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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]

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]

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]