Restoration and construction of tidal marshes has been encouraged to recover ecosystem structure and function in response to wetland loss. While natural and restored marshes often achieve similar structure, they may not function similarly. Further, the effects of nutrient loading on ecological functions may vary between natural and restored or constructed marshes, with consequences for marsh resilience to sea-level rise. As coastal restoration and creation projects become more common, it is important to understand the similarities and differences in tidal marsh responses to environmental changes, including nutrient enrichment, and their impacts on ecosystem functions and resilience. To test the effects of nutrients on marsh responses, we conducted a controlled nutrient enrichment mesocosm experiment (control-none; low: 20 g N m-2 yr-1 and 1.25 g P m-2 yr-1; high: 40 g N m-2 yr-1 /2.5 g P m-2 yr-1) utilizing intact, vegetated sods of Juncus roemerianus collected from natural and constructed tidal marshes along the Fowl River in Alabama. During the 9-month experiment, we measured surface elevation change, sediment characteristics, porewater chemistry, aboveground and belowground biomass, net primary production (NPP), litter decomposition, and nitrogen removal potentials.
Results/Conclusions
Both aboveground and belowground biomass were significantly higher in the natural than the constructed marsh; NPP tended to differ between nutrient treatments in both marshes, with low nutrient additions having the lowest NPP. Both marsh type and nutrient treatment significantly affected decomposition of J. roemerianus shoot litter such that low nutrient loading increased mass loss more than either high nutrients or the control. Decomposition of belowground plant tissue was similar between the two marsh types, but given relatively lower belowground biomass in the constructed marsh, decomposition could have a greater overall impact on elevation in the constructed marsh compared to the natural marsh. Nitrogen removal processes also tended to differ between marshes, with increased nutrients promoting denitrification in both marshes and carbon additions stimulating dissimilatory nitrate reduction to ammonium in the natural marsh only. While all mesocosms lost elevation over time, the control retained the most elevation in the natural marsh while the high nutrient treatment retained the most elevation in the constructed marsh. Such differences between marsh types have important implications for coastal restoration success under increasing eutrophic conditions, and collectively, indicate that nutrient additions may contribute to reduced resilience to sea-level rise in these systems.