Effects of a chronic impact on Cymodocea nodosa community carbon metabolism and dissolved organic carbon fluxes.

Seagrass communities have been degraded worldwide experiencing elevated shoot density reduction by anthropogenic chronic pressures. This study aims to assess how a chronic (i.e., low intensity but long-lasting) impact that promotes reduced shoot density in a temperate seagrass population may affect community components and functioning. To this end, shoot density was reduced (0, 40, and 75 %) for three months in contrasting seasons (winter and summer), and assessed its effects on biotic components (i.e., seagrasses, macroalgae, macrofauna, and microphytobenthos), as well as on community carbon metabolism, dissolved organic carbon (DOC) fluxes and sediment organic matter (OM) content. Lower shoot densities enhanced the presence of macroalgae and microphytobenthos in the community, while macrofauna remained unchanged. Net community production was significantly reduced with the simulated reduction in shoot density in both seasons (up to 10-fold lower), which shifted the community in winter from being largely autotrophic (CO sink) to heterotrophic (CO source). This was due to the expected reduction in gross primary production, but also to the unexpected increase in community respiration (up to 2.2-fold higher). Since OM in the sediment was reduced in the simulated shoot density reduction treatments, the increase in sediment bacterial activity may help explain the increase in community respiration. DOC fluxes were also greatly reduced in both seasons (up to 5.5-fold lower), which coupled with the reduced net community production and loss of OM in the sediment may have a continued silent effect on blue carbon capture and storage capacity in this chronically stressed community. This study therefore highlights the importance of chronic impacts that promote the degradation of seagrass communities that may reduce their ability to provide highly valuable ecological services, including the ability to cope with the effects of climate change.