Ragay Gulf, Luzon Island
V. Dupra and S.V. Smith
Study area description
Ragay Gulf is located in the south-eastern part of Luzon Island (13.00°-14.00°N, 122.25°-123.40°E) (Figure 1). The gulf is a relatively large triangular embayment with a total area of 3,600 km2 and with average depth of 179 m. At the mouth of the gulf is Burias Island, which effectively divides the mouth into two. The eastern half forms the northern boundary of Burias Pass that extends southwards for about 50 km without any significant change in depth. The western part opens into Sibuyan Sea. At depths greater than about 120 m, the gulf is separated from the ocean by a shallow sill (approximately 120 m), which extends from the northern tip of Burias Island to the southeastern tip of the Bondoc Peninsula. The presence of the sill across the gulf mouth plays a major role in the hydrographic properties and the circulation patterns in the gulf. Water in the gulf below the sill depth is supplied by sill overflow from the adjacent open sea. The vertically homogenous temperature and salinity properties of the gulf waters below the sill depth are defined by the properties of the water outside the gulf at the level of sill depth. In contrast, the temperature and salinity characteristics of the top 150 m exhibit strong seasonal variability, which is influenced by the monsoons.
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There are eight major rivers surrounding Ragay Gulf, with a total river drainage area of 1,083 km2. Average annual rainfall is 2,593 mm, annual rainfall in 1994 was 2,680 mm. Based on the water quality observations conducted by the Bureau of Fisheries and Aquatic Resources (BFAR 1994), the gulf may be considered as relatively undisturbed water body in spite of anthropogenic-linked perturbations in some parts (BFAR 1994).
The gulf was budgeted following the LOICZ Biogeochemical Modelling Guidelines (Gordon et al. 1996) to assess the net system metabolic processes of the gulf. Data collected in 1994 by BFAR were used.
Water and salt balance
Figure 2 presents the two-layer water and salt budget for Ragay Gulf. River runoff gauged from the eight rivers surrounding the bay was 2x109 m3 yr-1 (VQ). The calculated river runoff applying the method described by David, Appendix X was closed to the gauged river runoff. Precipitation (VP) was 10x109 m3 yr-1 and evaporation (VE) was calculated to be 80% of the total precipitation. Assuming that other freshwater fluxes were insignificant, the net freshwater input (residual flow, VR’) was 2x109 m3 yr-1. Volume of water intrusion (VD) calculated from the residual flow and salinity difference was 457x109 m3 yr-1. From the salt balance, volume of mixing (VZ) was calculated to be 225x109 m3 yr-1. Water exchange time for the gulf was calculated to be more than 1 year. Because of the very large volume of the gulf, the water exchange time is expected to be long. The unique feature of the gulf, the presence of sill, is manifested in this budget by the fast water exchange of the bottom layer (Figure 2).
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Figure 2. Two-layer water and salt budgets for Ragay Gulf. The box outlined with dashed lines represents the lower layer of the system. Volume in 109 m3, water fluxes in 109 m3 yr-1, salt fluxes in 109 psu-m3 yr-1 and salinity in psu.
Budgets of nonconservative materials
DIP and DIN balance
Figure 3 and 4 illustrate the DIP and DIN budgets for Ragay Gulf. Assuming that nutrient loads into the gulf were delivered via rivers and that other inputs were insignificant, the budgets show that the gulf almost balances the uptake and release of DIP: DDIP = +1x106 mol P yr-1 (0.00 mmol P m-2 day-1); and is slightly net releasing DIN: DDIN = +115x106 mol N yr-1 (0.09 mmol N m-2 day-1).
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Figure 3. Two-layer dissolved inorganic phosphorus budget
for Ragay Gulf. The box outlined with
dashed lines represents the lower layer of the system.
Fluxes in 106 mol yr-1 and concentrations in mmol m-3.
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Figure 4. Two-layer dissolved inorganic nitrogen budget for Ragay Gulf. The box outlined with dashed lines represents the lower layer of the system. Fluxes in 106 mol yr-1 and concentrations in mmol m-3.
Stoichiometric calculations of aspects of net system metabolism
The system seems to balance production and respiration and behaves as slightly fixing nitrogen. The budget is consistent with the conclusion derived from the BFAR report that the land-derived nutrients still seem to be within the assimilative capacity of the gulf. However, it is important to note that the budgeted area is large and that the impact of waste loads may be more apparent when budgets are done on smaller regions of the gulf.
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Last Updated 21 May 2006 by DPS