Abstract
AbstractAgricultural, forestry‐impacted and natural catchments are all vectors of nutrient loading in the Nordic countries. Here, we present concentrations and fluxes of total nitrogen (totN) and phosphorus (totP) from 69 Nordic headwater catchments (Denmark: 12, Finland:18, Norway:17, Sweden:22) between 2000 and 2018. Catchments span the range of Nordic climatic and environmental conditions and include natural sites and sites impacted by agricultural and forest management. Concentrations and fluxes of totN and totP were highest in agricultural catchments, intermediate in forestry‐impacted and lowest in natural catchments, and were positively related %agricultural land cover and summer temperature. Summer temperature may be a proxy for terrestrial productivity, while %agricultural land cover might be a proxy for catchment nutrient inputs. A regional trend analysis showed significant declines in N concentrations and export across agricultural (−15 μg totN L−1 year−1) and natural (−0.4 μg NO3‐N L−1 year−1) catchments, but individual sites displayed few long‐term trends in concentrations (totN: 22%, totP: 25%) or export (totN: 6%, totP: 9%). Forestry‐impacted sites had a significant decline in totP (−0.1 μg P L−1 year−1). A small but significant increase in totP fluxes (+0.4 kg P km−2 year−1) from agricultural catchments was found, and countries showed contrasting patterns. Trends in annual concentrations and fluxes of totP and totN could not be explained in a straightforward way by changes in runoff or climate. Explanations for the totN decline include national mitigation measures in agriculture international policy to reduced air pollution and, possibly, large‐scale increases in forest growth. Mitigation to reduce phosphorus appears to be more challenging than for nitrogen. If the green shift entails intensification of agricultural and forest production, new challenges for protection of water quality will emerge possible exacerbated by climate change. Further analysis of headwater totN and totP export should include seasonal trends, aquatic nutrient species and a focus on catchment nutrient inputs.
Highlights
Reconciliation of increasing reliance on agricultural and forestry products with water quality protection is becoming more urgent under the green growth policies that are being developed in Europe
Long-term assessments focus on a dominant land use, either agricultural (Blann, Anderson, Sands, & Vondracek, 2009; Pengerud et al, 2015), forestry-impacted (Kreutzweiser et al, 2008) or natural catchments (Vuorenmaa et al, 2018)
Significant proportions of nutrient loadings from Nordic countries to marine recipients originate from agriculture, natural and semi-natural ecosystems (HELCOM, 2018; Lepisto, Granlund, Kortelainen, & Raike, 2006) suggesting that changes in nutrient runoff from both managed and unmanaged ecosystems is pertinent to the ecological status of receiving waters
Summary
Agriculture is a dominant land use in Denmark and across the Nordic region, nutrient loadings from agriculture is a large concern for eutrophication of freshwater and marine ecosystems (Frigstad et al, 2020; Karlson, Rosenberg, & Bonsdorff, 2002) These waters are valuable natural resources and essential for Nordic societies, economies and human wellbeing as they provide multiple ecosystem services (Kronvang et al, 2008; Marttila et al, 2020; Ulen et al, 2007). Long-term changes in diffuse nutrient fluxes from managed catchments are strongly influenced by a complex combination of temporal and spatial factors, such as fluctuating climatic and hydrological conditions, land cover, soil characteristics, crop cycles and land-use practices in forestry and agriculture (Bechmann et al, 2014; Kyllmar, Carlsson, Gustafson, Ulen, & Johnsson, 2006; Tattari et al, 2017; Vuorenmaa, Rekolainen, Lepisto, Kenttamies, & Kauppila, 2002). Because nutrient runoff is potentially sensitive to differences in national mitigation measures (Hellsten et al, 2019; Kronvang et al, 2008; Ulen et al, 2007), we analyse for patterns by country in addition to examining patterns within land-use categories across the Nordic region
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