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Organisms well suited for the study of ecotype formation have wide distribution ranges, where they adapt to multiple drastically different habitats repeatedly over space and time. Here we study such ecotypes in a Crustacean model, Asellus aquaticus, a commonly occurring isopod found in freshwater habitats as diverse as streams, caves and lakes. Previous studies focusing on cave vs. surface ecotypes have attributed depigmentation, eye loss and prolonged antennae to several south European cave systems. Likewise, surveys across multiple Swedish lakes have identified the presence of dark‐pigmented "reed" and light‐pigmented "stonewort" ecotypes, which can be found within the same lake. In this study, we sequenced the first draft genome of A. aquaticus, and subsequently use this to map reads and call variants in surface stream, cave and two lake ecotypes. In addition, the draft genome was combined with a RADseq approach to perform a quantitative trait locus (QTL) mapping study using a laboratory bred F2 and F4 cave × surface intercross. We identified genomic regions associated with body pigmentation, antennae length and body size. Furthermore, we compared genome‐wide differentiation between natural populations and found several genes potentially associated with these habitats. The assessment of the cave QTL regions in the light–dark comparison of lake populations suggests that the regions associated with cave adaptation are also involved with genomic differentiation in the lake ecotypes. These demonstrate how troglomorphic adaptations can be used as a model for related ecotype formation. [ABSTRACT FROM AUTHOR]

A number of mechanisms result in a feedback between water clarity and macrophytes and, consequently, the occurrence of alternative stable states in shallow lakes. We hypothesize that bottom-up mechanisms and interactions within the benthic food web are more important in a charophyte-dominated clear-water state, while top-down mechanism and interactions in the planktonic food web prevail at angiosperm dominance. Charophytes, which dominate at lower nutrient concentrations and develop higher densities than most angiosperms, can have a higher influence on sedimentation, resuspension, and water column nutrients. During dominance of dense submerged vegetation like charophytes, zooplankton can be hampered by low food quality and quantity and by high predation pressure from juvenile fish, which in turn are favoured by the high refuge potential of this vegetation. Grazing pressure from zooplankton on phytoplankton can therefore be low in charophytes, but the main feedback in angiosperm-dominated ecosystems. Charophytes offer a higher surface than most angiosperms to periphyton, which favors benthic invertebrates. These support macrophytes by grazing periphyton and constitute a central link in a trophic cascade from fish to periphyton and macrophytes. To test these hypotheses, more experiments and field measurements comparing the effect of charophytes and angiosperms on water clarity are needed. [ABSTRACT FROM AUTHOR]

In the present study, we aim at relating Forbes' remarkable paper on 'The lake as a microcosm', published 125 years ago, to the present status of knowledge in our own research group. Hence, we relate the observations Forbes made to our own microcosm, Lake Krankesjön in southern Sweden, that has been intensively studied by several research groups for more than three decades. Specifically, we focus on the question: Have we made any significant progress or did Forbes and colleagues blaze the trail through the unknown wilderness and we are mainly paving that intellectual road? We conclude that lakes are more isolated than many other biomes, but have, indeed, many extensions, for example, input from the catchment, fishing and fish migration. We also conclude that irrespective of whether lakes should be viewed as microcosms or not, the paper by Forbes has been exceptionally influential and still is, especially since it touches upon almost all aspects of the lake ecosystem, from individual behaviour to food web interactions and environmental issues. Therefore, there is no doubt that even if 125 years have passed, Forbes' paper still is a source of inspiration and deserves to be read. Hence, although aquatic ecology has made considerable progress over the latest century, Forbes might be viewed as one of the major pioneers and visionary scientists of limnology. [ABSTRACT FROM AUTHOR]

The question of how diverging populations become separate species by restraining gene flow is a central issue in evolutionary biology. Assortative mating might emerge early during adaptive divergence, but the role of other types of reproductive barriers such as migration modification have recently received increased attention. We demonstrate that two recently diverged ecotypes of a freshwater isopod ( Asellus aquaticus) have rapidly developed premating isolation, and this isolation barrier has emerged independently and in parallel in two south Swedish lakes. This is consistent with ecological speciation theory, which predicts that reproductive isolation arises as a byproduct of ecological divergence. We also find that in one of these lakes, habitat choice acts as the main barrier to gene flow. These observations and experimental results suggest that migration modification might be as important as assortative mating in the early stages of ecological speciation. Simulations suggest that the joint action of these two isolating barriers is likely to greatly facilitate adaptive divergence, compared to if each barrier was acting alone. [ABSTRACT FROM AUTHOR]

