Legacy Phosphorus Storage and Remobilization in a Dynamic Floodplain

Issue

Legacy nutrients are nutrients introduced by human activities like agriculture or industry, but transferred through and accumulating in the environment. Phosphorus (P) is a limiting nutrient in many aquatic ecosystems and thus fuels harmful algal blooms and Gulf hypoxia. P readily sticks to fine sediment particles, so it can be transported and stored in stream corridors along with alluvial sediments. Floodplains are consequently major reservoirs of legacy P, and on annual to decadal timescales can act as either sources or sinks for P. To better manage P delivery to receiving surface waters, we need a better understanding of the rate of P transfers into and out of valley bottom reservoirs through deposition and erosion of P-laden sediment.

Objective

We seek to estimate the amount of legacy P stored within the valley bottom sediments throughout a study watershed, along with the rates of P remobilization (by erosion) and storage (by deposition) throughout the stream corridor. These estimates combined with mapping of the morphology and land cover in the stream corridor, can help to identify landscape positions and land cover that best promote long-term storage of P.

Approach

This study will take advantage of Iowa’s 2009 and 2020 publicly-available statewide LiDAR (Light Detection and Ranging) elevation datasets to assemble a map of elevation change throughout the study area. This data will be checked against dendrochronological estimates of elevation change from tree trunks partly buried in alluvial sediment, and direct erosion measurements using erosion pins. This data will be used to identify parts of the floodplain that serve as sources and sinks for P, and thereby estimate the rate of release of legacy P to surface waters.