Composition and bioavailability of dissolved organic matter in a semi-enclosed mariculture bay: Insights from spectral and stable carbon isotope analyses.

Abstract

Characterizing the composition, sources, and apparent bioavailability of coastal dissolved organic matter (DOM) is essential for understanding carbon cycling in semi-enclosed mariculture systems. In this study, field observations and laboratory incubation experiments were conducted in the Haizhou Bay (HZB), South Yellow Sea, during July 2024. Using measurements of dissolved organic carbon (DOC) concentration, DOM absorption and three-dimensional excitation-emission matrix fluorescence spectroscopy, stable carbon isotope (δ13C) analysis, and a Bayesian mixing model, the composition, sources, and microbial transformation of DOM were assessed. Three fluorescent components were identified, including microbial humic-like (C1), tryptophan-like (C2), and terrestrial humic-like (C3) substances. The DOM pool exhibited strong terrestrial characteristics, with the Bayesian model estimating a terrestrial contribution of 73.05 ± 15.55%, mainly associated with riverine inputs. Among fluorescent components, protein-like C2 appeared to represent an apparent bioavailable fraction and was preferentially removed during microbial degradation. Dark incubation experiments revealed a two-phase degradation pattern, with an average short-term microbial degradability of 24.53 ± 9.85%. Microbial processing selectively depleted labile DOM components while increasing the relative contribution of humic-like fluorescence, aromaticity, and humification degree. Higher apparent BDOC values near aquaculture zones suggested potential local modulation by aquaculture activities, although terrestrial inputs and microbial community differences may also contribute. This study highlights terrestrial dominance, selective microbial transformation, and the complex regulation of DOM cycling in semi-enclosed mariculture bays.