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Effects of dissolved humic matter on phytoplankton

von Einem, Jessica LU (2011)
Abstract (Swedish)
Popular Abstract in Swedish

Många sjöar i Sverige och i andra delar av den norra hemisfären blir allt brunare. Detta beror på att mer färgat organiskt material (humösa ämnen) sköljs ut från den omkring liggande marken och transporteras sen in i sjöarna. I min avhandling har jag undersökt om detta leder till att det blir mörkare i sjöarna, och hur detta i så fall påverkar växtplankton, dvs. små mikroalger som finns fritt fördelade i vattnet. I min första studie fann jag att det som väntat är mörkare i bruna sjöar jämfört med klara sjöar. Storleken på sjön är dock också viktig, eftersom temperatursprångskiktet ligger djupare i större sjöar. Under sommaren är sjöar skiktade och det finns då ett övre varmt skikt där... (More)
Popular Abstract in Swedish

Många sjöar i Sverige och i andra delar av den norra hemisfären blir allt brunare. Detta beror på att mer färgat organiskt material (humösa ämnen) sköljs ut från den omkring liggande marken och transporteras sen in i sjöarna. I min avhandling har jag undersökt om detta leder till att det blir mörkare i sjöarna, och hur detta i så fall påverkar växtplankton, dvs. små mikroalger som finns fritt fördelade i vattnet. I min första studie fann jag att det som väntat är mörkare i bruna sjöar jämfört med klara sjöar. Storleken på sjön är dock också viktig, eftersom temperatursprångskiktet ligger djupare i större sjöar. Under sommaren är sjöar skiktade och det finns då ett övre varmt skikt där växtplanktonen lever. Om detta övre skikt blir tjockare genom att temperatursprångskiktet ligger djupare så försämras ljusklimatet i sjön. I två olika experiment kunde jag bekräfta att ljusförhållandena i bruna sjöar påverkar växtplankton i stor utsträckning. När det inte finns tillräckligt med ljus i sjön produceras också mindre växtplanktonbiomassa. Vid ljusbrist p g a hög tillförsel av humusämnen från land producerar växtplankton också mindre syrgas och respirationen blir större än fotosyntesen, varvid sjöarna avger koldioxid till atmosfären. Däremot har jag inte hittat några samband mellan brunhet och artsammansättningen av växtplanktonsamhället. Det fanns alltså inga arter som gynnades eller missgynnades speciellt i bruna sjöar. Sammanfattningsvis har jag med min forskning visat att det verkar troligt att mängden växtplankton kan minska i sjöarna när vattnet blir brunare. Detta innebär att det finns mindre mat för växtätande organismer, och i förlängningen kan detta även leda till att det produceras färre och mindre fiskar i sjöarna. (Less)
Abstract
Water colour is currently increasing in thousands of lakes in the northern hemisphere due to an increased input of terrestrial dissolved organic carbon (DOC), and more specifically coloured disscolved humic matter (DHM). I studied how water colour affects the light climate in the epilimnion and, as a consequence, phytoplankton biomass, species composition and production in lakes in southern Sweden. In my first study, I found that epilimnion depth (ze) did not, contrary to my expectations, decrease with increasing water colour. Instead, ze increased when fetch (the distance wind can blow uninterrupted over a lake, calculated as the square root of lake area) increased, independently of water colour. Consequently, light intensities in the... (More)
Water colour is currently increasing in thousands of lakes in the northern hemisphere due to an increased input of terrestrial dissolved organic carbon (DOC), and more specifically coloured disscolved humic matter (DHM). I studied how water colour affects the light climate in the epilimnion and, as a consequence, phytoplankton biomass, species composition and production in lakes in southern Sweden. In my first study, I found that epilimnion depth (ze) did not, contrary to my expectations, decrease with increasing water colour. Instead, ze increased when fetch (the distance wind can blow uninterrupted over a lake, calculated as the square root of lake area) increased, independently of water colour. Consequently, light intensities in the epilimnion were in general lowest in large lakes with high water colour. Values for average light intensities in the epilimnion (Ē%) suggested that light may be a limiting factor for phytoplankton in some lakes. This was confirmed in an experimental study, where small volumes of lake water were incubated at different depths in lakes. Decreasing light intensities, i.e. increasing incubation depths, led to decreasing phytoplankton biomass (chlorophyll a) even when nutrient availability was sufficient to support higher biomasses. However, effects of water colour on phytoplankton species composition in this study were minor. In my third study, I investigated specifically differences between