V. Dupra2, S.V. Smith2, M.L. San Diego-McGlone1 and R.A. Valmonte-Santos3
1Marine Science Institute, University of the Philippines, Quezon City 1101, Philippines
2Department of Oceanography, University of Hawaii, Honolulu, Hawaii 96822, USA
3ICLARM, MCPO Box 2631, Makati, Metro Manila 0718, Philippines
15 September 1999
Note: This single box budget will be later modified to systems in series budget. The systems in series budget will include waters north of the bay.
Carigara Bay is situated between 11.30° and 11.42°N, and 124.53° and 124.83°E in the Visayan Islands of the Philippines (Figure 1). The bay is a broad (width » 27 km), relatively large (area » 500 km2) and shallow (depth » 40 m) crescent-shaped embayment (Calumpong et al., 1994). The bay is bounded on both its northern rims by steep hills, with a shallow floodplain along its southern coast. Five major river systems drain into the bay, and there are also several small tributaries within the floodplain area. The five rivers are Naugisan, Carigara, Canomantag, Himanglos and Sapiniton Rivers. The rivers run through fairly heavily populated areas (ca. 130,000 population). There are five municipalities with coastal jurisdiction: Capoocan, Carigara, Barugo, San Miguel and Babatngon. Land use status of the municipalities within the watersheds of the five rivers flowing into the Carigara bay are categorized as Alienable and Disposable (47%), Forest (26%), Timberland (16%), and Reservation (10%).
Figure 1. Map of Carigara Bay. Solid line represents the boundary of the budgeted systems
There are three distinct seasons affecting weather patterns in the bay: a dry season which coincides with the southwest monsoon from January to May, a rainy season which coincides to the northeast monsoon from June to September, and a storm or transitional season from October to December (Calumpong et al., 1994; Valmonte-Santos et al., 1996).
Carigara Bay is heavily stressed and damaged as a result of the increasing population pressure and economic depression. The inhabitants of the bay, having few options, continue to exploit the resources. Several major problems have been identified within the bay: overfishing and the use of destructive fishing method, siltation from poor land use practices and loss of marine habitats.
This study aims to estimate nitrogen and phosphorus non-conservative fluxes and infer from these fluxes the biogeochemical processes occurring in the system. The bay was budgeted applying the LOICZ Biogeochemical Budget Modelling guidelines (Gordon et al., 1996) as single box model with horizontally and vertically mixed water. This involved the used of water-salt-nutrient linked budgets. Data used were mostly from Resource and Ecological Assessment (Valmonte-Santos et al., 1996) for Carigara Bay monitored during the rainy season (September 1996) and dry season (March 1996). Average salinity and nutrient concentrations for the two sampling periods were calculated to represent annual values. Nutrient concentrations for rivers and the adjacent ocean used in this budget were measurements from Valmonte-Santos et al., 1996. Nutrient loads from sewage were estimated from the 130,000 population and considered in this budget. Conversions of 9.5 mole P per person per year and 140 mole N per person per year were used (McGlone, Appendix X ).
Figure 2 shows the steady-state water and salt budgets for Carigara Bay. The net freshwater input was calculated from precipitation (VP = 1 x 109 m3 yr-1), evaporation (VE = 1 x 109 m3 yr-1) and river discharges (VQ = 6 x 109 m3 yr-1), other freshwater sources were assumed insignificant. Precipitation and evaporation rates were from Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA). River discharges coming from the five rivers were from REA-BFAR Report (1994). The presence of many rivers emptying into the bay makes the rivers the dominant sources for freshwater. The net freshwater input, which is equal to the residual flow, is 6 x 109 m3 yr-1 (VR). The residual flow needs water to mix between the bay and adjacent ocean equal to 63 x 109 m3 yr-1 (VX) to balance salt flux. The water exchange between the bay and adjacent oceanic water is (t)106 days.
Figure 2. Water and salt budgets for Carigara Bay. Water fluxes in 109 m3 yr-1 and salinity in psu
Figure 3 presents the dissolved inorganic phosphorus (DIP) budget for Carigara Bay. Processes in the system seems to balance the release and uptake of DIP, DDIP = 0. Sewage nutrient loads, inorganic phosphorus (VODIPO) was estimated by multiplying the conversion factors of 9.5 mole P person-1 yr-1 to the 130,000 population living along the bay. Sewage conversion factors were estimated from a load of 20 kg BOD person-1 yr-1. Inorganic phosphorus load, VODIPO = 1.2 x 106 mole yr-1 was calculated.
Figure 3. Dissolved inorganic phosphorus budget for Carigara Bay.
Fluxes in 106 mol yr-1 and concentrations in mmol m-3.
Figure 4 illustrates the dissolved inorganic nitrogen (DIN) budget. The bay seems to be a net sink for nitrogen, DDIN = -17 x 106 mole N yr-1 or -0.09 mmole N m-2 day-1. Sewage nutrient loads, inorganic nitrogen (VODINO) was estimated using the conversion 140 mole N person-1 yr-1. Inorganic nitrogen load, VODINO = 18 x 106 mole yr-1 was estimated.
Figure 4. Dissolved inorganic nitrogen budget for Carigara Bay.
Fluxes in 106 mol yr-1 and concentrations in mmol m-3.
Nutrients for river discharges were not measured at zero salinity thus underestimated the actual nutrient loads. DIN considered for these budgets were only NO2- and NO3-. Disregarding ammonia (NH4+) further intensifies underestimation of DIN input in the river loads. To some extent compensating this problem, nutrient loads may be overestimated by the sewage inputs that actually incorporated with the river loads. This could be resolved with measurements of river nutrient concentrations upstreams.
Non-conservative fluxes of nutrients derived for the bay were low and within the above mentioned uncertainties. The problem of the loads and the rapid exchange of water between the embayment and adjacent water necessitate for a systems in series model, which involves extension of the budget towards north.
From stoichiometric analysis of the non-conservative nutrients, the system is very slightly denitrifying, (nfix-denit) = -0.1 mmole N m-2 day-1. Production and respiration was balanced, (p-r) = 0. Primary production derived from chlorophyll values ranged from 2 to 8 mmole C m-2 day-1.
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Last Updated 24 Jul 2000 by DPS