Bandon Bay

Gullaya Wattayakorn

Study area description

Bandon Bay (9.20^oN and 99.67^oE) is located in Surat Thani Province, southern Thailand. The watershed of the bay is 12,220 km², of which an important part consists of agricultural lands and aquaculture.  The population living within the watershed numbers approximately 830,000 with fisheries, aquaculture and tourism as their main activities.  In the coastal area, there are several socio-economic activities such as fisheries, oyster culture and shrimp farming that represent an income for the people living in the area.   Mangroves (Rhizophora spp, Sonneratia alba, Xylocarpus spp, Avicennia alba and Bruguiera spp.) cover the shores of the bay and the upland area.

Figure 1.  Location and map of Bandon Bay.

 Tide in Bandon Bay is diurnal, with the average tidal range of 1 metre.  The inner bay, from Chaiya District to Kanchanadit District (Figure 1), covers an area of 480 km² with 80 km of coastline.  The coastal area has a gradual slope and the water is shallow.  A large band of mudflats, resulting from high sedimentation in the bay area, extends along the coast to about 2 km from the shore.  Water depths vary from below 1 m to 5 m near the mouth of the bay, with a mean depth of 3 m, with respect to mean sea level.  The volume of the bay is estimated to be 1,440x10⁶ m³.  

The climate is intermediate between equatorial and tropical monsoon types and is characterized by constant high temperature and high rainfall without extremes of heat, with average rainfall and evaporation of 4 mm day^-1.  Mean relative humidity is 81% (range from 61-95%).  Mean monthly temperature ranges from 26°C, usually in the wettest month, to 32°C, usually in April.  Meteorological data for Bandon Bay in 1997 and 1998 is based on the data compiled from the Surat Thani weather station.  The wet season starts in May and lasts until December; during this season monthly rainfall ranges from 77 mm to 412 mm.  The highest amount of rainfall was in August.  Lower rainfall, 50 mm, and a higher evaporation rate characterises the dry season, from January to April.   Annual rainfall for 1997 was 1,500 mm, which is less than the mean annual rainfall for 1951-1996 (1,690 mm).

 Most of the surface freshwater discharge into Bandon Bay is from the Tapi-Phumduang River watershed.   The Phumduang River basin covers an area of 6,125 km² and the Tapi River Basin covers about 5,460 km².   These two rivers join about 15 km west of Surat Thani, forming an extended delta as they discharge into the Gulf of Thailand.  Annual runoff in Tapi and Phumduang rivers normally amounts to more than 10 billion m³.  The hydrological characteristics of the Tapi and Phumduang rivers in 1997 are summarised in Table 1.   The ratio of the maximum and minimum discharges appears to be high, which implies that monthly discharges fluctuate according to seasonal precipitation.

Table 1.  Hydrological characteristics of Tapi and Phumduang rivers in 1997.

Source: Royal Irrigation Department

The water quality of the Tapi River is greatly affected by human activities and land usage in the fairly densely populated Surat Thani province.  The present population of Muang and Phun Phin Districts is about 215,780.  Furthermore, about twenty large factories, mainly food-related industries, petroleum and oil depots and a distillery plant, are located along the river.  Further upstream, a rockfill dam with clay core (Chiew Larn Multipurpose Dam), 94 metres high, was constructed across the Klong Saeng River, a tributary of the Phumduang River.  The dam helps to ensure adequate water supply for irrigable area downstream, alleviation of flooding in southern provinces and salinity control in the Tapi River. 

 Average salinity in the bay range from 21.7 psu in the dry season to 17.4 psu in the wet season.  In the sea, salinity ranges from 32.3 psu in the dry season to 29.0 psu in the wet season.  Salinity of freshwater from Tapi-Phumduang rivers and rainfall are assumed to be 0 psu.

 Three surveys have been carried out so far for data for C, N and P budgets in Bandon Bay.  Data in April were collected to represent the dry season and October to characterise the wet season within the study area. 

 Water and salt balance

Salt and water budgets for April 1997 are illustrated in Figure 2.  Similar procedures were applied for October 1997 and April 1998, as summarised in Table 3.  Freshwater flow (V_Q) was estimated by adding the flows of the Tapi and Phumduang rivers.  Since only part of the watershed area draining into Bandon Bay is gauged, the total freshwater input to the bay was estimated using the ratio of measured to the rest of the watershed area.  The Tapi-Phumduang River basin has problems with saline intrusion into the groundwater, so groundwater inflows (V_G) can be considered to be zero.  Other inflows like sewage (V_O) are not available and are assumed to be zero.

