A Model for Estimating Organic Carbon Storage When Tidal Flooding Is Returned to Drained Agricultural Salt Marshes

Abstract:

Drainage of coastal marshes for agriculture has kept these wetlands from serving as a blue carbon sinks for decades, and in many locations centuries. These lands, now below sea level, are becoming increasingly more expensive to maintain as sea level continues to rise, driving an interest in their restoration. When tidal flooding is returned to these lands restoration of the ecosystem can be rapid, particularly on macrotidal coastlines. The limited research published on the rate at which carbon sequestration occurs indicates that, at least on the Bay of Fundy, a surprising amount of organic carbon (OC) is stored within just the first 9 years. To apply for credits for restoration on the carbon market, however, we must be able to estimate the magnitude and value of the renewed OC stock for a much longer period. Based on extensive field research, we developed a model to estimate OC accumulation rates over time as tidal flooding is returned to agricultural marshes. We examined OC stocks in 13 marshes, previously drained for agriculture, that had been reflooded for periods ranging from 9-92 yr. The marshes, located on the Bay of Fundy and Gulf of St Lawrence, had tidal amplitudes ranging from 2 to >10 m. In all but the marsh with the 9 yr recovery period, we cored within Spartina patens, which we used as an indicator of full vegetation recovery. Overall, we collected >56 cores. Using the depth at which an “agricultural horizon” was visible we found that the average thickness of the sediment accumulation in the marshes ranged from 28-96 cm. Although the thickness of sediment accumulation increased with recovery period the rate of OC accumulation significantly decreased with recovery period (R²=0.86). OC accumulation was weakly related to tidal amplitude (R²=0.40), but a multiple regression did not produce more significant results than when using recovery period alone.