Holocene environmental change in Tonle Sap, Cambodia based on fossil diatoms

Abstract

Tonle Sap Lake (“Great Lake”, Cambodia) is a biggest inland freshwater body. The size of the lake is changed dynamically following monsoon via connected to the Mekong river, especially the flood pulse. The flood pulse on Tonle Sap has affected considerably the lake’s ecological property as well diatom assemblages. The present study aimed to assess the impact of the flood pulse to diatom assemblages by time. Two short sediment cores from Tonle Sap Lake with the depth of 1.54 m and 1.27 m respectively below the lake floor were collected in May 2013 and 2015 and one short sediment core with the depth of 1.68 m was collected from the confluence of the Mekong River and Tonle Sap River in May, 2013. The sedimentations were dated by using radiometric dating (210Pb and 137Cs). Succession of fossil diatom assemblages was calculated by Rarefaction index (ES) and species richness is by Hill's N2 index. A total of 70 diatom species was released, and the diversity of diatom assemblages was extremely fluctuated in function of time (p-value = 0.0045***). Especially, 6 diatom taxa: Aulacoseira distans, Aulacoseira granulata, Aulacoseira granulata var. angustissima, Gyrosigma acuminatum, Gyrosigma attenuatum and Paralia sulcata characterized by the highest relative abundance (>1 %). In term of ecology, these species are the epipelic diatoms living commonly in eutrophication and high suspendid solid conditions. In fact, it is clear that the onset of flood pulses affected considerably the studied diatom assemblages in particular, and made sense to bio-community in general; also the lacustrine environment of Tonle Sap lake was changed very strongly in response to this shift of hydrological regime.