The importance of lake ecosystems for waterfowl remains a topic of debate. In order to assess how temporal variations in lake features, specifically shifts between alternative stable states, may interact with the waterfowl fauna, we performed a long-term (22 years) study of the shallow Lake Krankesjön, southern Sweden. Lower total numbers of waterfowl occurred during periods with low macrophyte cover and turbid water, than when submersed macrophytes flourished and the water was clear. Some specific functional groups of waterfowl, such as herbivores, invertebrate, and fish feeders, showed a positive relation to clear water and high macrophyte cover. Hence, our data suggest that some migratory waterfowl may select lakes based on water quality, thereby adjusting their large-scale migratory routes. On the other hand, omnivorous waterfowl exhibited their highest abundances during turbid conditions. Furthermore, waterfowl not primarily relying on food from the lake showed no response to fluctuations in turbidity or macrophyte cover, but followed regional trends in population dynamics. In our study lake, L. Krankesjön, we estimated that waterfowl remove less than 3% of the macrophyte biomass during a stable clear-water state with lush macrophyte beds. However, during transition periods between alternative stable states, when macrophyte biomass is lower and the plants already stressed, the consumption rate of waterfowl may have a stronger effect on lake ecosystem functioning. [ABSTRACT FROM AUTHOR]

Divergent natural selection is often thought to be the principal factor driving phenotypic differentiation between populations. We studied two ecotypes of the aquatic isopod Asellus aquaticus which have diverged in parallel in several Swedish lakes. In these lakes, isopods from reed belts along the shores colonized new stonewort stands in the centre of the lakes and rapid phenotypic changes in size and pigmentation followed after colonization. We investigated if selection was likely to be responsible for these observed phenotypic changes using indirect inferences of selection ( FST– QST analysis). Average QST for seven quantitative traits were higher than the average FST between ecotypes for putatively neutral markers (AFLPs). This suggests that divergent natural selection has played an important role during this rapid diversification. In contrast, the average QST between the different reed ecotype populations was not significantly different from the mean FST. Genetic drift could therefore not be excluded as an explanation for the minor differences between allopatric populations inhabiting the same source habitat. We complemented this traditional FST– QST approach by comparing the FST distributions across all loci ( n = 67–71) with the QST for each of the seven traits. This analysis revealed that pigmentation traits had diverged to a greater extent and at higher evolutionary rates than size-related morphological traits. In conclusion, this extended and detailed type of FST– QST analysis provides a powerful method to infer adaptive phenotypic divergence between populations. However, indirect inferences about the operation of divergent selection should be analyzed on a per-trait basis and complemented with detailed ecological information. [ABSTRACT FROM AUTHOR]

During the past century, Lake Tåkern and Lake Krankesjön, southern Sweden, have shifted repeatedly between a state of clear water and abundant submerged vegetation, and a state of turbid water and sparse vegetation. Long-term empirical data on such apparently alternative stable state dynamics are valuable as complements to modeling and experiments, although the causal mechanisms behind shifts are often difficult to identify in hindsight. Here, we summarize previous studies and discuss possible mechanisms behind the shifts. The most detailed information comes from monitoring of two recent shifts, one in each lake. In the 1980s, L. Krankesjön shifted to clear water following an expansion of sago pondweed, Potamogeton pectinatus. Water clarity increased when the pondweed was replaced by characeans. Zooplankton biomass in summer declined and the concentration of total phosphorus (TP) was reduced to half the previous level. The fish community changed over several years, including an increasing recruitment of piscivorous perch ( Perca fluviatilis). An opposite directed shift to turbid water occurred in Lake Tåkern in 1995, when biomass of phytoplankton increased in spring, at the expense of submerged vegetation. Consistent with the findings in L. Krankesjön, phyto- and zooplankton biomass increased and the average concentration of TP doubled. After the shift to clear water in L. Krankesjön, TP concentration has increased during the latest decade, supporting the idea that accumulation of nutrients may lead to a long-term destabilization of the clear water state. In L. Tåkern, data on TP are inconclusive, but organic nitrogen concentrations oscillated during a 25-year period of clear water. These observations indicate that intrinsic processes cause gradual or periodic changes in system stability, although we cannot exclude the possibility that external forces are also involved. During such phases of destabilization of the clear water state, even small disturbances could possibly trigger a shift, which may explain why causes behind shifts are hard to identify even when they occur during periods of extensive monitoring. [ABSTRACT FROM AUTHOR]

