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Brownification of freshwaters - the role of dissolved organic matter and iron

Ekström, Sara LU (2013)
Abstract (Swedish)
Popular Abstract in Swedish

Under de senaste årtiondena har vattenfärgen i många svenska sjöar och vattendrag ökat. Detta innebär att vattnet blir brunare, och detta fenomen benämns därför brunifiering. Brunifiering är inget unikt för Sverige, utan förekommer på åtskilliga ställen på norra halvklotet. Brunifieringen påverkar det akvatiska ekosystemet och vattnets potentiella samhällsnytta. Ur en ekologisk synvinkel kan brunare vatten till exempel minska tillväxten av växtplankton och undervattensväxter i sjön, eller missgynna fiskar som är beroende av synen för att finna sitt byte. Ur ett mänskligt perspektiv kan brunare vatten innebära att det blir svårare att rena och använda det som dricksvatten, och många finner en sjö... (More)
Popular Abstract in Swedish

Under de senaste årtiondena har vattenfärgen i många svenska sjöar och vattendrag ökat. Detta innebär att vattnet blir brunare, och detta fenomen benämns därför brunifiering. Brunifiering är inget unikt för Sverige, utan förekommer på åtskilliga ställen på norra halvklotet. Brunifieringen påverkar det akvatiska ekosystemet och vattnets potentiella samhällsnytta. Ur en ekologisk synvinkel kan brunare vatten till exempel minska tillväxten av växtplankton och undervattensväxter i sjön, eller missgynna fiskar som är beroende av synen för att finna sitt byte. Ur ett mänskligt perspektiv kan brunare vatten innebära att det blir svårare att rena och använda det som dricksvatten, och många finner en sjö med brunt vatten mindre trevlig att bada i. Därför är det viktigt att förstå varför vattenfärgen ökar.



Brunifieringen brukar tillskrivas ökade koncentrationer av löst organiskt material (OM) – ofullständigt nedbrutna växtdelar från omgivande mark. Detta material ger vattnet en brun färg och i de flesta fall sammanfaller ökningen i vattenfärg med en ökning i OM-halt i vattnet. De ökade halterna av OM har härletts till bland annat klimatförändringar och förändringar i markanvändning. En annan teori är att det är den minskade försurningen som ligger bakom brunifieringen. Fram till åttiotalet skedde det stora utsläpp av svavel till atmosfären från bl.a. förbränning av fossila bränslen så som kol. I luften omvandlades svavlet till svavelsyra vilket sänker pH och försurar regnet. Tack vare lagstiftning har utsläppet av svavel minskat i Europa, och försurningen via regn har därmed minskat. Samtidigt som försurningen har minskat har vattenfärgen och halterna av OM i sjöar ökat, vilket gör att man tror att den minskade försurningen kan vara en av orsakerna till brunifieringen.



I ett fältexperiment där jag bevattnade skogsmark med vatten med olika svavelinnehåll visade jag att minskad försurning, dvs högre pH i regnet, gör det organiska materialet i marken mer lösligt och rörligt, vilket betyder att mer OM kan följa med regnvatten från marken till närliggande bäck eller sjö. Förutom den ökade rörligheten såg jag förändringar i det organiska materialets beskaffenhet. Lägre försurning gav OM som bl.a. hade mer färg och bestod av större molekyler. Dessa förändringar i kvaliteten hos det organiska materialet kan påverka hur det bryts ner i sjön. Två viktiga processer för nedbryting av OM är fotooxidation, där OM bryts ner av solljus, och bakteriell nedbrytning, där bakterier använder OM som energikälla. Jag såg att OM under lägre försurning var mer tillgängligt för nedbrytning av solljus, men mindre tillgängligt för nedbrytning av bakterier. Detta kan komma att innebära att den ena processen blir viktigare än den andra för nedbrytningen av OM i det akvatiska systemet. I den utsträckning som minskad försurning är orsaken till brunifieringen kommer den att avstanna eftersom svavelhalterna i nederbörden börjar att närmar sig de nivåer som fanns innan försurningen startade.

Förutom OM så är järn i vattnet en viktig faktor bakom vattenfärgen. Dock har järn inte tidigare studerats i samband med brunifiering. Jag visar att samtidigt som vattenfärgen och OM har ökat, så har halten av järn ökat allra mest i våra ytvatten. Vattenfärgen har ökat avsevärt mer än OM, och teoretiskt sett skulle en stor del av brunifieringen kunna bero på ökade järnhalter.



