| 1. |
- Albert, Séréna, 1992-
(författare)
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Benthic-pelagic coupling in a changing world : Structural and functional responses of microbenthic communities to organic matter settling
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Marine soft sediments form the second largest habitat on the planet. Organisms residing in this environment represent a vast reservoir of biodiversity, and play key roles in ecosystem processes. Most benthic organisms depend on organic matter (OM) inputs from phytoplankton in the overlying water column as food supply, but human impacts such as eutrophication and climate change are profoundly altering natural ecosystem dynamics. The consequences of changes in benthic-pelagic coupling for the biodiversity and functioning of soft-sediment communities have yet to be resolved. The aim of this thesis is to assess the role of OM settling on soft-sediments microeukaryotic (small organisms < 1 mm) and bacterial communities. The intents are two-fold, to investigate impacts on (1) community structure and diversity (chapters I, II and IV); and (2) ecosystem functioning, notably in relation to nitrogen (N) cycling (chapters I and III). Our results show that settling OM quantity and quality both had a significant impact on microeukaryotic alpha-diversity. We observed a decrease in alpha-diversity following settling of diatom-derived spring bloom OM, possibly as a result of competitive exclusion, while cyanobacteria-derived summer bloom OM did not affect alpha-diversity (chapters I and IV). We also found that high biomass of diatoms and others fast sinking phytoplankton groups in the water column led to lower microeukaryotic alpha diversity after this material settled on the seafloor (chapter IV). Presumably, following this large sedimentation event, sediment oxygen (O2) demand was strongly stimulated, excluding O2-sensitive taxa. Overall, we propose that the assembly of microeukaryotic communities was primarily mediated by OM settling quantity (chapter IV), while differences in OM quality led to significant but more subtle changes, occurring at fine taxonomic level (chapter I). The response of bacterial communities to OM settling was less pronounced, and probably restricted to the uppermost sediment layer (chapters I and IV). We did, however, observe a significant effect of OM quality on bacterial communities assembly at the sediment-water interface, with taxa favored either by diatom- or by cyanobacteria-derived OM (chapter II). This study also showed that feedback mechanisms from nutrient recycling in the sediment could play a role in this response. Finally, our results indicated a substantial influence of OM quality on N cycling at the sediment-water interface. We found that settling of fresh OM (i.e. low C:N ratio) stimulated denitrification activity (chapters I and III), while simultaneously promoting more N recycling to the water column than settling of degraded OM (i.e. high C:N ratio) did (chapter III). Altogether, our results indicate that current changes in OM settling dynamics in marine systems will likely impact microeukaryotic and, to some extent, bacterial biodiversity in soft sediments. Alterations in settling OM quality, in particular, may also affect crucial microbial processes involved in N cycling. This thesis highlights the importance of considering benthic-pelagic coupling mechanisms to better understand likely future changes in marine ecosystems.
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| 2. |
- Tarimo, Barnabas Alphonce, 1983-
(författare)
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Patterns of fish larvae and zooplankton distribution in mangrove-seagrass seascapes of East Africa
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mangroves and seagrass meadows create coastal seascapes acting as breeding and nursery grounds for many fish species in tropical areas. Despite increased awareness on the role of these coastal habitats to fish, less knowledge is given to fish larvae distribution and population dynamics, limiting the understanding of major drivers of fish population dynamics and fisheries resources management, particularly in understudied regions such as the Western Indian Ocean. This thesis aimed at understanding seasonal and spatial patterns of fish larvae populations and community composition in relation to key environmental variables and spatiotemporal patterns of zooplankton in mangrove-seagrass seascapes of coastal East Africa. To address the specific goals of the thesis, four studies were performed and presented (Papers I-IV). Paper I focused on seasonal patterns of fish larvae distribution across mangrove-seagrass seascapes at the scale of Zanzibar (Tanzania). In Paper II, we studied seasonal variability of fish larvae distribution and association to environmental characteristics in seagrass meadows (at the scale of East Africa). In Paper III, we investigated seasonal patterns of zooplankton abundance and community composition across mangrove-seagrass seascapes of Zanzibar. Paper IV addressed how zooplankton community composition and water physicochemical properties influence fish larvae abundances across monsoon seasons in mangrove-seagrass seascapes of Zanzibar.The findings of Paper I showed that abundance and richness of fish larvae varied across different months and among habitats (mangrove creeks, nearshore seagrass meadows and inshore seagrass meadows), whereas there were no significant differences between the