Headwater streams: neglected ecosystems in the EU Water Framework Directive. Implications for nitrogen pollution control
Introduction
The global biogeochemical cycle of nitrogen (N) has been deeply altered, and the boundary within which humankind can operate safely has long been crossed (Rockstrom et al., 2009). N point and diffuse sources, such as human sewage, atmospheric deposition and agriculture, increase N loads in streams, resulting in the cultural eutrophication of aquatic ecosystems (Camargo and Alonso, 2006). Agriculture is currently recognised as the main driver of N pressures in many of the major catchments of the European Union (EU) (EEA, 2005, Grizzetti et al., 2008).
River drainage networks are hierarchically organized systems in which 1st and 2nd order streams, commonly referred as headwater streams, make up at least 70% of total stream length (Leopold et al., 1964, Meyer, 2007). Most of the N flowing through the whole hydrological network is estimated to come from the headwater catchments (Alexander et al., 2007). By contrast, significant in-stream retention of this transported N can occur in the headwater streams due their intense water–streambed interaction (Alexander et al., 2000, Peterson et al., 2001, Wollheim et al., 2001, Martí et al., 2006). Furthermore, there is a high degree of permanent loss of this N to the atmosphere through denitrification (Mulholland et al., 2009). In addition to the important role played by headwater streams in N cycling, they provide other important ecosystem services (Lowe and Likens, 2005). Headwater streams, however, particularly those in agricultural areas, are subjected to different pressures such as channelling, impoundment or burial (Veliz and Richards, 2005, Freeman et al., 2007).
Since 1972, legislation in the United States of America protects the nation's navigable waters under the Clean Water Act (CWA). At first, any tributary of a navigable-in-fact river was protected, but this consideration changed in 2001. After a 5-year debate, it was considered that the CWA must protect all waters with a “significant nexus” with the hydrological system (Nadeau and Rains, 2007, Leibowitz et al., 2008). In the European Union (EU), the Water Framework Directive (WFD; 2000/60/EC) aims to achieve the “good status” of EU waters by 2015 through the implementation of measures at basin scale by means of River Basin Management Plans (RBMPs). The WFD requires the identification of “bodies of surface water” (BSWs) as discrete and significant elements to be used for quality diagnosis and management. Due to their small size, headwater streams are not usually identified as BSWs and are therefore excluded from the measures implemented by RBMPs, regardless of their vulnerability to anthropogenic activities and their significant influence on downstream water quality.
Herein we present a case study focusing on a small catchment of the Ebro River affected by N pollution caused by agriculture. Study of such specific cases is necessary with regard to highlighting the importance of headwater streams in the management of water quality at large catchment level, in order for them to be considered in future RBMPs. The Ebro River Basin (NE, Spain) is the largest Spanish catchment. Similarly to other basins in EU countries, control of point source effluents has efficiently reduced P levels in surface waters, whereas less success has been had in abating the concentration of N compounds (Ibáñez et al., 2008), particularly nitrate, which is often the major constituent of the N pool in rivers. A study considering the overall Ebro River Basin (Lassaletta et al., 2009) recently reported that nitrate pollution has increased in many of the monitored sites of the Ebro River Basin over the last few decades. Moreover, agricultural land cover was closely related to nitrate concentrations recorded at different sites across the basin. These results could lead to non-compliance with WFD aims in 2015. From the management perspective it is important to ascertain the impact of headwaters upon downstream water quality. For this reason, we have selected a small agricultural catchment to compare N levels and pressures in surface waters with those observed at a higher scale.
The following hypotheses were contrasted: (1) stream nitrate response to agriculture varies with the spatial scale considered; (2) stream nitrate levels at the mouth of this small catchment (non-BSW) are similar to those observed downstream at sites (BSW) also influenced by agricultural activities. The hydrographical network of the Ega River Catchment was classified according to stream hierarchies and to considerations of the WFD. The aim of this study is to assess whether current EU legislation adequately considers the significance of headwater streams in the achievement of specific water quality targets.