Figure 2. Water and salt budgets for Bandon Bay in April 1997.  Volume in 10⁶ m³, water fluxes in 10⁶ m³ day^-1, salt fluxes in 10⁶ psu-m³ day^-1 and salinity in psu.

Data in Table 2 were used to calculate the water balance for each season according to equation (3) of Gordon et al. (1996).   Residual volume (V_R), the process responsible for adding and removing water, can then be estimated (Table 3).  The system shows substantial net residual outflow of water, as expected from the freshwater inputs to the system, higher during the wet season than during the dry season.  The salt balance of the system and the exchange or mixing volume (V_X) between the Gulf of Thailand and Bandon Bay can then be estimated according to equation (8) of Gordon et al. (1996).    

Table 2.  Runoff (V_Q), precipitation (P), and evaporation (E) data for Bandon Bay, April 1997 - April 1998.

 The exchange time of water in the bay can be calculated as the total volume of the bay divided by the sum of V_X  , mixing between bay and ocean, plus the absolute value of residual flow  |V_R |.   It can be seen that the water exchange rate in Bandon Bay is faster in the wet season (9 days) than in the dry season (17 to 80 days). Apparently, tides seem to be the main force driving water exchange with the adjacent Gulf of Thailand during the dry season, particularly in 1997.

Table 3.  Water circulation, residual flow (V_R ),  water exchange rates (V_X ) and water exchange time as calculated from the water and salt budgets for Bandon Bay.  The subscript “Q” indicates river; “P” is precipitation and “E” is evaporation.

Date	VQ  (106 m3 day-1 )	VP  (106 m3 day-1 )	VE * (106 m3 day-1 )	VR * (106 m3 day-1 )	VX (106 m3 day-1 )	Water exchange time, days
Apr-97	6	2	-2	-6	12	80
Oct-97	58	0	-2	-56	112	9
Apr-98	26	0	-3	-23	64	17
* minus sign for VE and VR indicates an output from the system

Budgets of nonconservative materials

Table 4 gives the nutrient waste loads calculated from BOD values, and Table 5 shows the chemical composition of Bandon Bay, the Tapi River and the adjacent sea.  The non-conservative fluxes of C, N and P in Bandon Bay were calculated from data in Tables 4 and 5 using a simple box model following Gordon et al. (1996), and are presented in Table 6.

 DIP balance

Dissolved inorganic phosphorus (DIP) values ranged from 0 to -16x10³ mol day^-1 (0 to -0.03 mmol m^-2 day^-1) in Bandon Bay.  This indicates that the system is a slight net sink for DIP especially in the wet season.  If phosphate desorption from sediments does not contribute significantly to DDIP, then this DIP sink probably results from primary producer uptake. 

 Dissolved organic phosphorus (DOP) was also removed from the water column in both seasons.  Overall, the system is a net sink for dissolved phosphorus (DIP + DOP) for both seasons but much more so in the wet season.

Table 4.  Nutrient loads calculated from BOD values for Bandon Bay.  BOD data were from Pollution Control Department, 1998.

Nutrients	Apr 1997	Oct 1997	Apr 1998
BOD, 103 mol day-1	75	85	78
DIO  DIP, 103 mol day-1	2	2	2
DOP, 103 mol day-1	2	2	2
NH4+, 103 mol day-1	7	8	7
NO3-, 103 mol day-1	5	5	5
DIN, 103 mol day-1	12	13	12
DOC, 103 mol day-1	128	145	133

 DIN balance

In the dry season, there is a release of NH₄⁺ and uptake of NO₃^-  with a net release of dissolve inorganic nitrogen (DIN).  This appears to represent net respiration of organic matter in the dry season.  

DIN was taken up in the wet season which is consistent with DIP + DOP uptake, while NH₄⁺ release reflects nitrogen recycling.  Some nitrogen and phosphorus were, of course, lost by being bound into refractory organic matter in the sediments (adsorption to particles). 

 DON was also removed from the water column in both seasons.  Overall, the system is a net sink of dissolved nitrogen (DIN + DON) for both seasons and this is also more strongly demonstrated in the wet season. 

Table 5.  Average chemical composition of Bandon Bay, Tapi River and adjacent Gulf of Thailand water samples.

