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Understanding the spatial scales at which organisms can adapt to strong natural and human-induced environmental gradients is important. Salinization is a key threat to biodiversity, ecosystem functioning and the provision of ecosystem services of freshwater systems. Clusters of naturally saline habitats represent ideal test cases to study the extent and scale of local adaptation to salinization. We studied local adaptation of the water flea Daphnia magna, a key component of pond food webs, to salinity in two contrasting landscapes—a dense cluster of sodic bomb crater ponds and a larger-scale cluster of soda pans. We show regional differentiation in salinity tolerance reflecting the higher salinity levels of soda pans versus bomb crater ponds. We found local adaptation to differences in salinity levels at the scale of tens of metres among bomb crater pond populations but not among geographically more distant soda pan populations. More saline bomb crater ponds showed an upward shift of the minimum salt tolerance observed across clones and a consequent gradual loss of less tolerant clones in a nested pattern. Our results show evolutionary adaptation to salinity gradients at different spatial scales, including fine-tuned local adaptation in neighbouring habitat patches in a natural landscape. [ABSTRACT FROM AUTHOR]

Habitat loss and fragmentation are growing global threats to natural habitats and their networks, posing significant challenges to biodiversity conservation. Globally, ponds are sharply declining in number because their small size makes them highly vulnerable to land use changes. While it is generally agreed that connectivity in habitat networks is crucial for sustaining biodiversity, the effect of connectivity on biodiversity patterns over small‐scaled habitat networks has so far received less attention given the general assumption that metacommunities lack spatial structuring on small scales. In this study, we tested whether this holds for multiple passively and actively dispersing organism groups in a well‐delineated pond metacommunity of 54 bomb crater ponds situated within 1 km. We investigated the influence of space and environment on species richness and metacommunity structure in these ponds, which share similar age, size, and shape and are subject to strong environmental gradients, making it an ideal study system. We specifically examined the impact of network centrality on taxonomic richness and eigenvector‐based spatial arrangement on metacommunity structure across different organism groups, including prokaryotes, microeukaryotes, zooplankton, macroinvertebrates, and amphibians. We found that while environmental filtering is the primary driver of community dynamics, there is also a significant spatial signal, particularly for passively dispersing groups, demonstrating the role of the central‐peripheral connectivity gradient. These findings highlight the importance of studying and protecting ponds as parts of a network rather than focusing on individual ponds. [ABSTRACT FROM AUTHOR]

Climate change‐related heatwaves are major threats to biodiversity and ecosystem functioning. However, our current understanding of the mechanisms governing community resistance to and recovery from extreme temperature events is still rudimentary. The spatial insurance hypothesis postulates that diverse regional species pools can buffer ecosystem functioning against local disturbances through the immigration of better‐adapted taxa. Yet, experimental evidence for such predictions from multi‐trophic communities and pulse‐type disturbances, like heatwaves, is largely missing. We performed an experimental mesocosm study to test whether species dispersal from natural lakes prior to a simulated heatwave could increase the resistance and recovery of plankton communities. As the buffering effect of dispersal may differ among trophic groups, we independently manipulated the dispersal of organisms from lower (phytoplankton) and higher (zooplankton) trophic levels. The experimental heatwave suppressed total community biomass by having a strong negative effect on zooplankton biomass, probably due to a heat‐induced increase in metabolic costs, resulting in weaker top‐down control on phytoplankton. While zooplankton dispersal did not alleviate the negative heatwave effects on zooplankton biomass, phytoplankton dispersal enhanced biomass recovery at the level of primary producers, providing partial evidence for spatial insurance. The differential responses to dispersal may be linked to the much larger regional species pool of phytoplankton than of zooplankton. Our results suggest high recovery capacity of community biomass independent of dispersal. However, community composition and trophic structure remained altered due to the heatwave, implying longer‐lasting changes in ecosystem functioning. [ABSTRACT FROM AUTHOR]

Chytrid fungal parasites increase herbivory and dietary access to essential molecules, such as polyunsaturated fatty acids (PUFA), at the phytoplankton–zooplankton interface. Warming enhances cyanobacteria blooms and decreases algae-derived PUFA for zooplankton. Whether chytrids could support zooplankton with PUFA under global warming scenarios remains unknown. We tested the combined effect of water temperature (ambient: 18°C, heat: +6°C) and the presence of chytrids with Daphnia magna as the consumer, and Planktothrix rubescens as the main diet. We hypothesized that chytrids would support Daphnia fitness with PUFA, irrespective of water temperature. Heating was detrimental to the fitness of Daphnia when feeding solely on the Planktothrix diet. Chytrid-infected Planktothrix diet alleviated the negative impact of heat and could support Daphnia survival, somatic growth and reproduction. Carbon stable isotopes of fatty acids highlighted a ~3x more efficient n-3 than n-6 PUFA conversion by Daphnia feeding on the chytrid-infected diet, irrespective of temperature. The chytrid diet significantly increased eicosapentaenoic acid (EPA; 20:5n-3) and arachidonic acid (ARA; 20:4n-6) retention in Daphnia. The EPA retention remained unaffected, while ARA retention increased in response to heat. We conclude that chytrids support pelagic ecosystem functioning under cyanobacteria blooms and global warming via chytrids-conveyed PUFA toward higher trophic levels. [ABSTRACT FROM AUTHOR]

