Lake Songkhla
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LAKE SONGKHLA Fish culture on Thale Sap Songkhla Photo: T. Kira A. LOCATION Phattahalung, Songkhla and Nakhon Si Thammarrat, Thailand. 7°08′ - 50′N, 100°07′- 37′E; neary 0 m above sea level. B. DESCRIPTION Lake Songkhla, a coastal lagoon produced by sand-bar formation includes three lakes, Thale Noi (2,800 ha), Thale Luang (78,280 ha) and Thale Sap Songkhla (17,600 ha), from north to south, which are inter-connected by narrow channels. A narrow strait (minimum width 380 m) connects Thale Sap with the sea (Gulf of Thailand) at its southeastern end. A grandient of salinity exists, therefore, between the brackish water of Thale Sap and the pure freshwater of Thale Noi. The middle lake, Thale Luang, approaches a freshwater condition during the rainy season (October - January), but is influenced by the invasion of seawater in other month. The lakes are shallow throughout and moderately eutrophic, with a mean water depth of 1.2 -1.3 m in relatively dry seasons, but the water level rises by about one meter during the winter months. Thale Sap is known for its production of fish, shrimp and crabs, and is also intensively utilized for the aquaculture of seabass (Lates calcarifer). The fry produced and supplied by the National Institute of Coastal Aquaculture in Songkhla is grown in net cages by fishermen families. Thale Noi and the eastern branch of Thale Luang, called Kukut, have been designated as an area for waterfowl protection. The catchment area of about 8,020 km2 consistes mostly of lowland rice fields, rubber plantations and forest - covered hills, and contains such rapidly growing cities as Hat Yai and Songkhla. The waste water from certain manufacturing and factories has caused local pollution of the lake and some damage to the fisheries, but the complete and rapid turnover of water in the rainy season seems to prevent severe problems (Q, 1, 2). C. PHYSICAL DIMENSIONS Surface area [km2] 1,082 1.6 (MSL)** Volume [km3] 3.8 (1.5 mMSL)** Maximum depth [m] 2.0 Mean depth [m] * 1.4 Luang 1.8 Sap Songkla 1.4 Normal range of annual water 0.6-2.2 level fluctuation (unregualtied)[m] Residence time [yr] 0.3-0.5 Catchment area [km2] 8,020 *The depth of the channels connecting Thale Sap with the Gulf of Thailand and with Thale Luang is approximately 6 m. The average of monthly water depth observations at 35 stations in Thale Sap and Thale Luang (Oct. 1976-Sep. 1978) was 1.53 m (2). ** mean surface level D. PHYSIOGRAPHIC FEATURES D1 GEOGRAPHICAL (Q) Sketch map of the lake system: Fig. ASI-02-01. Main Islands (4 groups): Ko Yo ; Ko Si, Ko Ha ; Ko Khop, Ko Mak, Ko Nok; Mo Nu, Ko Maco. Outfowing rivers and channels (number and names) ; 2 (channnel connecting Thale Sap Songkhla with the Gulf of Thailand, and Ranot Canal). D2 CLIMATIC Climatic data at Songkhal (Q,2) (cf. Fig. ASI-02-02) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann. Mean temp.* [deg C] 26.9 27.4 28.0 28.7 28.4 28.1 27.8 27.8 27.5 27.0 26.6 26.6 27.6 Precipitation **[mm] 141 39 42 56 131 102 121 114 122 305 541 437 2149 *1951-1980 **1951-1970 Fig.ASI-2-1 Sketch map of L. Songkhla and surrounding areas, with isopleths of mean salinity (2). Fig. ASI-2-2 Distribution of mean annual rainfall (1959-1983) in the catchment area (Q). Number of hours of bright sunshine: 2,625.0 hr yr-1. Water temperature [deg C] (Q,4) Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Thale Noi* 26.9 - - - - - - - 31.8 - - - Thale Luang** 28.7 28.1 30.0 31.8 31.1 29.4 30.0 28.4 29.8 30.7 27.4 27.6 Thale Sap** 27.3 28.3 29.6 31.0 31.3 29.8 29.6 29.4 28.5 30.4 26.5 27.0 *May. 1980 -Jan.1981 **Oct. 1976- Sep.1978 Notes on water mixing and thermocline formation (Q): Extensive mixing in Thale Luang ; no thermocline formation. E. LAKE WATER QUALITY E1 TRANSPARENCY [m] , Oct. 1976-Sep.1978 (2) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Thale Luang 0.5 0.5 0.6 0.6 0.5 0.5 0.6 0.5 0.4 0.7 0.6 0.4 Thale Sap 0.6 0.7 0.8 0.9 0.9 0.6 0.9 0.8 0.7 0.7 0.4 0.3 E2 pH ,1978-1979, average at 0.3 m depth (Q) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 7.8 7.8 8.1 7.9 7.2 7.3 7.8 7.3 7.5 6.9 6.9 7.2 E3 SS [mg l-1] of surface water, May 1980- Jan 1982 (4) Jan May Sep Thale Noi 26.2 176.7 49.9 Thale Luang (Kukut) 35.1 33.1 32.9 E4 DO [mg l-1] , 1978-1979, average at 0.3 m depth (Q) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 5.9 7.1 6.8 7.1 6.9 7.0 5.3 6.6 7.2 4.7 5.3 6.7 E5 COD [mg l-1] , 1983-1984, surface water of Thale Sap : 1-2 (Q) E6 CHLOROPHYLL CONCENTRATION [μg l-1] , 1978-1979, average at 0.3 m depth (Q) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 1.65 6.30 2.58 10.0 - - - 7.58 0.77 0.69 2.27 - E7 NITROGEN CONCENTRATION [mg l-1] Total-N of surface water, May 1980-Jan.1981 (6) Jan May Sep Thale Noi 0.64 3.75 1.39 Thale Luang (Kukut) 0.46 0.89 0.96 NO3-N (mean for whole lake, 1973-1977)(1). Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 2.20 1.96 2.58 2.59 - - - - 1.95 1.97 2.29 2.33 E8 PHOSPHORUS CONCENTRATION [mg l-1] PO4-P, 1978-1979, average at 0.3 m depth (Q) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0.05 0.08 0.06 0.07 0.02 0.08 - - 0.15 0.12 0.04 0.07 E9 SALINITY [%] , 1978-1979, average at 0.3 m depth (Q) (cf. Fig. ASI-02-01) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 2.9 3.3 5.9 6.9 1.2 1.1 1.3 3.1 9.3 11.0 3.5 2.2 F. BIOLOGICAL FEATURES F1 FLORA Mangrove plants: Kukut (Sonneratia caseolaris, Nipa fruticans, Acrostichum aureum) ; Thale Sap (Rhizophora apiculata) (6). Emerged macrophytes: Thale Noi (Phragmites communis, Scleris oryzoides, Cyperus grossus and spp., Eleocharis dulcis); Kukut (paspalum sp., Eleocharis dulcis, Scirpus mucronatus, Cyperus tegetiformes and spp., Phragmites connunis) (6). Floating macrophtes: Eichhornia crassipes (Q); Thale Noi (Nymphoides indica, Salvinia cucullata); Kukut (Azolla sp., Lemna sp.) (6) Submerged macrophytes: Thale Noi (Chara sp., Blyxa echinosperma, Utricularia flexuosa, Ceratophyllum demersum); Kukut (Chara spp., Ceratophyllum demersum, Najas marina) (6). Phytoplankton: Kukut (Phormidium spp., Oschillatoria spp., Anabaena spp., Nitzchia spp., Navicula s pp., Diatoma spp., Scenedesmus spp., Spirogyra spp.) (6); Thale Luang ; (Pediastrum sp., Spirulina sp., Surirella sp.) (2); Thale Sap (Chaetoceros spp., Nitzchia spp., Rhizosolenia spp., Coscinodiscus spp.) (2) F2 FAUNA Zooplankton: Thale Noi and Kukut (Arcella spp., Difflugia spp., Brachionus spp., Lecane spp., Cladocera, Podople a) (6). Cladocerans tend to be found in the upper part of the lake system, and copepods, urochorodates, nauplius larvae and chaetognatha in the lower part, while fish larvae and malacostraca are more widely distributed (Q). Benthos: Thale Noi and Kukut (Polychaeta (Nereis, Dendronereis, Nephtys, Sabella, Meglomma), Mollusca (Melanoides, Idiopoma, Pila, Clea, Corbicula, Modiolus), Crustacea (Palaemon spp., Macrobrachium spp., Gammalus sp.)); Thale Sap (Polychaeta (Nephtys, Nereidae, Orbiniidae, Lambrinereis, Arbella), Crustacea (Upogebia, Cyatura, Apseudes, Gammalus, Penaeus, Metapenaeus)) (2,6). Arthropods are found at high density in the uppere lake system, reaching the maximum level in dry season. Polychaetes increase their density toward the sea, flourishing in dry season. Nematodes are found at high density near the end of each lake, while nemerteans are abundant in the middle lake system (Q). Fish(caught by fishing gears): Thale Luang (Tachysurus spp., Mystus spp., Fluta sp., Puntius spp., Ophicephalus spp., Omp ox sp.); Thale Sap (Mugil dussunieri and spp., Lates calcarifer, Epinephelus spp., Eleuhteromema sp.,, Gerres sp., Plotosus sp.) (2). G. SOCIO-ECONOMIC CONDITIONS G1 LAND USE IN THE CATCHMENT AREA (Q, 7) Area [km2] [%] Natural landscape 1,726 24.7 Undisturbed 1,084 15.5 Diturbed 487 7.0 Swamp & Mangrove 155 2.2 Agricultural land 5,110 73.0 Paddy field (rice) 2,474 35.3 Rubber plantation 2.636 37.7 Residential area 163 2.3 Total 6,999 100.0 Main types of woody vegetation (main species): Tropical rain forest; secondary forests at various stages of regeneration; mangrove forest (Rhizophora apiculata, Sonneratia caseolaris, Nipa fruticans); rear mangrove (Melaleuca Leucadendron); brackish swamp forest; freshwater swamp forest (Alstonia spathulata, Eugenia grata); scrub on limestone hills, etc. Main types of herbaceous vegetation: Grass swamps (Paspalum scrobiculatum, Phragmites spp., Cyperus, Scirpus, Scleria, etc.). Levels of fertilizer application on crop fields: Light Trends of change in land use (1974-1982) (1) 1974 1978 Land use type 1982[%] [%] [%] Forested area Undisturbed forest -Tropical evergreen forest 19.7 16.2 15.3 -Mangrove and swamps - - 2.2 -Scrub - - 0.2 Diturbed forest -Old clearing area 5.9 - 2.7 -New deforesed area - - 4.2 -Rubber plantation mixed with forest 10.8 - - Non-forest area Rice field - 83.8 - Rubber plantation 22.6 - 35.3 Residential area 6.0 - 2.3 Total 100.0 100.0 100.0 G2 INDUSTRIES IN THE CATCHMENT AREA AND THE LAKE Kind of industry Units No.