Dissolved organic carbon can alter coastal sediment phosphorus dynamic: Effects of different carbon forms and concentrations.

Coastal waters are receiving increasing loads of dissolved organic carbon (DOC), differing in structural complexity and molecular weights with potential different effects on the phosphorus (P) dynamics in these waters. This study conducted an in-situ investigation in Xiangshan Harbor, China, to explore the patterns of P release in response to DOC inputs. To further elucidate the underlying mechanisms behind the DOC-affected sediment P release, a two-month mesocosm experiment was undertaken with coastal sediment (Xiangshan Harbor) to which acetate, glucose, and humic acid (representing the fermentation product, the simple available carbon, and the refractory humic-like carbon sources, respectively) were separately added to the overlying water at dosages of 0, 5, 10, and 20 mg C L. We found that: i) sediment P release showed a non-linear increase with DOC input, a pattern likely due to the diverse forms of DOC in coastal zones, which had varying impacts on P release; ⅱ) significant P release for labile DOC (acetate- and glucose-amended) treatments but retention for humic acid treatments, and the magnitude of P changes mainly depended on the amount of DOC addition; ⅲ) acetate and glucose shared similar P-release-promotion mechanisms, i.e., decreased dissolved oxygen, increased ppk genes in water, and increased P bacteria and alkaline phosphatase activity were the dominant factors behind the P release for both carbon sources, as indicated by piecewise structural equation modelling; ⅳ) humic acid-inhibitory effects on sediment P release, which likely reflect increasing "P-humic acid" complexes that favor P adsorption and sedimentation and form stable "humic acid-enzyme" complexes that reduce the catalytic activity of alkaline phosphatase. Our findings provide new understanding of relationships between loading of DOC with different form/concentration and sediment P dynamics in coastal areas.