Various ecosystems, including shallow lakes, are suggested to possess alternative stable state dynamics. The response of such systems to environmental change is non-linear and not fully reversible, which calls for identification of feedback mechanisms and subtle changes connected to structural stability. Here, we used a 25-year data series on water chemistry to make inferences on processes prior to a recent shift from a clear to a turbid state in Lake Tåkern, Sweden. Before the shift, annual concentration of total organic nitrogen (TON) described a cyclic pattern, with a periodicity of eight years. Annual TON was negatively correlated with the magnitude of a summer decline in calcium carbonate, treated as a proxy of the seasonal production of submerged vegetation. Cross-correlations of TON and the north Atlantic oscillation (NAO) indicated a possible connection to climate. The strongest correlation was obtained by a three-year lag of the NAO index. Using a set of linear time series models, the most parsimonious model was a 3rd order autoregressive model with NAO, delayed three years, as a covariate. Analyses of seasonality indicated that the delayed NAO signal was strongly correlated with summer TON. Also, the autocorrelation function was very similar for these two time series, and autoregressive models including NAO as a covariate were strongly supported (sum of Akaike weights=0.93). These results indicate that climate may have contributed to the regime shift through lowered macrophyte production at the time of the shift, and therefore most likely also a depleted resilience of the clear water state. The delayed effect of climate is suggested to result from indirect and inter-year dependent response of submerged vegetation to fish kills during harsh winters. [ABSTRACT FROM AUTHOR]

1. In this study, we examine how a 7-year period of expanding submerged stonewort ( Chara spp.) vegetation during a shift from turbid to clear water in a shallow lake influenced individual growth and population size structure of perch ( Perca fluviatilis). We expected that a shift from phytoplankton to macrophyte dominance and clear water would improve feeding conditions for perch during a critical benthivorous ontogenetic stage, and enhance the recruitment of piscivorous perch. 2. Growth analysis based on opercula showed that growth during the second year of life was significantly higher in years with abundant vegetation than in years with turbid water and sparse vegetation. Growth was not affected during the first, third and fourth year of life. Stable isotope analyses on opercula from 2-year-old perch showed that the increase in growth coincided with a change in carbon source in the diet. Stable nitrogen ratio did not change, indicating that the increased growth was not an effect of any change in trophic position. 3. Following the expansion of submerged vegetation, perch size range and abundance of piscivorous perch increased in central, unvegetated areas of the lake. In stands of stoneworts, however, mainly benthivorous perch were caught, and size range did not change with time. 4. Our findings provide empirical support for the notion that establishment of submerged vegetation may lead to increased recruitment of piscivorous perch, because of improved competitive conditions for perch during the benthivorous stage. This is likely to constitute a benthic-pelagic feedback coupling, in which submerged vegetation and clear water promote the recruitment of piscivorous perch, which, in turn, may increase water clarity through top-down effects in the pelagic. [ABSTRACT FROM AUTHOR]

Pigmentation in the freshwater isopod Asellus aquaticus (Crustacea) differed between habitats in two Swedish lakes. In both lakes, isopods had lighter pigmentation in stands of submerged vegetation, consisting of stoneworts (Chara spp.), than in nearby stands of reed (Phragmites australis). Experimental crossings of light and dark isopods in a common environment showed that pigmentation had a genetic basis and that genetic variance was additive. Environmental effects of diet or chromatophore adjustment to the background had minor influence on pigmentation, as shown by laboratory rearing of isopods on stonewort or reed substrates, as well as analyses of stable isotope ratios for isopods collected in the field. In both study lakes, the average phenotype became lighter with time (across generations) in recently established stonewort stands. Taken together, these results indicate that altered phenotype pigmentation result from evolutionary responses to local differences in natural selection. Based on the assumption of two generations per year, the evolutionary rate of change in pigmentation was 0.08 standard deviations per generation (haldanes) over 20 generations in one lake and 0.22 haldanes over two generations in the other lake. This genetic change occurred during an episode of population growth in a novel habitat, a situation known to promote adaptive evolution. In addition, stonewort stands constitute large and persistent patches, characteristics that tend to preserve local adaptations produced by natural selection. Results from studies on selective forces behind the adaptive divergence suggest that selective predation from visually oriented predators is a possible selective agent. We found no indications of phenotype-specific movements between habitats. Mating within stonewort stands was random with respect to pigmentation, but on a whole-lake scale it is likely that mating is assortative, as a result of local differences in phenotype distribution. [ABSTRACT FROM AUTHOR]