Salinity is a major environmental predictor of phytoplankton species richness and composition. We hypothesize that the variation in phytoplankton richness along coastal salinity gradients is reflected in essential ecosystem functions like resource use efficiency (RUE)—the proportion of limiting resource that is converted into biomass. To test the hypothesis, we analyzed time series of phytoplankton and environmental variables from the Chesapeake Bay and the Baltic Sea. We analyzed the relationship among salinity, diversity, and RUE in bivariate and interaction association, and in structural equation model (SEM)—a form of path analysis to resolve multivariate relationships among interrelated variables. We concluded that the intrusion of marine water will lead to rapid change in species diversity on the fresh side. Despite diversity drop, individual community functions, like RUE, remain relatively resilient, reflecting functional redundancy. We propose that the salinity gradient also reflects trophic complexity, allowing stable resource use at reduced diversity. [ABSTRACT FROM AUTHOR]

Chytrid fungal parasites convert dietary energy and essential dietary molecules, such as long‐chain (LC) polyunsaturated fatty acids (PUFA), from inedible algal/cyanobacteria hosts into edible zoospores. How the improved biochemical PUFA composition of chytrid‐infected diet may extend to zooplankton, linking diet quality to consumer fitness, remains unexplored.Here, we assessed the trophic role of chytrids in supporting dietary energy and PUFA requirements of the crustacean zooplankton Daphnia, when feeding on the filamentous cyanobacterium Planktothrix.Only Daphnia feeding on chytrid‐infected Planktothrix reproduced successfully and had significantly higher survival and growth rates compared with Daphnia feeding on the sole Planktothrix diet. While the presence of chytrids resulted in a two‐fold increase of carbon ingested by Daphnia, carbon assimilation increased by a factor of four, clearly indicating enhanced carbon transfer efficiency with chytrid presence.Bulk carbon (δ13C) and nitrogen (δ15N) stable isotopes did not indicate any treatment‐specific dietary effects on Daphnia, nor differences in trophic position among diet sources and the consumer. Compound‐specific carbon isotopes of fatty acids (δ13CFA), however, revealed that chytrids bioconverted short‐chain to LC‐PUFA, making it available for Daphnia. Chytrids synthesised the ω‐3 PUFA stearidonic acid de novo, which was selectively retained by Daphnia. Values of δ13CFA demonstrated that Daphnia also bioconverted short‐chain to LC‐PUFA.We provide isotopic evidence that chytrids improved the dietary provision of LC‐PUFA for Daphnia and enhanced their fitness. We argue for the existence of a positive feedback loop between enhanced Daphnia growth and herbivory in response to chytrid‐mediated improved diet quality. Chytrids upgrade carbon from the primary producer and facilitate energy and PUFA transfer to primary consumers, potentially also benefitting upper trophic levels of pelagic food webs. [ABSTRACT FROM AUTHOR]

Zoosporic fungi of the phylum Chytridiomycota are ubiquitous parasites of phytoplankton in aquatic ecosystems, but little is known about phytoplankton defense strategies against parasitic chytrid attacks. Using a model chytrid-phytoplankton pathosystem, we experimentally tested the hypothesis that the mucilage envelope of a mucilage-forming desmid species provides protection against the parasitic chytrid Staurastromyces oculus. Mucilage-forming Staurodesmu s cells were not accessible to the chytrid, whereas physical removal of the mucilage envelope rendered the same Staurodesmus sp. strain equally susceptible to chytrid infections as the original non-mucilage-forming host Staurastrum sp. Epidemic spread of the parasite only occurred in Staurastrum sp. whereas non-mucilage-bearing Staurodesmus sp. allowed for co-existence of host and parasite, and mucilage-bearing Staurodesmus sp. caused parasite extinction. In addition to the mucilage defense barrier, we also demonstrate the ability of both Staurastrum sp. and Staurodesmus sp. to resist infection by preventing chytrid development while still remaining viable and being able to reproduce and thus recover from an infection. This study extends our knowledge on phytoplankton defense traits and the functional role of mucilage in phytoplankton as a physical barrier against fungal parasites. [ABSTRACT FROM AUTHOR]

Summary: Mixotrophy usually is considered with respect to the advantages gained and the associated trade‐offs of this form of nutrition, compared to specialized competitors, strict photoautotrophs and heterotrophs. However, we currently have an incomplete understanding of the functional diversity of mixotrophs and the factors controlling niche differentiation in different mixotrophic species. Here we experimentally studied the light‐dependent niche differentiation in two chrysophyte species. We show that the newly isolated Ochromonas sp. is an obligate phototroph and possibly an obligate mixotroph. In contrast, Poterioochromonas malhamensis is a facultative mixotroph; photosynthesis and heterotrophy in this species represent substitutable routes of resource acquisition. We further hypothesize that the variable plasticity in the considered traits of the here tested species may result in different niche differentiation with regard to a vertical light gradient. Ochromonas sp. should perform better in stable stratified surface water layers, where light is available, but prey abundances might be low. However, P. malhamensis should be able to also successfully grow in deeper water layers, benefiting from higher bacterial production. This study represents a first step towards understanding competition between mixotrophs engaging in different physiological strategies, and consequently their potential co‐occurrence due to niche differentiation. [ABSTRACT FROM AUTHOR]

