Salinization of freshwater ecosystems threatens global aquatic biodiversity. There is a need for studies that follow populations in situ during salinization to understand the effects on species and ecosystems. We follow 170 years of Daphnia dynamics in the sediment ephippia archive of a small urban lake near St. Paul, Minnesota, to characterize effects of severe recent salinization on lake Daphnia. We found modest changes in the flux of ephippia in this lake; all three key Daphnia functional groups remained in the assemblage throughout the period of salinization. Reconstruction of the size distribution of Daphnia pulicaria demonstrated a consistent increase in body size in opposition to the expected trajectory during salinization. Our findings highlight that in hard‐water lakes, the effects of salinization are nuanced and require further investigation to better understand overall impacts of salinization on hard‐water lake Daphnia assemblages.

Speciation genomic studies have revealed that genomes of diverging lineages are shaped jointly by the actions of gene flow and selection. These evolutionary forces acting in concert with processes such as recombination and genome features such as gene density shape a mosaic landscape of divergence. We investigated the roles of recombination and gene density in shaping the patterns of differentiation and divergence between the cyclically parthenogenetic ecological sister‐taxa, Daphnia pulicaria and Daphnia pulex. First, we assembled a phased chromosome‐scale genome assembly using trio‐binning for D. pulicaria and constructed a genetic map using an F2‐intercross panel to understand sex‐specific recombination rate heterogeneity. Finally, we used a ddRADseq data set with broad geographic sampling of D. pulicaria, D. pulex, and their hybrids to understand the patterns of genome‐scale divergence and demographic parameters. Our study provides the first sex‐specific estimates of recombination rates for a cyclical parthenogen, and unlike other eukaryotic species, we observed male‐biased heterochiasmy in D. pulicaria, which may be related to this somewhat unique breeding mode. Additionally, regions of high gene density and recombination are generally more divergent than regions of suppressed recombination. Outlier analysis indicated that divergent genomic regions are probably driven by selection on D. pulicaria, the derived lineage colonizing a novel lake habitat. Together, our study supports a scenario of selection acting on genes related to local adaptation shaping genome‐wide patterns of differentiation despite high local recombination rates in this species complex. Finally, we discuss the limitations of our data in light of demographic uncertainty.

Ecologists and evolutionary biologists are increasingly cognizant of rapid adaptation in wild populations. Rapid adaptation to anthropogenic environmental change is critical for maintaining biodiversity and ecosystems services into the future. Anthropogenic salinization of freshwater ecosystems is quickly emerging as a primary threat, which is well documented in the northern temperate ecoregion. Specifically, many northern temperate lakes have undergone extensive salinization because of urbanization and the associated increase in impervious surfaces causing runoff, and the extensive use of road deicing salts (e.g., NaCl). It remains unclear whether increasing salinization will lead to extirpation of species from these systems. Using a “resurrection genomics” approach, we investigated whether the keystone aquatic herbivore, Daphnia pulicaria, has evolved increased salinity tolerance in a severely salinized lake located in Minnesota, USA. Whole-genome resequencing of 54 Daphnia clones from the lake and hatched from resting eggs that represent a 25-y temporal contrast demonstrates that many regions of the genome containing genes related to osmoregulation are under selection in the study population. Tolerance assays of clones revealed that the most recent clones are more tolerant to salinity than older clones; this pattern is concomitant with the temporal pattern of stabilizing salinity in this lake. Together, our results demonstrate that keystone species such as Daphnia can rapidly adapt to increasing freshwater salinization. Further, our results indicate that rapid adaptation to salinity may allow lake Daphnia populations to persist in the face of anthropogenic salinization maintaining the food webs and ecosystem services they support despite global environmental change.

Global climate change is expected to both increase average temperatures as well as temperature variability. Increased average temperatures have led to earlier breeding in many spring‐breeding organisms. However, individuals breeding earlier will also face increased temperature fluctuations, including exposure to potentially harmful cold‐temperature regimes during early developmental stages. Using a model spring‐breeding amphibian, we investigated how embryonic exposure to different cold‐temperature regimes (control, cold‐pulse, and cold‐press) affected (a) compensatory larval development and growth, (b) larval susceptibility to a common contaminant, and (c) larval susceptibility to parasites. We found: (a) no evidence of compensatory development or growth, (b) larvae exposed to the cold‐press treatment were more susceptible to NaCl at 4‐days post‐hatching but recovered by 17‐days post‐hatching, and (c) larvae exposed to both cold treatments were less susceptible to parasites. These results demonstrate that variation in cold‐temperature regimes can lead to unique direct and indirect effects on larval growth, development, and response to stressors. This underscores the importance of considering cold‐temperature variability and not just increased average temperatures when examining the impacts of climate disruption.

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