Microbiomes are increasingly recognized as key contributors to host fitness, yet their role in mediating stressor effects, especially across metamorphosis, remain poorly understood. To address this knowledge gap, we strongly reduced the gut microbiome of Ischnura elegans damselfly larvae using antibiotics followed by inoculation with a donor gut microbiome or not, and afterwards exposed the larvae to the pathogen Escherichia coli. Both host fitness and gut microbiome diversity and community composition were assessed during the larval and adult life stages. Despite marked differences in gut microbiome community composition between both life stages, partial retention of larval taxa in adult microbiota suggests incomplete microbiome turnover in these hemimetabolous insects. Furthermore, microbiome disruption significantly increased larval mortality, an effect mitigated by microbiome inoculation, underscoring the functional importance of microbial associations in damselflies. Moreover, pathogen exposure elevated larval mortality and tended to induce delayed mortality in the adult stage, revealing carry-over effects across metamorphosis. Yet, we did not find evidence of the gut microbiome mediating these carry-over effects. Together, our results highlight the critical role of the microbiome in determining host fitness, and the importance of considering both immediate and delayed stressor effects in animals with complex life cycles.

Cyanobacterial blooms represent one prevalent stressor in aquatic ecosystems worldwide, exposing aquatic animals to complex mixtures of live cells and dissolved bioactive compounds, including cyanotoxins. The gut microbiota, which plays fundamental roles in digestion, immunity, and metabolic regulation, has been recognized as a key interface between environmental stressors and host health. Exposure to cyanobacteria occurs primarily through ingestion, making the gut the main site of interaction with live cells, cyanobacterial metabolites, and associated bacteria. While dissolved bioactive compounds can also penetrate via gills and skin, their role is secondary in most animals. Understanding how cyanobacteria interact with the host gut microbiota is therefore essential to fully assess their impacts on animal health. This review summarizes current knowledge of gut microbiota responses to cyanobacterial stress in crustaceans, mollusks, and chordates, highlighting dynamic and context-dependent changes in microbial communities. We discuss the cascading effects on host physiology, immune function, and metabolic homeostasis, and explore how the microbiota may modulate host resilience, including potential roles in cyanotoxin degradation.

The space-for-time substitution (SFTS) approach is widely used to predict evolutionary trait responses to global warming. The current approach ignores the explicit role of the gut microbiome in shaping the future host phenotype, despite its strong influence on thermal adaptation and its ability to show more rapid adaptation than the host. We propose integrating reciprocal gut microbiome transplants into SFTS and identify a set of contrasts between treatment combinations to disentangle host and gut microbiome contributions to thermal adaptation under future warming. To illustrate our approach, we apply the proposed contrasts between treatment groups to reinterpret data on immune functioning of Ischnura elegans damselfly nymphs of which the gut microbiome was reciprocally transplanted between nymphs from warm-adapted low-latitude and cold-adapted high-latitude populations reared at both the cold (high-latitude) and warm (low-latitude) thermal regime. By disentangling the contributions of the host and its gut microbiome, our conceptual approach shows that gut microbiome adaptation can buffer against immune suppression in the high-latitude populations under future warming, while adaptation of the host will not. Incorporating the gut microbiome into SFTS may enhance realism in predicting species resilience to climate change and better inform conservation strategies under future climates.

