Dinamika Perairan Danau Pasang-Surut Dalam Perspektif Pengelolaan Danau Siombak

Abstract

Siombak Lake is one of the tidal lakes in Indonesia. This lake is located on the coast of Medan City, North Sumatra Province. This lake is unique and distinctive. The uniqueness and peculiarity of this lake is the lake water system which is estuarine. The river flow that enters the 7 km long lake from the Belawan River Estuary (Malacca Strait) is still affected by the ebb and flow of seawater. On the lake border, some mangroves grow on the edge of the lake. The aquatic biota in Siombak Lake consists of groups of freshwaters, brackish, and marine organisms. Siombak Lake functions as a water catchment, flood control, fishing ground, as well as a tourist spot. The area around the lake is also home to various community activities such as fishponds, residential areas, industry, and landfills of Medan City. The Siombak Lake Ecosystem is also one of the Green areas in Medan City. The existence of utilization activities in the waters of Siombak Lake and its surroundings will put ecological pressure on the lake ecosystem. Proper management needs to be done to maintain the sustainability of this tidal lake. Thus, lake management studies by integrating conservation, fisheries, and ecotourism need to be carried out as an integrated and sustainable management effort. The purpose of this study is to develop a tidal lake management concept based on tidal dynamics based on resource conservation. Data collection in the field was one year (September 2018 – August 2019). The data collected consists of data on the physical shape of the lake, water quality, aquatic and terrestrial organisms, and water levels (tides). Lake water level elevation is obtained from tidal data which is measured every hour for 15 days. Water quality data and aquatic organisms are taken every month during a full moon. Data analysis was carried out descriptively, statistically, and spatially. Tidal lake water quality index (TL WQI) as a novelty was developed to determine the level of pollution in lake waters. TL WQI was developed from the National Sanitation Foundation - Water Quality Index (NSF-WQI) which was adapted to the characteristics of tidal lakes. Marxan and Quantum GIS were used to design and locate locations for lake management zoning. Siombak Lake has an area of 29.72 ha with an average lake depth ranging from 1.63 – 4.53 m when it rains (MSL = 3.27 m) and 1.61 – 4.38 when dry (MSL = 3.25 m). In general, changes in water level in Siombak Lake occur every 6 hours (in 1 day there are 2 high tides and 2 low tides), except for neap tides. Throughout the year the water level of Siombak Lake fluctuates following the tidal cycle. The highest water level in Lake Siombak was in November 2018 to reach 227 cm. The lowest water level occurred in February 2019 with a height of -24 cm. The water quality characteristics of Siombak Lake are characterized by high concentrations of salinity and TDS. In the rainy season, the salinity concentration on the surface ranges from 2-7 ppt, at the bottom, it reaches 10 to 20 ppt. In the dry season, the surface salinity can reach 10-12 ppt and at the bottom, it can reach 15 ppt to 24 ppt. TDS concentration ranged from 9,678.75 - 14,257.50 mg/L. The pollution status of Siombak Lake based on the tidal lake water quality index (TL WQI) is in the low

to moderately polluted category. The water quality of Lake Siombak at low tide is worse than at high tide and in the dry season is also worse than in the rainy season. Siombak Lake has high biodiversity consisting of 54 species of phytoplankton, 12 species of zooplankton, nine families of larvae and juveniles, ten species of mollusks, and 18 species of crustaceans, 27 species of fish, 14 species of mangroves, 12 species of birds, and two species of primates. The biological character of the lake that is rarely found elsewhere is on the various sides (borders) of the lake overgrown with various mangrove trees dominated by Nipa Palm (N. fruticans) and Mangrove Apple (S. caseolaris). Some other unique and distinctive species in Lake Siombak include Indo-Pacific tarpon (M. cyprinoides), Shortfin Eel (A. bicolor), and Jellyfish (A. aurita). The biological community of Lake Siombak consists of freshwater, brackish, and marine organisms. Aquatic organisms in Lake Siombak also consist of resident and migrant organisms (seasonal). Ecologically Siombak Lake acts as a feeding and nursery ground. Economically Siombak Lake can be developed to capture fisheries and ecotourism activities. For the sustainability of capture fisheries, arrangements are made to the size of the allowed nets. The size of the net allowed is adjusted to the size of the first mature gonad, namely: size 22.38 cm for Mozambique tilapia, size 24.01 cm for Nile tilapia, 38.42 cm for Indo-Pacific tarpon, 19.75 cm for Spangled gudgeon, and 10 cm for mangrove crab. The development of Siombak Lake ecotourism includes leisurely sitting tours, boating tours, and fishing tours. The carrying capacity of lake tourism is set at 1000 people per day for leisurely sitting tours, 400 people for boating activities, and 250 people per day for fishing activities. Setting boating time, boat size, boat speed, and boating routes must be done to avoid conflicts with local fishers. Integrated management of fisheries and ecotourism in Siombak Lake by zoning lake management. The zoning consists of a conservation zone (consisting of a lake reserve area, lake border, and lake rehabilitation), a fishery zone, and a tourism zone. The lake reserve area is a zone for the protection of spawning and rearing fish and other aquatic biotas. The lake border area is a lake protection zone from the mainland. Habitat restoration and rehabilitation in the form of cleaning the lake from garbage, making garbage traps, rehabilitating mangroves on lake borders, and controlling illegal settlements. The novelties of this research are: 1) The method of determining the pollution status of coastal lake waters known as the Tidal Lake Water Quality Index (TL WQI); 2) Lake utilization models for fishing and tourism activities; and 3) the concept of integrated lake management by integrating fisheries and conservation-based eco-tourism.