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Productivity and carbon transfer in pelagic food webs in response to carbon, nutrients and light
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Some of the major problems we face today are human induced changes to the nitrogen (N), phosphorus (P) and carbon (C) cycles. Predicted increases in rainfall and temperature due to climate change, may also increase dissolved organic matter (DOM) inflows to freshwater ecosystems in the boreal zone. N, P, C and light, are essential resources that most often limit phytoplankton (PPr) and bacterial production (BP) in the pelagic zone of lakes. PPr and BP not only constitute the total basal C resource for the pelagic aquatic food web, but also influence ecosystem function and biogeochemical cycles.

In this thesis I studied how N, P, C and light affect the relative and absolute rates of PPr and BP, along a wide latitudinal and trophic gradient using published data, and in two in situ mesocosm experiments in a clear water oligotrophic lake. In the experiments I manipulated bottom-up drivers of production and top-down predation to examine how these factors interact to affect pelagic food web structure and function.

The most important predictors of PPr globally (Paper I) were latitude, TN, and lake shape. Latitude alone explained the most variation in areal (50%) and volumetric (40%) PPr. In terms of nutrients PPr was primarily N-limited and BP was P-limited. Therefore bacteria and phytoplankton were not directly competing for nutrients. BP:PPr was mostly driven by PPr, therefore light, N, temperature and other factors affecting PPr controlled this ratio. PPr was positively correlated with temperature, but not BP, consequently, higher temperatures may reduce BP:PPr and hence the amount of energy mobilised through the microbial food web on a global scale.

In papers II and III interaction effects were found between C-additions and top-down predation by young-of-the-year (YOY) perch. Selective predation by fish on copepods influenced the fate of labile C-addition, as rotifer biomass increased with C-addition, but only when fish were absent. Interaction effects between these top-down and bottom-up drivers were evident in middle of the food web, which is seldom examined in this type of study. Although the energy pathway from bacteria to higher consumers is generally longer than from phytoplankton to higher trophic levels, increased BP still stimulated the biomass of rotifers, calanoid copepods and YOY fish. However, this appeared to be mediated by intermediate bacterial grazers such as flagellates and ciliates.

Light was an important driver of crustacean zooplankton biomass (paper IV), but the light:nutrient hypothesis was inadequate to predict the mechanisms behind the decrease in zooplankton biomass at low light. Instead, it appeared that reduced edibility of the phytoplankton community under low light conditions and reduced BP most strongly affected zooplankton biomass. Thus, the LNH may not apply in oligotrophic lakes where PPr is primarily N-limited, Daphnia is rare or absent and mixotrophic phytoplankton are abundant.

N, P, C and light manipulations have very different effects on different parts of the pelagic food web. They influence the relative rates of PPr and BP, affect phytoplankton community composition, alter the biomass of higher trophic levels and change pathways of energy transfer through the pelagic food web. This thesis adds valuable information as to how major changes in these resources will affect food web structure and function under different environmental conditions and future climate scenarios.

Place, publisher, year, edition, pages
Umeå: Institutionen för ekologi, miljö och geovetenskap, Umeå universitet , 2011. , 33 p.
Keyword [en]
bacterial production, phytoplankton production, mesocosms, food webs, carbon, nutrients, light
National Category
Ecology
Research subject
Limnology
Identifiers
URN: urn:nbn:se:umu:diva-43467ISBN: 978-91-7459-191-0OAI: oai:DiVA.org:umu-43467DiVA: diva2:414096
Public defence
2011-05-26, KBC-huset, Stora Hörsalen, Umeå Universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Projects
Lake ecosystem response to environmental change
Available from: 2011-05-05 Created: 2011-05-02 Last updated: 2011-05-02Bibliographically approved
List of papers
1. Effects of nutrients and physical lake characteristics on bacterial and phytoplankton production: a meta-analysis
Open this publication in new window or tab >>Effects of nutrients and physical lake characteristics on bacterial and phytoplankton production: a meta-analysis
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We performed a meta-analysis compromising of field (300 studies) and experimental data (249 studies) from a wide range of lake trophic states and locations. We examined the effects of nitrogen (N), phosphorus (P), carbon (dissolved organic matter) (C (DOM)), temperature, latitude, and lake morphometry on the absolute and relative rates of primary production (PPr) and bacterial production (BP). We compared areal and volumetric rates of PPr, BP and BP:PPr, and differences between experimental and natural systems. Both field studies and experimental results showed tight agreement with regard to N and P as predictors of volumetric PPr and BP respectively. This was despite the large variation in study length, size and nutrient addition rates in experimental systems, and indicates that bacteria and phytoplankton do not seem to be competing for the same nutrients. Areal production measurements were more difficult to model and were more dependent on physical lake characteristics than nutrients. Temperature was positively correlated with PPr, but not with BP. BP:PPr was stable across experiments regardless of N, P, DOM, or glucose additions. In contrast, BP:PPr ratios varied greatly in the field data set and were highest in systems with low total N and at high latitudes. This pattern was driven by reduced PPr, not increased BP, therefore experimenters may need to manipulate PPr to change BP:PPr. Collectively, our results indicate that increased temperatures and N availability due to climate change will lead to higher PPr and lower BP:PPr, potentially decreasing the importance of energy mobilized through the microbial food web on a global scale.

