Nitrogen movement through the braidplain

Abstract

Rivers drive the earth system by moving life-critical elements like nitrogen and carbon from land to sea. Understanding exactly how, and how much, nitrogen rivers export is increasingly needed for both science and management: humans have now tripled the global flux of reactive nitrogen, and the vast majority of this extra reactive nitrogen is applied directly to land. But resolving this apparently straightforward question about nitrogen transport becomes profoundly complex with the dynamic four-dimensional hydrology of braided rivers. Braided rivers’ fluctuating water levels can rapidly change how much nitrogen moves laterally across channels, vertically between the shallow groundwater, surface water, and atmosphere, or longitudinally downstream. These physical movements in turn will affect how the rivers’ biological communities retain or emit nitrogen. In this talk I will use historical water quality data collected from the main stem of eight Canterbury rivers and the outcomes from a cross-braidplain pilot study to outline the challenges and opportunities for quantifying how nitrogen moves into and out of braided rivers. Resolving basic questions about how nitrogen moves through braided rivers will pave the way for understanding environmental issues like how increasing groundwater nitrate levels in Canterbury are affecting both the rivers themselves and the coastal ecosystems into which they flow.