HIDROLOGY AND MORPHOMETRY CHARACTERISTIC

HIDROLOGY AND MORPHOMETRY CHARACTERISTIC CONSIDERATION ON DETERMINING LAKE TOBA CARRYING CAPACITY FOR CAGE AQUACULTURE By. Lukman Carrying capacity of aquatic system on receiving material loading, especially nutrient, is determined by the water residence time (retention time), whereas the residence time is determined by the supply of water from the catchment area (CA) and the volume of water body (V). Lake Toba is the lake which has multipurpose activity, one of them is cage system aquaculture (CSA). Area (A) and volume (V) of Lake Toba is 1107.65 km2 and 256.19 x 109m3, respectively, an has a basin that are physically separated to north and south basin by the presence of Samosir Island. North basin (A: 577.58 km2; V: 155.67 x 109m3) and south basin (A: 529.96 km2; V: 100.52 x 109m3), which is bordered by an imaginary line on the east side in Parapat toward to Samosir Island, and to west region of lake in Pangururan. Meanwhile, the hydrological condition in the catchmen is characterized by an uneven distribution of rivers as inlets of Lake Toba. The inlet discharge in south basin of the lake is higher than the north, and the outlet of the lake only through Asahan River in southern basin with mean discharge 100 m3/sec. River Silang is the largest river as one inlet of Toba Lake, located in south basin with mean discharge 10.7 m3/sec., or nearly 10% of the outlet discharge. Under these conditions, indicating that the southern part of the lake will be more dynamic than the north, water retention time in south basin will be lower, so that carrying capacity of basin to receive the nutrient material will be higher. Consequence from that situation, for north basin has to be more careful on management point and development of CSA has to be more limited. Key words: Lake Toba, hydrology, morphometry, retention time, carrying capacity. INTRODUCTION Carrying capacity is the production attainment on maximum level of a resource that is expected to survive in a sustainable manner (Beveridge, 1987). Understanding of carrying capacity is an overview of production, whereas on the other side, the carrying capacity of natural ecosystems is how much an ecosystem able to support of biotic production continuously. Each waters ecosystem has a variety of carrying capacity that is affected by physical and chemical characteristics it has. In the interest of an aquaculture process in the waters, especially the development of fish culture in floating net (CSA; Cage System Aquaculture) in the lake ecosystem, carrying capacity is a major concern to guarantee the optimization and sustainability of fish production. There are two things that determine carrying capacity, namely: i) Carrying capacity to impact of nutrients addition form culture process which does not cause the process to eutrophication and deterioration of water quality; and ii) Carrying capacity of availability reserve oxygen (DO reservoir) on hypolimnion column (column of bottom waters) in receiving and degrade organic waste from food remains and feces. Prosiding Simposium Nasional Ekohidrologi. “Integrity Ecohydrological Principles for Good Water Governance” APCE. Komisi Nasional Indonesia untuk UNESCO. UNESCO – LIPI. Jakarta 25 September 2011. Hal. 185 - 187

First concern is related to other interests in the waters of the lake that require high water quality, while the second factor is that will determine the availability of dissolved oxygen in the CSA that are still sufficient for the life of the fish. Carrying capacity will be determined primarily by the physical characteristics of aquatic environments (hydromorphology and water regime), which is very specific to each lake. Carrying capacity formulation of waters have been developed by Vollenweider (1975), based on nutrients input-output relationship, particularly of N (Nitrogen) and P (Phosphorous). Vollenweider has linked the rate of load (the annual supply of nutrients per unit area of the lake) to the mean depth and produce value for nutrient content (N and P) are allowed based on the mean depth of the lake. As is known, the levels of total phosphorus (TP) in water often established as one of trophic status criteria of lake, beside total nitrogen (TN) level, a chlorophyll a concentration and Sechi depth (Vollenweider & Kerekes, 1980). Carrying capacity is concern to the availability of phosphorus levels in water and the supply of phosphorus from the outside. Total supply of phosphorus into the lake comes from natural sources i.e. from the catchment area (CA) and direct fall from waterborne rain, and artificial supply (non-natural) from human activities such as waste from the CSA. Meanwhile, levels of total (loading) phosphorus (P) in the waters, beside is determined by the supply of P from the outside is also influenced by the dimensions of the lake, flushing rate, and the fraction of P lost permanently to the bottom waters. Lake Toba is waters that have a multifunctional role, and tourism destination as major activity. On the other hand, the CSA is also currently develop in Lake Toba, get much attention from the public, related to the controversy between the socio-economic needs of community and the environment conservation, also between the achievement of production and waters carrying capacity. The number of CSA that had been operating on Lake Toba in 1999 is 2400 units (Arifin, 2004).