southeast monsoon (SEM) and northeast monsoon (NEM) seasons. The abundance of fish larvae was particularly high during the SEM season in mangrove creeks at one of the two studied seascapes. Assemblage composition of fish larvae did not change over time in any habitat. The findings suggest that mangroves and seagrass meadows are connected systems with similarity in assemblage compositions. Paper II displayed significant seasonal variations in abundance and community composition of fish larvae in two of the studied areas. It also showed site-specific influences of temperature, salinity, chlorophyll a and current velocities, on fish larvae abundance. Paper III showed that zooplankton abundance and community composition strongly varied across the two monsoon seasons and among habitats, with a significant difference between mangrove creeks and two different seagrass habitats. There was little influence of key environmental parameters during both monsoon seasons, with the exception of salinity (during precipitation peaks in SEM) and chlorophyll a, which influenced zooplankton abundance in mangrove creeks (in the Chwaka Bay seascape). The findings suggest that local hydrodynamics and seasonal variations have strong influences on zooplankton abundance and community composition in mangrove-seagrass seascapes. Paper IV revealed that fish larval abundances and diversity were generally strongly influenced by salinity and water temperature. In the Chwaka Bay seascape, fish larvae abundances were positively associated with abundances of copepods and invertebrate larvae groups of zooplankton in both monsoons and all three habitats. In the Fumba seascape, fish larvae abundances were strongly (positively) associated with temperature, chlorophyll a and zooplankton diversity, and to some degree also (negatively) related to abundances of non-copepod zooplankton (e.g cirripeds larvae).This thesis comprehends essential information on how fish larvae vary in time and space of tropical coastal seascapes, and how these patterns relate to spatiotemporal distributions of zooplankton and contemporary environmental conditions. Area-specific information integrating the monsoonal influence and spatiotemporal dynamics should be considered in conservation efforts of fish larvae and their habitats in coastal East Africa.
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| 3. |
- Kari, Elina, 1986-
(författare)
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Light conditions in seasonally ice-covered waters : within the Baltic Sea region
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Seasonal ice cover is a major driver of seasonality in aquatic ecosystems in the Baltic Sea region. Ice cover influences the underwater light conditions directly by limiting the light transfer and indirectly by modifying the mixing and circulation under the ice. Light conditions and stratification are key factors controlling the onset of the phytoplankton spring bloom. Therefore, the seasonal ice cover has an important role in setting the time frames for the primary production and in influencing the seasonality of the ecological processes. This thesis investigates the optical properties of the ice cover and the bio-optical substances in the water column.Bio-optical substances, suspended particulate matter (SPM), Coloured dissolved organic matter (CDOM) and Chlorophyll-a (Chl-a), determine the availability and spectral distribution of light. Measuring turbidity is quick and easy compared to the gravimetrical determination of the SPM concentration. Paper I provides a new model to estimate the concentration of SPM from turbidity. The new SPM-turbidity model predicts SPM concentrations well, despite the high CDOM absorption and the optical differences in the coastal northwestern and southeastern Baltic proper. Therefore, the new SPM-turbidity model offers a cost-effective and reliable method to monitor SPM concentration.The light transfer through the snow and ice cover was studied both in freshwater lake ice and in brackish sea ice (Papers II and III). Additionally, the seasonal evolution of light transmission through lake ice was investigated during spring. The crystal structure of the ice cover was analysed both in the coastal fast ice zone and in drift ice in the open Baltic Sea. The snow and ice cover was found not only to reduce the amount of light, but also to change its spectral and directional distribution. The light field under ice depended strongly on the snow cover. In addition, the bio-optical substances were analysed within sea ice and in the underlying water, as well as their effect on the light conditions.The seasonal sea ice cover also limits the wind-driven mixing of the water column. The development of stratification was investigated in a coastal bay in the northwestern Baltic proper (Paper IV). The preconditions for an under-ice plume development were defined along with the spatial and temporal dimensions of the stratification pattern. Furthermore, an under-ice plume was found to cause a delay in the onset of the phytoplankton spring bloom, but the timing of the Chl-a maximum was not affected. The results also show that although diatoms dominate the phytoplankton community with and without under-ice plume, the dynamic conditions without under-ice plume seem to favour the motile photosynthetic ciliate Mesodinium rubrum. Overall, this thesis contributes to better understanding of the current role of seasonal ice cover on the light conditions and consequently on to the ecosystem.
