The Diatom Stratigraphy of Rawapening Lake, Implying Eutrophication History

Abstract

Problem statement: The use of diatoms to reconstruct past ecological conditions in lakes is well established. Diatoms are microscopic algae that forms siliceous frustules which allow them to preserve well in sediments. Rawapening Lake is one of 15 Indonesian lakes identified as 2010-2014 National Priority Lakes. Naturally, Rawapening is a tectono-volcanic lake. In the early 1900s, the sole outlet of the lake, Tuntang River, was impounded for hydroelectricity, irrigation and fisheries. Since then Rawapening had become a semi natural lake. The main problem of Rawapening Lake is blooming of water hyacinth that reduce lake function. This research was conducted in order to reconstruct the nutrient history of Rawapening Lake, Java. Approach: Sediment samples were taken from four sites and were sliced every 0.5 cm for diatom analysis and bulked across 2-5 cm for ²¹⁰Pb radiometric dating of sediment. Diatom analysis consisted of three steps: the digestion process to separate the diatoms from the sediment; preparation and mounting of diatom residues onto slides and identification-enumeration. Results: The diatom-inferred condition of Rawapening Lake may be divided into four phases represented by zone 1 (1967-1974), zone 2 (1974-1983), zone 3 (1984-1990) and zone 4 (1990-2008). The predominance of Synedra from 1967 to present indicates that Rawapening Lake has been fresh and meso-eutrophic throughout. Zone 1 is also characterized by Fragilaria capucina Desm, Luticola goeppertiana (Bleisch) Mann, Mayamae atomus (Kutzing) Lange-Bertalot, Navicula radiosa Kutzing, Nitzschia palea (Kutzing) W. Smith and in one site, Tryblionella apiculata Gregory, that reflect eutrophic, but clear waters. An increase in epiphytic Gomphonema spp. in zone 2 marks an increase in aquatic macrophyte plants, perhaps in response to high nutrient levels. This change is followed promptly by an increase in acidophilous Eunotia spp. reflecting high organic production. A transition to a diatom community dominated by planktonic forms occurs c. 1983. This community was initially dominated by more clear water, oligotrophic species such as Discostella stelligera (Cleve and Grunow) Houk and Klee and Aulacoseira distans (Ehrenberg) Simonsen, but transitions in 1990 to one dominated by A. granulata (Ehrenberg) Simonsen and ultimately Aulacoseira ambigua (Grunow) Simonsen. This is interpreted as a shift to a turbid water phase that has advantaged phytoplankton, at the expense of benthic or epiphytic taxa that require clear water. Conclusion: The dominance of A. granulate (Ehrenberg) Simonsen since the 1990s indicates the lake experienced hypertrophic conditions with pH > 9. A high proportion of the taxa in Rawa Pening sediments are not represented in the European data set, so the development of data set of tropical lakes is recommended to provide stronger inferences in local settings.