Phenotypic plasticity is the ability of a genotype to express different phenotypes in different environments. Despite its common occurrence, few have investigated differences in plasticity between populations, the selection pressures responsible for it, and costs and constraints associated with it. In this thesis, I investigated this by studying local adaptation and phenotypic plasticity in populations of the common frog Rana temporaria, inhibiting islands with different pool types (temporary, permanent or both). The tadpoles develop in these pools, and have to finish metamorphosis before the pool dries out. I found that the tadpoles were locally adapted both in development time and in phenotypic plasticity of development time. Tadpoles from islands with temporary pools had a genetically shorter development time than tadpoles from islands with permanent pools. The population differentiation in development time, estimated as QST, was larger than the population differentiation in neutral molecular markers (FST), which suggest that divergent selection among the populations is responsible for the differentiation. Moreover, tadpoles from islands with more variation in pool drying regimes had higher phenotypic plasticity in development time than tadpoles from islands with only one pool type present. Interestingly, increased migration among populations did not select for increased plasticity, rather it was the local environmental variation that was important. This adaptation has occurred over a short time scale, as the islands are less than 300 generations old. In temporary pools, it is adaptive to finish development before the pool dries out. This could be achieved by entering the metamorphosis at a smaller size, as a smaller size takes shorter time to reach. However, I found that there is a minimum threshold size below which tadpoles’ cannot enter metamorphosis, and that there had been no evolution of this threshold size in populations living in temporary environments. That suggests that this developmental threshold is tightly linked to physiological constraints in the developmental process. Despite their expected importance as constrains on the evolution of plasticity, costs of plasticity are often not found in nature.  However, theories of why they are absent have not been tested empirically. In this thesis, I show that fitness costs of phenotypic plasticity are only found in populations with genotypes expressing high levels of phenotypic plasticity, while in populations with low-plastic genotypes, I find costs of not being plastic. This suggests that costs of plasticity increase with increased level of plasticity in the population, and that might be a reason why costs of plasticity are hard to detect.

One of ecology’s primary goals is to comprehend biodiversity and its patterns of distribution over space and time. Since microorganisms play a pivotal role in key ecological processes, the diversity of microbial communities may have important implications for the stability and functioning of Earth’s ecosystems. Thus, it is of utmost importance to develop a theoretical foundation but also a conceptual understanding for the mechanisms that generate and maintain microbial diversity. The aim of this thesis is to investigate to what extent local freshwater bacterioplankton diversity, i.e. richness and community composition, is structured by local environmental interactions and/or regional processes. The key objective is to identify ecological linkages between lake bacterioplankton and bacterial communities in connected streams and the surrounding terrestrial landscape, thereby applying a metacommunity approach. To do so, I studied several natural lake bacterioplankton assemblies within different regions of Sweden and assessed both local environmental properties and regional parameters (e.g. dispersal, landscape position). The genetic composition of freshwater bacterioplankton diversity was determined by means of terminal-restriction fragment length polymorphism or 454 pyrosequencing. From the review on the biogeography of bacterioplankton in inland waters it became clear that microbial diversity and its spatial distribution are governed by a complex interplay of both local and regional drivers. In one case, freshwater bacterioplankton communities were structured by local environmental conditions rather than by regional dispersal processes. These local environmental conditions seemed to be equally important in controlling both the total bacterioplankton community and its active fraction. In a study of bacterioplankton communities from five different regions, locally abundant aquatic bacteria were shown to be also regionally widespread, a pattern predicted by neutral theory. Yet, this degree of similarity decreased with increasing environmental heterogeneity. In another study, bacterioplankton richness was controlled mostly by nutrient content, indicating that productivity exerted influence on bacterioplankton richness. However, landscape position and productivity covaried, suggesting that the landscape dictates environmental properties, which then directly structure local bacterioplankton richness. Finally, a review synthesising results from empirical metacommunity approaches and comparing these to theory showed that yet a gap between empirics and theory exists. To conclude, local bacterioplankton diversity appeared to be mainly structured by local environmental properties. However, signatures of neutral processes driving local bacterioplankton community assembly were also recorded.

