Environ Geol (2009) 58:447–454 DOI 10.1007/s00254-008-1516-5

ORIGINAL ARTICLE

Coastal pollution due to increasing nutrient flux in aquaculture sites C. P. C. David Æ Y. Y. Sta. Maria Æ F. P. Siringan Æ J. M. Reotita Æ P. B. Zamora Æ C. L. Villanoy Æ E. Z. Sombrito Æ R. V. Azanza

Received: 17 April 2008 / Accepted: 12 August 2008 / Published online: 2 September 2008 Ó Springer-Verlag 2008

Abstract The supply of nitrogen and phosphorus in coastal zones through time is reflected in the nutrients’ concentration in the sediment record. Five aquaculture sites in the Philippines were investigated in an effort to establish how long-term changes in land and coastal water use could have led to biogeochemical modifications affecting the coastal ecosystem. Samples from study sites show a narrow concentration range for nitrogen and did not reveal any significant trend through time. In contrast, phosphorus concentrations in most sites start at less than 20 ppm in sediments 30 years and older. The phosphorus value continuously increase in younger sediments, with each site having a different magnitude change as well as timing of when the major increase happened. The uppermost 10 cm, representing the last 15 years in sites with age control, typically show a 2- to 3-fold increase in P load values. Historical increase in nutrient load also coincides with harmful algal bloom events in each area; when effective P input exceeded 130 kg/km2 per year. Lastly, the observed increase may be attributed to several factors including

C. P. C. David (&) National Institute of Geological Sciences, University of the Philippines Diliman, 1101 Quezon City, Philippines e-mail: [email protected]; [email protected] Y. Y. Sta. Maria
F. P. Siringan
J. M. Reotita
P. B. Zamora
C. L. Villanoy
R. V. Azanza Marine Science Institute, University of the Philippines Diliman, 1101 Quezon City, Philippines E. Z. Sombrito Philippine Nuclear Research Institute, Department of Science and Technology, Commonwealth Avenue Diliman, 1101 Quezon City, Philippines

physical attributes of the area, urbanization of coastal zones, but most importantly in the proliferation of aquaculture activities. Keywords Sediments

Algal blooms
Aquaculture
Phosphorus


Introduction Ocean upwelling provides the necessary nutrients to sustain primary productivity in the near-shore environment (Bunt 1973). However, oversupply of limiting nutrients such as nitrogen (as nitrate or ammonia) and phosphorus (as phosphate) in the coastal zone can result in eutrophication and more frequent hypoxia events (Turner and Rabalais 1994; Turner et al. 2006), large phytoplankton blooms (Azanza and Miranda 2001; Phillips et al. 2006; Beman et al. 2005), lower species diversity and richness (Verlecar et al. 2006), and general modification of the ecosystem (Yap et al. 2004; Vuorio et al. 2005). Major sources of nutrients come from sewage outfalls, agriculture runoff, increased erosion and intensive aquaculture activities. Because of these and the consequent decline in aquatic resources, coastal area management is being pushed to reverse such predicament (Chua 1992). A vital piece of information that may aid in coastal management is the determination of nutrient loading in each area to establish natural baseline values and trace nutrient input of anthropogenic origin through time. There are numerous processes that can affect the fate of nutrients in the marine system (Thouvenot et al. 2007) but in general, coastal sediments have been shown as an important sink for a host of elements including nitrogen and phosphorus (Vaalgamaa and Conley 2008). Several

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studies have used nutrient concentrations in sediments to trace the historical input of these elements into the coastal region (Church et al. 2006; Teodoru et al. 2007). Lastly, the importance of nutrients in sediments far extends from being a surrogate for nutrients in the water column as the sediment fraction’s exchangeability and availability persist in the marine environment and therefore may easily transfer between media at their interface (Coelho et al. 2004). The Philippines is estimated to produce more than 2.48 million metric tons of fish every year as fish remains as the population’s main source of protein in its diet (FAOSTAT, unpublished report). Moreover, the contribution of captured fisheries is not expected to grow much, in fact it is declining (Alin˜o, unpublished report), such that aquaculture production will likely shoulder the burden of the increasing demand (Stobutzki et al. 2006). Unfortunately, excess undigested and uneaten feeds and feces from intensive fish farming result in a direct input of carbon, nitrogen and phosphorus into the system. In Bolinao, Pangasinan, half of the estimated daily feed input of 2 kg/ m2 directly contributes to sedimentation (Reichardt et al. 2006). Moreover, fish farming also increases the population in these coastal zones which brings about more land-based activities that also contribute to the flux of nutrients into the marine ecosystem.

