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- Lefébure, Robert, 1980-
(författare)
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Effects of temperature and terrestrial carbon on fish growth and pelagic food web efficiency
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Both temperature and terrestrial dissolved organic carbon (TDOC) have strong impacts on aquatic food web dynamics and production. Temperature affects vital rates of all organisms and terrestrial carbon has been shown to alter the dynamics of phytoplankton and bacterial production and affect the trophic structure of planktonic food webs. As climate change predictions for the Baltic Sea suggests future increases in both terrestrial carbon run-off and increases in temperature, the aim of thesis was to adopt a system-ecological approach and study effects of these abiotic variables, not only on interactions within planktonic food webs, but also on the growth and consumption rates of one of the most common zooplanktivorous fish in the Baltic Sea, the three-spined stickleback Gasterosteus aculeatus. Results showed that three-spined sticklebacks display a high degree of resilience against increasing temperatures, as both growth rates as well as consumption rates on zooplankton were high at temperatures well over 20 °C. Furthermore, it was shown that the minimal resource densities required to sustain individual and population growth, actually decreased with increasing temperatures, implying that sticklebacks around their optimum temperature for growth at 21 °C will actually have an increased scope for growth. As stickleback population densities have increased over the last decade in the Baltic Sea and are now suggested to out-compete other coastal fish species for shared zooplankton resources, the results presented in this thesis suggest that increased water temperatures would only serve to increase sticklebacks competitive advantage. As the structuring role of this small zooplanktivore on pelagic communities might be considerable, further studies investigating competitive interactions as well as patterns of population abundances are definitely warranted. TDOC was overall shown to stimulate bacterial production and the microbial food web. Because of the longer trophic pathways required to transport carbon from bacterial production to higher trophic levels, the addition of TDOC always reduced food web transfer efficiency. However, it became apparent that the full effect of TDOC additions on pelagic food webs was complex and depended heavily not only on the existing trophic structure to which the carbon was introduced, but also on ambient temperature levels. When three-spined sticklebacks were part of food webs with significant TDOC inputs, the presence of fish, indirectly, through predator release of lower trophic levels, amplified the magnitude of the effects of carbon addition on bacterial production, turning the base of the system significantly more heterotrophic, which ultimately, impacted negatively on their own production. However, when a pelagic food web containing sticklebacks was simultaneously subjected to realistic increases in temperature and TDOC concentrations, food web efficiency and fish production increased compared to present day conditions. These results were explained by a temperature dependent increased production potential of zooplankton, sustained by an increased production of heterotropic microzooplankton via TDOC additions, which lead to higher fish production. Although the increased number of trophic linkages in heterotrophic food webs should have reduced energy transfer efficiency, these negative effects seem here to have been overridden by the positive increases in zooplankton production as a result of increased temperature. These results show that heterotrophic carbon transfer can be a viable pathway to top-consumers, but also indicates that in order to understand the full effects of climate change on trophic dynamics and fish production, abiotic variables cannot be studied in isolation.
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| 2. |
- Dahlgren, Kristin, 1979-
(författare)
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Food web structures and carbon transfer efficiencies in a brackish water ecosystem
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two differently structured food webs can be distinguished in the pelagic habitat of aquatic systems; the classical one (autotrophic) with phytoplankton as a base and the microbial food web (heterotrophic) with bacteria as a base. Energy (produced at the basal trophic level) reaches higher trophic levels, i.e. zooplankton, directly in the classical food web in contrast to the microbial food web where it passes through additional trophic levels before reaching zooplankton. Energy is lost between each trophic level and therefore less energy should reach higher trophic levels in the microbial food web than in the classical food web. However, factors such as edibility of prey, temperature and properties of the predator, might also influence the food web structures and functions.In this thesis I studied which factors are important for an efficient carbon transfer and how a potential climate change might alter the food web efficiency in pelagic and pelagic-benthic food webs in the Baltic Sea. Furthermore, one of the most dominant zooplankton in the northern Baltic Sea, Limnocalanus macrurus, was studied in order to establish the seasonal pattern of lipid reserves in relation to food consumption.My studies showed that the carbon transfer efficiency during summer was not directly connected to the basal production, but factors such as the ratio between heterotrophs and autotrophs, the relationship between cladocerans and calanoid copepods and the size and community structure of both phytoplankton and zooplankton were important for the carbon transfer efficiency. In a climate change perspective, the temperature as well as the relative importance of the microbial food web is likely to increase. A temperature increase may have a positive effect on the pelagic food web efficiency, whereas increasing heterotrophy will have a negative effect on the pelagic and pelagic-benthic food web efficiency, reduce the fatty acid content of zooplankton and reduce the individual weight of both zooplankton and the benthic amphipod Monoporeia affinis. During the seasonal study on the calanoid copepod L. macrurus, I found that this species is mainly a carnivore, feeding on mesozooplankton during most of the year but switches to feeding on phytoplankton when these are abundant. Furthermore, when food is scarce, it utilizes lipids that are built up during the course of the year.From these studies I can draw some major conclusions; there are many factors that influence how efficient carbon is transferred in the food web and different factors are probably of various importance in different areas. In order to determine the carbon transfer efficiency, the various strategies exerted by different organism groups have to be considered, as for example that some zooplankton utilize lipid reserves instead of feeding all year around. Also, in a climate change perspective, the pelagic-benthic food web efficiency will decrease, as will the quality of zooplankton and M. affinis, possibly having implications for higher trophic levels such as fish.
