Figure 1. Map of Lingayen Gulf showing the boundaries of the budgets.

The gulf has an average depth of 46 m and volume of 85x10⁹ m³.  It has three major coastal types.  The western section is dominated by fringing reefs surrounding two large islands (Santiago and Cabarruyan Is.) and several smaller ones.  The southern section has a mainly muddy bottom and is where most of the river systems of the gulf are located.   Agno River, the largest river contributing about 67% of the gulf’s surface water discharge of 10x10⁹ m³ yr^-1, is in this section.  Most of the other rivers (e.g., Naguilian/Bauang, Aringay) connect to Lingayen Gulf’s eastern margin (lined mainly by sandy beaches) and constitute approximately 16% of the total surface water discharge.  Altogether six major river systems drain into the gulf.  Groundwater input into Lingayen Gulf is approximately 10% of the reported river discharge rate.   Over 50% of the groundwater discharge comes from the western section of the gulf.

Due to economic growth of the provinces linked by Lingayen Gulf, the water quality of the gulf and that of the rivers that drain into it are deteriorating.  In 1995, all the six major rivers in the gulf were classified by the Department of Environment and Natural Resources as fit only for uses such as fishery, industry, and agriculture and not suitable for contact recreation (e.g., bathing).  The various economic activities (e.g., agriculture, domestic sewage, livestock) along its perimeter have also contributed waste loads of N and P into gulf waters.  A discussion on estimating waste loads from these activities is given below. 

Methodology

In general, the LOICZ Biogeochemical Modelling Guidelines (Gordon et al. 1996) were used to calculate the stoichiometrically linked water-salt-nutrients budgets.   In these mass balance budgets, complete mixing of the water column is assumed and only dry season mean nutrient concentrations are considered. 

Particular attention is paid to the issue of waste loading into Lingayen Gulf, since these are important inputs to the system.  The waste loads of N and P were estimated from relevant economic activities in the gulf.  The steps followed in doing waste load calculations from economic activities are given in a separate section in this report (Appendix II).  Briefly, after identifying economic activities, total discharge of effluents were approximated using the rapid assessment method utilised by WHO (1993).   Results are given in Table 5.1.  From point of origin to the coastal waters, a 40% assimilation factor was applied, thereby implying that approximately 60% of the N and P from waste loads make it to the gulf.  Literature (e.g., Howarth et al. 1996) has cited a higher assimilation rate (80%) of waste before entering coastal waters but this estimate may be too high for the gulf because most of the waste may be directly discharged into the water.  Since the derived N and P in effluents are Total N and Total P, conversions were made to determine the inorganic fraction using the DIP/TP (0.5) and DIN/TN (0.27) ratios given in San Diego-McGlone, Smith and Nicholas (1999).

Lingayen Gulf was divided into three boxes: Nearshore box, Bolinao box, and Upper Gulf box (Figure 5.10).  The Nearshore box is 10% of the total area of the Gulf, while the Bolinao and Upper Gulf boxes are 6% and 84% of the total area, respectively.  The large river systems of the gulf are located in the Nearshore box, the major habitats (coral reef and seagrass beds) are located in the Bolinao box, and the open area of the gulf that directly interacts with the South China Sea is included in the Upper Gulf box.