The study sites Five major aquaculture areas in the Philippines were investigated to determine nutrient levels and see if these active fish farming areas show any significant change in nutrient concentrations through time. The study areas include: the Malampaya Sound and Honda Bay in Palawan, Manila Bay, Bolinao in Pangasinan, and Milagros Bay in Masbate Island (Fig. 1). While each of these areas has coastal zones developed for aquaculture, significant differences in fish cage volume, land habitation, and coastal morphology exist. The Malampaya Sound is a narrow 30-km embayment facing the South China Sea in the island of Palawan. It is a major fishery (capture fisheries) and shellfish producing region but its coastline remains relatively uninhabited. Honda Bay has slightly less aquaculture development and differs morphologically from Malampaya in that it is a less enclosed bay, surrounded by shallow coral reef platforms and islands and opens towards the Sulu Sea. Manila Bay has a surface area of about 1,800 km2 and is adjacent to the urban centers of Metro Manila, Bulacan and Cavite. Fish cages are estimated to cover an area of 39 km2 of the coastal region (UNDP, unpublished report). About 250 km north of Metro Manila is Bolinao, Pangasinan.

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Fig. 1 Map of the study sites

This aquaculture site where more than 1,100 fish cages are set up mainly to culture milkfish (Chanos chanos) is along a narrow strait between mainland Luzon and Santiago Island. Lastly, Milagros Bay in Masbate is located near the inner Visayan Islands and is likewise shielded from the Pacific Ocean to the east. It is a developing aquaculture site where shellfish is the major produce. Its embayment is also wider compared to the other sites. All sites have experienced at least one harmful algal bloom event in the past. In addition, Manila Bay and Bolinao have seen several fish kill events due to hypoxia.

Environ Geol (2009) 58:447–454 Table 1 Sediment cores used for this study and the overlying water residence time in the area where the samples were collected

449

Sample #

Location

Length of core (cm)

Depth (m)

Estimated residence time (days) [50

MS-2

Malampaya Sound

30

4

MS-8

Malampaya Sound

34

14

40–50

OMS-11

Malampaya Sound

28

33

20–40

HB-5

Honda Bay

44

4

10–30

HB-SALV

Honda Bay

32

8

30–40

Tambac-1

Bolinao

50

3

24–27

BOL-1

Bolinao

64

11

9–12

BOL-3

Bolinao

40

4

18–21

BOL-6

Bolinao

40

22

CAV-2

Manila Bay

40

9

Not available

MB-14 MAS-14

Manila Bay Masbate

34 26

35 11

Not available 60–80

Methodology Sampling in the study areas were done from 2005 to 2007 (Table 1). Sediment cores were taken from depths ranging from 3 to 35 m. As much as possible to ascertain the representativeness of each core, all sampling stations were selected to be at least 100 m away from any cultured fish cages in order not to be affected by localized nutrient spikes from fish feeds and feces. For shallow sampling areas in Bolinao, an improvised PVC push core was used while a PVC-lined 1.2-m gravity corer was utilized for the rest of the stations. All cores were extruded on site and sampled every 2 cm (every 1 cm for the topmost 20 cm). The sediment samples were stored in polyethylene bags and transported at 4°C. In the laboratory, each sample was wet sieved through a 63-lm mesh to remove any chemical bias due to grain size distribution. These are then oven-dried at 30°C. Available phosphorus and total nitrogen were analyzed using the standard Kjeldahl method at the Bureau of Soils and Water Management, Department of Agriculture. The sedimentological work involved the use of loss on ignition (LOI) analysis in the determination of bulk density, and the organic/carbonate/silicate fractions in sediments. For bulk density computation, each sample was homogenized, after which a 5-ml syringe was used to extrude a cylindrical subsample of equal volume, which was then weighed. The samples were oven-dried at 60°C until constant weight is achieved. The dried subsamples were redried at 105°C, pulverized and burned at 550°C for 4 h and then at 950°C for an hour to determine respectively the total organic matter and carbonate content of the sediments following the procedure of Heiri et al. (2001). Water residence time is estimated using hydrodynamic tide models for each of the study areas. Virtual particles were scattered uniformly throughout the model domain and allowed to be advected by the tidal currents. The period of