	April 1997			October 1997			April 1998
	River	Bay	Sea	River	Bay	Sea	River	Bay	Sea
Salinity, psu	0	19.5	32.3	0	17.4	29.0	0	21.7	31.2
DIP, mM	0.3	0.6	0.5	0.4	0.3	0.4	0.6	0.5	0.4
DOP, mM	0.5	0.3	0.2	0.5	0.2	0.2	0.1	0.1	0.1
NH4 , mM	5.5	7.5	2.8	7.3	5.2	2.2	9.4	5.9	2.3
NO3 , mM	5.4	2.7	1.6	5.6	1.7	0.4	3.4	1.4	0.4
DIN, mM	10.9	10.2	4.4	12.9	6.9	2.6	12.8	7.3	2.7
DON, mM	31.6	29.4	32.3	12.4	8.7	11.9	29.3	27.5	26.9
TA, meq	1.2	1.2	1.9	1.0	1.6	2.2	1.7	2.3	2.7
DIC, mM	0.5	0.6	2.1	0.1	0.2	1.7	1.2	2.1	2.5
DOC, mM	0.2	0.7	0.4	0.8	0.5	0.3	0.8	0.6	0.2

 

Table 6.  Non-conservative carbon, nitrogen and phosphorus fluxes for Bandon Bay.

	April 1997		October 1997		April 1998
	103 mol day-1	mmol m-2 day-1	103 mol day-1	mmol m-2 day-1	103 mol day-1	mmol m-2 day-1
DDIP	0	0	-16	-0.03	-2	0
DDOP	-2	0	-20	-0.04	-3	-0.01
DNH4	+47	+0.10	+112	+0.23	+73	+0.15
DNO3	-11	-0.02	-125	-0.26	-8	-0.02
DDIN	+37	+0.08	-13	-0.03	+64	+0.13
DDON	-40	-0.08	-500	-1.04	-98	-0.20
DTA	-6*	-10	-19*	-40	-12*	-30
DDIC	-13**	-30	-121**	-250	-4**	-10
DDOC	+6**	+10	-2**	-4	+4**	+30

* fluxes in 10⁶ meq day^-1  

** fluxes in 10⁶ mol day^-1

 Stoichiometric calculations of aspects of net system metabolism

The net nitrogen fixation minus denitrification (nfix-denit) of the system can be calculated as the difference between observed (DDIN + DDON) and expected (DDIN + DDON).  The preliminary result on the average C:N:P ratio of reactive organic matter in the Tapi River was 324:27:1. The budgetary estimates yield an estimated (nfix-denit) equivalent to about +0.11 to +0.21 mmol m^-2  day^-1  in the dry season and 0.96 mmol m^-2  day^-1 in the wet season.  Hence, Bandon Bay is a coastal system where nitrogen fixation exceeds denitrification in both seasons (Table 7). 

 The calculation of the net ecosystem metabolism, which is, the difference between organic carbon production (p) and respiration (r) within the system (p-r), based on the particulate C:P ratio (324:1), indicate that Bandon Bay is a slightly net autotrophic system in the dry season: (p-r) = +1 to +3 mmol m^-2  day^-1 ;and greatly autotrophic in the wet season: (p-r) = +24 mmol m^-2  day^-1.  This assumes that lost DIP and DOP were both eventually consumed by primary producers.  Simultaneously, Piumsomboon and Paphavasit (1998) measured phytoplankton biomass as chlorophyll_a and estimated the phytoplankton production for Bandon Bay to be 105, 218 and 191 mg C m^-3 day^-1  (or 25, 52 and 45 mmol C m^-2 day^-1 ) for April 1997, October 1997 and April 1998 respectively.   The respiration rate and p:r ratio in Bandon Bay can then be estimated (Table 8).  The system consumes the produced organic matter about 93 to 96% in the dry season and 24% in the wet season.

 Table 7.  Stoichiometric calculations of aspect of net system metabolism  for Bandon Bay.

	April 1997	October 1997	April 1998
	mmol m-2 day-1	mmol m-2 day-1	mmol m-2 day-1
(p-r)1	0	+11	+1
(nfix-denit)2	+0.08	+0.87	+0.25

              ¹  based on particle C:P of 324; ²    based on particle N:P of 27

Table 8. Primary production, estimated respiration and photosynthesis/respiration ratio in Bandon Bay.

Date	Primary production (mmol C m-2 day-1 )	(p-r)estimated	restimated (mmol C m-2 d-1 )	p:r
Apr 1997	2	0	25	1.00
Oct 1997	52	+11	41	1.27
Apr 1998	45	+1	44	1.02

 

Figure 3. Dissolved inorganic phosphorus budget for Bandon Bay in April 1997.   Fluxes in 10³ mol day^-1 and concentrations in mmol m^-3.

Figure 4.  Dissolved inorganic nitrogen budget for Bandon Bay in April 1997.   Fluxes in 10³ mol day^-1 and concentrations in mmol m^-3.

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Last Updated 21 Jul 2000 by DPS