Microbiomes are key determinants of host health, yet empirical evidence demonstrat-ing their influence on host-parasite interactions is limited. We conducted a proof-of-principle experiment using the water flea Daphnia magna and its virulent yeast para-site Australozyma monospora (formerly Metschnikowia bicuspidata) to test if microbi-ome composition alters infection outcomes. Axenic hosts were inoculated with a con-trol microbiome (homogenates of laboratory-cultured Daphnia reared in natural fresh-water) or a single bacterial strain (Rhizobium sp.), and exposed to the parasite. Host survival differed markedly between microbiome treatments and depended on parasite exposure. Prior to parasite exposure, hosts of one genotype exhibited high juvenile mortality when inoculated with the control microbiome (79%), compared to Rhizobium (19%) (the other genotype: 48% vs. 50%). Parasite exposure reduced survival, but the extent varied with microbiome composition: survival of hosts with a control micro-biome (averaged across genotypes) declined from 66% to 0%; survival of those inocu-lated with Rhizobium sp. declined from 35% to 10%. In contrast, microbiome composi-tion did not influence parasite infectivity or transmission. Our results indicate that mi-crobiome effects on host survival are genotype- and context-dependent, differing be-tween parasite-free and parasite-exposed conditions. Microbiome composition may therefore impact host-parasite dynamics primarily by influencing host viability rather than parasite performance. ### Competing Interest Statement The authors have declared no competing interest. Swiss Federal Institute of Aquatic Science and Technology, https://ror.org/00pc48d59 SNF, 310030 L 166628 Deutsche Forschungsgemeinschaft, https://ror.org/018mejw64, WO 1587/6-1 KU Leuven, https://ror.org/05f950310, C16/17/002

BACKGROUND:Host-associated microbiomes play an important role in the ecology and fitness of organisms. Given their significance, it is much debated to what extent these associations are widespread and even obligatory. Such frequent associations are captured by the concept of the core microbiome. The cladoceran Daphnia is a pivotal genus in freshwater ecosystems occupying a central position in the food webs of standing waters. With its unique standing in pelagic waters, Daphnia serves as a key grazer, regulating algal populations and nutrient cycling, making its microbiome essential to understanding ecosystem function and stability. In recent years, Daphnia has become an increasingly popular study system for exploring host‒microbiota interactions. There is, however, limited knowledge on the baseline taxa that consistently inhabit this host and potentially contribute to its fitness. Identifying whether such a host-associated "core microbiome" exists for Daphnia and, if so, which microbial taxa it comprises is important both for enhancing our ecological understanding of this genus and its ecosystem function and for interpreting future experiments. RESULTS:We compiled a dataset on Daphnia magna microbiome based on 12 published studies, comprising gut and whole microbiome samples of both laboratory-cultured and field-grown animals across five countries spanning three continents. To identify core taxa, we employ quantification metrics based on prevalence and a combination of prevalence and relative abundance. Our analysis demonstrates that the D. magna microbiome is highly variable, yet, a consistent association with specific taxa, notably Limnohabitans planktonicus, is observed especially under laboratory conditions. However, this pattern is tempered by the observation that field-grown animals exhibit a more diverse microbiome with a weaker presence of L. planktonicus, challenging its status as a core member. CONCLUSIONS:Our analysis suggests that the D. magna microbiome is defined by its high variability and few conserved associations, with L. planktonicus being the most stable taxon in laboratory settings but not necessarily a core member in natural environments. These findings underscore the need for caution when using laboratory results to interpret natural microbiome compositions and emphasize the need for further research on field-grown animals to better understand the structuring of microbial communities under natural settings.

Cyanobacteria blooms pose a substantial threat to freshwater systems globally. While zooplankton grazers such as Daphnia can have an important role in suppressing cyanobacteria blooms, cyanobacteria can adversely impact Daphnia fitness and even kill them. Earlier work has shown an evolutionary increase in tolerance to cyanobacteria across years and strong genotype × genotype interactions determining the interaction between Daphnia and the cyanobacterium Microcystis. Here, we test the hypothesis that Daphnia magna can adapt during 1 growing season to changes in dominant strains of Microcystis. Over 2 consecutive years, we collected D. magna clonal lineages and Microcystis strains from a single pond early and late in the growing season and we assessed whether Daphnia survival differed when exposed to Microcystis strains from either the same or a different time point within the growth season. Our findings reveal important Daphnia genotype × Microcystis genotype interactions, with Daphnia survival being higher when exposed to Microcystis from the same time point than when exposed to Microcystis of a different time point. Our results extend earlier findings to variation within 1 single natural system and growth season, and suggest an important impact of rapid (co)evolutionary dynamics shaping the tolerance of zooplankton grazers to cyanobacteria.

