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How network structure can affect nitrogen removal by streams
Author(s) -
Helton Ashley M.,
Hall Robert O.,
Bertuzzo Enrico
Publication year - 2018
Publication title -
freshwater biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.297
H-Index - 156
eISSN - 1365-2427
pISSN - 0046-5070
DOI - 10.1111/fwb.12990
Subject(s) - streams , environmental science , drainage basin , cycling , hydrology (agriculture) , stream restoration , nitrogen , soil science , ecology , chemistry , geology , computer science , geography , geotechnical engineering , biology , computer network , cartography , archaeology , organic chemistry
Streams and rivers can be highly reactive sites for nitrogen (N) transformation and removal. Empirical and model‐based research show how location in a stream network affects rates of N removal. Because the structure of stream networks can vary widely and N cycling in headwater streams may affect N cycling in downstream reaches, we hypothesised that network structure may affect whole stream network processing of N. We generated three stream networks with the same catchment area but differing shapes, based on optimal channel network theory. We applied a model of nitrate ( NO 3 − ) transport and denitrification, and implemented model scenarios to examine how network shape affects NO 3 − removal with (1) increased NO 3 − loading from the catchment, (2) altered spatial distributions of NO 3 − loading and (3) decreased drainage density (i.e. loss of headwater streams). For all stream networks, the fraction of total NO 3 − removed decreased with increasing NO 3 − loading from the catchment. Stream networks in narrow catchments removed a higher fraction of NO 3 − , particularly at intermediate NO 3 − loading rates. Network shape also controlled the distribution of removal in small versus large streams, with larger streams removing a higher fraction of the total NO 3 − load in narrower networks. The effects of network shape on NO 3 − removal when the spatial distribution of NO 3 − loading was altered varied with the magnitude of NO 3 − loading. At low loads, NO 3 − was entirely removed when added to distal parts of the stream network, and about 50% removed when added near the outlet; there was no effect of network shape. At intermediate and high loads, the fraction of total NO 3 − load removed by the narrow stream network was 1.5× higher than the rectangular and square networks when NO 3 − was added to distal parts of the networks. Network shape did not have an effect when NO 3 − load occurred near the outlet, regardless of the magnitude of the NO 3 − load. The fraction of total NO 3 − removed by the stream network was up to 5% lower when drainage density was reduced from 1.0 to 0.74 km −1 , with the least change for the narrow network. Reducing the drainage density also altered the role of small relative to large streams, with the net effect of moving the location of NO 3 − removal downstream. Overall, effects of network shape contributed up to 20% of the variation in the fraction of NO 3 − removed by stream networks. Network shape was most important at intermediate to high NO 3 − loads and when NO 3 − was loaded to distal parts of the catchment. The narrow network removed more NO 3 − across model scenarios, with elevated removal in larger streams explaining most of the difference. We suggest the shape of the catchment may modulate the degree to which large streams contribute to whole network NO 3 − removal.