Those are two of the very contrary

lake utilization, regarding the needs to quite high of water quality as viewed from the aspect of transparency as tourism activity need and aquaculture which does not need it. In temperate regions, the management of the lake waters would be very concerned about the minimum transparence level in the summer which is a representation of chlorophyll a levels in the water column.

Prosiding Simposium Nasional Ekohidrologi. “Integrity Ecohydrological Principles for Good Water Governance” APCE. Komisi Nasional Indonesia untuk UNESCO. UNESCO – LIPI. Jakarta 25 September 2011. Hal. 185 - 187

Thus, the carrying capacity of waters in the development of CCS, as the case of the Lake Toba, will be determined by the characteristics of hydrological and those morfometry. CRACTERISTICS OF HYDROLOGY Lake Toba is a volcanic type of lake, is located in the North Sumatra Province, at an altitude of about 904 m above sea level, stretches from northwest to southeast along the 87 km and wide 27 km with a maximum depth about 505 m. Catchment area (CA) of Lake Toba approximately 3658 km2 (365 800 ha) and lake area (A) is 1 103 km2 (Nishimura et al. 1994; Hehanussa 2000). Meanwhile, according to Lukman & Ridwansyah (2010), based on calculating of Automatic Watershed delineation (AWD) by using application of SWAT model hydrologic, Lake Toba area is 2 486 km2 (248 600 ha). The composition of land covers on Lake Toba catchment consist with of bare ground and dry land agriculture which the highest proportion, 20.6% and 27.6%, respectively (Table 1). As known land cover in catchment is one of the factors that affect the quality and quantity of water inflow to the lake. Table 1. Land covers composition in catchment of Lake Toba Land cover Area (ha) Percentage (%) Lowland forest 171.8 0.1 Highland forest 25321.6 10.4 Industrial forest plantation 31452.2 12.9 Settlement 876.3 0.4 Dry land agriculture 67496.4 27.6 Dry land agriculture mixed shrub 43018.2 17.6 Swamp 1940.2 0.8 Paddy field 11247.9 4.6 Bush/shrub 12474.8 5.1 Open land 50374.0 20.6 Totally 244373.5 100.0 Source: Barumun Watersehed Manag. Board, Forestry Department (2008). (unpublished) Open land scattered in many regions of Samosir Island, west and southwest side of the catchment of Toba Lake, while the dry land agriculture spread in the southern island of Samosir, in the north, east and south of Lake Toba catchment. Rice field activities are spread in the river valleys, especially in the west part, southwest, south and southeast of catchment (Figure 1). Dry land agriculture mostly cultivated on a flat place, a Prosiding Simposium Nasional Ekohidrologi. “Integrity Ecohydrological Principles for Good Water Governance” APCE. Komisi Nasional Indonesia untuk UNESCO. UNESCO – LIPI. Jakarta 25 September 2011. Hal. 185 - 187

small portion in the sidelines of a steep hill, on still fertile land. Paddy fields are usually found on flat areas or on the sidelines of hill near the river and looked very dominant in the southeast area of the catchment, especially around Balige and around the outlet of Lake Toba.

Figure 1. Map of land use Lake Toba catchment area Source: Manag.Board of Barumum Watershed, Forestry Depart (2008). (unpublished) The pattern of river flows as inlet of the lake to form specific regime that is specify to each lake, and contribute to the patterns and dynamics of pollutants from a variety of activities in the lake area utilization. According to Sly (1978) catchment area of the lake has a major influence on the supply of sedimentary material and other material. The pattern of water flow of inlet in Lake Toba is dominated by small rivers, with a total of 289 rivers, but only 71 is permanent rivers and the rest is seasonal (intermittent) river. From the Samosir Island flowing 122 rivers and 177 river flow from mainland Sumatra (Soedarsono, 1989). Meanwhile, based on Meigh et al (1990) there are 295


rivers that flow into Lake Toba, most of which is a relatively small catchment area, have an average area of 8.2 km2 , and only some of which has an area of more than 100 km2 . Based on observations at several major rivers in the catchment, inlet of Lake Toba, the river which has the largest discharge is the River Silang/Simangira (+10 m3/sec.), middle river discharge being the Naborsahan (+2 m3/sec.), Balige (+2 m3/sec.), and Sipultakhuda (+1.4 m3/sec.). Most of the rivers are located in the southern part of the lake, while the outlet is Asahan River is also located in the south of the lake (Figure 2).

