Abstract
AbstractLarge Mississippi River (MR) diversions (peak water flow >1416 m3/s and sediment loads >165 kg/s) have been proposed as part of a suite of coastal restoration projects and are expected to rehabilitate and rebuild wetlands to alleviate the significant historic wetland loss in coastal Louisiana. These coastal wetlands are undergoing increasing eustatic sea‐level rise, land subsidence, climate change, and anthropogenic disturbances. However, the effect of MR diversions on wetland soil organic carbon (SOC) sequestration in receiving basins remains unknown. The rate of SOC sequestration or carbon burial in wetlands is one of the variables used to assess the role of wetland soils in carbon cycling and also to construct wetland carbon budgets. In this study, we examined the effects of MR water and sediment diversions on landscape‐scale SOC sequestration rates that were estimated from vertical accretion for the next 50 yr (2010–2060) under two environmental (moderate and less optimistic) scenarios. Our analyses were based on model simulations taken from the Wetland Morphology model developed for Louisiana's 2012 Coastal Master Plan. The master plan modeled a “future‐without‐action” scenario as well as eight individual MR diversion projects in two of the hydrologic basins (Barataria and Breton Sound). We examined the effects that discharge rates (peak flow) and locations of these individual diversion projects had on SOC sequestration rates. Modeling results indicate that large river diversions are capable of improving basin‐wide SOC sequestration capacity (162–222 g C·m−2·yr−1) by up to 14% (30 g C·m−2·yr−1) in Louisiana deltaic wetlands compared to the future‐without‐action scenario, especially under the less optimistic scenario. When large river diversions are placed in the upper receiving basin, SOC sequestration rates are 3.7–10.5% higher (6–24 g C·m−2·yr−1) than when these structures are placed in the lower receiving basin. Modeling results also indicate that both diversion discharge and location have large effects on SOC sequestration in low‐salinity (freshwater and intermediate marshes) as compared to high‐salinity marshes (brackish and saline marshes).
Highlights
Large Mississippi River (MR) diversions have been proposed for coastal restoration to rehabilitate and rebuild wetlands to alleviate significant historic wetland loss in coastal Louisiana (Day et al 2009, Allison and Meselhe 2010, Allison et al 2014)
Discharge rate effect Our results indicated that simulated soil organic carbon (SOC) sequestration rates varied significantly (P < 0.001) with hydrologic basin, future environmental scenario, diversion discharge, and their interactions (Table 3)
Our results show that increasing discharge from 142 to 1416 m3/s could significantly enhance SOC sequestration rates in Barataria and Breton Sound basins
Summary
Large Mississippi River (MR) diversions have been proposed for coastal restoration to rehabilitate and rebuild wetlands to alleviate significant historic wetland loss in coastal Louisiana (Day et al 2009, Allison and Meselhe 2010, Allison et al 2014). Coastal wetland ecosystems are known to sequester carbon efficiently because of their high rates of primary productivity and low rates of SOM decomposition as well as accumulating soil and sediment carbon 40 times faster than the average terrestrial forest (Mcleod et al 2011) This ecosystem service allows coastal wetlands to be entered into the voluntary carbon market, once a SOC sequestration rate is quantified (Chmura et al 2003, Mitra et al 2005, Crooks et al 2011, Callaway et al 2012, DeLaune and White 2012, Pendleton et al 2012, Mitsch et al 2013). One major question is whether large MR diversions can maximize SOC sequestration and land-building potential simultaneously
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