Temperature Modulates Recalcitrant Dissolved Organic Carbon Production by Coastal Macrophytes: An Underestimated Blue Carbon Pathway

Marine macrophytes play a significant role in the marine carbon cycle by releasing dissolved organic carbon (DOC), including a recalcitrant fraction with potential for long-term carbon sequestration. Here, we investigated how warming and the presence of an invasive species affect DOC dynamics in different native temperate macrophyte communities (Zostera noltei, Cymodocea nodosa and Caulerpa prolifera) from the south of the Iberian Peninsula, a transitional habitat between Atlantic and Mediterranean marine regimes. Additionally, we introduced a standardized framework to link DOC release to internal carbon content, facilitating comparisons of blue carbon pathways among macrophyte communities across diverse ecosystems. Controlled mesocosm experiments across three temperatures (24, 26 and 28 °C) revealed that the presence of the invasive seagrass Halophila stipulacea did not significantly alter the carbon metabolism or DOC fluxes of native macrophytes. However, temperature significantly affected both the quantity and composition of the released DOC. In particular, recalcitrant DOC decreased by 28%, while labile DOC increased by a similar proportion as temperature rose, and bioavailable DOC decay rates also declined significantly at higher incubation temperatures of the tested macrophytes. These results suggest that warming may enhance both net and labile DOC production, while the remaining DOC is less bioavailable than that produced at lower temperatures. This clearly indicates that warming restructures DOC composition, potentially reducing coastal carbon storage capacity and the role of recalcitrant DOC. By applying our proposed standardization, we estimate that the recalcitrant fraction produced in the tested macrophyte communities was comparable in magnitude, although 1.41 higher, to the carbon burial rates in the sediment measured in the same communities, which underscores the potential contribution of recalcitrant DOC produced by macrophyte communities to the long-term carbon storage. This standardized approach positions recalcitrant DOC as a crucial climate-sensitive blue carbon pathway that should be integrated into global carbon budget estimates.