US Long-Term Ecological Research Network

Human alterations of nutrient cycles have jeopardized the status of many water bodies.  Currently, there is a need to understand how individual water bodies- which range widely in shape, size, and hydrology- may contribute differentially to nutrient transport and transformation through river networks.  For example, wetlands can be important sites of nutrient cycling and organic matter settling, but the role of wetlands depends partly on internal hydrologic connections between nutrient sources and sinks.  We conducted a comparative study of stream and wetland nitrogen (N) cycling in northern Wisconsin to quantify possible differences in aquatic nitrogen uptake efficiency, and examine the influence of hydrologic connectivity on nitrogen uptake in these two distinct habitat types (Powers et al. in review). 

The morphology of stream-wetland study sites is shown in Fig.1. Our results emphasize lower nitrogen uptake efficiency in flow-through wetlands relative to streams during the growing season (May-August).  This surprising result is nonetheless explainable by the spatial differences in nitrogen uptake and nutrient supply within the ecosystem.  For example, there was a wide range in the nitrogen uptake rate of slack water (transient storage) zones relative main channel (MC) zones across both stream and flow-through wetland systems (Fig. 2).  Despite TS uptake variability, MC zones consistently accounted for the majority of NO3N uptake by the ecosystem.  Our results emphasize that aquatic nutrient retention is the outcome of a balance between: 1) ecosystem-level nutrient uptake rate and water velocity; 2) TS zone uptake rate and the strength of TS-MC hydrologic connection.  These tradeoffs restrict the influence of hydrologically disconnected habitat upon nutrient transport, and may apply to a wide range of ecosystem types and sizes.

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