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| 4. |
- Karlsson, Konrad, 1983-
(författare)
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Local adaptation in life history traits and population size estimation of aquatic organisms
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Human society is dependent on healthy aquatic ecosystems for our basic needs and well-being. Therefore, knowledge about how organisms respond and interact with their environments is pivotal. The Baltic Sea is highly affected by human activity and future populations living in its catchment area will have to respond to multiple set of changing abiotic and biotic predictors.The first two papers of this thesis focus on local adaptation, adaptive capacity, and the response to changing temperature, salinity, and food conditions of different Eurytemora affinis populations, a ubiquitous zooplankton species in the Baltic Sea. Development time of zooplankton is an important trait and relates to how fast a population can increase in number. Common garden experiments showed that E. affinis populations from warmer southern areas had shorter development time from nauplii to adult at high temperature compared to populations from colder areas, which indicates an adaptation to temperature. The adaptation was explained by a correlation in development time between higher temperatures, 17 and 22.5 °C, while development between a colder temperature, 12 °C, and the two higher temperatures was uncorrelated. This implies that adaption to short development time at high temperature is unlikely for populations originating from cold temperatures. Hence, global warming will be disadvantageous for northern, compared to southern populations. However, development time is heritable and may change under selection, and may improve the competitive advantage of northern populations. The population with the shortest development time had comparably lower survival at high temperature and low food quality. This represents a cost of fast development, and emphasizes the importance of including multiple stressors when investigating potential effects of climate change.E. affinis inhabits a broad range of habitats from an epi-benthic life in freshwater lakes and river mouths, to pelagic life in estuaries. Paper III aims to link the morphology of different populations to habitat and resource utilization. Results showed that the individuals of a pelagic population were smaller in size and more slender, compared to a littoral population of larger and more fecund individuals. In experimentally constructed benthic and pelagic algae communities, the littoral population produced less offspring than the pelagic population when filamentous benthic diatoms were included. This suggests that filaments disturb their feeding and that littoral populations of E. affinis stay epi-benthic. As pelagic fish typically select larger prey, living close to the bottom probably allows the littoral population to grow larger than the pelagic. These results link morphology to habitat specialization, and show contrasting ecological effects of two E. affinis populations.Paper IV focuses on the recreational angler’s potential role as a citizen scientist. The pike Esox lucius has a stabilizing role in ecosystems as a top consumer and is highly valued by recreational anglers in European lakes and estuaries. Results showed that recreational angling could be used to estimate population size and connectivity of E. lucius in spatial capture-recapture models. The only prerequisite is that anglers practice catch and release, retain spatial data, and take photos of their caught fish. These results show that data from recreational angling can be of potential use for fisheries managers and researchers.
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| 5. |
- Serandour, Baptiste, 1995-
(författare)
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Ecological niche dynamic, lessons from plankton
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Abiotic conditions shape biological communities around the globe. Through spatial and temporal heterogeneity, environments impact plankton physiology, phenology and distribution. Understanding the dynamic relationships between biotic and abiotic components is essential to assess the resistance and resilience of biological communities to environmental changes. In an intricate interaction network, plankton organisms are channeling energy to higher trophic levels. However, the relative importance of the different energy sources and trophic pathways within the pelagic food web remains to be quantified. A better comprehension of the ecological niches of the main plankton species, and the influence of abiotic conditions on their interaction network is needed to enhance our understanding of fluxes and predicting the response of oceans to environmental change. In this thesis, we employed diverse approaches to explore the influence of environmental conditions on feeding interactions and spatial distribution of Baltic Sea plankton species. Using metabarcoding tools, we show the broad trophic niche of mesozooplankton species and shed light on the diet variability across species belonging to a similar size class. Additionally, we observed for some species, the ability to change trophic behavior on a spatial and temporal scales. These new insights were incorporated in flux models to quantify energy pathways, which revealed the essential role of cyanobacteria in supporting the pelagic food web. However, biotic interactions are sensitive to abiotic conditions, therefore, expected environmental changes could lead to modifications in the marine network. We assessed the influence of changes in abiotic parameters on the spatial and seasonal distribution of key Baltic Sea species by projecting their suitable habitat areas in both current and future conditions. In this doctoral project, we unveil the future loss in habitat suitability of several important zooplankton species of the Baltic Sea food web, potentially leading to cascading effects. In addition, we mapped the distribution of suitable habitats of the newly introduced Cnidarian species Blackfordia virginica to the Baltic Sea. We show that a notable proportion of coastal areas present favorable levels of environmental parameters for the growth of this species, which could alter the pelagic communities in these regions.Overall, this thesis refines our comprehension of the trophic interactions, illustrates the role of cyanobacteria in the Baltic Sea and projects potential modifications in zooplanktonic communities, through changes in habitat suitability levels and invasion, due to expected changes in abiotic conditions.
