Rawa Pening Lake is one of the lakes in Indonesia that have the problems of sedimentation and water quality degradation. Within 22 years (1976-1998) Rawa Pening Lake’s volume decreased by 29.34%. It is predicted if the sedimentation and erosion in the upstream do not change, in year 2012 Rawapening will turn into a land.
"Ecologically, Rawapening Lake has undergone many changes as indicated by uncontrolled growth of aquatic weeds that are generally associated with the process of eutrophication that causes silting," said Dra. Tri Retnaningsih Soeprobowati, M.App.Sc., lecturer of Faculty of Biology, Diponegoro University, during an open examination of the doctoral program of Environmental Science study program on Tuesday (21/12) at the UGM Graduate School.
Soeprobowati said while defending her dissertation entitled Indonesia Protocol of Diatom Analysis for Rawapening Lake Reconstruction, Central Java, Indonesia before the board of examiners, research on Rawapening Lake has already been done quite frequently, but there has never been a study evaluating the reconstruction of this lake. Reconstruction was based on the organism bookmark, which is stored in layers of sediment, thus reflecting the environmental conditions when the organism was deposited. Organism that can indicate the location, status, and quality of water is known as a bioindicator. "Diatoms, algae, have potential as a bioindicator because the cell wall is composed of silica which is preserved during the deposition process. Diatoms are algae that can be found in any fresh waters that have a very important role in silica sikulus and carbon," the lady born in Blora, April 29 1964 explained.
In Indonesia, Soeprobowati continued, research of bioindicator of water quality has been conducted in seven rivers and mangrove ecosystems in Central Java's north coast; however, diatom analysis refers to a method of analysis developed by other countries, because there is no other method in Indonesia that can be implemented.
The diatom analysis that is conducted by the wife of Drs. Sugeng Maryanto, M. Kes., was done in three stages, namely, the digestion to separate the diatoms from sediment particles, construction preparations for diatoms, and identification of enumeration. Based on 294 samples, diatom efficiency above 80% is obtained in the number of 200 valva and above 90% in the number of 300 valva.
Based on her reconstruction of the Rawapening Lake’s past conditions, it can be identified that the conductivity, pH, and total phosphor showed a trend of change associated with rainfall. The Asinan research location with the thickest sediments samples obtained (63cm) is a representation of Rawapening Lake ecological conditions. The four zones that are formed indicate the increase of total phosphorus content of waters from 1990 up to the present.
Furthermore, Soeprobowati said the land use changes in Rawapening Lake catchments areas, such as agriculture intensification with the use of fertilizers and low rainfall between the years of 1984-1990, have induced the higher increase of total phosphorus concentration. “An integrated and sustainable management can be conducted based on the reconstruction of past ecological condition so that predictions of changes in the future may be anticipated,” said the UGM's 1,317 doctor who passed cum laude.