Vulnerability of blue carbon to sea-level rise in coastal freshwater ecosystems

Coastal freshwater ecosystems are economically and ecologically important and provide multiple environmental services worldwide. They sequester carbon at rates ten times faster, and store five times more carbon per unit area than do mature tropical forests. Vulnerability of these carbon sinks to marine inundation, however, is expected to increase in response to global sea-level rise (GSLR). To better understand the implications of future GSLR, we investigated the geochemical and biological consequences of episodic Holocene marine incursions into Lake Izabal, a large coastal freshwater ecosystem on the Caribbean coast of Central America. About 8300 cal yr BP, marine incursion transformed Lake Izabal into a sulfur-rich anoxic waterbody, altered its biogeochemical cycles, eliminated several aquatic species, and reduced sediment organic carbon (OC) concentration by as much as 90 %. After that Early Holocene seawater incursion, it took almost 5000 years for the lacustrine ecosystem to return to low-salinity status. And even when it did, the system did not fully recover to pre-inundation conditions. Some freshwater taxa failed to return, and sediment carbon content remained lower than pre-inundation values. A subsequent, but less intense marine incursion ca. 1900 cal yr BP led to the formation of a sulfur-rich, hypoxic, brackish-water ecosystem that triggered a similar biodiversity loss and further sediment OC decline. These findings suggest that future marine incursions into coastal freshwater ecosystems, driven by ongoing GSLR, could have dramatic consequences, leading to losses of environmental services, including the ability of these systems to maintain high rates of blue carbon storage.