Composition, distribution, and source of organic carbon in surface sediments of Erhai Lake, China

Lake sediment is an important organic carbon (OC) sink. Nevertheless, few studies have been conducted on sediment organic carbon (SOC) in lakes, and the effects of environmental variables on SOC pools remain poorly understood. We combined physicochemical and spectroscopic analyses to investigate the composition, distribution, and source of OC in surface sediments of Erhai Lake, southwest China, and explored the relationships between environmental variables and its SOC pool. The SOC pool consists of relatively high proportions of labile organic carbon fractions, mainly from algal production, which are rapidly decomposed and exhibit high turnover rates. The relative content of humus carbon ranges from13.5% to 20.5%, with fulvic acid carbon predominating (average 52.95%), indicating weak humification and a relatively active humus carbon pool. The dissolved organic matter in water column and sediments of Erhai Lake is largely influenced by endogenous production, with a great contribution from phytoplankton. Surface sediments contained more protein-like components than overlying waters (80.0 % vs. 63.0 %), attributed mainly to abundant algal deposition and intense bacterial metabolism. Among environmental variables, sediment chlorophyll a showed the strongest relationship with the SOC pool, and was associated with rapid decomposition and promotion of the humification process, which supported the conclusion that algae had an important influence on the SOC pool. The SOC pool in the southern region of the lake is mainly contributed by algae, other microorganisms, and sewage, exhibiting a greater potential to release organic matters into the water column. The center and northern SOC pools show relatively stable characteristics and stronger OC sink capacity, mainly because of the input of terrestrial refractory organic matters from runoff. Our data shed light on the OC storagemechanisms in the surface sediments of Erhai Lake and provide theoretical bases for enhancing the OC sink of sediments in the lake.

Automated summary

Lake sediment is an important storage area for organic carbon (OC), but research on sediment organic carbon (SOC) is limited and the impact of environmental variables on SOC pools is not well understood. This study used physicochemical and spectroscopic analyses to investigate the composition, distribution, and sources of OC in surface sediments of China's Erhai Lake, as well as the relationship between environmental variables and SOC pools. The findings revealed that the SOC pool in the lake primarily consists of labile organic carbon fractions generated by algal production. Humus carbon content was relatively low, with fulvic acid carbon being dominant. The dissolved organic matter in both water column and sediments was largely influenced by endogenous production, with a significant contribution from phytoplankton. Sediment chlorophyll a showed the strongest correlation with the SOC pool, indicating the important influence of algae on SOC. The southern region of the lake, which receives contributions from algae, other microorganisms, and sewage, has a greater potential to release organic matter into the water column. On the other hand, the central and northern regions have stable SOC pools and stronger capacity to store OC due to the input of terrestrial refractory organic matter from runoff.