Marine macrophytes, including seagrasses and seaweeds, are major contributors to the marine carbon cycle through the release of dissolved organic carbon, a fraction of which is recalcitrant (resistant to microbial degradation for weeks to months), thereby supporting long-term carbon storage. Here we tested how warming and invasion by a non-native seagrass affect carbon dynamics in temperate macrophyte communities from southern Iberia using controlled mesocosm experiments across three temperatures. The invasive seagrass did not substantially alter carbon metabolism or dissolved organic carbon release. However, warming altered the amount and composition of released carbon. The recalcitrant fraction decreased by 28% with increasing temperature, while labile carbon (readily degradable) increased proportionally. When standardized to macrophyte carbon stocks, the recalcitrant fraction produced was comparable to sediment carbon burial rates in the same communities. These results suggest that warming restructures dissolved organic carbon composition, reducing coastal carbon storage capacity and affecting global carbon budget estimates. Ocean warming can influence both the quantity and composition of dissolved organic carbon released from large coastal macrophytes such as seagrasses and eelgrasses, according to analyses of cross-ecosystem mesocosm experiments.
Yamuza-Magdaleno et al. (Thu,) studied this question.