Syftet med denna avhandling är att öka förståelsen av hur grovt organiskt material, sk grovdetritus, tex löv och makrofytrester, bryts ner i sjöars strandzoner. Motivet bakom intresset för detta är att små näringsfattiga sjöar i skogsrika områden ofta är beroende av tillförsel av energi utifrån och att nedbrytningen av grovdetritus är relativt lite undersökt i sjöar. Utifrån tillförd detritus, i löst och partikulär form kan utgöra extra energikälla för sjöekosystem, men bara om några organismer kan tillgodogöra sig detta organiska material. I sötvatten kan sådana organismer vara bakterier, svampar och ryggradslösa djur. Under nedbrytningsförloppet produceras en mängd mellanprodukter som kan användas av andra konsumenter tex fisk. På så sätt återcirkuleras energi och näring från detritus utan fullständig nedbrytning. Vidare har det föreslagits att hög biodiversitet kan resultera i hög ekosystemfunktion, t ex effektiv nedbryning. Därför kan studier av både samhällen och enskilda arter av nedbrytare öka förståelsen av förändringar i hela ekosystemets funktion. Inom ramen för avhandlingen har jag studerat vilka arter som deltar i nedbrytningen av grovdetritus och vilka möjliga vägar för det fortsatta nyttjandet av detta material som finns. Jag har särskilt betonat det säsongsmässiga mönstret för omsättningen av grovdetritus och för tillgängligheten av mellanprodukter. Resultaten visar att både mikroorganismer och ryggradslösa djur successivt processar tillgängligt grovt organiskt material under hela året. Viktminskningsmönstret över året hos löv och makrofytrester är nära kopplat till förekomst och tillväxt hos olika arter av nedbrytande ryggradslösa djur, sk fragmenterare. Av de vanligaste förekommande tio arterna fragmenterare, var nio nattsländelarver. Därför drar jag slutsatsen att dessa arter av nattsländelarver spelar en avgörande roll för omsättningen av grovdetritus i sjöstränder i sydöstra Sverige. Samtidigt visar resultaten att sötvattengråsuggan spelar mindre roll för nedbrytningen av grovdetritus än förväntat. Denna art föredrar andra födoämnen åtminstone under våren och undviker de grunda bottnarna där grovdetritus finns under vintern. I laboratorieexperiment har jag visat att nedbrytning av löv i närvaro av fragmenterare huvudsakligen ger upphov till två olika produkter: löst organiskt material och finpartikulära rester. Därmed har jag visat att fragmenterare kan omsätta grovdetritus snabbare än mikroorganismer. Slutsatsen blir att mikroorganismernas kvantitativa bidrag till nedbrytning av grov grovdetritus är beroende av mängden fragmenterare. Jag har också visat att olika kombinationer detritussorter och fragmenterararter kan påverka när olika nedbryningsprodukter blir tillgängliga för andra organismer. Sammanfatningsvis har jag visat att grovdetritus är en attraktiv födoresurs för ryggradslösa djur i näringsfattiga sjöars strandzoner, precis som i små bäckar. Dessutom visar jag att de arter av fragmenterare som är speciella för sjöar, tillsammans omsätter tillgängligt grovdetritus i ett finskaligt tidsmönster över året. Detta mönster har utvecklats genom anpassning till trädens lövfällning och nedvissningen av vattenväxter som sker på hösten i vårt klimat. Jag vill också betona att ökade kunskaper om nedbrytning av grovdetritus krävs för att kunna förutsäga effekter av olika störningar i dessa ekosystem. Slutligen rekommenderar jag att nedbrytningshastigheten för grovdetritus testas som mått på ekosystemfunktion också i sjöar. / The general aim of this thesis was to increase the understanding of the mechanisms behind coarse detritus turnover in oligotrophic lake littoral zones. The reason for this interest is that small lakes located in forested areas often display dependence on detritus based energy sources, both external and internal. Additionally, in lake ecosystems the coarse detritus resource has been comparatively less studied than in streams. Both dissolved and particulate external detritus constitute extra energy contribution to ecosystems, but only if some organisms can assimilate the provided matter. The most common capable organisms to do so in freshwaters are bacteria, fungi and invertebrates, and they return a variety of intermediary products available for other consumers. Further, it has been suggested that a positive relation between species diversity and ecosystem function exists. Therefore the study of both community and individual species of decomposers may provide information significant to understand changes in ecosystem function. Within the outline of the thesis, I studied what species are involved in the processing of coarse detritus carbon and what are the key routes for the further use of this littoral carbon. I specially emphasize the temporal pattern of the detritus turnover and the availability of decomposition products to other organisms in the lake ecosystem. The results showed that microorganisms and invertebrates in the lake littoral zone successively process carbon from coarse detritus during the whole year. The weight loss from coarse detritus displayed a seasonal distinctive pattern temporally connected to shredder appearance and growth. Therefore, I emphasize the key role of certain trichopteran shredder species for litter processing in lakes in this region, southeast Sweden. At the same time, the shredder function of Asellus aquaticus was less important than expected. A. aquaticus was shown to utilize of a quite broad range of food sources and to prefer other habitats during the main decomposition period for leaf litter. In the laboratory I discovered that the decomposition of leaf litter principally results in two products, DOM from passive leakage and FPOM from active leaf litter processing by shredders. The experiment showed that shredders have the potential to consume leaf litter resources before microbial decomposition influence litter weight considerably. Thus, the quantitative importance of microorganisms (fungi and/or bacteria) for leaf litter turnover is suggested to directly depend on density of shredders in lake littoral zones of the kind studied. I have also shown that the combination of detritus types and invertebrate processor species influence the temporal accessibility of intermediate decomposition products. I conclude that coarse detritus is an attractive food resource for invertebrates also in small oligotrophiclakes like it is in smaller streams. I also conclude, that the lake specific set of shredder species recycle this carbon resource yearly in a fine-scaled temporal sequence - adapted to the predictable seasonal variation of detritus quality on these latitudes. Further, I stress the importance of increased knowledge of decomposing processes before predicting the effects of disturbance on this kind of widespread freshwater systems. Finally, I propose that the decomposition rate of coarse detritus should be tested as an assessment tool for integrating disturbances on ecosystem functions in small oligotrophic lakes, respecting the unique lake characteristics.

