Blue Carbon Vulnerability Revealed by Damming- and Eutrophication-Induced Source Shifts in River-Connected Marshes

C-depleted aged terrestrial carbon that briefly accounted 70% of total inputs. As terrestrial OC inputs fell by 70%, eutrophication-fueled marine productivity subsequently filled the gap, restoring ∼40% of the lost accumulation and permanently shifting the dominant OC source from terrestrial to marine. This dynamic tug-of-war between damming and eutrophication suggests that widespread river fragmentation, compounded by continued decline in global anthropogenic nutrient inputs, could profoundly disrupt the coastal blue-carbon sink.