Aim: Waterbirds are important dispersal vectors of multicellular organisms; however, no study to date has focused on their potential role in dispersing aquatic microbial communities. We explicitly studied endozoochory of prokaryotes and unicellular microeukaryotes by waterbirds using DNA metabarcoding. By directly comparing the dispersed set of organisms to the source pool of a natural metacommunity, we aimed at a realistic estimate of the importance of waterbird zoochory for natural microbial communities. Location: Temporary saline soda pans in Austria and Hungary. Taxon: Prokaryotes and unicellular microeukaryotes. Methods: In 2017 and 2018, water samples were collected from a network of 25 temporary ponds along with fresh droppings of five waterbird species including the dominant greylag goose (Anser anser). Prokaryotic and microeukaryotic communities were identified via 16S and 18S rRNA gene amplicon sequencing. After quality filtering of sequence reads, pro‐ and microeukaryotic amplicon sequence variant (ASV) compositions were compared between the aquatic and dropping samples, across years and waterbird species. Results: 28% of the dominant aquatic prokaryotic and 19% of the microeukaryotic ASVs were transported by A. anser. ASV richness was lower, but compositional variation was higher in A. anser droppings than in aquatic communities, probably resulting from stochastic pick‐up from multiple aquatic habitats. The composition of prokaryotic ASVs in bird droppings differed among the 2 years and reflected the actual aquatic communities. The dispersed set of microbes were largely similar among the waterbird species except for the planktivore filter‐feeder northern shoveler (Spatula clypeata), which dispersed more microeukaryotes than the other waterbirds. Main conclusions: Using an amplicon sequencing approach to characterize aquatic microorganisms in waterbird droppings and in the associated environment, our study provides strong evidence for endozoochory of natural communities. These results imply that waterbirds may be crucial in maintaining ecological connectivity between aquatic habitats at the level of microbial communities. [ABSTRACT FROM AUTHOR]

In metacommunity ecology, a major focus has been on combining observational and analytical approaches to identify the role of critical assembly processes, such as dispersal limitation and environmental filtering, but this work has largely ignored temporal community dynamics. Here, we develop a "virtual ecologist" approach to evaluate assembly processes by simulating metacommunities varying in three main processes: density‐independent responses to abiotic conditions, density‐dependent biotic interactions, and dispersal. We then calculate a number of commonly used summary statistics of community structure in space and time and use random forests to evaluate their utility for inferring the strength of these three processes. We find that (i) both spatial and temporal data are necessary to disentangle metacommunity processes based on the summary statistics we test, and including statistics that are measured through time increases the explanatory power of random forests by up to 59% compared to cases where only spatial variation is considered; (ii) the three studied processes can be distinguished with different descriptors; and (iii) each summary statistic is differently sensitive to temporal and spatial sampling effort. Including repeated observations of metacommunities over time was essential for inferring the metacommunity processes, particularly dispersal. Some of the most useful statistics include the coefficient of variation of species abundances through time and metrics that incorporate variation in the relative abundances (evenness) of species. We conclude that a combination of methods and summary statistics is probably necessary to understand the processes that underlie metacommunity assembly through space and time, but we recognize that these results will be modified when other processes or summary statistics are used. [ABSTRACT FROM AUTHOR]

The importance of mixotrophic algae as key bacterivores in microbial food webs is increasingly acknowledged, but their effects on the next trophic level remain poorly understood. Their high stoichiometric food quality is contrasted by anti‐grazing strategies.We tested the quality of freshwater mixotrophs as prey for zooplankton, using four non‐colonial chrysophyte species and a cryptophyte as a high quality reference food. We (1) analyzed the stoichiometric and biochemical (fatty acid) composition of the mixotrophs, and (2) quantified their dietary effects on Daphnia longispina survival.Survival of D. longispina significantly depended on the identity of species provided as food, ranging from higher to lower as compared to starvation. This was not reflected in differences in cellular stoichiometry or fatty acid profiles of the mixotrophs. We suggest that toxicity may be the driver for the observed differences.Generalization of the dietary effects of mixotrophic chrysophytes does not appear straightforward. Besides fundamental species‐specific differences, potential toxic effects may vary depending on environmental cues or physiological strategies. Notably in our study, Ochromonas tuberculata, a species previously reported to be deleterious, turned out to be a beneficial food source in terms of enabling high survival of D. longispina.We challenge the generality of the assumption that chrysophytes are of low value as food for zooplankton. We recommend that future studies test how environmental conditions and physiological strategies shape the quality of mixotrophs as food for consumers at higher trophic levels, specifically focusing on effects of dietary toxicity. [ABSTRACT FROM AUTHOR]