Influence of land cover changes on organic carbon and organic nitrogen concentrations in Swedish rivers

Abstract: There is increasing pressure on freshwater systems due to excessive nutrient loading by anthropogenic sources. In Sweden, freshwaters have become browner due to increasing exports of organic matter from soils into streams, rivers, and lakes. This study aimed at detecting temporal patterns in Total Organic Carbon (TOC) and Total Organic Nitrogen (TON) concentrations in 44 Swedish rivers, and to link these trends to land cover changes in the catchments. It was hypothesized that increasing TOC trends would be governed by afforestation, while TON dynamics would be determined by urbanization and/or agricultural expansion in the catchments. Annually averaged water quality time series for TOC (1987-2012), TON (2002-2011) and the calculated TOC:TON ratio were retrieved from the database WaterBase, whereas land cover and associated changes were based on three CORINE land cover raster layers (2000, 2006 and 2012) from the European Earth observation programme Copernicus. Net land cover changes indicated that 1.02% of Sweden changed land cover from 2000 to 2012, with directional land cover changes highlighting the role of both forest gains and losses. The analysis of water quality revealed temporal trends, with a general increase in TOC concentrations by 0.106 ± 0.082 mg C L -1 yr-1 and a simultaneous decrease in TON concentrations by 0.015 ± 0.017 mg N L-1 yr-1. Comparison of temporal trends in TOC, TON and TOC:TON grouped by land cover change revealed linkages between certain land cover dynamics and water chemistry trends. However, differences in TOC trends could not be connected to afforestation within the catchment. Catchments with increasing impervious surfaces showed a significantly stronger negative trend in TON (median=-0.015 mg L-1 yr-1) compared to catchments with unchanged (median=-0.007 mg L-1 yr-1) or decreasing (median=-0.009 mg L-1 yr-1) urban areas, although the two latter categories were not significantly different from each other. These trends contrast the traditional view on urban and industrial areas as sources of TON and suggest considerably improved wastewater treatment. TOC dynamics significantly differed depending on agricultural land cover changes. The TOC data exhibited stronger positive trends for changing catchments, regardless of the direction of change. Both catchments with increasing (median=0.145 mg L-1 yr-1) and decreasing (median=0.100 mg L-1 yr-1) agricultural extent had stronger increases in TOC compared to catchments with unchanged agricultural extent (median=0.053 mg L-1 yr-1). Potentially, these patterns are associated with changes in management that lead to more soil organic carbon (SOC) being lost in runoff. Conversion to forest leads to increases in SOC and thus more subsequent leakage. Transformation from natural land cover (pasture, grassland) to agriculture represents a disturbance that causes losses in SOC, possibly expressed in temporarily stronger TOC trends. This study advanced the understanding of organic matter loading by establishing novel linkages between land cover change and trends in riverine TOC and TON in a country dominated by boreal vegetation. The presented findings showed a decoupling of browning trends from TON concentrations. The strong increase in TOC:TON ratio suggests a shift in organic matter origin, from dominance of (autochthonous) in-water sources toward more (allochthonous) terrestrial sources.