Salt marshes buffer coastlines and provide critical ecosystem services from storm protection to food provision. Worldwide, these ecosystems are in danger of disappearing if they cannot increase elevation at rates that match sea-level rise. However, the magnitude of loss to be expected is not known. A synthesis of existing records of salt marsh elevation change was conducted in order to consider the likelihood of their future persistence. This analysis indicates that many salt marshes did not keep pace with sealevel rise in the past century and kept pace even less well over the past two decades. Salt marshes experiencing higher local sea-level rise rates were less likely to be keeping pace. These results suggest that sea-level rise will overwhelm most salt marshes' capacity to maintain elevation. Under the most optimistic IPCC emissions pathway, 60% of the salt marshes studied will be gaining elevation at a rate insufficient to keep pace with sea-level rise by 2100. Without mitigation of greenhouse gas emissions this potential loss could exceed 90%, which will have substantial ecological, economic, and human health consequences.

Ocean uptake of carbon dioxide (CO 2) is causing changes in carbonate chemistry that affects calcification in marine organisms. In coastal areas, this CO 2-enriched seawater mixes with waters affected by seasonal degradation of organic material loaded externally from watersheds or produced as a response to nutrient enrichment. As a result, coastal bivalves often experience strong seasonal changes in carbonate chemistry. In some cases, these changes may resemble those experienced by aquacultured bivalves during translocation activities. We mimicked these changes by exposing juvenile hard clams (500 μm, Mercenaria mercenaria) to pCO 2 in laboratory upwellers at levels resembling those already reported for northeastern U.S. estuaries (mean upweller pCO 2 = 773, 1274, and 1838 μatm) and then transplanting to three grow-out sites along an expected nutrient gradient in Narragansett Bay, Rhode Island (154 bags of 100 clams). Prior to the field grow-out, clams exposed to elevated pCO 2 exhibited larger shells but lower dry weight per unit volume (dw/V). However, percent increase in dw/V was highest for this group during the 27-day field grow-out, suggesting that individuals with low dw/V after the laboratory treatment accelerated accumulation of dw/V when they were transferred to the bay. Treatments also appeared to affect shell mineral structure and condition of digestive diverticula. Although treatment effects diminished during the field grow-out, clams that were preexposed for several weeks to high pCO 2 would likely have been temporarily vulnerable to predation or other factors that interact with shell integrity. This would be expected to reduce population recovery from short-term exposures to high pCO 2 .

No abstract