Water and salt balance

Figure 2 represents the water and salt budgets.  The water budget for each of the boxes in Lingayen Gulf is determined mainly by the average precipitation over the gulf area (V_P), the average evaporation (V_E), the average freshwater discharge from the rivers (V_Q) and the average groundwater discharge (V_G).  River discharge for the Nearshore box was estimated to be 8x10⁹ m³ yr^-1 (National Wetlands Research Center (NWRC) Philippines 1976).   In the Bolinao box, the river discharge was 0.2x10⁹ m³ yr^-1, while in the Upper Gulf box the discharge is 2x10⁹ m³ yr^-1 (NWRC Philippines 1976).  A mean annual pan evaporation of 2,060 mm was obtained from the local weather office (Philippine Atmospheric, Geophysical and Astronomical Services Administration, PAGASA) in San Manuel, Pangasinan.  This rate was multiplied with the area in each box to get V_E.  No pan correction factors were used.  Mean annual precipitation (2,250 mm), based on 1965-1970 data from PAGASA stations in Dagupan City, Mabini (both in Pangasinan), and Tubao, La Union, when multiplied by the area of each box gave the V_P.   Freshwater from groundwater (V_G) was estimated using Darcy’s law (WOTRO 1998).  Freshwater input from sewage is assumed to be 0.  To balance inflow and outflow of water in each box, there must be a residual outflow (V_R) of -8x10⁹ m³ yr^-1 from the Nearshore box to the Upper Gulf box, -1x10⁹ m³ yr^-1 from the Bolinao box to the Upper Gulf box, and -11x10⁹ m³ yr^-1 from the Upper Gulf box to the South China Sea.

Figure 2.  Water and salt budgets for Lingayen Gulf.  Volume in 10⁹ m³, water fluxes in 10⁹ m^-3 yr^-1, salt fluxes in 10⁹ psu-m³ yr^-1 and salinity in psu.

The salinity outside the gulf (34.4 as an average value in the top 50 m) was taken from a hydrographic station in the South China Sea closest to the mouth of the gulf (San Diego-McGlone et al. 1995).  Inside the boxes, average salinity values were obtained from the data set of WOTRO (1997, 1998).   The residual fluxes of salt (V_RS_R) from the three boxes indicate advective export.  Exchange of gulf water with ocean water must replace this exported salt by V_X1(S₃-S₁) = +260x10⁹ psu-m³ yr^-1 from the Nearshore box to the Upper Gulf box, V_X2(S₃-S₂) = +34x10⁹ psu-m³ yr^-1 from the Bolinao box to the Upper Gulf box, and V_X3(S_Ocn-S₃) = +376x10⁹ psu-m³ yr^-1 from the Upper Gulf box to the South China Sea.  The water exchange flow (V_X) is then determined to be +87x10⁹ m³ yr^-1, 68x10⁹ m³ yr^-1, and 940x10⁹ m³ yr^-1 for the Nearshore box, Bolinao box, and Upper Gulf box, respectively.  The total exchange time (flushing time) of the Upper Gulf box is longest at 27 days since the volume of this box is the largest.  The flushing time of the Nearshore box is 12 days, while the Bolinao box is only 2 days.  Flushing time for the whole gulf is 32 days.

Table 1.  Effluents from economic activities in Lingayen Gulf (in 10⁶ mole yr^-1).

Economic Activity	Nitrogen	Phosphorus
Household activities	1,754	202
-  domestic sewage	1,595	91
-  solid waste	159	11
-  detergents	-	100
Urban Runoff	126	5
Agricultural Runoff	3,465	174
-  crop fertilization	1,820	157
-  cropland erosion	1,645	17
Livestock	29	2
-  commercial piggery	25	2
-  poultry	4	-
Aquaculture	22	2
Total	5,396	385

