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

Iwan, Julia LU (2021) In Student thesis series INES NGEK01 20202
Dept of Physical Geography and Ecosystem Science
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... (More)
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. (Less)
Popular Abstract
For us humans, freshwater may be the most important resource on the planet. However, human activity also poses the largest threat to water quality of surface waters. This refers to eutrophication, which occurs when freshwaters receive an ‘overdose’ of nutrients from human sources (e.g., agricultural runoff, urban wastewater), which overly stimulates productivity and oxygen demand in the water until most/all oxygen is depleted.
In Sweden, especially organic nutrient concentrations are on the rise in surface waters, which causes darkening (‘brownification’). There are multiple consequences of browning: browner surface waters emit more CO2 to the atmosphere, lead to more expensive (and potentially less efficient) drinking water treatment,... (More)
For us humans, freshwater may be the most important resource on the planet. However, human activity also poses the largest threat to water quality of surface waters. This refers to eutrophication, which occurs when freshwaters receive an ‘overdose’ of nutrients from human sources (e.g., agricultural runoff, urban wastewater), which overly stimulates productivity and oxygen demand in the water until most/all oxygen is depleted.
In Sweden, especially organic nutrient concentrations are on the rise in surface waters, which causes darkening (‘brownification’). There are multiple consequences of browning: browner surface waters emit more CO2 to the atmosphere, lead to more expensive (and potentially less efficient) drinking water treatment, and cause more acidification in water systems.
This study uses a novel approach to link human land use to water quality and browning, as it compares changes (in land cover) with changes (in water quality parameter). Water quality data comes from the European database WaterBase, with information on two prevalent organic nutrients and their ratio, namely total organic carbon (TOC), total organic nitrogen (TON) and TOC to TON (TOC:TON). Land cover data for 2000, 2006 and 2012 was provided by the European Earth observation programme Copernicus.
These three variables were analysed for potential trends between 1987 and 2012 (TOC), or between 2002 and 2011 (TON, TOC:TON). Then, these chemistry changes were grouped by the direction of land cover change between 2000 and 2012 (Increase, Decrease, No Change).
The 44 study sites showed clear temporal trends in all three chemical parameters. TOC has predominantly increased between 1987 and 2012. This aligns with previously observed browning trends, where stable TOC cannot be degraded and remains unprocessed in the water. With more TOC entering surface waters, water colour becomes browner. On the other hand, most measurement sites recorded a decrease in TON since 2002, which suggests that the browning trend in Sweden is not related to TON in the water. The ratio between carbon and nitrogen generally increased between 2002 and 2011 in the catchments, which implies that organic matter entering the water is more stable and less degradable. This organic matter causes less eutrophication, as nutrients cannot be freed and taken up by aquatic organisms.
Changes in agricultural and urban areas influenced water chemistry trends in the 44 rivers and surrounding areas. Urbanization led to a stronger decrease in TON than unchanged or decreasing urban areas. This can be explained by stricter filtering regulations for new wastewater treatment plants, causing expanding urban areas to release less TON.
Catchments with changing agricultural coverage had a stronger increase in TOC than unchanged areas. The possible reasons depend on the direction of land cover change. When agriculture decreases by transforming into forest, the soils generate more organic matter, which could potentially leach into freshwaters. The increase of agriculture at the expense of pasture and grassland releases large quantities of soil organic carbon into freshwater systems, which cease when a new steady state is reached.
This study advanced the understanding of organic matter by successfully linking trends in TOC and TON to changes in land cover in Sweden. As TON concentrations decreased while browning increased, it could be assumed that browning was not caused by TON developments. The strong increase in TOC:TON ratio implied a change in organic matter source. The surrounding land seemingly contributed a lot more organic matter than could be generated by microorganisms in the water. (Less)
Please use this url to cite or link to this publication:
author
Iwan, Julia LU
supervisor
organization
alternative title
Påverkan av förändringar av landskapsförändringar på koncentrationer av organiskt kol och kväve i Svenska älvar
course
NGEK01 20202
year
type
M2 - Bachelor Degree
subject
keywords
Total Organic Carbon (TOC), Total Organic Nitrogen (TON), Land Cover Change, Sweden, WaterBase
publication/series
Student thesis series INES
report number
536
language
English
id
9038646
date added to LUP
2021-01-28 11:42:41
date last changed
2021-01-28 11:42:41
@misc{9038646,
  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.}},
  author       = {{Iwan, Julia}},
  language     = {{eng}},
  note         = {{Student Paper}},
  series       = {{Student thesis series INES}},
  title        = {{Influence of land cover changes on organic carbon and organic nitrogen concentrations in Swedish rivers}},
  year         = {{2021}},
}