Section snippets
Headwater streams in the EU legislation
The “good status” condition defined by the WFD is reached by a water body when both its “ecological status” and “chemical status” are considered to be at least “good”. The “ecological status”, a great improvement of the Directive, is an expression of the quality of the structure and functioning of aquatic ecosystems associated with surface waters. Evaluation of this quality involves consideration of biological elements and other quality elements supporting these, such as physical–chemical and
Materials and methods
Herein we compare nitrate data in stream waters obtained in nested hydrological systems (Fig. 1) in order to establish whether or not stream nitrate concentration responds similarly at different scales. Our case of study centres on the Galbarra Stream Catchment (Fig. 1c), a small watershed devoted to cereal cultivation, intensively sampled by our team during 2002 and 2003. Available public data on higher spatial scales of Ega River (Fig. 1b) and Ebro tributaries and the Ebro River mouth (Fig. 1
Results
Nitrate was the dominant form of Dissolved Inorganic Nitrogen (DIN). None of the sites presented an averaged nitrate/ammonium molar ratio lower than 1 (Table 2). With the exception of GT-1 and GT-2, with no agriculture or urban presence in their catchments, nitrate was 1 or 2 orders of magnitude higher than ammonium. GT-3 was the only site with an ammonium mean concentration higher than 0.15 mg/L, which was due to the wastewater effluent from a small village upstream from the sampling site.
Discussion
The response of nitrate concentration in surface waters to the agricultural cover of the catchment was similar regardless of spatial scale. The strong relationship between nitrate levels and agricultural cover found in Galbarra headwater catchments has also been reported in several studies conducted at different scales (Liu et al., 2000, Kyllmar et al., 2006, Lassaletta, 2007, Broussard and Turner, 2009). Mean instant yield of nitrate was also closely related to agricultural cover in the
Conclusions
Nitrate concentration in the headwater streams (1st and 2nd order streams) studied herein may be higher than downstream river concentration, and clearly responds to percentage of agricultural land use in their catchment. The effect of agriculture on nitrate concentration was very high and independent from the size of the embedded catchments considered in this study (0.1–7.3 km2 and 223–3113 km2 catchment area, respectively), thus refuting our initial hypothesis. The nitrate concentration in the
Acknowledgements
This research was funded by an agreement between the Spanish Ministry of Environment and Rural and Marine Affairs (Ministerio de Medio Ambiente, Rural y Marino), CIEMAT, and the Universidad Complutense de Madrid on “Critical loads and levels”. This research was also funded by contract FGUCM-94/2000 between the Environmental Department of the Navarre Regional Autonomy and Madrid's Fundación General de la Universidad Complutense (FGUCM). We wish to thank the Ebro Basin Confederation for providing
Luis Lassaletta is a research scientist at the Department of Ecology in the Universidad Complutense de Madrid (UCM). He holds a PhD in Biology from this university, awarded in 2007. He was the Head of the Bureau of Environmental Management of Alcalá de Henares University, Spain. He is currently participating in projects related to “nitrogen trends and critical loads” as a member of the Pollution and Aquatic Ecosystems Research Group of the UCM.
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Luis Lassaletta is a research scientist at the Department of Ecology in the Universidad Complutense de Madrid (UCM). He holds a PhD in Biology from this university, awarded in 2007. He was the Head of the Bureau of Environmental Management of Alcalá de Henares University, Spain. He is currently participating in projects related to “nitrogen trends and critical loads” as a member of the Pollution and Aquatic Ecosystems Research Group of the UCM.
Héctor García-Gómez is a PhD student at the Department of Ecology in the Universidad Complutense de Madrid (UCM). He obtained his BSc in Biology in 2006 and his MSc in Ecology and Environmental Sciences in 2008. His research interest focuses on aquatic ecosystem pollution and the nitrogen cycle in agroecosystems. He is a member of the Pollution and Aquatic Ecosystems Research Group of the UCM.
Benjamín S. Gimeno researches the ecotoxicology of pollutants. He has developed most of his career at CIEMAT and has been very active in the UN-ECE Convention on Long-Range Transboundary Air Pollution. At present he is involved in the management of science at the Spanish Ministry of Science and Innovation.
José V. Rovira is a lecturer of Ecology and Ecosystems Pollution at the Department of Ecology, Universidad Complutense de Madrid (UCM), Spain. He received his PhD in Ecology and Environmental Sciences in 1991. He works in the environmental pollution area (limnology, ecotoxicology, heavy metals and nutrients). He is currently a researcher and manager of the Pollution and Aquatic Ecosystems Research Group of the UCM.