Lakes as Nutrient “Sources” forWatersheds: A Landscape Analysis of the Temporal Flux of Nitrogen through Sub-Alpine Lakes and Streams

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Verhandlungen der Internationalen Vereinigung fürTheoretische und Angewandte Limnologie





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lakes, nutrient sources, watersheds, landscape analysis, temporal flux, nitrogen, sub-alpine lakes, streams

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"In many watersheds, streams and lakes are linked in complex spatial patterns. Simple watersheds may have only dendritic stream channels, whereas more complex ones can have several lakes linked with streams in complex networks. Although individual streams and lakes are usually studied in isolation from one another, their relative position in the landscape can have important impacts on water bodies further downstream. The position effect on water chemistry and potential for colorization has recently been recognized for lakes(WEBSTER et al. 1996, HILLBRICHT-ILKOWSKA 1999, MAGNUSON & KRATZ 1999, SORANNO et al. 1999, HERSHEY et al. 1999). Lake-stream studies have largely focused on insect communities in lake outflows (ROBINSON & MINSHALL 1990, RICHARDSON & MACKAY 1001), or on hypotheses of how reservoirs influence river properties below dams (STANFORD &WARD 2001). Studies that consider streams and lakes as integrated wholes are generally absent from the literature. At annual time scales, lakes are normally thought of as sinks for nutrients - fewer nutrients leave them than enter (COOKE et al. 1993). Nutrients are lost by burial in the sediments and via denitrification. In watersheds with lakes that have residence times of several years, nutrient sedimentation can impoverish downstream systems (PAULSON & BAKER 1981). However, the annual time scale may not be relevant for many aspects of nutrient transport and biological responses. In many watersheds, nutrients are introduced in sharp pulses during spates or snowmelt (DAHM et al. 2003). If they pass quickly through a simple watershed, productivity may also be pulsed and relatively low after nutrients flush from the system. In contrast, physical retention and recycling of nutrients in complex watersheds with lakes may allow productivity to be sustained at higher levels in streams and lakes further down the watershed. That is, lakes may attenuate strong nutrient pulses and make them available for longer periods for downstream biota. We tested these ideas using watersheds in the Sawtooth Mountains of Idaho (USA)."

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