Human impacts on sedimentation and nutrient sequestration in the Upper Mississippi River floodplain

Eroded topsoil and agricultural fertilizers are currently the greatest non-point sources of phosphorus and nitrogen to most downstream bodies of water. High concentrations of these nutrients in conjunction with organic carbon result in excessive algal growth and subsequent dissolved oxygen depletions in local lakes and rivers and in downstream coastal waters. This research expands our present spatial and temporal knowledge of large river floodplain nutrient storage through high resolution stratigraphic and chemical analyses of sediment cores taken from upper Mississippi River (UMR) backwaters.;Pre-European-American sedimentation rates averaged 0.06 cm yr -1 (540 g m-2 yr-1) and increased to 0.25 cm yr-1 (2000 g m-2 yr-1) at study sites with measurable deposition during the period following ca. 1850 settlement to 1937. Lock and dam closure in 1937 inundated large areas of UMR Pools 8 and 11 floodplains and increased sedimentation at all study sites to an average of 0.72 cm yr-1 (4500 g m-2 yr-1) between 1937 and 1963. Post 1963 to present sedimentation rates, by mass, slowed at all but two sites to a mean of 3500 g m-2 yr-1, though sedimentation rates, by depth, continued at 0.71 cm yr-1. Carbon (C), nitrogen (N), and phosphorus (P) accumulation in the backwaters was found to be strongly tied to sediment deposition, though local iron oxide concentrations and organic matter production can accelerate nutrient accumulation. Pre-settlement to post-settlement rates of C, N and P sequestration increased on average from 18.9, 1.6, and 0.2 g m-2 yr-1 to 66.5, 5.8, and 2.4 g m-2 yr-1, respectively. River connectivity to floodplains increased with lock and dam closure and led to C, N, and P sequestration rates of 162.2, 14.0, and 5.7 g m-2 yr-1 when averaged across all study sites for the period 1937 to 1963. Despite the widespread application of N and P fertilizers beginning in the late 1950s, C and N deposition increased only marginally to 168.8 and 15.5 g m-2 yr -1 as backwater productivity increased and P deposition slightly decreased to 5.2 g m-2 yr-1 as sedimentation rates decreased. Current estimates of annual N and P retention in association with backwater sediment from the total inputs to Pool 8 are only 0.2% for N and 1.2% for P, suggesting the backwaters do not play a major role in downstream nutrient reduction despite the accelerated sediment and nutrient accumulation occurring within them.;Sequestration of organic matter in the backwaters is associated with 15% of the total sedimentary P and 96% of the total sedimentary C. The strong correlation between organic matter and total N concentrations suggests that virtually all sedimentary N is organic N. Based on delta15N, delta 13C and C:N values of the sediment, it is evident that the majority of the organic matter deposited at the study sites is of C3 terrestrial plant and/or macrophyte origin, though C4 terrestrial plant inputs increased with settlement and plankton increased with lock and dam closure. The isotopic and C:N signature of the surface sediment is nearly identical to downstream main channel particulate organic matter, suggesting they share the same sources.