Doctoral Dissertation Research: Floodplain Sedimentation and Nutrient Sequestration, Upper Mississippi River

淡水和海洋生态系统因沉积物和营养物质而超载，造成了许多生态和健康问题。虽然已有研究研究了河流在营养物质运输和循环中的作用，但关于大型河流泛滥平原在减少这些水生污染物向下游输送方面所起的作用的研究相对较少。最近的研究表明，洪泛平原可以在个别洪水或年份储存大量的氮、磷和碳，但很少有人关注扩大洪泛平原在十年到世纪的时间尺度上保留这些营养物质的有效性。通过对威斯康星州拉克罗斯和卡斯维尔之间的密西西比河泛滥平原多个区域的沉积物取心，以及通过高时间分辨率的沉积物分析，这项研究将1)量化密西西比河泛滥平原从前欧美殖民地到现在与沉积物相关的沉积物和营养物质浓度的变化；2)表征密西西比河泛滥平原的水文连通性和地貌如何影响营养物质的保持和通量；以及3)确定滞留在泛滥平原上的磷的主要形态。磁化率和颗粒大小特征将与磷、氮和碳分析结合使用，以解决研究目标。沉积年代学将结合使用加速器质谱仪、放射性碳测年、铯放射性核素活度和与欧美定居有关的仙人掌花粉来建立。预计密西西比河泛滥平原保留了大量的沉积物和氮、磷和碳，这些在现有的营养收支中没有被计算在内，否则这些营养物质就会被冲下河流。这项研究是确定泛滥平原环境在从单一洪水到世纪尺度的时间尺度上隔离营养物质的重要性的必要步骤。氮和磷导致河流、湖泊和海洋生态系统的大规模藻类水华，可能导致水质下降和水生栖息地退化。通过更好地了解洪泛平原如何保留沉积物和营养物质，以及通过更好地了解它们在泛滥平原上的空间位置，资源管理者将能够更有效地管理这些地区，以减少下游营养物质的输送及其不利影响。需要了解洪泛区的养分储存如何受到流域土地利用和洪涝灾害增加的影响，特别是考虑到玉米生产乙醇的增加可能会导致营养物质和河流泥沙的输送增加。随着气候研究人员继续寻找降低大气二氧化碳浓度的方法，这项研究的预期结果将是重要的，这项研究记录了河流沉积物中的碳储量如何随着泛滥平原的水文而变化。

The overloading of freshwater and marine ecosystems with sediment and nutrients causes numerous ecological and health problems. While studies have researched the role of rivers in transporting and cycling nutrients, relatively little research has been conducted on the role large river floodplains play in reducing downstream conveyance of these aquatic contaminants. Recent studies have shown floodplains can store significant quantities of nitrogen, phosphorus and carbon during individual floods or years, but few have looked at the effectiveness of aggrading floodplains to retain these nutrients over decadal to century timescales. Through sediment coring of multiple areas of the Mississippi River floodplain between La Crosse and Cassville, Wisconsin and through high temporal resolution sediment analyses this study will 1) quantify how sedimentation and nutrient concentrations associated with sediments have changed on the Mississippi River floodplain from pre-European-American settlement to present; 2) characterize how hydrologic connectivity and geomorphology of the Mississippi River floodplain affect nutrient retention and fluxes; and 3) identify the dominant forms of phosphorus retained on the floodplain. Magnetic susceptibility and grain size characteristics will be used in conjunction with phosphorus, nitrogen and carbon analyses to address the research objectives. Depositional chronology will be established using a combination of accelerator mass spectrometry radiocarbon dating, cesium radionuclide activity, and ambrosia pollen associated with European-American settlement. It is expected that the floodplain of the Mississippi River retains significant quantities of sediment and nitrogen, phosphorus, and carbon unaccounted for in existing nutrient budgets that would otherwise be flushed downstream.This research is a necessary step toward determining the significance of floodplain environments in sequestering nutrients over timescales ranging from singular floods to century scale. Nitrogen and phosphorus contribute to large algal blooms in rivers, lakes and marine ecosystems that can lead to a reduction in water quality and degradation of aquatic habitat. Through a greater understanding of how floodplains retain sediment and nutrients and through improved knowledge of where they are retained spatially on the floodplain, resource managers will be able to manage these areas more effectively to reduce downstream nutrient transport and its adverse impacts. Knowledge concerning how floodplain nutrient storage is affected by drainage basin land use and increased flooding is needed, especially as increased corn production for ethanol will likely lead to increased nutrient and sediment transport by rivers. As climate researchers continue to search for ways to reduce atmospheric carbon dioxide concentrations, the expected results of this research which documents how carbon storage in river sediment varies with floodplain hydrology will be important.