RESEARCH ARTICLES CURRENT SCIENCE, VOL. 111, NO. 1, 10 JULY 2016 168 *For correspondence. (e-mail: hachyuthan@yahoo.com) Organic matter source in the freshwater tropical lakes of southern India R. Vijayaraj and Hema Achyuthan* Department of Geology, Anna University, Chennai 600 025, India In this article, sediment texture, calcium carbonate, organic matter (OM), total organic carbon (TOC), total nitrogen (TN) and C/N ratio analysis of lake floor sediments of the Berijam and Kukkal lakes located nearly 2000 m amsl in the Palani Hills, Tamil Nadu, India are presented. This study was carried out to decipher the spatial distribution, origin of OM and the relationship among the components. TOC, TN% and C/N ratio of the Berijam lake sediments indicate a mixed source of OM (terrestrial and in situ lake algae and other biota), while for the Kukkal lake it is due to algae, aquatic weeds and plant detritus that grow along its margins. The lake margins and lake-bed morphology control the distribution of sediment parti- culate matter and texture. Keywords: Algae, freshwater lakes, organic matter, particulate matter, PCA analyses, sediment texture. PLANT detritus and sediments are an integral part of the lake system, that influence the nutrient budget and reflect upon the ecology, trophic status and rate of evolution of lakes 1,2 . Lake floor sediments play a significant role as a trap and offer a surface for sediment deposition. They act as a natural regulator for biological processes in the lakes 3,4 . They are also a source of nutrients to the water column that lead to benthic–pelagic mixture, which influ- ences the principal productivity 3,4 . Spatial changes in the floor sediment types often carry signatures of variations in the hydrological processes that operate within the lakes and the local sediment sources 5,6 . Sediments produced due to the physical and chemical erosion of rocks in the catchment area are transported by streams and deposited in the lakes. There is relatively little information pub- lished about the lake floor sediment characteristics 5,6 . Further, information pertaining to contributions from the catchment area and distribution pattern of sediments in the lake, origin and source of total organic carbon (TOC) is scarce 3,4 . The primary source of organic matter (OM) and TOC in lake sediments is often considered to be derived from the particulate detritus of plants and only a small percentage is from animals and other sources 7,8 . Lakes are also major sinks for carbon and hence it is important to know the origin of OM and TOC in the lakes and to categorize major biogeochemical processes that operate within the lake systems 8 . In this study, we present data on OM, TOC, total nitrogen (TN), C/N and texture of lake floor sediments collected from two lakes, Berijam and Kukkal, Palani Hills, Tamil Nadu, India 9 , to identify the source of OM and understand the spatial distribution and interrelation among various parameters. Study area Berijam lake (101100N: 772344E) and Kukkal lake (1016N: 7722E) are located respectively at an altitude of ~2165 (ref. 9) and ~1887 m amsl in the Palani Hills, about 21 and 33 km from Kodaikanal town. The Palani Hills are an eastward extension of the Western Ghats range in southern India (Figure 1). Both are freshwater lakes having zig-zag lake margins (Figures 1 and 2). The Berijam lake has an catchment area of 77.8 sq. km with a maximum length of 3 km and a surface area of 24 ha; it has a capacity of 77 million cubic metres of water. By comparison, Kukkal lake is small, with a maximum length of ~1 km, covering an area of 18 ha and can hold ~30–35 million cubic metres of water. The underlying bedrock for both the lakes is Precambrian charnockite in association with hornblende–biotite gneiss, granite and quartzite covered by red vertisols and laterite. Amara- vathi, Kudhiraiyar, Palar, Porandalar, Vardhamanadhi, Manjalar and Maruthanadhi are the watersheds that cover the Berijam lake and for the Kukkal lake, it is Kudiraiyar. The area receives mean annual rainfall of 1690 mm from both the SW and NE monsoon, with contribution from the former being dominant. The dry season is from December through March. The summer and winter temperatures vary between 19.8C (max) and 11.3C (min), as well as 17.3C (max) and 8.3C (min) respectively. Both lakes are surrounded by the Shola forests. The soil is clay to clay-loam, black in colour due to the accumulation of humus 10 , and acidic in nature with high percentage of iron and alumina. Materials and methods In the present study, 30 lake floor sediment samples (15 samples from each lake) were collected using a scoop