Resilience to changes in lake trophic state: Nutrient allocation into Daphnia resting eggs

Interactions between two functionally distinct aquatic invertebrate herbivores complicate ecosystem- and population-level resilience

Resilience, the capacity for a system to bounce-back after a perturbation, is critical for conservation and restoration efforts. Different functional traits have differential effects on system-level resilience. We test this experimentally in a lab system consisting of algae consumed by zooplankton, snails, or both, using an eutrophication event as a perturbation. We examined seston settlement load, chlorophyll-a and ammonium concentration as gauges of resilience. We find that Daphnia magna increased our measures of resilience. But this effect is not consistent across ecosystem measures; in fact, D. magna increased the difference between disturbed and undisturbed treatments in seston settlement loads. We have some evidence of shifting reproductive strategy in response to perturbation in D. magna and in the presence of Physa sp. These shifts correspond with altered population levels in D. magna, suggesting feedback loops between the herbivore species. While these results suggest only an ambiguous connection between functional traits to ecosystem resilience, they point to the difficulties in establishing such a link: indirect effects of one species on reproduction of another and different scales of response among components of the system, are just two examples that may compromise the power of simple predictions.

A sterol-mediated gleaner-opportunist trade-off underlies the evolution of grazer resistance to cyanobacteria

The human-caused proliferation of cyanobacteria severely impacts consumers in freshwater ecosystems. Toxicity is often singled out as the sole trait to which consumers can adapt, even though cyanobacteria are not necessarily toxic and the lack of nutritionally critical sterols in cyanobacteria is known to impair consumers. We studied the relative significance of toxicity and dietary sterol deficiency in driving the evolution of grazer resistance to cyanobacteria in a large lake with a well-documented history of eutrophication and oligotrophication. Resurrecting decades-old Daphnia genotypes from the sediment allowed us to show that the evolution and subsequent loss of grazer resistance to cyanobacteria involved an adaptation to changes in both toxicity and dietary sterol availability. The adaptation of Daphnia to changes in cyanobacteria abundance revealed a sterol-mediated gleaner-opportunist trade-off. Genotypes from peak-eutrophic periods showed a higher affinity for dietary sterols at the cost of a lower maximum growth rate, whereas genotypes from more oligotrophic periods showed a lower affinity for dietary sterols in favour of a higher maximum growth rate. Our data corroborate the significance of sterols as limiting nutrients in aquatic food webs and highlight the applicability of the gleaner-opportunist trade-off for reconstructing eco-evolutionary processes.

Reversed evolution of grazer resistance to cyanobacteria

Exploring the capability of organisms to cope with human-caused environmental change is crucial for assessing the risk of extinction and biodiversity loss. We study the consequences of changing nutrient pollution for the freshwater keystone grazer, Daphnia, in a large lake with a well-documented history of eutrophication and oligotrophication. Experiments using decades-old genotypes resurrected from the sediment egg bank revealed that nutrient enrichment in the middle of the 20th century, resulting in the proliferation of harmful cyanobacteria, led to the rapid evolution of grazer resistance to cyanobacteria. We show here that the subsequent reduction in nutrient input, accompanied by a decrease in cyanobacteria, resulted in the re-emergence of highly susceptible Daphnia genotypes. Expression and subsequent loss of grazer resistance occurred at high evolutionary rates, suggesting opposing selection and that maintaining resistance was costly. We provide a rare example of reversed evolution of a fitness-relevant trait in response to relaxed selection.

Anthropogenic changes, such as eutrophication from lake pollution, can lead to rapid evolution. Comparing Daphnia resurrected from generations adapted to historical pollution to contemporary, post-cleanup populations finds that Daphnia rapidly reversed their evolved resistance to harmful cyanobacteria.

Next-generation sequencing of DNA from resting eggs: signatures of eutrophication in a lake's sediment

Hatching resting stages of ecologically important organisms such as Daphnia from lake sediments, referred to as resurrection ecology, is a powerful approach to assess changes in alleles and traits over time. However, the utility of the approach is constrained by a few obstacles, including low and/or biased hatching among genotypes. Here, we eliminated such bottlenecks by investigating DNA sequences isolated directly (i.e. without hatching) from resting eggs found in the sediments of Lake Constance spanning pre-, peri-, and post-eutrophication. While we expected genome-wide changes, we specifically expected changes in alleles related to pathways involved in mitigating effects of cyanobacterial toxins. We used pairwise F_ST-analyses to identify transcripts that showed strongest divergence among the four different populations and a clustering analysis to identify correlations between allele frequency shifts and changes in abiotic and biotic lake parameters. In a cluster that correlated with the increased abundance of cyanobacteria in Lake Constance we find genes that have been reported earlier to be differentially expressed in response to the cyanobacterial toxin microcystin and to microcystin-free cyanobacteria. We further reveal the enrichment of gene ontology terms that have been shown to be involved in microcystin-related responses in other organisms but not yet in Daphnia and as such are candidate loci for adaptation of natural Daphnia populations to increased cyanobacterial abundances. In conclusion this approach of investigating DNA extracted from Daphnia resting stages allowed to determine frequency changes of loci in a natural population over time.