6–9

time from release of a particle until advection out of the model domain through any of the model open boundaries was used as the residence time estimate for the particular area where the particle was released. The final output of the model is a map showing the residence times as a function of space. Areas closer to the open boundaries have lower residence times compared to areas in the interior of the embayments. To constrain sedimentation rates in each study site, radiometric dating of the cores was conducted by the Philippine Nuclear Research Institute (PNRI). Dried 1-g samples are acid digested and spiked with a 208Po tracer for chemical yield measurement and followed by spontaneous plating onto a silver disc (Sombrito et al. 2004). 210 Po and 208Po were detected by counting in an alpha particle spectrometry system using a surface barrier silicon detector for a minimum of 24 h. The 210Pb of a sample was determined by measuring its daughter nuclide, 210Po, which decays by alpha particle emission. Sedimentation rate calculations were made using the Constant Initial Concentration (CIC) Model which assumes that at each stage in accumulation, the initial concentration of 210Pb in the sediment is constant despite any variation that may have occurred in the sediment accumulation rate. In undisturbed cores, 210Pb concentration values must decline monotonically with depth (Goldberg 1963; Robbins and Edgington 1975). The sedimentation rates were determined from the slope of the least squares fit for 210Pb excess values plotted versus depth.

Results and discussion Sediments in investigated sites are predominantly siliciclastics ([75% by weight) which imply a terrestrialdominated source for sediments. All sites also record a very

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slight increase in organic content in younger sediments reflective of the increase in various anthropogenic input into the coastal region. Using water circulation modeling and sediment transport vector calculation (Gao and Collins 1992), an inward transport of sediments predominate in the Malampaya Sound. The same techniques prove that in Bolinao, water and sediments predominantly come into the Caquiputan Strait and Tambac Bay from the Lingayen Gulf during high tide (Zamora, unpublished report). Sedimentation rates vary between the study areas due to the expected difference in sediment input and sources as well as in deposition dynamics (in part due to the morphology of the bays). Variation in sedimentation rates is also evident between stations within each study area wherein sampling stations near the coast and/or near a major river will expectedly show higher sediment deposition than in deeper portions of the bay. Areas with extensively developed aquaculture sites also have relatively higher sedimentation rates due to the addition of fish pellets and increased organic activity. The sediment cores exhibited regular and congruent geochronology based on Pb-210 radionuclides which imply a complete section and fairly regular sedimentation regimes can be identified. Sedimentation rates range from 0.7 to 1.8 cm/year and therefore cores collected typically extend to at least 30 years (the 0.64 m push cores from Bolinao extend to about 80 years). Inside fish cages in Bolinao, sedimentation rates are measured to be *3 cm/ year with minor resuspension occurring except in very shallow areas (Reichardt et al. 2006). Most core sites also record relatively long water residence times ([10 days) typical of embayments which support continuous sedimentation and less sediment reworking. Still, bulk density analysis of sediments shows that the upper layer (up to 2 cm in Malampaya) is composed mostly of low density, high organic content materials which are prone to resuspension. In all cores taken this floc-dominated layer is excluded in the reported data. Nitrogen concentration in sediments for all sites shows a narrow range from 0.05 to 0.20% (Fig. 2). Moreover, N concentration does not show any distinct trend in most cores sampled. Variations are thought to be caused by annual difference in nutrient input, however, homeostatic regulation affects N concentrations in both water and sediment substrate (Smayda 2002) and that denitrification processes may reduce nitrogen input by 55% (Verlecar et al. 2006). Lastly, although nitrogen and phosphorus follow different biogeochemical cycles, N and P loading is tightly coupled—following a fixed ratio in the ocean (Smith et al. 2003). Still, the long core retrieved in Bolinao shows evidence of an increased and more variable N concentration starting in the 1980s. This is also corroborated by increased nitrogen concentration in the Bolinao

