Euro-limpacs Deliverables

ABSTRACT - DELIVERABLE 54

Report describing outcome of pilot study to quantify in-stream nutrient retention

Nutrient retention in stream ecosystems is dependent on hydrologic, biological and chemical retention. Hydrologic retention is influenced both by the hydrologic regime and the hydraulic and morphological properties of the stream channel, which may determine the extent of the transient water storage relative to the free flowing water. The transient water storage is driven by stream compartments or structures that delay the water transport relative to the average water velocity in the channel. The magnitude of transient water storage zones can be quantified by solute transport models applied on data from solute tracer additions. Stream components influencing the water transient storage can be relevant for nutrient retention, since the interaction between stream biota and available nutrients is greater in transient storage zones than in the free flowing water. We expect, therefore, that the influence of in−stream processes on stream water chemistry will be greater when transient water storage is larger. In temperate streams with well developed riparian forests, leaf litter inputs during fall may constitute an episodic natural driver of channel hydraulic properties, which is superimposed onto the channel geomorphological features. However, the leaf fall effect may also result in a decrease in the surface water−sediment interaction, which could result in a decrease of the relevance of hyporheic processes to overall ecosystem nutrient retention. In addition, leaf litter constitute an external input of energy and resources to the stream communities, acting also as new substrata for the development of microbial communities.

This report describes a study that aimed a) to examine how the accumulation of leaves in the stream during leaf fall modified the hydraulic properties of the channel, including transient water storage, and b) to evaluate stream nutrient retention response over this period and examine its relationship with leaf accumulation and hydraulic properties. The study was conducted in a stream located in the NE of Barcelona (Spain). Although the local climate of the stream catchment favours stream features similar to those of streams in temperate zones, the general climate is characteristic of Mediterranean regions. Within these conditions, leaf fall inputs are usually coupled with frequent floodings, which may counterbalance their effects on nutrient retention. Mediterranean regions are one of the most vulnerable to current climate change. It is expected temperatures in this region will increase and precipitation will decrease but, most importantly, the seasonality of precipitation will become less pronounced with more rain and more episodic events during the rainy seasons and more intense droughts in summer. These changes will alter the current hydrologic regime of streams, which ultimately may result in greater or lower coupling between the timing of leaf fall inputs and possible floodings. In this context, our questions become of critical relevance to envisage stream nutrient dynamics under different climate change scenarios.

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