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| 6. |
- Burian, Alfred, 1984-
(författare)
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Impact of food quality on aquatic consumers : Behavioral and physiological adjustments
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Food quantity and quality together determine growth rates of consumers and the utilisation efficiencies of available resources in aquatic and terrestrial ecosystems. The effect of food quality on the performance of consumers is dependent on both, its direct influence on ingestion and assimilation rates, and on the behavioural and physiological adjustments of consumers to their food environment. The main target of this thesis was to investigate the nature and scope of behavioural and physiological adjustments in consumers and assess the resulting consequences for consumers’ fitness and ecosystem-wide nutrient flows.In paper I, we investigated the extent of elemental homeostasis across several taxonomic groups of planktonic herbivores. We found that adjustments in elemental ratios (C:N:P) in body tissues are an important physiological response of heterotrophic flagellates, but that in ciliates and multi-cellular organisms C:N:P ratios varied much less than in their algal prey. Hence, alternative regulatory mechanisms determine the reactions of metazoan zooplankton to decreases in food quality. In paper II, we developed a theoretical model to explore regulation in behaviour and digestive physiology of consumers to changes in the food environment. Our results demonstrate that feeding and digestion of consumers are determined by trade-offs between benefits and costs of investments in these processes. We revealed that the flexibility in consumers’ behaviour and physiology had strong influences on assimilation rates and efficiencies and thereby affected growth rates and a wide range of ecosystem functions. In paper III, we investigated the scope and consequences of adjustments in feeding and assimilation rates of copepods exposed to different diets. An important finding was that consumers can use resources, which are available in surplus, to increase the uptake of a limiting nutrient. Such nutrient interconversion led to co-limitation, the simultaneous limitation of copepods by two different nutrients. Finally, in paper IV, we aimed to test the effect of food quality on population dynamics in the field. We investigated zooplankton populations in tropical soda-lakes, an environment with a surplus of planktonic food sources that thus provides an ideal setting for investigations of food quality. However, we found that the hatching of resting eggs from lake sediments was the main driver of zooplankton bloom formation resulting in non-cyclical dynamics that were not related to food quality.These findings contributed to our understanding under which circumstance and by which mechanisms food quality affects the performance of consumers. My results highlight that food quality has not only direct effects on consumers’ growth but also triggers behavioral and physiological responses in consumers to maximize their fitness.
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| 7. |
- Caputo, Andrea, 1988-
(författare)
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Genomic and morphological diversity of marine planktonic diatom-diazotroph associations : a continuum of integration and diversification through geological time
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Symbioses between eukaryotes and nitrogen (N2)-fixing cyanobacteria (or diazotrophs) are quite common in the plankton community. A few genera of diatoms (Bacillariophyceae) such as Rhizosolenia, Hemiaulus and Chaetoceros are well known to form symbioses with the heterocystous diazotrophic cyanobacteria Richelia intracellularis and Calothrix rhizosoleniae. The latter are also called diatom-diazotroph associations, or DDAs. Up to now, the prokaryotic partners have been morphologically and genetically characterized, and the phylogenetic reconstruction of the well conserved nifH gene (encodes for the nitrogenase enzyme) placed the symbionts in 3 clusters based on their host-specificity, i.e. het-1 (Rhizosolenia-R. intracellularis), het-2 (Hemiaulus-R. intracellularis), and het-3 (Chaetoceros-C- rhizosoleniae). Conversely, the diatom-hosts, major representative of the phytoplankton community and crucial contributors to the carbon (C) biogeochemical cycle, have been understudied.The first aim of this thesis was to genetically and morphologically characterize the diatom-hosts, and to reconstruct the evolutionary background of the partnerships and the symbiont integration in the host. The molecular-clock analysis reconstruction showed the ancient appearance of the DDAs, and the traits characterizing the ancestors. In addition, diatom-hosts bearing internal symbionts (with more eroded draft genomes) appeared earlier than diatom-hosts with external symbionts. Finally a blast survey highlighted a broader distribution of the DDAs than expected.The second aim of this thesis was to compare genetic and physiological characteristics of the DDAs symbionts with the other