Contributions of Organic and Mineral Matter to Vertical Accretion in Tidal Wetlands across a Chesapeake Bay Subestuary

Jenny R Allen,Jeffrey C Cornwell,Andrew H Baldwin

doi:10.3390/jmse9070751

Abstract

Persistence of tidal wetlands under conditions of sea level rise depends on vertical accretion of organic and inorganic matter, which vary in their relative abundance across estuarine gradients. We examined the relative contribution of organic and inorganic matter to vertical soil accretion using lead-210 (210Pb) dating of soil cores collected in tidal wetlands spanning a tidal freshwater to brackish gradient across a Chesapeake Bay subestuary. Only 8 out of the 15 subsites had accretion rates higher than relative sea level rise for the area, with the lowest rates of accretion found in oligohaline marshes in the middle of the subestuary. The mass accumulation of organic and inorganic matter was similar and related (R2 = 0.37). However, owing to its lower density, organic matter contributed 1.5–3 times more toward vertical accretion than inorganic matter. Furthermore, water/porespace associated with organic matter accounted for 82%–94% of the total vertical accretion. These findings demonstrate the key role of organic matter in the persistence of coastal wetlands with low mineral sediment supply, particularly mid-estuary oligohaline marshes.

Highlights

Tidal wetlands are under an increasing threat from rising sea levels
There was no linear relationship between accretion rate and bulk density or organic matter content (p = 0.56, p = 0.96, respectively)
A previous study done on the Nanticoke River using surface elevation tables (SET) to measure elevation changes found that the mid-estuarine sites are losing elevation despite high rates of accretion [30]

Summary

Introduction

Tidal wetlands are under an increasing threat from rising sea levels. On average, global mean sea level rose approximately 1.2 mm yr−1 between 1901 and 1990 and 3.1 mm yr−1 between 1993 to 2017 [1,2,3,4]. Along the east coast of the US, a 1000 km stretch of coastline north of Cape Hatteras has been identified as a “hotspot” for accelerated sea level rise [5]. This includes the Chesapeake Bay region, where relative sea level rise (RSLR) rates ranged from 3.24–5.11 mm yr−1 between 1969 and 2014 and have been accelerating by. The ability of tidal wetlands to maintain surface elevation under accelerated sea level rise is critical for their persistence and depends on many factors including accretion of mineral and organic matter, rates of decomposition, plant community structure, and productivity [8,9,10,11]. These processes may vary across the range of tidal wetlands types and within individual wetlands as the sediment load changes and as wetland plant communities shape these dynamics [14,15,16]

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