Slutligen har jag studerat varför järnet i våra vattendrag ökar. Järnet kommer från omgivande mark, och variationen i järnkoncentration inom åar tycks till stor del bero på förekomsten av syrefria förhållanden i marken. När det blir syrefritt i marken blir järnet lösligt och kan transporteras ut till vattendragen. Syrefria förhållanden i marken bildas när hög markfuktighet sammanfaller med höga temperaturer. De pågående klimatförändringarna med mer nederbörd och högre temperatur kan komma att öka förekomsten och utbredningen av syrefria förhållanden, och därmed öka transporten av järn från mark till vatten. Det är därför troligt att brunifieringen kommer att fortgå även i framtiden. (Less)
Abstract
The term brownification refers to the trend of increasing water color, i.e. the water becoming browner, which has been observed throughout the northern hemisphere over the last decades. Brownification has both ecological and societal implications. From an ecological point of view the impaired light climate may e.g. reduce aquatic primary production and affect predator-prey interactions within the water, while from a societal point of view brownification may obstruct drinking water purification and reduce the recreational value of lakes.



Traditionally, the increasing water color has been ascribed to increasing concentrations of dissolved organic matter (DOM) from the catchment, as DOM concentrations and water color often... (More)
The term brownification refers to the trend of increasing water color, i.e. the water becoming browner, which has been observed throughout the northern hemisphere over the last decades. Brownification has both ecological and societal implications. From an ecological point of view the impaired light climate may e.g. reduce aquatic primary production and affect predator-prey interactions within the water, while from a societal point of view brownification may obstruct drinking water purification and reduce the recreational value of lakes.



Traditionally, the increasing water color has been ascribed to increasing concentrations of dissolved organic matter (DOM) from the catchment, as DOM concentrations and water color often correlate both spatially and temporally. Several mechanisms have been proposed as the driver behind the increasing DOM concentration, e.g. decreasing acidification, land-use changes and climate change with increasing precipitation and temperature. Interestingly, in many cases the water color has increased more than the DOM concentration, implying that increasing DOM concentration alone is not sufficient to explain the increase in water color. Thus, there must be other factors also affecting the water color.



In this thesis I show, in a field experiment, that lower acid load results in a higher net charge of the organic matter in soils and thereby a higher solubility and mobility, which should facilitate a higher transport of DOM from the terrestrial to the aquatic system. Moreover, concurrent with the increase in mobility, there was a change in the quality of the DOM, where DOM from a lower acid load was relatively more colored, aromatic and of higher molecular weight. Thus, a reduction in acid load may contribute to brownification by increasing the export and the color of terrestrially derived DOM to the aquatic system. Experiments were performed to test if the altered quality of the mobile soil DOM may affect its reactivity in the aquatic system. It was found that the susceptibility to photodegradation increases, while the susceptibility to bacterial degradation decreases. The relative importance of each turnover process may hence be altered due to the decreasing acidification.



Another factor affecting the water color of freshwaters is the concentration of iron (Fe) in the water. Still, the potential role of Fe to brownification has not previously been addressed. Data from long-term monitoring showed that water color of most Swedish rivers have increased significantly since the early 1970’s. More surprisingly, most rivers also exhibit strongly increasing iron concentrations (up to 470 %). Increases is DOM concentration were significantly lower than the increase in water color and theoretically, variations in Fe concentration could explain on average 25 % and up to 75 % of increasing water color.



Fe plays a key role in aquatic systems, affecting the biogeochemical cycling of many major elements, such as carbon, nitrogen and phosphorus. Thus the increasing Fe concentrations may have profound consequences. By analyzing within-year variations in water chemistry, air temperature, and dis-

charge of three Swedish rivers, I explored what control Fe concentrations. It appears that variations in Fe concentrations are primarily driven by redox dynamics in the catchment. High discharge and high temperature create conditions that favor bacterial activities and reductive dissolution of the largely insoluble Fe(III) to the more soluble Fe(II). Fe(II) may then be transported from the soil to the aquatic system. Once in the oxic stream water, interactions with DOM maintain the Fe in solution. Furthermore, long-term trends of increasing air temperature and discharge in these catch-ments may have extended the periods of reducing conditions by increasing microbial activity and soil saturation, and thus have facilitated Fe transport to the aquatic system.



