Viruses has a relatively recently been discovered in high abundances in aquatic systems. Their possible importance has therefore been largely overlooked. In aquatic food webs there should be large differences in carbon and nutrient fluxes depending on whether the main cause of bacterial mortality is viral infection or grazing, where lysis following viral infection should result in a release of organic carbon and nutrients from the lysed bacteria and hence not reach higher trophic levels. Recent research on aquatic viruses has mainly focused on marine environments and the number of studies on freshwater viral ecology is limited. Hence, there is a need for more studies on the importance and functioning of viruses in freshwater systems. The aims of this thesis were to explore the functioning and diversity of viruses that infects bacteria (phages) in freshwater systems. To effectively address this I conducted two experiments and three field studies in 23 lakes in different parts of Sweden. The results show that viral infection and subsequent lysis of the host cell can partly explain the formation of non-nucleoid-containing bacteria and further that viruses can respond to increases in phosphorus concentrations without any net increase in bacterial abundance. Generally, a larger part of bacterial production in lakes were grazed by flagellates than lysed by viruses, but a larger fraction of the total bacterial mortality can be attributed to viruses in hypolimnion compared to in epilimnion. Further, the largest impact of phages on bacterial production may be in humic lakes, which have a relatively high frequency of visibly infected bacterial cells, but low flagellate abundance. Community composition of bacteria and viruses were only weakly coupled in the studied systems. The most important factors for predicting viral community composition were temperature and concentrations of dissolved organic carbon, total phosphorus and soluble reactive phosphorus. The viral community composition changed over the season and temperate phages could be detected by incubations with mitomycin C showing that a large fraction of the viruses detected appeared to be temperate phages. The most important environmental factor co-varying with viral community composition was again concentrations of total phosphorus. To summarize, bacteriophages, as a bacterial mortality factor, are important in freshwater microbial food webs and phosphorus supply has a potential central function in the regulation of the importance of bacteriophages and additionally for viral diversity.

<p>This thesis concerns the predator-prey interactions of three raptor species in a Swedish arctic community: the gyrfalcon (<i>Falco rusticolus</i>), the rough-legged buzzard (<i>Buteo lagopus</i>) and the golden eagle (<i>Aquila chrysaetos</i>).</p><p>The gyrfalcon behaved like a highly specialised ptarmigan (<i>Lagopus spp.</i>) predator. Gyrfalcon’s functional response to ptarmigan was close to density independent, and ptarmigan remained the dominating prey even in areas with the lowest ptarmigan density. The gyrfalcon did not respond functionally to microtine rodents (i.e. lemmings and voles) and it was clear that the gyrfalcon did not use microtines as an alternative prey category to ptarmigan. As the gyrfalcons did not switch to any alternative prey when ptarmigan was scarce, their reproductive success seemed to be directly dependent on the amount of ptarmigan available in the breeding territories. Of the two ptarmigan species in the study area, rock ptarmigan (<i>L. mutus</i>) dominated gyrfalcon’s diet. Locally, the proportion of rock ptarmigan in gyrfalcons’ diets showed a positive relationship to the expected availability of rock ptarmigan in the breeding territories, indicating a density dependent utilisation. </p><p>The rough-legged buzzard behaved like a highly specialised microtine rodent predator and Norwegian lemming (<i>Lemmus lemmus</i>) was its preferred microtine species. The buzzards showed a type 2 functional response to lemmings. Surprisingly though, they also had a type 3 functional response to grey-sided voles (<i>Clethrionomus rufocanus</i>). We present an optimal diet model where a central place forager, during good food conditions, benefits from partial prey preference, which renders separate functional responses to each prey category. We discuss how the double functional responses of the buzzard affect the population dynamics of sympatric vole species, on both temporal and spatial scales.</p><p>The golden eagle behaved like a generalist predator, and it preyed on all major prey categories in the study area: microtines, ptarmigan, mountain hare, (<i>Lepus timidus</i>) and reindeer (<i>Rangifer tarandus</i>). It seemed to respond functionally to microtine rodent fluctuations with an increased consumption of lemmings during a peak year in the microtine rodent cycle. The golden eagle showed a numerical response to its main prey, the ptarmigan. </p><p>Ptarmigan, microtine rodents and hares seemed to have synchronized population fluctuations in the study area. Such synchronized population fluctuations are believed to be generated by predation. Although the three raptors are the main predators of their community, their predation patterns fail to explain the observed prey population dynamics in the study area. </p>