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water column during this time (San Diego-McGlone and Ranches 2003). Changes in sediment nitrogen values may not reflect year-to-year variations but may show long-term adjustments in nitrogen concentration in coastal regions. Unlike nitrogen, nutrient mass balance studies estimate that phosphorus in sediments represents 80–90% of the total input from its sources and therefore may be a better representation of nutrient loading in the area (Tyrrell 1999; Teodoru et al. 2007). P concentrations in the study sites range from 10 to 90 ppm, however, each site shows distinct trends unlike the N profiles (Fig. 3). The wide and relatively undeveloped Honda Bay exhibits a tight P concentration averaging about 22 ppm throughout the entire record with HB-5 even showing a decreasing P trend. A consistent P concentration is observed in the inner Malampaya Sound with an average of 15 ppm (MS-2). Increasing trends are observed in mid and outer Malampaya Sound (20–85 ppm), Bolinao (20–90 ppm), Manila Bay (20–60 ppm) and in Masbate (15–40 ppm). In all sites, the baseline value for phosphorus is about 15–20 ppm. The mid Malampaya Sound core (MS-8) show spikes in P values throughout the column but a definite increase is observed starting in the early 1970s, the inception of aquaculture in the area. Fluctuations in P concentrations are thought to be due to its proximity to nutrient sources (fish cages and river outlets) as well as a longer residence time which result to the preservation of short term nutrient fluctuations in the sediment record. In contrast, OMS-11 from the outer Malampaya Sound show a more uniform and relatively lower sediment phosphorus concentration that also increases through time. This profile is consistent with fewer sources of nutrients in the outer sound and a shorter residence time that tend to produce a more uniform sediment geochemistry due to dilution and efficient redistribution of sediments. Two algal bloom episodes (2000 and 2005) coincide with maximum P concentration. MS-2 in the inner sound shows little variation as this portion of the bay is not affected much by ocean circulation. The sediment cores from Bolinao show trends reflective of the stations’ subtle differences. TAMBAC-1 is situated near the southern end of Bolinao where abundant fish pens are set-up. Work on pollution dispersal in the area also suggest that nutrients from north of Caquiputan Strait can accumulate in Tambac (Magdaong, unpublished report). Several rivers also debouch into Tambac Bay. Core data indicate a rapid increase in sediment phosphorus in this area in the last decade (upper 10 cm). In contrast, BOL-1 located at the entrance of the larger Lingayen Gulf shows a gradual increase in P values starting around 50 years ago. Only sediments representing the last 5 years saw a rapid 1.5-fold increase in P values. Three recorded algal blooms in Bolinao (2002, 2003, and 2004) coincide with both cores’ recorded rapid P increase.

Environ Geol (2009) 58:447–454

451

Fig. 2 Nitrogen concentration (%) for core samples collected

BOL-3 was taken along the Caquiputan Strait which is another area with dense fish cages. Phosphorus in Caquiputan sediments was at the 15 ppm baseline value prior to 1993 which marked an unusual decrease in P values to

less than 10 ppm. The same trend is shown in the BOL-6 core which recorded the highest P average as this core site is very near to a cluster of fish cages. The reason for the resetting of P values in Bolinao at the height of aquaculture

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Fig. 3 Phosphorus concentration (ppm) for core samples collected. The gray region represents the years when harmful algal blooms occurred in the area

is still unresolved. However, this is consistent with observations by Holmer et al. (2003) that phosphorus flux at the sediment–water interface in very active fish pen sites tend to favor P release to the water column. Several factors were cited to contribute to this efflux including: sediment oxygen uptake, reduced phosphate mineralization, sediment binding capacity (release of P due to low oxygen) and

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nutrient uptake due to benthic microalgae proliferation. This unusual decrease in P concentration may also signify the point when reducing conditions were realized in the bottom sediments. Aigars (2001) and Belias et al. (2007) confirmed the release of sediment P back into the water column once reducing conditions are realized. Consistently low dissolved oxygen levels (\3 mg/l) have been reported