eukaryote-diazotroph symbiosis, i.e. prymnesiophyte-UCYN-A (or Candidatus Atelocyanobacterium thalassa). The genome comparison highlighted more genes for transporters in het-3 (external symbiont) and in the UCYN-A based symbiosis, suggesting that symbiont location might be relevant also for metabolic exchanges and interactions with the host and/or environment. Moreover, a summary of methodological biases that brought to an underestimation of the DDAs is reported.The third aim of this thesis was to determine the distribution of the DDAs in the South Pacific Ocean using a quantitative polymerase chain reaction (qPCR) approach and to outline the environmental drivers of such distribution. Among the het-groups, het-1 was the most abundant/detected and co-occurred with the other 2 symbiotic strains, all responding similarly to the influence of abiotic factors, such as temperature and salinity (positive and negative correlation, respectively). Globally, Trichodesmium dominated the qPCR detections, followed by UCYN-B. UCYN-A phylotypes (A-1, A-2) were detected without their proposed hosts, for which new oligonucleotides were designed. The latter suggested a facultative symbiosis. Finally, microscopy observations of the het-groups highlighted a discrepancy with the qPCR counts (i.e. the former were several order of magnitudes lower), leading to the idea of developing a new approach to quantify the DDAs. The fourth aim of this thesis was to develop highly specific in situ hybridization assays (CARD-FISH) to determine the presence of alternative life-stages and/or free-living partners. The new assays were applied to samples collected in the South China Sea and compared with abundance estimates from qPCR assays for the 3 symbiotic strains. Free-living cells were indeed detected along the transect, mainly at deeper depths. Free-living symbionts had two morphotypes: trichomes and single-cells. The latter were interpreted as temporary life-stages. Consistent co-occurrence of the 3 het-groups was also found in the SCS and application of a SEM model predicted positive interactions between the het groups. We interpreted the positive interaction as absence of intra-specific competition, and consistent with the previous study, temperature and salinity were predicted as major drivers of the DDAs distribution.
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| 8. |
- Novotny, Andreas, 1991-
(författare)
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Functional diversity of zooplankton in marine food webs : Integrating DNA metabarcoding and network modeling
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The oceans are important regulators of the Earth’s climate system by sequestering carbon from the atmosphere taken up by primary producers. Zooplankton, including protozoans and metazoans of different phyla and size classes, occupies several trophic niches and regulates energy flow between primary producers and fish. The structural configuration of the food web determines the rates at which primary production is either enriched to sustain organisms at higher trophic levels or exported to the ocean floor. However, limited knowledge about plankton interactions causes uncertainty of how the oceans will respond to climate changes. This thesis presents a framework for studying and modeling pelagic food webs using novel implementations of DNA metabarcoding. Study I shows that DNA metabarcoding of zooplankton sampled in their natural environment reveals a broader and more complex diet than zooplankton in classic grazing observations. We also show that differential feeding strategies facilitate species coexistence and that the zooplankton diet is largely dependent on prey availability. The approach was extended in Study II, where we include the smaller fraction of zooplankton that is often overseen in food web studies to broaden the perspective of functional diversity in pelagic food webs. We show that different populations have unique functions in channeling the primary production of different sources and especially highlight the role of filter-feeders in making detrital nutrients available for other organisms in the food web. In Study III, we shifted focus to trophic links between zooplankton and fish by comparing niche overlap between the three main planktivorous fish in the Baltic Sea - stickleback, sprat, and herring. The results from the three first studies were finally used to calculate selectivity indices between each predator and prey. This information was implemented in Study IV in a network model quantifying fluxes of energy through the food web. The model revealed cyanobacteria as the primary contributor to secondary production in the Baltic Sea food web and that the spring bloom of diatoms and dinoflagellates remains largely unutilized by the zooplankton. This is the first time DNA metabarcoding is used to compare niche differences of several zooplankton species in a pelagic guild and to quantify fluxes in a food web model. The thesis refines our knowledge of pelagic community and food web structure, and the framework presented here is a suitable entry point for food web modeling in other ecosystems.