<p>Two decades ago the recruitment of YOY perch (<em>Perca fluviatilis</em>) started to decline along the Swedish east cost of the Baltic Sea. Factors that influence recruitment are e.g. eutrophication that causes habitat losses and overfishing of cod (<em>Gadus morhua</em>) which causes cascading effects in the food web. Filamentous cyanobacterial blooms are often toxic and has increased in the Baltic Sea and its coastal waters. The aim of this field study was to evaluate the effects of salinity and cyanobacterial exposure on fitness related parameters of young of the year (YOY) perch (<em>Perca Fluviatilis</em>) in a natural environment. Our study was performed in the Curonian Lagoon (Lithuania) in August 2009. The lagoon offers a temporary salinity gradient (wind induced influxes from the Baltic Sea) ranging from 7 psu in the north to 0 psu in the south. Submerged enclosures containing YOY perch were set up at three different locations along the salinity gradient in the Lagoon (referred to as North, Middle, South). The duration of the experiment was 21 or 27 days, depending on treatment. Measurements of perch condition were specific growth rate, somatic condition index (SCI) and whole fish lipid and protein content. Average chl <em>a</em> values for the three stations during the experimental time were: north 180 ± 70 µg/l chl <em>a</em>, middle 133 ± 36 µg/l chl <em>a</em> and south 180 ± 52 µg/l chl <em>a</em>. The North and the Middle stations experienced two different salinity influxes reaching a maximum salinity of 6.5 psu at the northern station. The duration of each saline influx was approximately 4-6 days. The saline water did not reach the Southern station at any time. Results show that perch from the southern station were in best condition in terms of specific growth rate and contents of total lipids. Compared to the South the perch condition declined to the Middle station and was lowest at the Northern station which experienced the highest degree of fluctuation in terms of salinity and cyanobacterial exposure. Examination of the abundance of the main food resource at the different stations revealed no statistical differences, which suggest that availability of food was not a factor in explaining the differences in growth.  The results possibly indicate that a changing environment with the potential synergistic negative effects of salinity and cyanobacteria has a higher negative impact on YOY perch condition compared to constantly high concentrations of cyanobacteria.</p>

Resources are often unevenly distributed through the environment, resulting in a challenging task for insects to locate food, mates and oviposition sites. Consequently, there is an ongoing need to unravel how insects rely on behavioural and sensory traits while searching for resources in heterogeneous environments. In the first part of this thesis, I addressed this issue by studying how neighbouring resources can affect the likelihood of insects finding their preferred host resources. These effects of neighbouring resources are commonly referred to as associational effects, and are expected to result from limitations in the sensory physiology of insects. Such limitations constrain the insect’s ability to correctly evaluate resource quality at the different steps involved in insect search behaviour. Furthermore, I determined whether the physiological state of an insect, and sensory experiences made during larval stages, can affect host search behaviour in heterogeneous environments. By comparing the behaviour of Drosophila melanogaster in environments with single and multiple resources, I found that the presence of neighbouring recourses increased the selection rates for attractive resources, while it decreased the selection rates for less attractive resources. These effects are referred to as associational susceptibility and associational resistance respectively. Furthermore, by studying oviposition behaviour, I found that during these small-scale behavioural decisions, associational effects are mainly governed by gustatory mediated selection and less by olfactory mediated selection. The oviposition assay eliminated potential misinterpretations of resource quality along the different steps of search behaviour, hence the results suggested that associational effects rely on distinctive selection behaviour between resource types rather than on sensory constraints. In the second part of this thesis I determined whether natal experiences can be used by insects as sensory shortcuts to find host resources, and whether this leads to better larval performance on those selected host resources. For this purpose, I studied the interactions between the larval parasitoid Asecodes lucens and the oligophagous leaf beetle Galerucella sagittariae. The results showed that the relationship between oviposition preference and larval performance, of both insect species, depends on an interactive effect between the insects’ natal origin and the quality of the different host resources. Moreover, I found that the natal origin was a better predictor for the adult host preference, rather than for larval performance. This suggests that, aside from the actual quality of the host resources, locating any suitable host might be even more limiting for the female’s fitness. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Accepted. Paper 3: Submitted.</p>

Microtine rodents are a key component in the boreal ecosystem. Due to unique yearly fluctuation in population numbers, in addition to their central placement within boreal foodwebs, microtine rodent population dynamics control a wide variety of predator populations. Despite their grouping as microtine rodents, many individual species exhibit very different cycles, such as voles and lemmings. As such, the identification of species composition and ratio in each given habitat is of utmost importance. The purpose of this study was to compare the composition of species of small rodents between four different habitats (grassland, wetland, spruce dominated forest and pine dominated forest) and to statistically analyze if the different biotopes hosted significant differences in species composition. The project also aimed to examine differences in weight and numbers between populations of the same species in the different biotopes. To accomplish this, rodents were captured using non-lethal traps, and the resulting data was analyzed using RxC tables and ANOVA. The species captured were A. sylvaticus, C. glareolus and M. agrestis. The result of the analysis showed that the only difference in species composition existed between the grassland habitat and the rest. In addition, there existed no difference between different populations of A. sylvaticus in regards to weight. However, a difference was found between the populations of C.glareolus, with the individuals inhabiting spruce dominated forest being larger than those found in other habitats.

