Experimental degradation of Ulva prolifera in sand sediments: Implications for dissolved carbon and alkalinity export from green-tide macroalgae buried in sandy intertidal zones.

The degradation of massive Ulva prolifera (U. prolifera), generated during blooms of the macroalgae (i.e., green tides), exerts multi-faceted impacts on biogeochemical cycling in coastal seawater and sediments. The kinetics of U. prolifera degradation in permeable sediments and subsequent export of degradation products remain poorly understood. In this study, we conducted a 30-d experimental investigation of Ulva prolifera degradation in sandy sediments under conditions of periodic water exchange (WE) to characterize the degradation kinetics and quantify the export of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and total alkalinity (TA). The results demonstrated that U. prolifera degradation in the sandy sediments followed first-order kinetics, independent of WE frequency. Compared to control systems without WE, increasing WE frequency accelerated degradation rates by a factor of 2.5-3.6, resulting in 1.8-6.8-fold increases in the export of DOC, DIC, and TA. DOC derived from U. prolifera degradation was predominantly composed of labile protein-like components, though its lability gradually declined as degradation progressed. The production of DIC and TA was predominantly through anaerobic degradation of U. prolifera coupled to sulfate reduction and/or denitrification. The resulting DIC presented primarily as carbonate alkalinity, which may exert legacy effects on marine carbon sequestration potential. Our findings suggest that fine particulate organic matter (FPOM) represents a significant byproduct of U. prolifera degradation under dynamic conditions. The export and subsequent degradation of FPOM may substantially influence dissolved carbon dynamics in coastal ecosystems.