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| 3. |
- Md Amin, Roswati, 1983-
(författare)
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Copepods in Skeletonema-dominated food webs : Toxicity and nutritional quality as factors controlling copepod-diatom interactions
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- My thesis focuses on copepod-diatom interactions, specifically on the effects of food quality and toxicity on copepod feeding, reproductive success and behavior but as a frame, also includes a quantitative evaluation of copepod carbon requirements compared to other trophic plankton groups. My aim was to evaluate the function of copepods in diatom-dominated spring blooms. I thereby used a series of mesocosm and laboratory experiments. For a realistic extrapolation of the results to natural environments I used different strains of a diatom species, Skeletonema marinoi, which is a common spring blooming species in the Baltic Sea. This species is known to produce polyunsaturated aldehydes (PUA; mainly heptadienal, octadienal and decadienal), which have previously been identified as the potential reasons for the detrimental effects of diatoms on copepod reproduction. All strains varied in size, mineral and biochemical content, and PUA production. I tested the effects on different dominant copepod species from northern temperate waters; Acartia sp. (A. clausi and A. tonsa), Calanus finmarchicus, Pseudocalanus elongatus, and Temora longicornis, as well as the dominant species in the northern Baltic Sea, Eurytemora affinis. The specific contributions of respiratory carbon requiment of mesozooplankton and lower size fractions to carbon cycling during PUA-producing diatom blooms are poorly documented. My results show that nanoplankton and microzooplankton dominated the carbon cycling (> 50% of primary production) whereas the contribution of bacterioplankton varied. Mesozooplankton was always of minor importance with contribution of <6% of primary production. This illustrates the importance of lower size fractions during a phytoplankton spring bloom. Irrespective of their small contribution to the total community carbon cycling, copepods displayed non-selective and typically high feeding rate on different PUA-producing S. marinoi strains, indicating that there was no feeding deterrence. The effect of feeding on copepod reproductive success, however, varied between different strains, and depending on copepod species. In experiments with monospecific diatom diets reduced egg production rate and hatching success were mainly related to food quality measured as fatty acids and sterols, or algae growth rate, low assimilation efficiency or PUA production / ingestion. On the other hand, copepod reproduction and population development in the diverse diet, including a high concentration of S. marinoi and PUA (both particulate and dissolved), increased with increasing food concentration and was unaffected by the presence of toxic diatoms. I conclude that although a negative correlation between different reproductive variables and PUA production / ingestion may sometimes be observed in laboratory incubations, this is highly dependent on the strain / species used, and the effect of the algal strain can be stronger than the effect of the e.g., growth-stage dependent PUA production. Although copepod grazing might not be very important during a diatom spring bloom, even a highly PUA-producing S. marinoi can be considered an appropriate food source for copepods when occurring among the natural food assemblage, inducing a high reproductive output.
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| 4. |
- Wikner, Johan, Professor, 1961-, et al.
(författare)
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Report on boundary conditions for winter mesocosms
- 2023
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Ongoing climate change is projected to extend the warmer and therefore the biologically productive season, reducing ice cover, ice thickness, and quality, potentially influencing biodiversity, and productivity of aquatic ecosystems. Changed influence of dissolved organic matter is one factor that can contribute to those effects. Winter ecology is little studied, and the advancement of knowledge would benefit from controlled experiments on the mesocosm scale. To investigate the capability of mesocosm experimental infrastructures for winter ecological research, a 5-months long experiment during the sub-arctic winter in 2021/2022 was conducted in Umeå, Sweden. Simultaneously, the performance of an outdoor and indoor mesocosm facility with ice-forming capability at the same site was compared. Boundary conditions for hydrographic, chemical, and biological variables were determined.The facilities were operated successfully over winter and treatments caused similar effects in both systems, despite some differences presented below. Salinity and temperature were similar between the facilities throughout the experiment. Ice was markedly thicker on the sea compared to in the indoor facility. Further the ice inside the outdoor mesocosms, was significantly thicker than on the surrounding natural sea. Light irradiance indoors correlated with the outdoor facility, but light irradiance indoors could not reach the outside values in the lightest months of the experiment (after mid-March). Both dissolved organic carbon and dissolved nitrogen was higher in the outdoor facility, possibly caused by a pump effect increasing organic carbon and nitrogen concentrations. Most other nutrient levels remained similar. Chlorophyll-a was comparable between the facilities, while plankton respiration was twice the rate outdoors compared to indoors. Two substances were used to simulate browning, HuminFeed® (a commercially available leonardite) and soil extract, causing similar treatment effects in both facilities for 75% of measured variables. HuminFeed caused a marked increase in CDOM (coloured dissolved organic matter) and nitrite during spring. Treatment with soil extract resulted in slightly higher phosphorus concentrations.The indoor mesocosm facility was thus comparable to the outdoor facility regarding experimental effects, despite facility differences observed. The organic matter sources HuminFeed and soil extract differ in some experimental effects that need to be considered. These results should provide basic knowledge for improving experimental design in future winter mesocosm studies.
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