Latitudinal patterns in fitness-related traits within species are receiving increased attention as these inform how high-latitude populations may evolve in response to global warming. The underlying mechanisms for these latitudinal trait patterns remain poorly understood, and recently the gut microbiome has been suggested to be a potentially important proximate driver of these patterns. We investigated the novel idea of whether the gut microbiome drives latitudinal differences in immune function and pathogen load. To test this idea, we performed a reciprocal gut microbiome transplant between slow-paced, high-latitude (southern Sweden) and fast-paced, low-latitude (southern France) populations of Ischnura elegans damselflies. The transplants were conducted in the laboratory between high- and low-latitude larvae, at both a colder and warmer thermal regime, whereafter larvae were exposed to Escherichia coli, a widespread pathogenic bacterium in aquatic ecosystems. During the experiment, larval mortality, growth rate, phenoloxidase (PO) level (a measure of immune function), E. coli burden (a measure of pathogen load) and the gut microbiome diversity (α-diversity) and community compositions (β-diversity) were analysed. Exposure to the pathogen increased mortality, especially under warming. Our results confirmed latitude-associated thermal adaptation and a faster pace of life of the low-latitude larvae, which was associated with a lower immune function (lower activity of PO), consistent with previous findings and now showed this to be linked to a higher pathogen load (higher E. coli body burden). Moreover, our results provided the first experimental evidence that the gut microbiome causally contributed to latitudinal differences in the host's immune function and pathogen load. As latitudinal patterns in the microbiome are widespread, this may be an important yet ignored proximate driver of the latitudinal patterns in immune function and pathogen load.

Schistosomiasis is a snail-borne disease affecting over 200 million people worldwide. Despite dedicated control efforts and effective diagnostic tools, schistosomiasis remains prevalent. Novel and sustainable control measures are urgently needed. Bacteria might offer such a solution as links between bacteria, disease resistance and transmission potential of intermediate hosts have been established in other systems. To better understand the tripartite interaction potentially driving snail-schistosome compatibility patterns, microbial communities must be investigated throughout and across various parasite exposure conditions. Therefore, we studied Biomphalaria glabrata snails exposed to a high- and low-shedder population of Schistosoma mansoni and Schistosoma rodhaini in single and co-exposure experiments. Snails were sacrificed at different time points post-exposure and their bacterial communities and trematode (co-)infection status were determined through metabarcoding tools. Snails infected by low- and high-shedder S. mansoni populations were more likely to have bacterial community dysbiosis than those infected by S. rodhaini but this was also affected by miracidial load. Moreover, the single-infection hierarchical effect on the bacterial component of the microbiome is not maintained under co-infection with S. rodhaini, which appears to stabilize the snail’s bacterial profile even after being outcompeted by high-shedder S. mansoni. Finally, alpha diversity differed significantly between infected and uninfected snails around the onset period of shedding at 30 days post-miracidial exposure. The timing of this bacterial shift suggests an intricate parasite-snail interaction around key parasite development moments. Future studies investigating the tripartite interaction are advised to consider the effect of outcompeted or prepatent infections on the snail’s microbiome.

Recent research has shown that climate change can both induce and modulate the expression of plastic traits but our understanding of the role of phenotypic plasticity as an adaptive response to climate change is limited. In this review, we dissect the mechanisms and impact of phenotypic plasticity as a response to accumulating climatic pressures on the individual, species and community levels. (i) We discuss how plasticity can affect individuals, populations and community dynamics and how climate change can alter the role of plasticity. We hypothesise that some pathways to phenotypic plasticity such as irreversible and anticipatory organismal responses will be reduced under increasing climate change. (ii) We then propose an integrated conceptual framework for studying phenotypic plasticity to advance our understanding of the feedbacks between the different levels of biological organisation. (iii) By formulating as yet unaddressed research questions within and across levels of biological organisation, we aim to instigate new research on phenotypic plasticity and its role in climate change responses.