Figure 2. Pattern of water flow regime in Lake Toba According Meigh et al (1990), mean runoff from lake catchment was 78.6 m3/sec., which of them will flow through the rivers. Meanwhile, based on discharge data Asahan River (outlet of Lake Toba) were observed at the Siruar station, outlet discharge period from 1920 to 1932 was 110.4 m3/sec., from 1957 to 1975 was 104.4 m3/sec., and between 1976 -1988 period reached 90 m3/sec. (Sastromijoyo, 1990). Based on those data, range of outlet discharge between 90 m3/sec. to 110 m3/sec., so that the mean discharge of outlet Lake Toba will be at 100 m3/sec. Thus, there was a supply of water to Lake Toba from outside of the surface flow was + 20 m3/sect. Distribution of large rivers is generally located in the south basin; this appears to be associated to the catchment, in the northern part relatively more narrow than the south. Under these conditions the water supply is estimated from south catchment region will be higher than the north. Prosiding Simposium Nasional Ekohidrologi. “Integrity Ecohydrological Principles for Good Water Governance” APCE. Komisi Nasional Indonesia untuk UNESCO. UNESCO – LIPI. Jakarta 25 September 2011. Hal. 185 - 187

CHARACTERISTIC OF MORPHOMETRY Physical condition of Lake Toba is marked by Samosir Island in the middle part, and it has created two main basins. Based on calculations from bathymetry measurement and delineation of Landsat imagery provides morphometry characteristics of Lake Toba, including surface area, 1124 km2 (112 400 ha), lake volume of about 256.2 km3 (256.2 x 109 m3) with a maximum depth of 508 m (Table 2; Figure 3). Base on data of volume and mean discharge in outlet, Lake Toba will have a water residence (retention time) about 81.24 years. The water retention of Lake Toba is quite long, if compared to the water retention time of Lake Poso, which is only 7.21 years (Lukman & Ridwansyah, 2009). No. 1 2 3 4 5 6

Table 2. Morphometry characteristic of Lake Toba Parameters Dimension Lake area (A) (km2) 1.124 Maximum depth (m) 508 Lake volume (x 109 m3) 256,2 Mean depth (m) 228 Catchment Area(CA) (km2) 2.486 CA/A Ratio 2,21 Source: Lukman & Ridwansyah (2010)

source Landsat imagery Batimetrik map Bathymetry map Bathymetry map Landsat imagery Calculation

Figure 3. Batimetri map of Lake Toba (Source: Lukman & Ridwansyah, 2010)


Morphometric characteristics of Lake Toba which formed two large basins, show that north basin was relatively larger than the south basin and water volume in south basin also higher (Table 3). Table 3. Area and volume calculation of north and south basin of Lake Toba Area Volume Volume Basin Area (km2) proportion (x 109 m3) proportion (%) (%) North 586.16 52.15 155.67 60.8 South

537.84

47.85

100.52

39.2

Total

1 124. 54

100

256.2

100

Source: Lukman & Ridwansyah (2010) ATTENTION TO LAKE CARRYNG CAPACITY Reference of carrying capacity formulation for CSA activities, it is consider to supply and removal balance of phosphorus (P) in aquatic systems (Beveridge, 1996). It is known that phosphorus is one of the parameters determining the trophic state of lake (Vollenweider & Kerekes, 1980). Based on the approach used Dillon & Rigler (1975) the total supply of phosphorus (TP) into the waters, will be associated with catchment and morphometry conditions, and supported by the population density that inhabit it. Trophic state of aquatic system as the impact of phosphorus supply will be illustrated by chlorophyll a concentration (Figure 4). Geology

Hydrologic Budget

Land use Natural phosphorus load Precipitation Population density

Spring phosphorus concentration in lake*)

Artificial phosphorus load Lake morphometry

Kedalaman keping Sechi*)

Chlorophil a consentration (summer average*)

*) lake condition in temperate zone Prosiding Simposium Nasional Ekohidrologi. “Integrity Ecohydrological Principles for Good Water Governance” APCE. Komisi Nasional Indonesia untuk UNESCO. UNESCO – LIPI. Jakarta 25 September 2011. Hal. 185 - 187

