BIOGEOCHEMISTRY OF UPPER MISSISSIPPI RIVER FLOODPLAIN LAKE SEDIMENTS: GEOMORPHIC AND HYDROLOGIC IMPACTS ON NUTRIENT SEQUESTRATION

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.  While the value of wetland buffers and floodplain connectivity for reducing nutrient delivery from low order streams is generally understood, little is known about the role of large alluvial river floodplains and wetlands in storing nutrients in association with deposited sediment.  This research expands our present spatial and temporal knowledge of large river floodplain nutrient storage through high resolution stratigraphic and chemical analyses of nine sediment cores taken from the upper Mississippi River.  Post-lock and dam (ca.1937) sedimentation rates vary from 5-10 mm yr^-1_, depending on geomorphic setting and hydrologic connection to the main channel.  These rates represent an order of magnitude increase over long-term late-Holocene rates of deposition.  Post-1937 phosphorus burial rates ranged from 4.0 g m^-2 yr^-1 to 7.4 g m^-2 yr^-1.  Sedimentation depth is the best predictor for total phosphorus burial across the sites with an r² of 0.82.    Mass of sediment deposited at each site is a poorer predictor of total phosphorus burial at a site with an r² of 0.50.  Within individual sites, percent iron oxides (Fe_d) and percent organic matter (LOI₅₅₀) are best correlated with phosphorus concentrations.  While phosphorus, carbon, and nitrogen concentrations rapidly increased following lock and dam construction and continue to rise to the present, the greatest quantity of phosphorus sequestered per cm³ at most sites is located in a zone of high iron oxide concentrations associated with the pre-lock and dam average water table height.  The greatest quantity of total carbon and total nitrogen per cm³ is often associated with buried pre-settlement soils.  Large rivers with hydrologically connected floodplains play an important role in long-term sequestration of significant quantities of aquatic contaminants, with rates of sedimentation primarily controlling total nutrient sequestration.