The Declining Role of Organic Matter in New England Salt Marshes

Abstract

The Northeast USA is experiencing severe impacts of a changing climate, including increased winter temperatures and accelerated relative sea level rise (RSLR). The sediment-poor, organic-rich nature of many Southern New England salt marshes makes them particularly vulnerable to these changes. In order to assess how marsh accretion has changed over time, we returned to Narragansett Bay, RI where salt marsh vertical accretion rates were documented almost 30 years ago. Using radionuclide tracers ( 210 Pb and 137 Cs), we observe no significant change in overall accretion rates (0.27-0.69 cm year −1 ) compared to historical averages (0.24-0.60 cm year −1 ), but we document a shift in how these marshes maintain elevation. Organic matter now plays a smaller role in contributing to vertical accretion across all study sites, declining by 22 % on average. We attribute this reduction to potentially higher decomposition rates fueled by higher water temperature. Inorganic matter also contributes less to accretion (declining by 44 % on average at marshes located more internal to the estuary), likely due to diminishing sediment supply in this region. With organic and inorganic solids accounting for less of the total accretion, several of the marshes are experiencing symptoms of swelling, with water and porespace contributing more towards accretion compared to historical values. Accretion rates (0.27-0.45 cm year −1 ) at these organic-rich (>40 % sediment organic matter) marshes are predominantly lower than the current (30 years) rate of RSLR (0.41±0.07 cm year −1 ). These results, combined with the increased rate of RSLR and the hardened shorelines inhibiting landward migration, call into question the longterm survivability of these marshes.

Carbon Sequestration in a Pacific Northwest Eelgrass (Zostera marina) Meadow

“…Rates determined with the CRS model ranged from 0.08 to 0.31 cm yr -1 , with a mean of 0.19 ± 0.01 cm yr -1 ( Table 2). The CIC and CRS accretion rates were significantly correlated (r = 0.89, P < 0.01), similar to comparisons by Carey et al (2017) and Bricker-Urso et al (1989).…”

Modeling long‐term salt marsh response to sea level rise in the sediment‐deficient Plum Island Estuary, MA

“…When elevation decreases within the range supporting peak productivity of low marsh plants, organic accretion and increased carbon storage also enable marsh elevation to stabilize relative to sea level, as demonstrated in New England marshes in Cape Cod, MA (Gonneea et al 2019). However, similar to other sediment-poor, organic-rich New England marshes (Carey et al 2017), our model predictions suggest reduced sediment availability at PIE will prevent marsh elevation from stabilizing even as accretion rates increase across an expanding low marsh. Even if the maximum accretion rate predicted in 2100 (9.33 AE 1.10 mm yr −1 ) occurred at every position of the marsh platform, it would be insufficient to stabilize the elevation under a high rate of SLR.…”

Vegetation Dynamics in Rhode Island Salt Marshes During a Period of Accelerating Sea Level Rise and Extreme Sea Level Events

“…This is in agreement with Warren and Niering (1993), who also documented vegetation changes at low marsh elevations that are first exposed to rising sea levels. We expect these changes to continue because complementary research in Nag Marsh shows that a deficit now exists between salt meadow accretion rates and recent rates of sea level rise (Carey et al 2015) and that current inundation patterns are limiting the productivity of common Narragansett Bay salt marsh taxa including S. patens (Watson et al 2014. Taken together, this suggests that sea level rise is driving the loss of S. patens salt meadow in our Sentinel Sites marshes, which agrees well with studies in other southern New England marshes that have experienced similar changes (Donnelly and Bertness 2001;Warren and Niering 1993).…”