[Spatial and Temporal Variation of Dissolved Organic Matter and Its Biodegradability in Typical Algal Bay].

Abstract

The biodegradability of dissolved organic matter （DOM） is a crucial factor in assessing the carbon balance and ecosystem equilibrium of freshwater lakes. It is closely linked to regional hydrological characteristics and biochemical processes. Taking cyanobacterial bloom-dominated bays in Lake Taihu as the research object, combined with characterization by ultraviolet absorption, three-dimensional fluorescence spectroscopy, and bioassay experiments, this study analyzed the temporal and spatial variations of DOM concentration, structural composition, and biodegradable dissolved organic carbon （BDOC） during the rainy and bloom season and the dry and post-bloom season. The results showed that during the bloom season, the release of cyanobacterial-derived organic matter led to a significant increase in DOM concentration in Meiliang Bay and Gonghu Bay, which was mainly composed of protein components, such as tyrosine and tryptophan （>70%） with a lower aromaticity （SUVA₂₅₄）, molecular weight, and humification degree （HIX）. In comparison, the DOM samples in Zhushan Bay had higher absorbance （SUVA₃₅₀）, SUVA₂₅₄, HIX, relative molecular mass, and content of humic substances, which was associated with stronger input of terrestrial humic substances during the rainy season. As a result, the average BDOC in Zhushan Bay was lower （31.6%） compared to those in the other two bays （38.8%）. With the decline of cyanobacterial bloom and the decrease of terrestrial input in the dry and post-bloom season, in-situ degradation significantly increased SUVA₂₅₄ and humic-like components while decreasing protein-like components in DOM. This resulted in a decrease in average BDOC to 28.1%. Partial least squares analysis further revealed that DOM optical indices could effectively predict BDOC （R²=0.81）, with fluorescence peaks T/C and S_R indices being the most effective positive predictive factors and SUVA₂₅₄ being the most effective negative predictive factor. These findings suggest that cyanobacterial blooms and in-situ degradation are crucial processes that affect the biodegradability of DOM in freshwater lakes and propose an alternative method for assessing BDOC.