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Nitrogen effects on aquatic food web efficiency in the pelagic zone of unproductive lakes along a gradient of dissolved organic carbon
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.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Arcum)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Atmospheric nitrogen (N) deposition and terrestrial dissolved organic carbon (DOC) loadings are increasing in northern boreal lakes. However, consequences of increased N availability on food web efficiency (FWE) and consumer growth in N limited unproductive boreal lakes are unclear. Here, we performed in situ mesocosm experiments in late summer (2011; 2013) in six unproductive boreal Swedish lakes, paired across a DOC gradient, with one lake in each pair fertilized with N (2011: reference year; 2012, 2013: impact years). We assessed how zooplankton growth and FWE were affected by changes in pelagic energy mobilization (PEM), food chain length (PP:BP, i.e. phytoplankton: bacterial production ratio), and food quality (seston stoichiometry) in response to N fertilization. Although PP, PEM and PP:BP increased in low and medium DOC lakes after N fertilization, consumer growth and FWE in the low DOC lake were reduced, potentially due to a reduction in phytoplankton food quality (increased C:P; N:P). At high DOC, N fertilization caused modest increases in PP and PEM, with marginal changes in PP:BP and phytoplankton food quality, which combined led to a slight increase in zooplankton growth and FWE. We conclude that the background lake DOC level is critical in order to infer effects of enhanced inorganic N availability on pelagic productivity and FWE. In clear lakes increased N deposition will decrease FWE due to mismatches in food quality demand and supply. In humic lakes this mismatch will not occur, wherefore and zooplankton production and FWE will increase slightly following enhanced N deposition.

Keyword [en]
boreal lakes, climate change, nitrogen deposition, trophic transfer efficiency, zooplankton
National Category
Ecology Environmental Sciences
Research subject
Limnology
Identifiers
URN: urn:nbn:se:umu:diva-130338OAI: oai:DiVA.org:umu-130338DiVA: diva2:1066137
Funder
Swedish Research Council, 621-2010-4675
Available from: 2017-01-17 Created: 2017-01-17 Last updated: 2017-01-23
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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CiteExportLink to record
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Citation style
  • apa
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