phytoplankton communities in lakes in a water colour gradient using phytoplankton pigments to identify groups of phytoplankton. Only one phytoplankton pigment, chlorophyll c2 (affiliated with dinoflagellates, diatoms and chrysophytes), was affected directly by water colour, but Ē % and ze influenced the phytoplankton communities of the lakes. Finally, a mesocosm study revealed that primary production decreased with increasing water colour. In this study, increasing water colour was not accompanied by an increase in DOC, which explains why (bacterial) respiration did not increase as expected. If both production decreases and respiration increases as a consequence of increasing water colour in the future, lakes will become more heterotroph and thus emit more CO2 to the atmosphere. This effect may be enhanced by increasing temperature. In my mesocosm study, I found that respiration increased with increasing temperature but production did not. There were also no interaction effects of warming and browning on production and respiration. In summary, the results from these studies imply that increasing water colour is likely to lead to lower phytoplankton biomass and production in lakes in southern Sweden in the future. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Docent Bergström, Ann-Kristin, Department of Ecology and Environmental Sciences, Umeå University, Sweden
organization
publishing date
type
Thesis
publication status
published
subject
keywords
phytoplankton, PAR, DOC, Water colour, production
pages
104 pages
defense location
Nedre hörsalen, Palaestra
defense date
2011-03-03 09:30
ISBN
978-91-7473-089-0
language
English
LU publication?
yes
id
5e9d91eb-021a-491e-b342-e823a513e402 (old id 1782585)
date added to LUP
2011-02-04 11:48:09
date last changed
2016-09-19 08:45:18
@phdthesis{5e9d91eb-021a-491e-b342-e823a513e402,
  abstract     = {Water colour is currently increasing in thousands of lakes in the northern hemisphere due to an increased input of terrestrial dissolved organic carbon (DOC), and more specifically coloured disscolved humic matter (DHM). I studied how water colour affects the light climate in the epilimnion and, as a consequence, phytoplankton biomass, species composition and production in lakes in southern Sweden. In my first study, I found that epilimnion depth (ze) did not, contrary to my expectations, decrease with increasing water colour. Instead, ze increased when fetch (the distance wind can blow uninterrupted over a lake, calculated as the square root of lake area) increased, independently of water colour. Consequently, light intensities in the epilimnion were in general lowest in large lakes with high water colour. Values for average light intensities in the epilimnion (Ē%) suggested that light may be a limiting factor for phytoplankton in some lakes. This was confirmed in an experimental study, where small volumes of lake water were incubated at different depths in lakes. Decreasing light intensities, i.e. increasing incubation depths, led to decreasing phytoplankton biomass (chlorophyll a) even when nutrient availability was sufficient to support higher biomasses. However, effects of water colour on phytoplankton species composition in this study were minor. In my third study, I investigated specifically differences between phytoplankton communities in lakes in a water colour gradient using phytoplankton pigments to identify groups of phytoplankton. Only one phytoplankton pigment, chlorophyll c2 (affiliated with dinoflagellates, diatoms and chrysophytes), was affected directly by water colour, but Ē % and ze influenced the phytoplankton communities of the lakes. Finally, a mesocosm study revealed that primary production decreased with increasing water colour. In this study, increasing water colour was not accompanied by an increase in DOC, which explains why (bacterial) respiration did not increase as expected. If both production decreases and respiration increases as a consequence of increasing water colour in the future, lakes will become more heterotroph and thus emit more CO2 to the atmosphere. This effect may be enhanced by increasing temperature. In my mesocosm study, I found that respiration increased with increasing temperature but production did not. There were also no interaction effects of warming and browning on production and respiration. In summary, the results from these studies imply that increasing water colour is likely to lead to lower phytoplankton biomass and production in lakes in southern Sweden in the future.},
  author       = {von Einem, Jessica},
  isbn         = {978-91-7473-089-0},
  keyword      = {phytoplankton,PAR,DOC,Water colour,production},
  language     = {eng},
  pages        = {104},
  school       = {Lund University},
  title        = {Effects of dissolved humic matter on phytoplankton},
  year         = {2011},
}