Discriminating winter-drought and summer-flood regimes using CDOM optical proxies: A proof-of-concept analysis in...

Freshwater lake ecosystems are increasingly exposed to hydro-climatic extremes, with profound implications for water quality and carbon cycling. However, rapid diagnostic tools capable of resolving the collective impacts of these extremes remain limited. Here, we test the hypothesis that the optical properties of chromophoric dissolved organic matter (CDOM) provide a sensitive and integrative proxy for distinguishing the two hydro-climatic states. Using Poyang Lake (China) as a model system, we characterized CDOM dynamics under two contrasting coupled hydro-climatic states -winter extreme drought (January 2015) and summer extreme flood (July 2016) -through combined absorbance and fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC). Three fluorescent components were resolved, including two humic-like fractions (microbial-derived C1 and terrestrial-derived C2) and one protein-like fraction (tryptophan-like C3). The winter extreme drought state was associated with CDOM of lower molecular weight, reduced aromaticity, and a stronger terrestrial signature relative to the summer extreme flood state. Building on these contrasts, we developed a Fisher's linear discriminant model integrating four optical indices -S (275-295), FI (370), S_R, and C1:C3- which achieved perfect classification accuracy under leave-one-out cross-validation in discriminating between the two distinct coupled hydro-climatic states (winter extreme drought vs. summer extreme flood). Together, these findings establish a proof-of-concept framework demonstrating that CDOM optical properties can serve as rapid, high-resolution indicators of site-specific hydro-climatic regimes. This approach provides a scalable pathway for monitoring hydro-climatic extremes and their biogeochemical consequences in freshwater systems.