1. We studied vertical and horizontal distribution of macroinvertebrates within a dense stand of Chara spp. in Lake Krankesjön, southern Sweden. Invertebrates were sampled at three depths within the vegetation and at three distances from the vegetation edge during day and night in July and August. Corresponding samples of oxygen content of the water were taken. 2. The densities (number of invertebrates per unit plant biomass) of most invertebrate taxa were generally lower in the upper layers of the vegetation than in the layers close to the sediment. The densities of several taxa (Asellus aquaticus, Cloëon sp. and Polycentropodidae), as well as total density of invertebrates, were higher at the edge than in the innermost parts of the macrophyte stand, whereas snail densities generally were highest at the innermost sites. 3. Densities of A. aquaticus, Chironomidae larvae and Helobdella stagnalis generally increased at night. These taxa appear to undertake a diel vertical migration within the vegetation, towards, or even down to the sediment in daytime and up into the vegetation, in some cases to the vegetation surface, at night. Factors underlying the diel vertical migration are discussed, as are their ecological consequences. [ABSTRACT FROM AUTHOR]

Summary1. We measured the abundance and biomass of filter-feeding microcrustacean zooplankton and calculated their grazing impact on phytoplankton biomass during summer in five shallow, mesotrophic to eutrophic lakes. For three of the lakes data exist both from years with dense submerged vegetation and low turbidity (the clearwater state), as well as from years characterised by sparse vegetation and high turbidity (the turbid state). In the other two lakes data are available only for clearwater conditions. 2. In all lakes conditions of dense vegetation and clear water coincided with a low abundance of crustacean plankton during summer. In the three lakes that shifted, the calculated biovolume ingested by crustacean plankton (filtering rate) was 3–11 times lower during clearwater conditions compared with turbid conditions. Because phytoplankton biomass was lower during clearwater conditions, however, daily grazing pressure from microcrustacea (expressed as percentage of phytoplankton biomass) did not differ between states. In three of the five lakes, grazers were estimated to take less than 10% of the phytoplankton biomass per day, indicating filtration by zooplankton was not the most important mechanism to maintain clearwater conditions. 3. High densities of Cladocera were found in three of the lakes within dense stands of Charophyta. However, these samples were dominated by plant-associated taxa that even during the night were rarely found outside the vegetation. This indicates that plant-associated zooplankton has no major influence on the maintenance of water clarity outside the vegetation. 4. Spring peak abundance of Cladocera was observed in three of the lakes. In two of these, where seasonal development was studied in both the clearwater and the turbid state, spring peaks were lower during the clearwater state. 5. Predation, low food availability or a combination of both may explain the low zooplankton densities. Phytoplankton may be limited by low phosphorus availability in the lakes dominated by Charophyta. Our results indicate that the importance of zooplankton grazing may have minor importance for the maintenance of the clearwater state in lakes with dense, well-established submerged vegetation. [ABSTRACT FROM AUTHOR]

The mortality and physiological status (body water content) of Asellus aquaticus (Isopoda) and Gammarus pulex (Amphipoda) were measured after 25 days exposure in 20 natural streams with a pH range of 4.3-7.5 and a colour range of 8-280 mg Pt L. In addition, the effects of keeping the animals as single species or together were studied. The response of Gammarus to low pH was an increased mortality and lower physiological status of surviving individuals in streams with a pH lower than 6.0. In Asellus the physiological status was correlated with pH, while the mortality was not pH dependent. The effects of humus on the physiological status of Asellus was significant when fitted to a second order polynomial function. The influence of humus can, however, be regarded as small relative to pH. The interactions between the species could be described as asymmetric under optimal conditions of high pH and low humus concentrations, where the presence of Gammarus decreased the survival and physiological status of Asellus. Acid stress did not seem to reverse the direction of this asymmetry, but the presence of Gammarus improved the physiological status of Asellus at pH lower than 6.0. Since the presence of Asellus did not increase the mortality or decrease the physiological status of Gammarus, this could be explained by Asellus feeding on Gammarus that died from physiological stress solely. This mechanism suggests that food quality, and thus effects of diffuse competition, can be important for the ability to withstand acid stress. The results, though, give no support for the hypothesis that competition from Asellus is important for the disappearance of Gammarus during the acidification of streams. [ABSTRACT FROM AUTHOR]