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Biogeosciences, 9, 1985–1996, 2012 www.biogeosciences.net/9/1985/2012/ Biogeosciences doi:10.5194/bg-9-1985-2012 © Author(s) 2012. CC Attribution 3.0 License. Organic matter dynamics and stable isotope signature as tracers of the sources of suspended sediment Y. Schindler Wildhaber, R. Liechti, and C. Alewell Institute for Environmental Geosciences, Basel, Switzerland Correspondence to: Y. Schindler Wildhaber ([email protected]) Received: 22 December 2011 – Published in Biogeosciences Discuss.: 16 January 2012 Revised: 24 April 2012 – Accepted: 2 May 2012 – Published: 4 June 2012 Abstract. Suspended sediment (SS) and organic matter in river will increase, which will possibly affect brown trout rivers can harm brown trout Salmo trutta by affecting the negatively. health and fitness of free swimming fish and by causing sil- tation of the riverbed. The temporal and spatial dynamics of sediment, carbon (C), and nitrogen (N) during the brown trout spawning season in a small river of the Swiss Plateau 1 Introduction were assessed and C isotopes as well as the C/N atomic ratio were used to distinguish autochthonous and allochthonous All streams carry some suspended sediment (SS) under nat- sources of organic matter in SS loads. The visual basic pro- ural conditions (Ryan, 1991). An increase of SS due to an- 13 15 gram IsoSource with Ctot and N as input isotopes was thropogenic perturbation, however, has been observed in the used to quantify the temporal and spatial sources of SS. Or- last decades (e.g. Owens et al., 2005). Perturbation includes ganic matter concentrations in the infiltrated and suspended forestry, pasture and agricultural activities, which enhance sediment were highest during low flow periods with small soil erosion processes and hence the sediment delivery into sediment loads and lowest during high flow periods with high rivers (e.g. Pimentel and Kounang, 1998). In addition, it sediment loads. Peak values in nitrate and dissolved organic is expected that the frequency and intensity of heavy rain C were measured during high flow and high rainfall, prob- events in middle Europe will increase due to climate change ably due to leaching from pasture and arable land. The or- (IPPC, 2007), enhancing soil erosion triggered by water. Ac- ganic matter was of allochthonous sources as indicated by cording to model calculations, the sediment supply from the 13 the C/N atomic ratio and δ Corg. Organic matter in SS in- Alpine region into the Rhine basin, for example, is expected creased from up- to downstream due to an increase of pasture to increase by about 250 % (Asselman et al., 2003). Increas- and arable land downstream of the river. The mean fraction ing SS loads in rivers generally leads to a higher fine sed- of SS originating from upper watershed riverbed sediment iment infiltration rate in the riverbed gravel (Greig et al., decreased from up to downstream and increased during high 2005; Zimmermann and Lapointe, 2005; Schindler Wild- flow at all measuring sites along the course of the river. Dur- haber et al., 2012). SS and fine sediment infiltration (SI) ing base flow conditions, the major sources of SS are pas- can provide a serious threat to aquatic ecosystems includ- ture, forest and arable land. The latter increased during rainy ing phytoplankton, aquatic invertebrates, and salmonid fish and warmer winter periods, most likely because both trig- (for a review see Bilotta and Brazier, 2008). Salmonid fish gered snow melt and thus erosion. The measured increase can be affected by SS in several ways. Their eggs develop in in DOC and nitrate concentrations during high flow support so-called redds, a shallow depression created by the female these modeling results. Enhanced soil erosion processes on brown trout, where eggs and sperms are deposited. The fe- pasture and arable land are expected with increasing heavy male covers the fertilized eggs with gravel. While SS can di- rain events and less snow during winter seasons due to cli- rectly impact health and fitness of free swimming fish (New- mate change. Consequently, SS and organic matter in the combe and Jensen, 1996), fine sediment infiltration in the redds can induce siltation of the gravel resulting in a decrease Published by Copernicus Publications on behalf of the European Geosciences Union. 1986 Y. Schindler Wildhaber et al.: Tracing sources of suspended sediment Willisau the SS were found to be highly variable with minimum val- ª*#! ues around 1.5 % at high flow and maximum values around ª!*# ± ! ª!*# 10.5 % at low flow (Schindler Wildhaber et al., 2012). Site C An identification of SS sources is required to improve 583 m a. s. l. Switzerland site management and possibly restrain the described nega- ! tive consequences for brown trouts. Sediment tracer-based !