Figure 4. Scheme of empiric models to identified impact of human activity on trophic state of lake ( Source: Dillon & Rigler, 1975). Referring to the formulation of Vollenweider (1975), loading rate (the annual input of nutrient per unit lake area, L) in the lake related to morphometry conditions (mean depth, z) and the water retention time). The supply of natural sources phosphorus (JN) into water derives from the catchment (JE) and from direct precipitation (JPR). The overall of phosphorus supply (JT) to the lake waters come from natural events (JN) and the supply from human activities/artificial (JA). The water residence time influence on flushing rate (ρ) which is part of the discharge (Q) of water coming out at the outlet to lake volume (V), or Q/V. Flushing rate will affect the sedimentation rate (σ) of P in the waters. According to the formulation of Larsen & Mercier (1976), phosphorus sedimentation rate (σ) in natural lake waters are: 1 + 0.747 ρ0.507. Taking into account the hydrological conditions of Lake Toba which is characterized by narrow catchment in the north and rivers flowing is dominant in the south, while the outlet is also located Asahan River in the southeast of lake, it is indicated that water circulation pattern in south basin tend to be more dynamic. But with the quantity of the river in the south are also much more, so the supply of P from the catchment will be higher flowing to the lake through those rivers Morphometry conditions of Lake Toba separated between the north and south basin. The water volume in north basin (61%) much larger than south basin (39%) (Table 3) and the water flow pattern in the south is higher, so that it will create a different water retention time. The northern part of Lake Toba will have the retention time much longer than the south, and this should be a consideration on utilizing that area, related to different rate of accumulation of pollutants. CONCLUSION Based on a review on hydrology and morphometry of Lake Toba, northern basin waters will have a condition that is more susceptible to the accumulation of nutrients, especially P component associated to retention time tend to longer. The difference of water mass circulation patterns that tend to be higher in southern basin, implicated that lake management for north basin has to be more careful and development of CSA on that area has to be more limited. Prosiding Simposium Nasional Ekohidrologi. “Integrity Ecohydrological Principles for Good Water Governance” APCE. Komisi Nasional Indonesia untuk UNESCO. UNESCO – LIPI. Jakarta 25 September 2011. Hal. 185 - 187

REFERENCES Arifin, S., 2004. Pengelolaan Ekosistem Kawasan Danau Toba yang Berwawasan Lingkungan. Prosiding Lokakarya Danau Kedua Pengelolaan Danau Berwawasan Lingkungan di Indonesia. Forum Danau Indonesia (FDI) dan International Lake Environment Committee Foundation (ILEC). Hal. 89 – 95. Beveridge, M.C.M, 1987.Cage Aquaculture. Fishing New Books Ltd.England. 352p Dillon, P.J., & F.H. Rigler. 1975. A simple methods for predicting the capacity of a lake for development based on trophic status. J. Fish. Res. Bd. Canada, 32: 1519-1531 Hehanussa, P.E. 2000. Lake Toba, a Multiple Caldera Depression, North Sumatera, Indonesia. Report of Suwa Hydrobiological Station, Shinshu University, Japan. Larsen , D.P & H.T. Mercier. 1976. Phosphorus retention capacity of lakes. J. Fish. Res. Bd. Canada. 33: 1742 - 1750 Lukman & I. Ridwansyah. 2010. Kajian morfometri dan beberapa parameter stratifikasi perairan Danau Toba. Limnotek, Vol. XVII (2): Lukman & I. Ridwansyah. 2009. Telaah Kondisi Fisik Danau Poso dan Prediksi Ciri Ekosistem Perairannya. Limnotek, Vol. XVI (2): 64 – 73 Meigh, J., M. Acreman, K. Sene & J. Purba. 1990. The wáter balance of Lake Toba. International Conference on Lake Toba, May 1990. Jakarta – Indonesia. Nishimura, S., E. Abe, J. Nishida, T. Yokoyama, A. Dharma, P.E. Hehanussa, and F. Hehuwat. 1984. Gravity and Volcanostratigraphic Interpretation of Lake Toba Region, North Sumatra, Indonesia. In: Tectonophysics. Elsevier Science: Amsterdam. p. 253 -272. Vollenweider, R.A. 1975. Input-output models with special reference to the phosphorus loading concept in limnology. Schweiz. Z. Hydrol. 37: 53 - 84 Vollenweider, R.A & J. Kerekes, 1980. The loading concept as basis for controlling eutrophication philosophy and preliminary result of the OECD programme on eutrophication. Prog. Wat. Tech. Vol. 12 (2), Norway, pp. 5 -38. IAWPR/Pergamon Press Ltd. Great Britain.


HIDROLOGY AND MORPHOMETRY CHARACTERISTIC

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