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| 9. |
- Hedberg, Per, 1978-
(författare)
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Responses of benthic-pelagic coupling to environmental change
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Changing abiotic factors, like temperature and light are important drivers of seasonality, affecting aquatic ecosystems and organisms annually in a predictable pattern, structuring species composition, growth, reproduction and trophic interactions. As a consequence of human induced climate change, coastal ecosystems are seeing a disruption in the predictability of annual cycles. This in turn is affecting species composition at the base of the food web, possibly causing effects for higher trophic levels. Although the benthic and pelagic environments are coupled, still very little is known about the effect that changes in species composition will have on this interaction. In Study I, three species of common macrofauna from Baltic Sea soft sediment bottoms were for the first time subjected to a gradient of spring bloom related diatoms and summer bloom related cyanobacteria as food resource. We found a clear differentiation between the two food types regarding preference of the consumers. The results highlight that diatoms were consumed by all species with a strongly positive linear relationship with available food. Cyanobacteria was consumed, but with no clear pattern relating to amount available. In Study II, we investigated if seasonal phytoplankton succession affects the composition of essential compounds in benthic macrofauna, focusing on transfer of fatty acids (FAs) from phytoplankton via sediment to benthic consumers. The outcome showed for the first time large interspecific variation in FA composition and concentration, but low seasonal variation within species for five major invertebrate taxa over three seasons. We found only few convincing links between seasonal phytoplankton production and its succession of FAs in the seston, sediment and the animals. This suggests that demands for FAs differ substantially between taxa and that FAs of sedimenting organic material is not clearly reflected in the investigated species. In Study III, we quantified recruitment of phyto- and zooplankton from oxic sediment to the pelagic environment, under proposed changes to light and temperature caused by climate change. We found clear indications of both light and temperature having an effect on phytoplankton (mainly cyanobacteria and dinoflagellates) recruitment in spring. We found no effect on recruitment of phyto- or zooplankton in summer, but large differences in zooplankton recruitment in summer compared to spring. The phytoplankton taxa that showed strong recruitment responses in spring, could with proposed climate change scenarios potentially impact the species composition during spring, thus affecting food web dynamics. In Study IV we used advanced molecular tracing techniques of compound specific stable isotope analysis of carbon and nitrogen in amino acids in five taxonomic groups of phytoplankton, with the goal to advance methods of differentiating between groups in mixed samples of phytoplankton. By applying this method to amino acids, we were able to differentiate between all five taxonomical groups. The key in revealing the group specific finger printing on particular amino acids was based on amino acid synthesis pathways and thus depletion or enrichment of isotopes. The novel findings presented in this thesis further advances our knowledge about responses of benthic-pelagic coupling to environmental change and how to further develop tracing of food webs in order to predict the effects of a changing environment on aquatic organisms.
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| 10. |
- Nielsen, Jens Munk, 1983-
(författare)
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Species interactions and energy transfer in aquatic food webs
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Food webs are structured by intricate nodes of species interactions which govern the flow of organic matter in natural systems. Despite being long recognized as a key component in ecology, estimation of food web functioning is still challenging due to the difficulty in accurately measuring species interactions within a food web. Novel tracing methods that estimate species diet uptake and trophic position are therefore needed for assessing food web dynamics.The focus of this thesis is the use of compound specific nitrogen and carbon stable isotopes and molecular techniques for assessing predator-prey interactions and energy flow in natural aquatic ecosystems, with a particular focus on the species links between phytoplankton and zooplankton.The use of δ15N amino acid values to predict organism trophic position are evaluated through a meta-analysis of available literature which included measurements from 359 marine species (article I). Through a controlled feeding study isotope incorporation in aquatic organisms, across both plant-animal and animal-animal species linkages is further assessed (article II).These studies showed that δ15N amino acid values are useful tools for categorizing animal trophic position. Organism feeding ecology influenced nitrogen trophic discrimination (difference in isotope ratio between consumer and diet), with higher discrimination in herbivores compared to omnivores and carnivores (article I). Nitrogen isotope trophic discrimination also varied among feeding treatments in the laboratory study (article II). The combined findings from articles I & II suggest that researchers should consider using group specific nitrogen trophic discrimination values to improve accuracy in species trophic position predictions. Another key finding in the controlled laboratory study (article II) was consistently low carbon isotope discrimination in essential amino acids across all species linkages, confirming that these compounds are reliable dietary tracers.The δ13C ratios of essential amino acids were applied to study seasonal dynamics in zooplankton resource use in the Baltic Sea (article III). Data from this study indicated that zooplankton assimilate variable resources throughout the growing season. Molecular diet analysis (article IV) showed that marine copepod and cladoceran species ingested both autotrophic and heterotrophic resources.Evidence from both articles III & IV also revealed that zooplankton feed on a relatively broad range of diet items but not opportunistically on all available food sources. Mesozooplankton feeding patterns suggested that energy and nutritional flows were channelled through an omnivorous zooplankton food web including microzooplankton prey items. Overall the results of this thesis highlight that stable isotope ratios in specific compounds and molecular techniques are useful tracing approaches that improve our understanding of food web functioning.
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