Sediment and nutrient accumulation within lowland bottomland ecosystems: An example from the Atchafalaya River Basin, Louisiana

Sediment and nutrient deposition, storage, and transformations are important environmental functions of riverine forested wetland ecosystems, yet documentation and interpretation of sedimentation/nutrient processes remain incomplete. Our studies located in the Coastal Plain of southeastern USA, including the Atchafalaya Basin, La. (a distributary of the Mississippi River) serve as example for detailed discussion of sediment and nutrient accumulation in lowland systems. The Atchafalaya Basin is the largest contiguously forested riparian wetland in North America and is incurring high sediment loads and hypoxic zones in backswamp settings. We established several floodplain transects, located to reflect major depositional environments within the Basin, to monitor general and local sediment deposition patterns over a multi-year period. Deposition rate and loss on ignition (LOI) data were collected above artificial markers (clay pads) at multiple stations along each transect. Mean floodplain sedimentation rates ranged from about 2 to 42 mm/yr and mean percent organic material ranged from about 7 to 28 percent. The transects were categorized into statistically different deposition groups based on sedimentation rate; most of these groups could be coherently interpreted based on a suite of parameters that includes hydroperiod (elevation), source(s) of sediment-laden water, hydraulic connectivity, flow stagnation, and location in transect (levee versus backswamp). Low elevatic (long hydroperiod), high hydraulic connectivity to multiple sources of sediment-laden water, and hydraulic damming (flow stagnation) lead to the highest amounts of sediment trapping; the converse in any of these factors may diminish sediment trapping. Based on aerial extent of deposition groups, the study area (about 500 km 2) potentially traps 6.72+109 kg of sediment, annually, of which 12 percent or 8.20.108 kg are organic material. This accumulated sediment contains a coarsely estimated 5% and 27% of the annual nitrogen and phosphorus loads to the Atchafalaya Basin, respectively, and 3.7+108 kg C. Thus, the Atchafalaya Basin plays an important role in sediment storage, including the sequestration of carbon, nitrogen, and phosphorus.