No significant long-term enhancement of sedimentary carbon burial by intensive Porphyra farming: Evidence from Haizhou Bay, China.

Seaweed farming is widely advocated as a nature-based blue carbon solution, yet its long-term impact on sedimentary carbon burial remains poorly constrained. Here, we assess this effect in a high-density Porphyra yezoensis farming area in Haizhou Bay, China, by analyzing surface and core sediments from farming (FA) and non-farming (NFA) sites for total organic carbon (TOC), inorganic carbon (TIC), total nitrogen (TN), stable isotopes (δ13C and δ15N), and their relationships. TOC and TN were slightly elevated in FA surface sediments relative to NFA, whereas TIC exhibited the opposite pattern, suggesting short-term organic enrichment. In contrast, sediment cores revealed no significant FA-NFA differences in TOC, TIC, or TN, indicating that the enrichment signal does not translate into long-term carbon burial. δ13C indicated that sedimentary TOC was primarily of marine origin. Estimated carbon burial fluxes were similar between FA (TOC: 7.2 ± 1.6 g m-2 yr-1, TIC: 17.7 ± 3.5 g m-2 yr-1) and NFA (TOC: 7.0 ± 1.5 g m-2 yr-1, TIC: 16.7 ± 3.4 g m-2 yr-1). Mechanistically, Porphyra farming suppresses phytoplankton growth via nutrient competition, reducing phytoplankton-derived carbon input. Moreover, Porphyra debris is highly labile and may potentially trigger decomposition of native recalcitrant organic carbon through a priming effect, limiting long-term burial efficiency. Our findings demonstrate no significant long-term enhancement of sedimentary carbon burial by intensive Porphyra farming. Although the activity may confer blue-carbon benefits, the assumption that seaweed farming augments sedimentary carbon sinks should be evaluated on a case-by-case basis rather than generalized.