Freshwater diversions as an ecosystem management tool for maintaining soil organic matter accretion in coastal marshes

• Vertical accretion was related to the amount of soil organic matter accumulation.
• Marsh accretion data for maintenance of upper Barataria Basin marshes.
• Mississippi River freshwater reintroduction should enhance marsh stability.

Marsh soil accretion processes were examined at 12 locations in fresh and brackish marshes in Louisiana's northern Barataria Basin estuary. Soil accretion rate determined from 137Cs ranged from 0.59 to 1.03 cm yr− 1. Soil structure and accretion were controlled primarily by organic matter accumulation rather than mineral sediment deposition with water and entrapped gases occupying the majority of the soil volume ranging from 92 to 95%. Organic matter accumulation ranged from 328 to 580 g m− 2 yr− 1 while mineral accumulation ranged from 78 to 380 g m− 2 yr− 1 Such organic-based fresh and brackish marsh soil are more fragile than mineral based marsh soils and are subject to impact by salt water intrusion and hurricane storm surge forces. The Davis Pond diversion is a conduit between the Mississippi River and the coastal marshes of Barataria basin currently reintroducing river water into the basin leading to lower basin salinities This hydrologic management measure should benefit and extend the stability of the studied marsh sites encouraging continued vegetation growth and soil organic matter accumulation. Accrual of organic matter is necessary to prevent drowning of the marsh which is subject to both the global eustatic sea level rise (1–2 mm yr− 1) and significant local subsidence (~ 10 mm yr− 1). This high rate of relative sea level rise can lead to open water/ponding and subsequent erosion of the surrounding marsh due to flooding and salinity stress.