The impact of sea-level rise on organic matter decay rates in Chesapeake Bay brackish tidal marshes

M L Kirwan,J P Megonigal,J A Langley,G R Guntenspergen

doi:10.5194/bg-10-1869-2013

Abstract

Abstract. The balance between organic matter production and decay determines how fast coastal wetlands accumulate soil organic matter. Despite the importance of soil organic matter accumulation rates in influencing marsh elevation and resistance to sea-level rise, relatively little is known about how decomposition rates will respond to sea-level rise. Here, we estimate the sensitivity of decomposition to flooding by measuring rates of decay in 87 bags filled with milled sedge peat, including soil organic matter, roots and rhizomes. Experiments were located in field-based mesocosms along 3 mesohaline tributaries of the Chesapeake Bay. Mesocosm elevations were manipulated to influence the duration of tidal inundation. Although we found no significant influence of inundation on decay rate when bags from all study sites were analyzed together, decay rates at two of the sites increased with greater flooding. These findings suggest that flooding may enhance organic matter decay rates even in water-logged soils, but that the overall influence of flooding is minor. Our experiments suggest that sea-level rise will not accelerate rates of peat accumulation by slowing the rate of soil organic matter decay. Consequently, marshes will require enhanced organic matter productivity or mineral sediment deposition to survive accelerating sea-level rise.

Highlights

Coastal marshes are among the most productive ecosystems on Earth
We found no significant influence of inundation on decay rate when bags from all study sites were analyzed together, decay rates at two of the sites increased with greater flooding
We have made several important assumptions in our attempt to design a simple experiment that isolates the effect of inundation and sea-level rise on the decomposition rate of organic material

Summary

Introduction

Coastal marshes are among the most productive ecosystems on Earth. Net primary productivity in temperate regions commonly exceeds 800 g C m−2 yr−1 (Mendelssohn and Morris, 2013; 2012), though where see moderateMinacrraeSnaisoeesltiidanl.fl,Eo2ao0d0ri4tnh;gKleirawdatno et al, flushing of salts, delivery of nutrients, and enhanced productivity. Nyman and DeLaune (1991), for example, measured reduced soil respiration rates in more flooded environments, but several authors have concluded that flooding frequency and redox potential play only minor roles in determining organic decay rates (Valiela et al, 1982; Hackney, 1987; Blum, 1993; Blum and Christian, 2004). Such a conclusion is perhaps surprising when viewed in the context of experiments conducted in terrestrial ecosystems. Where decomposition takes place in dominantly aerobic conditions, periodic flooding may increase soil moisture and microbial activity, whereas excessive flooding creates anaerobic conditions that limit decomposition (Neckles and Neill, 1994)

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Results

Conclusion