Identifiers
urn:nbn:se:umu:diva-43462 (URN)
Available from: 2011-05-02 Created: 2011-05-02 Last updated: 2011-05-02Bibliographically approved
2. Bottom–up carbon subsidies and top–down predation pressure interact to affect aquatic food web structure
Open this publication in new window or tab >>Bottom–up carbon subsidies and top–down predation pressure interact to affect aquatic food web structure
2011 (English)In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 120, no 2, 311-320 p.Article in journal (Refereed) Published
Abstract [en]

Human impacts such as eutrophication, overexploitation and climate change currently threaten future global food and drinking water supplies. Consequently, it is important that we understand how anthropogenic resource (bottom–up) and consumer (top–down) manipulations affect aquatic food web structure and production. Future climate changes are predicted to increase the inputs of terrestrial dissolved organic carbon to lakes. These carbon subsidies can either increase or decrease total basal production in aquatic food webs, depending on bacterial competition with phytoplankton for nutrients. This study examines the effects of carbon subsidies (bottom–up) on a pelagic community exposed to different levels of top–down predation. We conducted a large scale mesocosm experiment in an oligotrophic clear water lake in northern Sweden, using a natural plankton community exposed to three levels of glucose addition (0, 420 and 2100 mg C l–1 total added glucose) and three levels of young-of-the-year perch Perca fluviatilis density (0, 0.56 and 2 individuals m–3). Bacterioplankton production doubled with glucose addition, but phytoplankton production was unaffected, in contrast to previous studies that have manipulated carbon, nutrients or light simultaneously. This suggests that carbon addition alone is not sufficient to reduce autotrophic production, at least in an oligotrophic lake dominated by mixotrophic phytoplankton. Larval perch grazing did not produce a classical trophic cascade, but substantially altered the species composition of crustacean zooplankton and ciliate trophic levels. Glucose addition increased the biomass of rotifers, thus potentially increasing energy transfer through the heterotrophic pathway, but only when fish were absent. This study illustrates that changes in community structure due to selective feeding by top-predators can determine the influence of bottom–up carbon subsidies.

Place, publisher, year, edition, pages
John Wiley, 2011
Identifiers
urn:nbn:se:umu:diva-39554 (URN)10.1111/j.1600-0706.2010.18683.x (DOI)
Available from: 2011-02-01 Created: 2011-02-01 Last updated: 2016-05-19Bibliographically approved
3. Transfer of bacterial production based on labile carbon to higher trophic levels in an oligotrophic pelagic system
Open this publication in new window or tab >>Transfer of bacterial production based on labile carbon to higher trophic levels in an oligotrophic pelagic system
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

It is debatable whether bacterial production (BP) based on labile carbon (C) is an important energy subsidy for higher trophic levels in the pelagic zone of lakes. Increased BP may reduce phytoplankton and basal production through competition for phosphorus. However, enhanced BP can also be regarded  as an additional basal food resource used directly by unselective filter feeding (cladocerans) or indirectly through grazing on the microbial food web (cladocerans and copepods). In a mesocosm experiment we traced the contribution of BP to crustacean zooplankton and planktivorous fish using stable isotopes and labile glucose-C as a biomarker. BP increased with glucose-C addition and all zooplankton and fish incorporated some glucose-C. Although cladocerans incorporated the most glucose-C, increased BP did not affect cladoceran biomass. Instead, calanoid copepod biomass increased with glucose addition. This suggests that the ability to select high quality food such as bacterial grazing protists capable of trophic upgrading (i.e. de novo synthesis of fatty acids), had a stronger positive effect on calanoids, than unselective grazing on bacteria and protists had on cladoceran biomass. Higher BP was associated with increased survival and population growth of young-of-the-year perch when stocked at high densities, which suggested that BP had a density dependant effect on fish growth. Although the total amount of energy mobilized did not affect fish growth, energy mobilized through the microbial food chain increased calanoid copepod biomass; the favored prey species of planktivorous fish in this system.

Identifiers
urn:nbn:se:umu:diva-43465 (URN)
Available from: 2011-05-02 Created: 2011-05-02 Last updated: 2011-05-02Bibliographically approved
4. Lower bacterial production and phytoplankton edibility reduces crustacean zooplankton biomass at low light
Open this publication in new window or tab >>Lower bacterial production and phytoplankton edibility reduces crustacean zooplankton biomass at low light
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Anthropogenic changes in the nitrogen (N), phosphorus (P), and carbon (C) cycles have altered nutrient concentrations and the light climate in freshwaters globally. These factors influence phytoplankton (PPr) and bacterial production (BP), which constitute the basal energy resource for higher trophic levels in the pelagic zone of lakes. The light:nutrient hypothesis (LNH) predicts that although basal production will decrease at low light, seston C:nutrient ratios also decrease, thus increasing food quality for crustacean zooplankton, which tend to have relatively fixed C:nutrient ratios. We tested the LNH in a mesocosm experiment by examining the effects of N, P and C additions and shading on PPr, BP, seston C:nutrient ratios and zooplankton biomass in an oligotrophic clear water lake. We found that zooplankton biomass was strongly reduced in shaded treatments. Although PPr was unaffected by shading, BP decreased with shading. Bacteria can be an important energy and P source for zooplankton when mobilized through intermediate trophic levels, and correlations between BP, bacterial biomass, ciliates and zooplankton support this. Seston C:nutrient ratios were not affected by shading, possibly due to a high abundance of mixotrophic phytoplankton across treatments. Shading shifted the phytoplankton community towards low light adapted, but potentially less edible phytoplankton species, such as colony forming Dinobryon (Chrysophyta) and gymnoid (Dinoflagellata) taxa, which were negatively correlated with zooplankton biomass. Thus, the LNH may be inadequate when predicting changes in crustacean zooplankton biomass in response to light and nutrients in oligotrophic systems, where Daphnia is rare and mixotrophic phytoplankton are abundant.

Identifiers
urn:nbn:se:umu:diva-43466 (URN)
Available from: 2011-05-02 Created: 2011-05-02 Last updated: 2011-05-02Bibliographically approved

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