CARBON STOCK AND ACCUMULATION RATES IN THE DELAWARE BAY TIDAL SALT MARSHES

Salt marshes provide numerous benefits and services essential for mitigation and adaptation to climate change and resilience along the coast. Salt marshes are large carbon (C)-storing ecosystems that sequester significant amounts of C, known as blue carbon, from the atmosphere and oceans, and store it in the below ground sediments for thousands of years (Murray et al. 2011). These ecosystems sequester C at a faster rate, and store it for a longer period of time, than terrestrial forests (McLeod et al. 2011). However, when these systems are degraded – through erosion or land use changes - they become a potential source of C emissions. Worldwide salt-marsh loss since the 1800’s is estimated at 25-35 % (Bridgham et al. 2006, Duarte et al. 2008). Delaware Bay salt marshes are being lost at a rate of an acre/day with an estimated net loss of 25% by 2100 (PDE, 2012). Although, the DE Bay is the 2^nd largest estuary in North America, with a tidal marsh area of ~ 830 km² (Reed et al. 2008; Titus et al. 2008), there are no studies that accurately estimate the amount of C stored in its salt marshes. Additionally, assessments of salt-marsh C pools, and salt-marsh C accumulation rate (CAR), typically focus on the top meter of sediment. Sediments accumulated at depths < 1 m usually represent < 100 years of salt marsh accumulation (Nikitina et al. 2014).

We reconstructed the history of salt marsh development along the NJ DE Bay at two sites using salt-marsh sedimentary archives and aerial photos. Both marshes have been developing for ~2000 years and share similar tidal range and vegetation. Estimated short and long-term CAR equal 2.3MgC/ha/yr., and 0.85 MgC/ha/yr., respectively. We documented variation in sediment and CAR through time due to changes in depositional environments and calculated C content through the entire sediment sequence. Estimates of C accumulation ranged from 355 MgC/ha (1 m depth) to 1,016 MgC/ha (3 m depth). Estimated C stock of the combined sites (65 hectares) is 54,000 MgC. The results show that the Delaware Bay salt marshes sequester significant amounts of C and suggest that C stock assessments focused on the top 1 m of sediment underestimate the total C stock by more than two-fold. We therefore propose that to achieve increased accuracy of C stock assessments, future studies should account for the entire salt-marsh sediment sequence.