Electronic supplementary material
Material and methods for Figures
Figure 2.
Retention of phosphate was measured at Latxe stream, a 2nd order stream in the Urumea river basin, and at a small derivation canal for the Bertxin hydropower plant in nearby Leitzaran stream, a 4th order stream in the Oria river basin (north Spain). Two sites were selected in Latxe stream, one with low physical complexity because of historical removal of large wood, and one where wood was added to restore channel complexity (Flores et al., 2011), and an additional site in the Bertxin canal, a concrete canal with trapezoidal cross-section.
Retention experiments were performed 6 times at each reach during spring of 2008, in different hydrologic conditions. A reach of 100 m was marked in the three study sites, and nutrient retention was measured following the short-term, constant-rate addition technique (Stream Solute Workshop, 1990). Basically, on each addition, a solution containing NH4Cl, Na(H2PO4)·H2O as nutrient sources and NaCl as a hydrologic tracer (Bencala et al. 1987) was added to the stream with a Masterflex L/S peristaltic pump (Cole-Parmer Instruments Co., Illinois, U.S.), in a place were fast mixing with stream water was ensured. The amount of reagents in the solution and the rate at which it was delivered was set to target a 3 times increase in background nutrient concentration at the mixing point. During the addition, conductivity was measured every 10 seconds at the lower end of the reaches with a conductivity meter (WTW 330i, Weilheim, Germany). Before starting the experiment and again when conductivity reached a plateau, water samples for nutrient analysis were taken bottles in pre-rinsed 250 mL polyethylene bottles at six points along the reaches. Samples for nutrient analysis were filtered in situ with pre-ashed glass fiber filters, stored at 4 °C, and transported to the laboratory for analysis. PO43--P was analyzed by standard colorimetric methods (APHA, 1998).
Phosphorus uptake length (Sw, defined as the average distance travelled by a nutrient molecule before being removed from the water column, Newbold et al., 1981), was calculated as the negative inverse of the slope of the regression of the ln-transformed and background corrected nutrient:conductivity ratio versus distance downstream from the addition point. Sw is an indicator of the nutrient retention efficiency at the reach scale (Webster & Valett, 2006).
References for the electronic supplementary material
APHA, 1998. Standard methods for the examination of water and wastewater. 20th edition. American Public Health Association, Washington, D.C.
Bencala, K.E., D. M. McKnight & G. W. Zellweger, 1987. Evaluation of natural tracers in an acidic and metal-rich stream. Water Resources Research23: 827-836.
Flores, L., A. Larrañaga, J. R. Díez & A. Elosegi, 2011. Experimental wood addition in streams: effects on organic matter storage and breakdown. Freshwater Biology 56: 2156-2167.
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Stream Solute Workshop, 1990. Concepts and methods for assessing solute dynamics in stream ecosystems. Journal of the North American Benthological Society9: 95-119.
Webster, J. R. & H. M. Valett, 2006. Solute dynamics. In: Methods in Stream Ecology (Eds F. R. Hauer & G. A. Lamberti), pp. 169–185. Academic Press, San Diego.