table 1). This result may be due to the dominance of recirculated SGD even during the wet season, despite the increase in non-recirculated SGD. Alternatively, the slight increase in SiO3 during the wet season might be due to the predominance of carbonate substrate in the area, as SiO3 is a product of limestone dissolution (Ford and Williams, 2007; Garcia-Solsona et al., 2010). In reefs fringed by non-carbonate rocks, the increase in silica from SGD is considerable. The DIN concentrations in Malilnep Channel (Fig. 2a) and north of Silaki Island were up to a magnitude higher than those in other reef areas where SGD was also suspected to be a significant source of nutrients (Table 2). In Discovery Bay, Southern Florida and Guajaruba Reef, Brazil the increase in nutrient availability due to SGD was associated with increases in macroalgae and turf in the coral reef area (Costa et al., 2000; Lapointe, 1997; Lapointe et al., 2004). Blanco et al. (2010) suggested that the increase in cyanobacteria in Shiraho Reef, Japan was due to the increase in SGD-derived nutrients. Nitrogen inputs to corals reefs around Ishigaki Island, Okinawa, Japan were found to be of anthropogenic origin and were higher where human and livestock population densities were higher and agricultural activities were more intense (Umezawa et al., 2002). In Bolinao, massive mortalities of coral transplants were recorded around Santiago Island, particularly in Malilnep Channel and northeast of the reef flat, apparently due to the presence of freshwater seepage after a period of heavy precipitation (Shaish et al., 2010). A shift in the benthic composition on the northern part of the reef flat was also observed from 1998 to 2008. By 2008, algae and the blue coral Heliopora coerulea dominated the reef area (Vergara et al., 2010). H. coerulea is known to be tolerant to less saline conditions (Licuanan and Gomez, 1988) and their dominance could be a response to SGD. The range of estimated SGD nutrient fluxes to the reefs of Bolinao is comparable to those from other parts of the world (Table 3). DIN fluxes in Bolinao were higher during the rainy season than the dry season and were within the same order of magnitude as those found in Eilat, Israel (Shellenbarger et al., 2006), North Inlet, South Carolina (Krest et al., 2000), and several coastal sites in Hawaii (Garrison et al., 2003; Street et al., 2008). However, even if the measured nutrient fluxes at the stations studied on the Santiago reef flat were not unusually high, there appear to be widespread (non-point) SGD sources on the reef flat (Cardenas et al., 2010) that may contribute substantially to nutrient loading in the area and, consequently, alter the oligotrophic conditions that currently characterize the coral reef area.

The NH4 and SRP fluxes from SGD (Table 3) were also compared with the nutrient fluxes that were measured from the sediments inside fish pens in Bolinao. Flux estimates inside the fish pens were 1–22 mmoles m 2 d 1 for NH4 and 0.2–4.7 mmoles m 2 d 1 for dissolved inorganic phosphorous (Holmer et al., 2002). The nutrients released from the organic-rich sediments of the fish pens can enhance the productivity of the otherwise oligotrophic waters of Bolinao (Holmer et al., 2002). The nutrient fluxes from SGD (Table 3) measured in the present study were at the lower end of the range of the nutrient fluxes inside the fish pens. Algal bloom occurrences due to eutrophication have already been reported in the mariculture areas of Bolinao (San Diego-McGlone et al., 2008). In the island communities of Santiago and Silaki, SGD may be contaminated by anthropogenic wastes derived from inadequate or absent sewage facilities. The proper disposal of sewage has to be addressed by local government officials because of the likelihood of introducing higher inputs of nutrients and contaminants to reef areas through SGD. This, in turn, could cause coastal waters to have higher dissolved N:P ratios and drive the coastal waters of Bolinao towards P-limitation within the coming decades, perhaps changing the present N-limited coastal primary production (Slomp and Van Cappellen, 2004). If the system shifts towards P-limitation, it could also cause the decline of seagrass cover, as has already occurred in some coastal regions (Armitage et al., 2005), and could compromise one of the most extensive seagrass beds in the Philippines. Moreover, at Bolinao the observed change to algae and blue corals from the previously massive and soft-coral dominated reef flat may have been exacerbated by the entrance of nutrient-enriched SGD into coastal waters. The quantity and quality of SGD in tropical reef areas also warrant further study, as knowledge about the impacts of SGD in the coastal zone can provide the basis for land-use planning and may justify limits on certain types of development, particularly in small island locations where coastal development and management of coastal resources are growing concerns.

Acknowledgements We gratefully acknowledge the GEF-World Bank Coral Reef Targeted Research & Capacity Building for Management Program and Prof. Edgardo D. Gomez for supporting this study. Dr. M. Bayani Cardenas’ contribution was supported by the Philippine Department of Science and Technology Balik Scientist Program.

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