*# 50 km ª*# Site B methods have been used to distinguish possible sources in 625 m a. s. l. watersheds. Stable carbon (C) and nitrogen (N) isotopes and Hergiswil as well as the carbon to nitrogen atomic ratio (C/Na) have *#! been found to be reliable tracers in recent studies (Onstad ª! et al., 2000; McConnachie and Petticrew, 2006; Fox and Pa- ª panicolaou, 2007; Gao et al., 2007; Fox et al., 2010). Stable isotope compositions as well as C/Na are affected by many factors including soil depth, vegetation, climate and cultural history (Kendall, 1998). By assessing the ratio of 13C to 12C, Site A 15N to 14N and Ca to Na in the SS as well as in surface soils 757 m a. s. l. of the catchment, conclusions concerning possible SS ori- *# !! gin can be drawn. The isotopic compositions of SS samples *# may represent a mixture of potential sources. The propor- tional contributions of different sources can be assessed by (! Riverbed S linear mixing models. The disadvantage of these models is *# Forest *# generally the limitation in detecting potential sources by the !*# + ! Pasture ! number of isotope tracers. With n isotope tracers, n 1 po- ª Arable land ! tential sources can be detected. The model is mathematically (! under-determined if the number of potential sources exceeds (! (!*# n+1, resulting in an equation system with less equations than 00.5 1 2 km *#(!*# unknown variables and therefore no single solution is possi- ble (Phillips and Gregg, 2001). Phillips and Gregg (2003) Fig. 1. Watershed of the river Enziwigger with the three field developed a a visual basic program called IsoSource to as- sites A, B and C including their altitude, soil sample spots and the sess potential source contributions if more than n+1 sources towns Willisau and Hergiswil (Canton of Lucerne, Switzerland). are present. The program examines in small increment steps all possible combinations of each source contribution, result- ing in several feasible solutions. This program was used in in hydraulic conductivity (Schalchli¨ , 1995). This affects the this study to quantify source contributions to SS during the oxygen supply to the developing salmonid embryos in the brown trout spawning season. redds negatively, and hence their survival (Greig et al., 2005, The objective of this study was (I) to assess the temporal 2007a; Heywood and Walling, 2007). The presence of high and spatial C and N dynamics during the brown trout spawn- organic matter in the IS and interstitial water can additionally ing season in suspended and infiltrated sediments and river disproportionately impact on spawning habits (Greig et al., as well as interstitial water, (II) the use of Corg isotopes as 2005). As respiration is strongly dependent on the availabil- well as C/Na to distinguish autochthonous and allochthonous ity of organic matter, oxygen demand within riverbeds will sources of the organic matter in the SS, and (III) the use of increase as the pool of organic matter increases (for a review Ctot and N isotopes as tracers to quantify the sources of SS see Greig et al., 2007b). Organic material is derived either with respect to time and space. from in-stream sources (autochthonous), for example macro- phyte vegetation or from external sources (allochthonous), for example leaf litter or runoff from agricultural fields (Sear 2 Materials and methods et al., 2008). Schindler Wildhaber et al. (2012) reported in a study on 2.1 Study site and general setup sediment dynamics in a small Swiss headwater river of the Swiss Plateau with a native brown trout Salmo trutta popu- The river Enziwigger is a small canalized river located near lation an SS increase from up- to downstream. This finding Willisau, Canton of Lucerne, Switzerland with a total wa- could be related to an increased shear stress attributable to tershed area of about 31 km2 (Fig. 1). The flow regime a higher water level down the stream and/or to a higher fine of the Enziwigger is not affected by hydro-power facili- sediment input from the arable land in the lower part of the ties and there is no waste water treatment plant located catchment. Furthermore, organic carbon concentrations of above Willisau. Like most rivers in the Swiss Plateau, its Biogeosciences, 9, 1985–1996, 2012 www.biogeosciences.net/9/1985/2012/ Y. Schindler Wildhaber et al.: Tracing sources of suspended sediment 1987 morphology is strongly modified: Only 5 % is close to natural site, six manure samples close to site B and C, and 5 riverbed or natural, 21 % is little affected and 74 % is strongly affected sediment samples from the upper most accessible reach of or even artificial, including terraces that have been inserted the river were collected. No surface water ran at the upper to prevent deep channel erosion and scouring of the bed dur- most accessible reach of the river during dry periods, which ing flood events (classified with the Swiss modular stepwise allowed sediment sampling with a simple corer.

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