In-lake processing counteracts the effect of allochthonous input on the composition of color dissolved organic matter in a deep lake.

Color dissolved organic matter (CDOM) plays a key role in lacustrine ecosystems and its composition is commonly mediated by the allochthonous input and autochthonous production. Deep lakes have a strong in-lake processing, which highly affects the sources, composition and cycle of CDOM. Here, the second deepest lake (Lake Fuxian) in China was selected to investigate the effects of allochthonous input and in-lake processing on lacustrine CDOM in deep lakes. Firstly, a detailed survey on CDOM composition across Lake Fuxian in the top water layer and inflowing rivers was carried out in the wet season representing the allochthonous input. In addition, CDOM in Lake Fuxian was compared with those in other lakes with distinct catchment characteristics and lake morphology. The results showed that compared to lacustrine CDOM in Lake Fuxian, the riverine CDOM contained much more humic-like substances, resulting in the humic-like fluorescence intensity peaked at the confluence of rivers into Lake Fuxian. In contrast, CDOM in Lake Fuxian was dominated by the protein-like substance. Comparison of CDOM composition among Lake Fuxian (well-vegetated catchment, deep lakes) with other diverse lakes in China (shallow/deep lakes with poor-vegetated catchment, and shallow lakes with well-vegetated catchment) showed similar CDOM quality in all type lakes, which were dominated by non-humified and autochthonous CDOM. Yet, CDOM quantity increased as the orders of deep lakes within poor-vegetated (Tibetan deep lakes) < the deep lake within well-vegetated catchment (Lake Fuxian) < shallow lakes within poorly-vegetated catchment (Tibetan shallow lakes) < shallow lakes within well-vegetated catchment (lakes along the middle and lower reaches of Yangtze River). Our results evidenced that the effect of allochthonous input on CDOM composition could be counteracted by in-lake processing in deep lakes. For deep lakes, a comprehensive understanding of in-lake processing of CDOM is critical for predicting lacustrine DOM composition and cycle.