Abstract
Salt marshes sequester substantial amounts of blue carbon, playing a critical role in coastal carbon cycling. How human pressures reshape the magnitude and stability of blue carbon remains an active area of research. Here, we collected and analyzed 15 sediment cores from 5 representative salt marshes in New England, United States, to investigate how river damming and eutrophication have altered organic carbon (OC) sources and accumulation over the last century. We find that Nell’s Island Marsh (Nell’s), strongly influenced by river input, accumulated OC at rates 5–12 times greater than nearby tidal-dominated marshes, highlighting the importance of river connectivity. We show that, however, this advantage is fragile: dam construction abruptly reduced Nell’s OC accumulation by nearly 80% and triggered a pulse of 14C-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.