Deadwood has increased over the last 25 years, but it remains unclear to what extent this is driven by forestry practices or storms. Therefore, I wanted to study the change in volume, decay stage and tree species during a 22-year period, to see if there was a correlation between increase of deadwood and storm. This study included data from southern Sweden, collected by the Swedish National Forest Inventory between 1994-2016. Deadwood in production forest have doubled over the last 25 years and almost quadrupled in protected forest. The increase does not depend on storm since much of the fallen wood was probably removed following year. In protected forest there was an increase in deadwood of broadleaved trees and a drastic decrease in Pinus sylvetstris. While in production forest, conifer trees dominate and there was no lasting effect due to the storm Gudrun (2005) on Picea abies. Hard deadwood decreased in production forest, possibly due to increased removal of branches and treetops, used as forest fuel in forest management. Possible reasons for the increase in deadwood could be the awareness in forestry, especially certification system and voluntarily set asides. Though, there is still necessary to increase the volume of deadwood in production forest, since it covers the largest parts of Swedish forests and does not seem to reach the national environment objective in 2030.

Man ser idag att det finns ett ökat intresse för ett mer hållbart levnadssätt hos konsumenter, vilket kan spela en stor betydelse för just hållbarare paketering av produkter (Steenis et. al, 2017:286). Ett problem som finns är dock att konsumenter inte har den vetskapen som kan behövas för att göra ett mer hållbart val i sin konsumtion, och väljer därför att förlita sig på de konsumtionsvanor som dom har vant sig vid (ibid). Om man ser på förpackningsdesign från en konsuments ögon så finns det vissa delar man kollar på när man kollar efter hållbar produkt. Förpackningsdesign består av olika element som talar om huruvida hållbar förpackningen är eller inte. Nämligen förpackningsmaterial, vilka grafiska element som har använts i utformningen av förpackningens design samt vilka informativa element som finns på förpackningen, som märkningar etc (Steenis et. al, 2017:286). De metoder som använts för att komma fram till resultatet med denna undersökning är semiotisk bildanalys samt enkätundersökning. Det huvudsakliga resultatet i denna studie är att färgvalen man gör samt hur man väljer att använda märkningar är viktigt om man vill förmedla budskapet om att ett vin är ekologiskt.

Arctic ecosystems are amongst the most vulnerable on Earth to ongoing climate change. While the responses to these changes are well studied on land, less is known about how aquatic communities may respond to a warmer arctic. For stream invertebrate communities, predicting such responses requires basic understanding of how and why different taxonomic groups fluctuate throughout the year. However, few studies have assessed the community dynamics of stream macroinvertebrates across seasons in the Arctic. In this project, I asked how macroinvertebrate community structure changes between months and across seasons in a small Arctic stream in northern Sweden. I expected that community change over time would reflect changes in the supply of organic matter (e.g., leaf litter and algae) to dominant consumers. A total of five transects were sampled for macroinvertebrates each month from July to April using Surber sampling. I used descriptive and multivariate analyses to evaluate changes in community structure between months and seasons. Marked differences in community composition were found between the seasons with detritivores (shredders) dominating the autumn months possibly reflecting input of birch litter and high abundances of grazers during and post winter, possibly reflecting primary production early in spring. Expected climate change effects in the Arctic include warmer temperatures and increases in the terrestrial plant productivity. My results suggest that these shifts could cause changes in stream community composition, driven by increases in deciduous litter inputs that promote shredders and/or by increases in primary production during spring that favour grazers and collector-gatherers, which feed on algae.

Interconnected freshwater habitats are essential for many fish populations, which allow fish to migrate between areas to fulfil their life-cycle and maximize their fitness. Currently, such populations are negatively affected by human habitat exploitation. As a result, the biodiversity in freshwater ecosystems is decreasing. In this study, I use data from surveys of two Asp (Leuciscus aspius) populations in Uppland to see how a representation of day length, water temperature and water current influence their spawning migrations. I also determine the difference in timing of migration between males and females, and if the individual body size matters. I found that day length, water temperature and water current affect the migratory behavior of Asp. I also found a significant difference in migratory timing between males and females, where males on average arrive 5-6 days earlier than females to spawning areas.

Coastal populations of perch (Perca fluviatilis) in the Baltic Sea has declined substantially the last decades while the populations of three-spined sticklebacks (Gasterosteus aculeatus) has increased rapidly during the same time period. Earlier studies have suggested that predation on perch larvae and or competition from sticklebacks are the causes behind the decline in perch. To test if predation from sticklebacks commonly occur on perch larvae as well as provide data on stickleback diets in general, diets of sticklebacks were examined by looking at the stomach content of collected samples of sticklebacks from different bays along the Swedish and Finnish coast. Results showed no evidence of stickleback predation on perch larvae as no perch larvae were found in any of the examined stomachs.  Three-spined sticklebacks generally had the same diet in all studied bays consisting mainly of Chironomidae and Asellus aquaticus. The diet results suggest that competition between perch larvae and sticklebacks is minor if any due to low proportions of zooplankton found in the stomachs of the sticklebacks while zooplankton is the main food source for perch larvae. In bays where three spined sticklebacks were found with nine-spined sticklebacks they generally had similar diets. Still, due to a larger size and gape size of three-spined sticklebacks they fed more on larger prey like Asellus aquaticus while nine-spined sticklebacks contained smaller prey such as benthic cladocerans. Due to few samples from bays where sticklebacks were found together with perch larvae, no conclusion regarding predation on larvae as the main the mechanism for declines in coastal perch population can be drawn from the results in this study.