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F. Changes in the Hydrodynamic of Rawa Pening Following Urbanisation

It is likely that there have been extensive changes in the hydrodynamic processes in this rawa since the arrival of Eichornia crassipes and the increased urban development in the catchment. This is a shallow lake with a relatively large surface area at a moderate altitude. Water circulation, and therefore aeration, should be adequate to ensure that oxygen is available at the bottom of the lake, although the lake is tropical.

1. Conditions about 50 years ago

2. Changes occur

3. Results

4. Eventual risk

There is a possibility that at times, in hot dry weather, with little wind action on the surface of the rawa, the bottom water will become de-oxygenated. Bacteria will have consumed all the available oxygen in the water. Under these conditions and even before these conditions are reached, hydrogen sulphide will appear in the water. It is a soluble gas. It is very toxic. It easily kills fish and crustaceans. Once fish and crustaceans have been killed, the added amount of organic matter in their dead bodies leads to further bacterial growth and more oxygen demand. Once this situation is reached, it may take many weeks for the lake to become aerated again.

No episodes of de-oxygenation have been reported in this lake. Goeltenboth (1979) did not detect any stratification during regular sampling of water chemistry on transects across the lake and consequently designated the lake as oligomictic. Stratification is often accompanied by de-oxygenation if it persists for any length of time in tropical conditions.

However, it is noteworthy that de-oxygenation of water in a lake or swamp can occur under mats of vegetation. De-oxygenation has been recorded in shallow water under mats of Eichornia crassipes in a swamp on the Gwydir River near Moree in Australia.

It is unlikely that Goeltenboth sampled the water under some of the extensive mats or "floating islands" in the lake. These form a solid compact mass of intertwined roots and vegetation on which one can stand and include small trees, bushes, and shrubs growing to a height of two or three metres. The roots may extend two or three metres or more into the water. It would be very difficult to drop a water sampling bottle through such a mat. The islands may be over a hectare in area. The vegetation on these islands and their structure has been described by Polak (1951) and by Notosoedarmo, et al (1978).

If de-oxygenation were to occur in this lake it would be likely to occur under such large mats or floating islands of mixed vegetation starting originally as mats of Eichornia Crassipes when the island become "grounded" in shallow water at the edge of the lake. It would be most likely to occur in the dry season during a prolonged drought if there were a spell of hot weather with no wind.

Development in the catchment during the last twenty years has led to changes which would all tend to increase the possibility of de-oxygenation. These changes include increases in urbanisation, in tourism, in road traffic, and in rice intensification, the construction of bottled water factories, and small scale industrialisation.

Regrettably, it appeared that none of the information necessary to assess the situation in Rawa Pening, at the present time, was available to the investigating team.

Extensive deoxygenation would wipe out the fauna. The fishing industry would cease to exist. The peat extraction industry would come to a halt because workmen digging out peat would find the smell of hydrogen sulphide too offensive for work. The gas is toxic to people too. The tourist industry would stop completely because tourists do not want to be exposed to nasty smells. It would have an unfortunate effect on the power station, because the change in pressure in the pipe on the way down to the power station would release much of the gas from solution. There would be a very high concentration of toxic gas at the bottom of the narrow valley in which the power station is situated. It would be most doubtful if the power station staff could continue without gas masks.

Obviously this is not a situation anyone would want to see occurring. Therefore the main long-term objective must be to manage the system to avoid such a disaster. The objective of ecological sustainability can be interpreted here as being to manage the system so as to hold the environmental situation as it is, to prevent any further deterioration, and then to improve the long term prospects for the rawa, despite the expected future increases in urbanisation and tourism.

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