In this thesis I have investigated the effects of invading top and intermediate consumers in a size-structured fish community, using a combination of field studies, a lake invasion experiment and smaller scale pond and aquaria experiments. The lake invasion experiment was based on introductions of an intermediate consumer, ninespine stickleback (Pungitius pungitius L.), in to allopatric populations of an omnivorous top predator, Arctic char (Salvelinus alpinus L.). The invasion experiment was performed in two tundra lakes and in two birch forest lakes to investigate the effect of climate on the invasion success. I found that the effect of sticklebacks on char was size dependent. Small char suffered reduced growth from resource competition with sticklebacks whereas the maximum size of adult char increased from the addition of a larger prey resource, stickleback. The negative effect of sticklebacks on the growth of small char suggests that sticklebacks may be a better resource competitor than char, which was also supported by the pond and aquaria experiments. The pond experiments also suggested that char were more efficient cannibals than interspecific predators on sticklebacks. Cannibalism in char may limit the recruitment of char and decrease both their predatory and competitive effect on coexisting species and thereby also promote the coexistence of char and sticklebacks. The successful invasion by sticklebacks and their subsequent increases in density suggest that the absence of sticklebacks in char lakes in this region is not caused by biotic interactions with char. Instead, it may be suggested that co-occurrence of sticklebacks and char in the region is limited by dispersal. The char – stickleback system resembles an intraguild predation system with char as the top consumer and stickleback as the intermediate consumer. The effects of the stickleback invasion is also contrasted with a field study of a northern pike (Esox lucius L.) invasion into a system with coexisting char and stickleback, where pike can be viewed as the top consumer and char as the intermediate consumer both feeding on sticklebacks. In this case pike excluded char. The identity of the invading species and the relative strength of the predatory and competitive interactions in the two contrasting systems are discussed in relation to coexistence in intraguild predation systems. I found that the identity of the invading species is of crucial importance for the response at the ecosystem level, and that the inherent size dependency of competitive and predatory interactions in fish communities is important for attaining a mechanistical understanding of the effects of invasive species in lake ecosystems.

