Vertical Distribution of Diatoms Analysis to Determine the Condition of Rawapening Lake in the Past through Cluster Analysis with the Bray-Curtis Model

Rawapening Lake is located in Semarang Regency, Central Java Province, Indonesia, and is the center of various activities, including agriculture, fisheries, sources of electrical energy, raw water, tourism, and culture. A 100-cm-long sediment sample was extracted from Rawapening Lake to reconstruct the dynamics of environmental change by observing the preserved diatoms. Diatoms are sensitive to changes in water quality, so their pattern of existence can reflect changes in lake conditions. The sampling location is located at the mouth of the Tuntang River, the only outlet of Rawapening Lake, which has 20 inlets as water sources. Sediment cores were sliced at 1-cm intervals for diatom analysis. Diatom slide mounts were prepared from about 5 grams of dry sediment using 10% hydrochloric acid followed by 10% peroxide to remove organic matter and carbonates. A total of 300 valva diatoms were counted for all samples in order to obtain the lowest level of diatom taxonomy. Vertical distribution analysis is used to determine the condition of Rawapening Lake in the past through cluster analysis using the Bray-Curtis model, which is based on depth similarities. The reconstruction of Rawapening Lake, which was carried out based on variations in the abundance and diversity of diatoms vertically, showed that the pH of the waters tended to increase. Indicators that strengthen this statement are the dominance of A. ambigua, A. granulate, S. ulna, and S. acus. This can occur due to two things: the occurrence of decomposition of organic material by microorganisms at the bottom of the lake or the use of lime in the manufacture of organic fertilizer from water hyacinth. When it rains, the lime will wash off and enter the lake. Total Nitrogen of 1.773 μg/l in Rawapening Lake indicates that the nutrient enrichment of these waters has reached hypereutrophic levels. The dominance of benthic indicators with high nutrient status indicates that Rawapening Lake has a long history of eutrophication. The condition of Rawapening Lake can be categorized into 3 zones, namely: (a) Zone 3 (90-100 cm layer) is dominated by diatoms N. palea, N. closterium, and T. apiculate, which indicate eutrophic water conditions; (b) Zone 2 (30-40 cm layer) is dominated by A. distans, N. closterium, T. apiculata, S. ulna, and N. radiosa species, where A. distans is widely distributed in clear and oligotrophic waters; (c) Zone 1 (1-20 cm layer) is found S. pupula of the genus Sellapora, which is an indicator of lake or river waters with mesotrophic-eutrophic conditions.