Exchange of energy between the aquatic environment and its adjacent terrestrial ecosystem is a continuous process and thus, they are influenced much by each other. Land-based activities around the aquatic ecosystem have a crucial effect on energy inputs to it. For proper functioning of an ecosystem, there must be a balance between inputs and use of energy. Exceptions to this would be damaging for both the ecosystems.
Gulshan-Baridhara Lake was declared an Ecologically Critical Area (ECA) in 2001. Continuation of all sorts of banned activities in the ECA has turned it into an ecologically dead lake. This has been manifested recurrently through the death of fishes, receiving media coverage vigorously each year.
Importance of the lake: In the context of very scarce recreational opportunity in Dhaka city, this lake could be an important recreational point for city dwellers. Despite being a source of ground water recharge, the lake has a very important cooling effect on the city environment. Also this lake is a source of drinking water for wild fowl and other animals in the area.
Physiography of lake: The lake is elongated in a north-south direction and surrounded mainly by residential areas and some industrial units as well. The lake is fragmented into four parts by culvert type structures. Openings of such structures appear to be very insufficient for free flow of water among different parts of the lake. This badly affects the dilution of pollution loads, dissolution of oxygen into lake water and as a result, the localised effect of pollution sometimes becomes so severe that it causes death of aquatic organisms, such as fish.
Water quality: Pollution load (Biological Oxygen Demand/BOD) measured in the lake has been much greater than the critical level for fisheries (Table 1). Also, there is spatial variation in BOD load in different parts of the lake. In the middle points of each part of the lake, BOD load is smaller compared to the end-points (in longitudinal direction) or close to culverts. A similar trend in Dissolved Oxygen (DO) levels is observed throughout the lake. These negative impacts might have originated due to fragmentation of lake habitat that hinders lake water movement and ultimately affects load accumulation capacity of the lake.
Temporal variations in DO level in lake water (considering a 24-hour cycle) are observed (Fig. 2). The deep green colour of water indicates the presence of an algal boom in the lake, owing to high pollution load. After daybreak, as the sun is high, the DO level in lake water reaches its maximum around noon, due to algal photosynthesis by which oxygen is released into lake water. The DO level begins to decline as the sun goes down. At night, through to early in the morning, high respirational demand for oxygen causes exhaustion of oxygen in the water, leading to a DO level near zero. Such a temporary anaerobic condition might have been responsible for the death of fishes in the lake.
Factors responsible for the imbalance:
* Direct discharge of untreated sewage and dumping of solid wastes into the lake.
* Surface runoff during the rainy season that carries pollutants from the catchment area of the lake.
* Fragmentation of lake ecosystem by culverts that hinders free movement of water.
* Discharge of industrial waste into the lake.
* Encroachment on the lake.
* Performing commercial and other banned activities in the ECA.
* Lack of effective conservation activities of the lake ecosystem, etc.
* A lake management authority comprising all stakeholders could be formed for the conservation of the lake.
* Delimiting the boundary of Gulshan lake to save it from encroachment.
* Manage household discharge: identification of pollution sources, quantification and selection of treatment option(s).
* Manage solid waste in the area to reduce pollution load through surface runoff.
* Ensure free movement of lake waters.
* Stop industrial wastewater discharge into the lake.
* Design fine scale activities for environmental conservation of the lake.
Dr. Md. Sohrab Ali is Deputy Director (Technical/CC), Department of Environment, Agargaon, Dhaka.