The aquatic isopod Asellus aquaticus can in some lakes be found as two different ecotypes; one in the habitat dominated by reed and one in stonewort stands. These ecotypes have been shown to differ in size, color and behavior. The reed ecotype is larger, darker and more active compared to the stonewort ecotype. In the two habitats there are different dominating predators: Invertebrate predators in the reed habitat and fish in the stonewort habitat. This project aims to examine how the presence of invertebrate predators and fish affect the two ecotypes of the isopod in regard to behavior and size composition in different substrates. To examine the effect on behavior the activity of isopods collected from Lake Tåkern was measured without and with chemical cues from perch and damselfly larva. The result of the behavior experiment showed no difference between the ecotypes nor the treatments. The lack of differences in the behavior indicates that there could be a variation between lakes. The effect on the size composition was examined by subjecting groups of isopods to predation by perch or damselfly larva in different substrate. The mean length of the group was measured before and after the trials. The size decreased significantly for the reed ecotype in stonewort substrate when subjected to predation by perch. The size for the stonewort ecotype increased significantly in reed substrate with damselfly larva as predator. The effect on size supports that the predators are the cause of the size difference between the ecotypes.

A constructed wetland (0.08 ha) receiving drainage water from a small agricultural catchment (22 ha) with clay soil, was investigated with respect to phosphorus dynamics and retention. The aim was to evaluate the function of the wetland with respect to phosphorus retention, and relate that to gross sedimentation as measured with sediment traps. Hydraulic load and phosphorus retention were estimated for 2003-2010 based on monitoring data. Furthermore, water quality dynamics was studied during three intensive sampling periods of 3-5 days during 2010. For each period, phosphorus retention was calculated and the relationship between flow and phosphorus concentrations analysed. Additionally, the gross sedimentation rate was estimated using sediment traps, and the phosphorus, carbon and nitrogen content analysed. The results suggested that there was no net retention of phosphorus during 2003-2010, except for 04/05. During the intensive sampling periods, release of phosphorus from the wetland mainly occurred during high flow. Sediment analyses showed that settling of inflow particles mostly occurred in the inlet pond, while the sediment found in a shallow vegetated area and outlet pond likely originated from internal processes rather than from the catchment. In fact, the gross sedimentation of phosphorus during April-July and July-August, respectively, exceeded the measured phosphorus inflow. The results showed that short periods with rapid flow increases were crucial for the wetlands function and thus high frequency sampling must be done during these periods. Furthermore, it seems that the particles lost from the catchment during high flows are too small to settle in the wetland.

I have done a follow up study of the restoration of two rivers, Röälven and Grundan, in order to evaluate the effects of the restoration on the endemic population of trout. Both rivers have been used for timber floating during many years throughout the 20th century, and because of this, had all obstacles such as rocks and wood parts removed. In 2004-2005 both rivers were restored, and rocks and wood were put back into the rivers. Spawning grounds were created and boulder dams were constructed to promote the streaming water. The result showed that the trout population of both rivers increased after the restoration, but also that the trout population of Röälven increased more than that of Grundan. My conclusion is that the way the restoration is of a river contributes to the result.

Phytoplankton produce allelochemicals; excreted chemical substances that are affecting other microorganisms in their direct environment. In my thesis, I investigated strain specific variability in the expression of allelochemicals of the harmful flagellate Prymnesium parvum, that is euryhaline but mainly bloom forming in brackish water. I found a large variation among strains, but further showed that all strains of P. parvum were more allelopathic in brackish water compared to marine water. In a marine microbial community, allelochemicals can affect prey, competitors and grazers both, directly and indirectly. For instance, in a food web where grazing controls prey abundance, the negative direct effect of allelochemicals on grazers will positive affect their prey. During my thesis, I investigated how marine microbial communities respond to the addition of allelochemicals. I performed field experiments with microbial communities from seawater collected from different places over Europe, and tested how this communities respond to the addition of allelochemicals from the dinoflagellate Alexandrium tamarense. Before I incubated the microbial communities for several days with A. tamarense algal filtrate, I evaluated the allelopathic efficiency of the algal filtrates with an algal monoculture of Rhodomonas spp. This allowed me to compare the effect of A. tamarense filtrate between the different microbial communities. In general, bacteria reached higher abundances when allelochemicals were present. As allelochemicals also inhibited nanoflagellates and ciliates, we concluded, that allelochemicals indirectly benefit bacteria by reducing grazing pressure. In microbial food webs with many heterotrophic grazers, allelochemicals further benefitted other phytoplankton by inhibiting grazers. It was also shown that bioavailable DOM is released from a microbial community when allelochemicals are present. As most DOM was released from the seawater fraction > 60 μm, we concluded, that larger microorganisms are more affected by allelochemicals than smaller microorganisms. The results can be explained by the surface to volume ratio of microorganisms: Larger organisms provide more contact surface for allelochemicals, and therefore, are probably more vulnerable towards allelochemicals. In conclusion, the effect of allelochemicals on a microbial community depends among others on the structure of the microbial food web, the amount of available DOM, the particle density in the seawater and the composition of the phytoplankton community.

