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Effects of inorganic nitrogen and organic carbon on pelagic food webs in boreal lakes
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Arcum)
2017 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Effekter av oorganiskt kväve och organiskt kol på pelagiska födovävar i boreala sjöar (Swedish)
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 [en]
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: urn:nbn:se:umu:diva-130340ISBN: 978-91-7601-640-4 (print)OAI: oai:DiVA.org:umu-130340DiVA: diva2:1066248
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
List of papers
1. Food quantity and quality in unproductive clear water and humic lakes and consequences for pelagic mesozooplankton
Open this publication in new window or tab >>Food quantity and quality in unproductive clear water and humic lakes and consequences for pelagic mesozooplankton
(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
DOC, phytoplankton production, temperature, light, seston stoichiometry
National Category
Ecology Environmental Sciences
Research subject
Limnology
Identifiers
urn:nbn:se:umu:diva-130335 (URN)
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
2. Pelagic food web response to whole lake N fertilization
Open this publication in new window or tab >>Pelagic food web response to whole lake N fertilization
Show others...
2017 (English)In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 62, no 4, 1498-1511 p.Article in journal (Refereed) Published
Abstract [en]

Anthropogenic activities are increasing inorganic nitrogen (N) loadings to unproductive boreal lakes. In many of these lakes phytoplankton are N limited, consequently N fertilization may affect ecosystem productivity and consumer resource use. Here, we conducted whole lake inorganic N fertilization experiments with six small N limited unproductive boreal lakes (three control and three N enriched) in an area receiving low N deposition with one reference and two impact years. Our aim was to assess the effects of N fertilization on pelagic biomass production and consumer resource use. We found that phytoplankton primary production (PP) and biomass, and the PP: bacterioplankton production ratio increased after fertilization. As expected, the relative contribution of phytoplankton derived resources (autochthony) that supported the crustacean zooplankton community increased. Yet, the response in the consumer community was modest with autochthony only increasing in one of the three major zooplankton groups and with no effect on zooplankton biomass. In conclusion, our findings imply that newly available phytoplankton energy derived from N fertilization was not efficiently transferred up to zooplankton, indicating a mismatch between producer energy supply and consumer energy use with potential accumulation of phytoplankton biomass as the result.

Keyword
autochthony, basal production, boreal, dissolved organic carbon, limitation, nitrate, phytoplankton, zooplankton
National Category
Ecology Environmental Sciences
Research subject
Limnology
Identifiers
urn:nbn:se:umu:diva-130332 (URN)10.1002/lno.10513 (DOI)000404993100013 ()
Funder
Swedish Research Council, 621-2010-4675
Available from: 2017-01-17 Created: 2017-01-17 Last updated: 2017-08-02Bibliographically approved
3. Phytoplankton response to whole lake inorganic N fertilization along a gradient in dissolved organic carbon
Open this publication in new window or tab >>Phytoplankton response to whole lake inorganic N fertilization along a gradient in dissolved organic carbon
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.

Keyword
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:nbn:se:umu:diva-130336 (URN)10.1002/ecy.1758 (DOI)000398175200011 ()28144934 (PubMedID)
Funder
Swedish Research Council, 621-2010-4675
Available from: 2017-01-17 Created: 2017-01-17 Last updated: 2017-06-15Bibliographically approved
4. Nitrogen effects on aquatic food web efficiency in the pelagic zone of unproductive lakes along a gradient of dissolved organic carbon
Open this publication in new window or tab >>Nitrogen effects on aquatic food web efficiency in the pelagic zone of unproductive lakes along a gradient of dissolved organic carbon
(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
boreal lakes, climate change, nitrogen deposition, trophic transfer efficiency, zooplankton
National Category
Ecology Environmental Sciences
Research subject
Limnology
Identifiers
urn:nbn:se:umu:diva-130338 (URN)
Funder
Swedish Research Council, 621-2010-4675
Available from: 2017-01-17 Created: 2017-01-17 Last updated: 2017-01-23

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