Budgets of nonconservative materials

DIP balance

Figure 3 illustrates the DIP budget for Lingayen Gulf.  The DIP concentrations inside the boxes were taken from the data set of WOTRO (1997, 1998).   These data represent dry season conditions in the gulf.  The average PO₄ concentration for the Nearshore box is 0.4 mmol m^-3, 0.4 mmol m^-3 for the Bolinao box, and 0.1 mmol m^-3 for the Upper Gulf box.  The average PO₄ concentration is 11 mmol m^-3 in the rivers of the Nearshore box, 6 mmol m^-3 for rivers in the Bolinao box, and 0.7 mmol m^-3 of PO₄ for rivers in the Upper Gulf box (LGCAMC 1998).  The oceanic PO₄ concentration is 0.0 mmol m^-3 (San Diego-McGlone et al. 1995).  Groundwater PO₄ concentration is 8 mmol m^-3 in the Nearshore box, 0.4 mmol m^-3 in the Bolinao box, and 2 mmol m^-3 in the Upper Gulf box.  These values are comparable to reported groundwater PO₄ concentration for similar systems (1-10 mmol m^-3: Lewis 1985; Tribble and Hunt 1996).  Waste load of PO₄ ( V_ODIP_O) in each box was determined from the waste load estimated for the entire gulf scaled down to the gulf’s coastline found within the box.  This assumes that most of this waste enters the gulf from along the coast and some from the rivers; waste carried by the rivers has been partially accounted for in the river flux (V_QDIP_Q).  Overall, waste load input dominates the DIP budget for the Bolinao and Upper Gulf boxes.  For the Nearshore box, river input of DIP is higher than waste load.  In order to balance the DIP contributed by the rivers, waste load and groundwater in the boxes with residual and exchange fluxes, nonconservative processes inside the boxes must fix or remove DIP.  The large input of DIP from the rivers and from waste load in the Nearshore box relative to what goes out of this box has resulted in a net removal of DIP (i.e. DDIP is negative) in this box.  The Bolinao box is also a net sink of DIP but the Nearshore box is a stronger net sink of DIP in the gulf.  On the other hand, the Upper Gulf box is a net source of DIP.

Figure 3.   Dissolved inorganic phosphorus budget for Lingayen Gulf.  Fluxes in 10⁶ mol yr^-1 and concentrations in mmol m^-3.

DIN balance

Figure 4 illustrates the DIN budget for the gulf.  DIN is defined as S NO₃^- + NO₂^- + NH₄⁺.  The DIN concentrations inside the boxes were taken from the data set of WOTRO (1997, 1998) and these data represent dry season conditions in the Gulf.  The average DIN concentration is 1.7 mmol m^-3  for the Nearshore box, 3.9 mmol m^-3 for the Bolinao box, and 0.8 mmol m^-3 for the Upper Gulf box.  The average DIN concentration is 16 mmol m^-3  for the rivers in the Nearshore box, , 22 mmol m^-3 for the rivers in the Bolinao box, and 4 mmol m^-3 of DIN for rivers in the Upper Gulf box (LGCAMC 1998).  The oceanic DIN concentration is 0.5 mmol m^-3 (San Diego-McGlone et al. 1995).  Groundwater DIN concentration is 53 mmol m^-3 for the Nearshore box, 55 mmol m^-3 for the Bolinao box, and 71 mmol m^-3 for the Upper Gulf box.  These values are comparable to reported groundwater DIN concentration for similar systems (37-72 mmol m^-3:   Lewis 1985; Tribble and Hunt 1996). 

Waste load of DIN (V_ODIN_O) in the boxes was estimated using similar methods as for (V_ODIP_O).  Again, the balance for DIN is strongly dominated by waste discharge in all the boxes.  Budgeting results show that the three boxes are net sinks of DIN (DDIN is negative).

Figure 4.  Dissolved inorganic nitrogen budget for Lingayen Gulf.  Fluxes in 10⁶ mol yr^-1 and concentrations in mmol m^-3.

Stoichiometric calculations of aspects of net system metabolism

As outlined by Gordon et al. (1996), net ecosystem metabolism (p-r) can be calculated from DDIP.  The basic formulation is as follows, where (C:P)_part represents the C:P ratio of organic matter which is reacting in the system.

            (p-r) =  -DDIP × (C:P)_part                                                                                    (1)

Because DDIP is negative in the Nearshore box, the qualitative conclusion to be drawn from Eq. (1) is that the system is net autotrophic (i.e., (p-r) is positive).  This implies that the DIP delivered by the rivers and from waste load is fixed in the Nearshore box as organic P in the dissolved form or trapped in the sediments.  Net (p-r) is also positive in the Bolinao box suggesting that this box is autotrophic, albeit not as strongly as the Nearshore box.  In the Upper Gulf box, the (p-r) is negative, indicating net heterotrophy thus implying that an external source of organic material is needed to support decomposition in this box.  This source material that is exported to the Upper Gulf box is the organic P fixed in both the Nearshore box and the Bolinao box.  The small DDIP flux and correspondingly the low (p-r) for the Upper Gulf box suggests that the system is very nearly in balance metabolically.  This means that waste materials delivered to the Gulf are broken down within the system, an indication of the efficiency of the Gulf in recycling organic material.

