Effects of agricultural projects on nutrient levels in Lake Bera (Tasek Bera), Peninsular Malaysia

Abstract

Lake Bera is the largest natural lake in Peninsular Malaysia and was designated as its first RAMSAR site in 1994. The lake has a total catchment area of 593.1km2, although approximately 340km2 of the original tropical rain forest cover has been converted to oil palm and rubber plantations since 1972. Research was conducted to determine the soil nutrient contents in the areas of developed land and to correlate historical variations in nutrient concentrations and eutrophication at the lake with anthropogenic activities. Thus, soil samples in areas of different land use in the catchment area were collected in addition to two cores in the bottom sediments of Lake Bera. In total, 132 samples were analyzed for total carbon (TC) and total nitrogen (TN) contents as well as fallout 210Pb and 137Cs radioisotope activities. Sediment profile dating was performed using the constant rate of supply (CRS) model; the resultant sediment ages were verified by 137Cs horizons. Soils in cleared forest areas exhibited the lowest average nutrient content and 137Cs inventory with an average loss of carbon, nitrogen and 137Cs, of 54.6%, 31.2%, and 74%, respectively, in comparison with soils in areas of undisturbed forest. Clear-felling and burning during forest conversion were identified as the two main mechanisms that disrupted the nutrient cycles in the lake catchment. The total concentrations of nutrients in the bottom sediment profiles in the main open water and in the north of Lake Bera decreased in the order of TOC>K>TN>S>Mg>C. The results highlight a clear correlation between variations of nutrient contents in the lake sediments with anthropogenic and natural events dates using the CRS model; the C/N ratio has remarkably increased four times since oil palm plantations were developed in 1981. This result indicates an upward increase in eutrophication during and following land-use changes. The results also suggest long-term increasing acidic conditions in Lake Bera, leading to a reduction in exchangeable cation contents (Ca, Mg, and K), organic matter preservation, and an incremental addition of SO4 (sulfate) and NO3 (nitrate) ions, particularly in the top layer of the sediment column. This situation will result in Lake Bera being on the verge of considerable ecological risk, as illustrated by very low dissolved oxygen contents, high levels of nitrate, and a reduction in the fish population.