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
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 systemsThere 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
