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Food quantity and quality in unproductive clear water and humic lakes and consequences for pelagic mesozooplankton
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.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

1. Small oligotrophic humic lakes are the most common type of lake in the boreal zone and are predicted to become more abundant with climate change. Humic lakes generally have lower whole lake primary and consumer production than clear water lakes, but research is inconclusive as to what controls pelagic production in these lakes.

2. We compared food quantity (primary production (PP) and phytoplankton biomass, pelagic energy mobilization, seston carbon (C)) and food quality (phytoplankton edibility, seston stoichiometry) for crustacean mesozooplankton in four humic (> 15 mg L-1 DOC) and four clear water lakes.

3. We found that PP was over three times higher in clear water lakes, and was controlled by nutrient concentrations and temperature, whereas in humic lakes PP was dependant on light availability. Nevertheless, total food quantity (i.e. pelagic energy mobilization, seston C concentrations) was similar between lake types, even though food composition differed. In humic lakes bacterial production based on allochthonous C contributed three times more to total pelagic energy mobilization and seston C consisted of 20% less phytoplankton biomass.

4. Food composition did not have any effect on total zooplankton biomass, however, cladoceran: copepod biomass ratios increased with DOC concentration, both in our lakes and in a subset of northern lakes from the Swedish lake monitoring program.

5. Our results imply that increased DOC concentrations in boreal lakes will reduce PP and phytoplankton biomass and can alter food composition. However, browning of boreal lakes is unlikely to reduce pelagic energy mobilization or total zooplankton biomass, but can promote a higher dominance of cladocerans relative to copepods, which may have consequences for pelagic planktivorous predators.

Keyword [en]
DOC, phytoplankton production, temperature, light, seston stoichiometry
National Category
Ecology Environmental Sciences
Research subject
Limnology
Identifiers
URN: urn:nbn:se:umu:diva-130335OAI: oai:DiVA.org:umu-130335DiVA: diva2:1066128
Funder
Swedish Research Council, 621-2010-4675Swedish Research Council, 637-2013-7449
Available from: 2017-01-17 Created: 2017-01-17 Last updated: 2017-01-20
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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