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Phytoplankton response to whole lake inorganic N fertilization along a gradient in dissolved organic carbon
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Arcum)
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Arcum)
2017 (English)In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 98, no 4, 982-994 p.Article in journal (Refereed) Published
Abstract [en]

Global change has increased inorganic nitrogen (N) and dissolved organic carbon (DOC; i.e. ‘browning’) inputs to northern hemisphere boreal lakes. However, we do not know how phytoplankton in nutrient poor lake ecosystems of different DOC concentration respond to increased N availability. Here, we monitored changes in phytoplankton production, biomass and community composition in response to whole lake inorganic N fertilization in six boreal unproductive Swedish lakes divided into three lake pairs (control, N enriched) at three DOC levels (low, medium, high), with one reference year (2011) and two impact years (2012, 2013). We found that phytoplankton biomass and production decreased with DOC concentration before N fertilization. Further, phytoplankton community composition also differed with respect to DOC, with a dominance of non-flagellated autotrophs at low DOC towards an increasing dominance of flagellated autotrophs with increased lake DOC concentration. The N fertilization increased phytoplankton biomass and production in all lakes, but did not affect phytoplankton community composition. However, the net response in biomass and production to N fertilization declined with increasing DOC, implying that the lake DOC concentration is critical in order to infer phytoplankton responses to N fertilization, and that the system switches from being primarily nutrient limited to becoming increasingly light limited with increased DOC concentration. In conclusion, our results show that browning will reduce phytoplankton production and biomass and influence phytoplankton community composition, whereas increased inorganic N loadings from deposition, forestry or other land use will primarily enhance phytoplankton biomass and production. Together, any change in the landscape that enhances inorganic N availability will increase phytoplankton production and biomass, but the positive effects of N will be much weaker or even neutralized in browner lakes as caused by light limitation.

Place, publisher, year, edition, pages
2017. Vol. 98, no 4, 982-994 p.
Keyword [en]
boreal lakes, climate change, DOC, N deposition, N limitation, phytoplankton community composition, phytoplankton production, whole lake enrichment experiment
National Category
Ecology Environmental Sciences
Research subject
Limnology
Identifiers
URN: urn:nbn:se:umu:diva-130336DOI: 10.1002/ecy.1758ISI: 000398175200011PubMedID: 28144934OAI: oai:DiVA.org:umu-130336DiVA: diva2:1066135
Funder
Swedish Research Council, 621-2010-4675
Available from: 2017-01-17 Created: 2017-01-17 Last updated: 2017-06-15Bibliographically approved
In thesis
1. Effects of inorganic nitrogen and organic carbon on pelagic food webs in boreal lakes
Open this publication in new window or tab >>Effects of inorganic nitrogen and organic carbon on pelagic food webs in boreal lakes
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Effekter av oorganiskt kväve och organiskt kol på pelagiska födovävar i boreala sjöar
Abstract [en]

Anthropogenic activities are increasing inorganic nitrogen (N) loadings to lakes in the northern hemisphere. In many boreal lakes phytoplankton are N limited, wherefore enhanced N input may affect the productivity of pelagic food webs. Simultaneously, global change causes increased inflows of terrestrial dissolved organic carbon (DOC) to boreal lakes. Between clear and humic lakes, whole lake primary and consumer production naturally differs. However, research is inconclusive as to what controls pelagic production in these lakes. Further, it is unclear how DOC affects the response of the pelagic food web to enhanced inorganic N availability. The overarching goal of this thesis was to study the effects of inorganic N and organic C for pelagic food webs in boreal lakes. In the thesis, I first identified the main drivers of pelagic production during summer in eight non-manipulated Swedish boreal lakes with naturally low or high DOC. Then I investigated how increased N availability affects the pelagic food chain, and how the response differs with DOC. Therefore, whole lake inorganic N fertilization experiments were conducted in six Swedish boreal lakes across a DOC gradient (low, medium, high) divided into three lake pairs (control, N enriched) with one reference and two impact years. In each lake, I also investigated the response of zooplankton growth using in situ mesocosm experiments excluding planktivores. I found that humic boreal lakes had lower phytoplankton production and biomass than clear water lakes. Further, phytoplankton community composition and food quality differed with DOC. However, high DOC did not reduce pelagic energy mobilization or zooplankton biomass, but promoted a higher dominance of cladoceran relative to copepod species. N addition clearly enhanced phytoplankton biomass and production in the experimental lakes. However, this stimulating N effect decreased with DOC as caused by light limitation. Further, the newly available phytoplankton energy derived from N addition was not efficiently transferred to zooplankton, which indicates a mismatch between producer energy supply and consumer energy use. Indeed, the mesocosm experiment revealed that decreased food quality of phytoplankton in response to N addition resulted in reduced food web performance, especially in clearer lakes. In humic lakes, zooplankton production and food web efficiency were clearly more resilient to N addition. In summary, my thesis suggests that any change in the landscape that enhances inorganic N availability will especially affect pelagic food webs in clear water lakes. In contrast, brownification will result in more lakes being resilient to eutrophication caused by enhanced N deposition.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2017. 33 p.
Keyword
autochthony, basal production, boreal, global change, dissolved organic carbon, food web efficiency, N deposition, phytoplankton, seston stoichiometry, whole lake enrichment, zooplankton
National Category
Ecology Environmental Sciences Physical Geography
Research subject
Limnology
Identifiers
urn:nbn:se:umu:diva-130340 (URN)978-91-7601-640-4 (ISBN)
Public defence
2017-02-10, Älgsalen, Uminova Science Park, Umeå, 09:30 (English)
Opponent
Supervisors
Available from: 2017-01-20 Created: 2017-01-17 Last updated: 2017-02-17Bibliographically approved

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