Environ Geol (2009) 58:447–454

453

in the Caquiputan Strait which have not been observed in the other study sites (this study; Magdaong, unpublished report). The increasing sediment P trend that followed this period of decreased P ensued in mid to late 1990s can be partly explained by the 33-fold increase in the number of fish cages that was recorded during this time (Padayao and San Diego-McGlone 2004). Increased aquaculture activity is accompanied by an increase in nutrient input (and consequently the deposition of nutrients in sediments) which may have already exceeded the P efflux, thus reflecting a sediment P increase despite of prevailing reducing conditions. A moderate increase of 20 ppm (from a 10 ppm baseline value) in sediment P is consistent with the relatively developing aquaculture area in Masbate. The same is observed in Manila Bay wherein a definite increase in sediment P is observed starting in 1993. Again this coincides with the intermittent algal blooms in the area from 1992 to 1996. Lastly, the Manila Bay samples show the highest P values in older sediments (before 1990), reflective of a longer history of elevated nutrient input in the area which potentially can be attributed to urban runoff. The internal nutrient pool as estimated from phosphorus loads prior to major aquaculture development in the 1970s is below 100 kg/km2 per year in Malampaya and Bolinao Table 2). More recent total P loading (assuming no environmental loss) in these aquaculture sites reached more than 200 kg/km2 per year in the last 15 years, an increase of 2–3 times the natural P load. The high P in the BOL-6 Table 2 Average phosphorus loading in kg/km2/yr for cores with age control Year

MS-8

2001–2005

OMS-11

BOL-1

BOL-3

BOL-6

195.4

204.2

159.9

452.5

1996–2000

144.5

167.0

191.1

90.1

474.7

1991–1995

134.2

157.2

130.0

95.4

649.9

1986–1990

132.7

141.6

169.9

167.1

1981–1985

120.7

133.8

125.5

152.6

1976–1980

68.6

123.7

111.1

1971–1975 1966–1970

121.6 35.6

116.3

142.2 105.1

1961–1965

25.6

core was calculated to have more than 400 kg/km2 per year. Except for the high P loading in Bolinao, these values are similar to what have been recorded in other aquaculture sites worldwide—230 kg/km2 per year, intensive shrimp farming in Brazil (De Lacerda et al. 2006); 120 kg/km2 per year, aquaculture in the Baltic Sea (HELCOM 1993). More importantly, the concurrent high P loads and algal bloom events in all Philippine sites strengthen the role of nutrient concentration in algal blooms. Lastly, these areas recorded algal bloom events at a time when more than 130 kg P/km2 per year effective input is realized. This value can be used as a working limit for phosphorus input into coastal zones to reduce the risk of algal blooms.

Conclusions Comparison between absolute sediment P concentrations between aquaculture areas is not possible due to site-specific differences. Still, in most sites investigated, higher than the natural phosphorus load is identified in the last 15 years when the volume of fish farming increased in these sites. The significant increase in nutrient input has an important effect in the physico-chemical and biological composition of the oceans and can result to undesirable implications such as the instigation of harmful algal blooms. Although it is established that anthropogenic seedings are not the sole cause for bloom stimulation events, nutrient input represents the first phase in a multiple-step process towards these events (Avnimelech and Wodka 1988). Nutrient flux and its corresponding problems will only increase as we develop our coastal resources further to satisfy the increasing demand for marine produce. The next step is to use the historical nutrient data coupled with new monitoring data to confirm the probable limits for nutrient loading of each area to be used for coastal management purposes. Acknowledgments This study is funded through the Department of Science and Technology, Philippine Council for Aquatic and Marine Research and Development Grant entitled, ‘‘Dinoflagellate cysts in selected mariculture sites: Implication to management’’.

59.9

1956–1960

75.3

1951–1955

66.8

1946–1950

58.9

1941–1945

75.7

1936–1940

57.4

1931–1935

55.2

1926–1930

50.1

1921–1925

43.8

1916–1920

72.6

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