In summary, it appears that in south Sweden, where the acidification has been high historically, the impact of decreasing acidification on organic matter mobility may be a major factor behind brownification, whereas in the north where the acidification has been much more restricted, increasing Fe concentrations may be more important. Climate change with increasing precipitation and temperature may increase the prevalence of reducing conditions in the soils, further facilitating the export of Fe to the aquatic system and causing a continuous brownification of the freshwaters. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • PhD Evans, Chris, Centre for Ecology and Hydrology, Bangor, UK
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Brownification, freshwater, water color, dissolved organic matter (DOM), iron (Fe), decreasing acidification, climate change
pages
126 pages
publisher
Department of Biology, Lund University
defense location
Blue hall, Ecology building, Sölvegatan 37, Lund
defense date
2013-11-01 09:30
ISBN
978-91-7473-683-0
project
BECC
language
English
LU publication?
yes
id
81763278-b990-4b8f-ba60-c7fb8e36714f (old id 4076293)
date added to LUP
2013-10-07 10:59:26
date last changed
2016-09-19 08:45:13
@misc{81763278-b990-4b8f-ba60-c7fb8e36714f,
  abstract     = {The term brownification refers to the trend of increasing water color, i.e. the water becoming browner, which has been observed throughout the northern hemisphere over the last decades. Brownification has both ecological and societal implications. From an ecological point of view the impaired light climate may e.g. reduce aquatic primary production and affect predator-prey interactions within the water, while from a societal point of view brownification may obstruct drinking water purification and reduce the recreational value of lakes. <br/><br>
<br/><br>
Traditionally, the increasing water color has been ascribed to increasing concentrations of dissolved organic matter (DOM) from the catchment, as DOM concentrations and water color often correlate both spatially and temporally. Several mechanisms have been proposed as the driver behind the increasing DOM concentration, e.g. decreasing acidification, land-use changes and climate change with increasing precipitation and temperature. Interestingly, in many cases the water color has increased more than the DOM concentration, implying that increasing DOM concentration alone is not sufficient to explain the increase in water color. Thus, there must be other factors also affecting the water color. <br/><br>
<br/><br>
In this thesis I show, in a field experiment, that lower acid load results in a higher net charge of the organic matter in soils and thereby a higher solubility and mobility, which should facilitate a higher transport of DOM from the terrestrial to the aquatic system. Moreover, concurrent with the increase in mobility, there was a change in the quality of the DOM, where DOM from a lower acid load was relatively more colored, aromatic and of higher molecular weight. Thus, a reduction in acid load may contribute to brownification by increasing the export and the color of terrestrially derived DOM to the aquatic system. Experiments were performed to test if the altered quality of the mobile soil DOM may affect its reactivity in the aquatic system. It was found that the susceptibility to photodegradation increases, while the susceptibility to bacterial degradation decreases. The relative importance of each turnover process may hence be altered due to the decreasing acidification. <br/><br>
<br/><br>
Another factor affecting the water color of freshwaters is the concentration of iron (Fe) in the water. Still, the potential role of Fe to brownification has not previously been addressed. Data from long-term monitoring showed that water color of most Swedish rivers have increased significantly since the early 1970’s. More surprisingly, most rivers also exhibit strongly increasing iron concentrations (up to 470 %). Increases is DOM concentration were significantly lower than the increase in water color and theoretically, variations in Fe concentration could explain on average 25 % and up to 75 % of increasing water color. <br/><br>
<br/><br>
Fe plays a key role in aquatic systems, affecting the biogeochemical cycling of many major elements, such as carbon, nitrogen and phosphorus. Thus the increasing Fe concentrations may have profound consequences. By analyzing within-year variations in water chemistry, air temperature, and dis-<br/><br>
charge of three Swedish rivers, I explored what control Fe concentrations. It appears that variations in Fe concentrations are primarily driven by redox dynamics in the catchment. High discharge and high temperature create conditions that favor bacterial activities and reductive dissolution of the largely insoluble Fe(III) to the more soluble Fe(II). Fe(II) may then be transported from the soil to the aquatic system. Once in the oxic stream water, interactions with DOM maintain the Fe in solution. Furthermore, long-term trends of increasing air temperature and discharge in these catch-ments may have extended the periods of reducing conditions by increasing microbial activity and soil saturation, and thus have facilitated Fe transport to the aquatic system. <br/><br>
<br/><br>
In summary, it appears that in south Sweden, where the acidification has been high historically, the impact of decreasing acidification on organic matter mobility may be a major factor behind brownification, whereas in the north where the acidification has been much more restricted, increasing Fe concentrations may be more important. Climate change with increasing precipitation and temperature may increase the prevalence of reducing conditions in the soils, further facilitating the export of Fe to the aquatic system and causing a continuous brownification of the freshwaters.},
  author       = {Ekström, Sara},
  isbn         = {978-91-7473-683-0},
  keyword      = {Brownification,freshwater,water color,dissolved organic matter (DOM),iron (Fe),decreasing acidification,climate change},
  language     = {eng},
  pages        = {126},
  publisher    = {ARRAY(0x9c411f0)},
  title        = {Brownification of freshwaters - the role of dissolved organic matter and iron},
  year         = {2013},
}