<p>In this thesis I investigate predator-induced effects on behavioural and life-history characteristics of prey. At any moment a given predator is capable of attacking a small number of prey. However, the mere presence of a predator may impact a much larger number of individuals, as prey implement various behavioural and developmental mechanisms to reduce the risk of predation. It has become increasingly clear that predator induced responses have the potential to affect patterns of species abundance and distribution as well as individual fitness of prey. I study these responses by incorporating field surveys, semi-field experiments and laboratory experiments. All experiments were done in an aquatic environment using fish or large odonate larvae as predators and damselfly-or diving beetle larvae as prey.</p><p>My work highlights the importance of monitoring prey behaviour when studying life-history characteristics. I show that fish presence is an important factor for determining species abundance and distribution of odonates, and that prey behaviour may be a good predictor for fish vulnerability. Larval damselflies react behaviourally to predator presence by reducing activity and/or restricting habitat use. I confirm that such anti-predator responses have positive effects on prey survival in the presence of a predator but negative effects on growth and development of prey. In addition, my results suggest that the increase in per capita food resources for surviving prey following a predation episode (i.e. thinning) can have a stronger positive effect on prey growth and development than the negative effect of anti-predator responses. I also show that the strength of an anti-predator response is dependent on resource availability of the prey, with prey responding less strongly when resources are scarce. My results also indicate that the strength of the anti-predator response of damselfly larvae depends on predator diet and larval age. Predators feeding on prey conspecifics induce a stronger behavioural response in young larva than predators that feed on prey heterospecifics do. This diet-effect was not found in larvae late in ontogeny, due to an increased activity of larva where predators consumed damselflies. Such increased larval activity can be explained as a reaction to a time-constraint. Finally, I found that activity of damselfly larvae is genetically determined and that this has lead to a behavioural syndrome that might limit larval plasticity to a certain activity-range. This phenomenon may have implications for how well larvae are able to react to both biotic and abiotic changes in the environment.</p>

In this thesis I investigate predator-induced effects on behavioural and life-history characteristics of prey. At any moment a given predator is capable of attacking a small number of prey. However, the mere presence of a predator may impact a much larger number of individuals, as prey implement various behavioural and developmental mechanisms to reduce the risk of predation. It has become increasingly clear that predator induced responses have the potential to affect patterns of species abundance and distribution as well as individual fitness of prey. I study these responses by incorporating field surveys, semi-field experiments and laboratory experiments. All experiments were done in an aquatic environment using fish or large odonate larvae as predators and damselfly-or diving beetle larvae as prey. My work highlights the importance of monitoring prey behaviour when studying life-history characteristics. I show that fish presence is an important factor for determining species abundance and distribution of odonates, and that prey behaviour may be a good predictor for fish vulnerability. Larval damselflies react behaviourally to predator presence by reducing activity and/or restricting habitat use. I confirm that such anti-predator responses have positive effects on prey survival in the presence of a predator but negative effects on growth and development of prey. In addition, my results suggest that the increase in per capita food resources for surviving prey following a predation episode (i.e. thinning) can have a stronger positive effect on prey growth and development than the negative effect of anti-predator responses. I also show that the strength of an anti-predator response is dependent on resource availability of the prey, with prey responding less strongly when resources are scarce. My results also indicate that the strength of the anti-predator response of damselfly larvae depends on predator diet and larval age. Predators feeding on prey conspecifics induce a stronger behavioural response in young larva than predators that feed on prey heterospecifics do. This diet-effect was not found in larvae late in ontogeny, due to an increased activity of larva where predators consumed damselflies. Such increased larval activity can be explained as a reaction to a time-constraint. Finally, I found that activity of damselfly larvae is genetically determined and that this has lead to a behavioural syndrome that might limit larval plasticity to a certain activity-range. This phenomenon may have implications for how well larvae are able to react to both biotic and abiotic changes in the environment.