Vertical profiles of low molecular weight organic acids in sediment porewaters of six Chinese lakes

SummaryThe composition and contents of low molecular weight organic acids (LMWOAs) were determined, their contributions to dissolved organic carbon (DOC) and vertical profiles were investigated in sediment porewaters of six lakes, China: Lake Sihai-Longwan (SHLW), Lake Bosten (BST), Lake Qinghai (QH), Lake Chenghai (CH), Lake Erhai (EH) and Lake Dianchi (DC). The sources and behaviors of LMWOAs during early diagenesis were also discussed. The results show that up to eight species of LMWOAs were detected in those sediment porewaters: lactic acid, acetic acid, formic acid, methanesulfonic acid (MSA), pyruvic acid, trifluoroacetic acid (TFA), sorbic acid and oxalic acid; acetic acid was the predominant species. The contents of organic acids in six lakes decrease in the rank order of QH > SHLW > BST > EH > DC > CH, with the total amount of organic acids in the northern lakes being about 27 times higher than those in the southern lakes. In total amounts of various organic acids, acetic acid were the highest and MSA was the lowest, they were 62.7 and 0.3 μM, respectively. The highest total amount of organic acids was in QH, with the average value of 92.0 μM and the contribution to DOC was 10.9% followed by SHLW, BST, EH and DC, which were 43.7 μM, 2.7%; 42.3 μM, 5.6%; 5.6 μM, 1.1%; 4.4 μM, 0.7%, respectively. Organic acid was the lowest in CH, with the average value of 0.5 μM and the contribution to DOC was just 0.04%. The main organic acids were acetic acid, pyruvic acid, sorbic acid and formic acid in SHLW, BST, DC and CH, lactic acid in both EH and QH. Vertical profiles of LMWOAs varied among different species and lakes due to the sources and complex microbial processes in early diagenesis. This study suggests LMWOAs played a significant role in the sources and transformation of DOM in the sediments in lakes.