Brinson Review: Perspectives on the Influence of Nutrients on the Sustainability of Coastal Wetlands

Among the solutions being proposed for reversing wetland loss in the Mississippi River Delta are the creation of diversions to reintroduce suspended sediment carried in the river. In areas of rapid relative sea-level rise, as in the Mississippi Delta, it is generally accepted that a supply of sediment in flood water and mineral sedimentation are critical to sustaining wetlands. But plans to create diversions have raised questions about the collateral effects of nutrients carried in the Mississippi River, effects that may contravene the benefits of sediment. This review finds the balance of empirical and theoretical evidence supports that nutrients benefit above- and belowground plant production and that fresh water and sediment diversions can be effective and beneficial for restoring wetlands in the Delta, especially if designed to maximize sediment inputs. The input of sediment, nutrients, and fresh water will change the community composition of some wetlands and their biogeochemical processes. Most of the nitrogen input should be assimilated or denitrified. Labile organic matter is likely to degrade more quickly, but labile organic matter does not add 'new' soil volume and its speed of decay is of little consequence. Additional research is needed before we fully understand the consequences of nutrients on the preservation of organic matter in sediment, but building on what is known of the activities of lignin-degrading fungi and their enzymes, it is likely that refractory organic matter should increase and contribute positively to sediment accretion. This is supported by long-term studies of sediment accretion in a New England salt marsh with peat sediment, and in a mineral-dominated southeastern salt marsh, that show elevations in fertilized plots gained as much or more than in reference plots.