The stoichiometric approach given by Gordon et al. (1996) can also be used to estimate (nfix-denit), the difference between nitrogen fixation and denitrification.  The general formulation (based on the inorganic nutrient budgets, in the absence of dissolved organic nutrient data).

            (nfix-denit) =   DDIN_obs - DDIN_exp = DDIN_obs   - DDIP × (N:P)_part                       (2) 

where DDIN_obs is the observed value for DDIN, and DDIN_exp is that value expected by the reaction of organic matter of a known N:P ratio (N:P)_part. Because the N:P ratio of both planktonic and waste-derived organic matter lies near 16:1, the ambiguity associated with the source organic matter does not exist.  The DDIN_obs for all three boxes indicate that these are sinks for DIN.  However the amount of DIN fixed with DIP in the Nearshore box and Bolinao box (DDIN_exp) via autotrophic processes exceed the net DIN calculated from the balance of inflow and outflow in these boxes.  Hence in these boxes, N fixation is in excess of denitrification.  In the Upper Gulf box, which was estimated to be net heterotrophic from (p-r), the DIN released and that due to DDIN_obs resulted in a negative (nfix-denit), indicating net denitrification.  The N fixed in the Bolinao box and Nearshore box is most probably exported as organic N into the Upper Gulf box and this could be the material that fuels denitrification in the Upper Gulf box.

In the Bolinao box, (nfix-denit) is estimated to be 2 mol N m^-2 yr^-1 in excess of denitrification.  Nitrogen fixation is known to provide most of the nitrogen requirement in coral reefs (e.g., Larkum et al. 1988; Shashar et al. 1994) and seagrass beds (e.g., Hanisak 1983).  The 200 km² of coral cover in the Bolinao area (McManus et al. 1992) and approximately 10 km² of seagrass beds (WOTRO 1996) within the Gulf may account for the predominance of nitrogen fixation over denitrification in this box.

A summary of nonconservative fluxes for the nutrients in Lingayen Gulf (based on the mean flux estimates) is given in Table 2. 

Table 2.  Summary of nonconservative fluxes in the three boxes of Lingayen Gulf. 

 

Process (Area, Vol.)	Nearshore Box (210 km2, 3.2 km3)		Bolinao Box (126 km2,0.3 km3)		Upper Gulf Box (1,764 km2,81 km3)		Whole System (2,100 km2,84.5 km3)
	106 mol yr-1	mol m-2 yr-1	106mol yr-1	mol m-2 yr-1	106mol yr-1	mol m-2 yr-1	106mol yr-1	mol m-2 yr-1
DDIP	-97	-0.46	-27	-0.21	+10	+0.01	-114	-0.05
DDIN	-313	-1.5	-180	-1.4	-310	-0.2	-803	-0.4
(p-r)	+10,282	+49	+2,862	+23	-1,060	-1	+12,084	+6
(nfix-denit)	+1,239	+5.9	+252	+2.0	-470	-0.3	+1,021	+0.5

Some ecological implications

One major concern in Lingayen Gulf is the growing number of human activities that input waste materials into Gulf waters.  The validity of this concern can be seen in the dominance of the P and N budgets by waste loading.  If the system were indeed autotrophic with inorganic nutrients primarily coming from decomposed organic wastes utilised to sustain production in the area, then of interest would be the carrying capacity of the Gulf under these conditions.  With present loads of organic waste, only 30% of 5mg L^-1 of dissolved oxygen (minimum level set by the Department of Environment and Natural Resources) is utilised for respiration.  Since the distribution and average of nutrient concentrations as well as the N:P ratio in the Gulf have not varied much over the years, this is an indication of the Gulf’s current assimilative capacity.  Thus any added load which will bring the dissolved oxygen concentrations to levels less than 2 mg L^-1 (limit for fish to survive) will compromise existing conditions. 

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Last Updated 21 May 2006 by DPS