Microbial communities are crucial in host adaptation to stressors, particularly in dynamic ecosystems. In aquatic environments, Daphnia magna is ideal for studying host-microbiome interactions due to its ecological importance and sensitivity. Adaptation to toxins, such as those produced by cyanobacteria, may involve both host and microbial gene repertoires. Yet, the influence of microbiota composition and function on host performance remains poorly understood. Because epigenetic mechanisms such as DNA methylation regulate gene expression and mediate adaptive responses, we also investigated whether these associations are reflected in DNA methylation levels. To address this, we conducted a fully factorial transplant experiment using microbiota-depleted Daphnia colonised with microbiota from the same or different genotype, previously exposed to toxic or nontoxic diets, or left uncolonised. We assessed life-history traits, microbial composition (16S rRNA genes), functional profiles (whole-genome-resequencing), and DNA methylation (colorimetric quantification). Daphnia fed nontoxic diets grew larger and reproduced more. Increased methylation occurred when microbiota donors differed from the host genotype and was strongest under toxic diet. Dysbiosis and reduced performance were noted in individuals colonised with toxic-diet microbiota from another genotype, where Limnohabitans spp. was reduced or absent. Signs of hormesis emerged when Daphnia received microbiota from their own genotype reared on nontoxic diets. DNA methylation of both host and microbiota was associated with functional pathways, including increased mitochondrial fatty acid biosynthesis. These findings highlight the importance of host-microbiota matching and microbial environmental history in shaping host performance and epigenetic responses, emphasizing the need to consider host-microbe-environment interactions in evolutionary and ecological studies.

Organisms are facing multiple, potentially interacting stressors in natural populations. The ability of populations coping with combined stressors depends on their tolerance to individual stressors and how stressors interact, which may not be correctly captured in controlled laboratory settings. One reason for this is that the microbial communities in laboratory settings often differ from the natural environment, which could result in different stressor responses and interaction patterns. In this study, we investigated the impact of single and combined exposure to a toxic cyanobacterium and an oomycete parasite on the performance of three Daphnia magna genotypes. Daphnia individuals were sterilized and subsequently exposed to a natural or a laboratory-derived microbial inoculum. Survival, reproduction and body size were monitored, and gut microbiomes were characterized. Our study confirmed that natural and laboratory microbial inocula and gut microbiomes are differently structured. An antagonistic interaction between the two biotic stressors was revealed with respect to survival when Daphnia, across all three genotypes, were exposed to the laboratory microbial inoculum, with a higher survival in the multiple stressor treatment than in the single stressor treatments. In contrast, no antagonistic interaction was detected in Daphnia exposed to a natural microbial inoculum, where the interaction effects were mainly host genotype-dependent. Our results provide the first causal evidence that host-stressor interaction patterns may be shaped by the gut microbiome and the uptake from certain strains from the environment. This raises concern that the many multiple stressor studies on lab-cultured animals with a differently structured microbiome may provide misleading results.

Microplastic pollution in aquatic environments is a growing global concern. Microplastics, defined as plastic fragments smaller than 5 mm, accumulate in freshwater reservoirs, especially in urban areas, impacting resident biota. This study examined the effects of microplastics (MP) on the performance and microbiome of Daphnia, a keystone organism in freshwater ecosystems, through both in situ sampling of freshwater ponds and a controlled 23-day in vitro exposure experiment. Using bacterial 16S ribosomal RNA gene amplicon sequencing and whole-genome shotgun sequencing, we analyzed the microbiome's composition and functional capacity in relation to microplastic pollution levels. Urban ponds contained higher microplastic concentrations in water and sediment than natural ponds, with distinct differences in plastic composition. Bacterioplankton communities, defined as bacterial assemblages in the water column, were more diverse and richer than Daphnia-associated microbiomes. Overall, the in situ study showed that the composition of the Daphnia-associated community was influenced by many factors including microplastic levels but also temperature and redox potential. Functional analysis showed increased relative abundances of polyethylene terephthalate degradation enzymes and antibiotic resistance genes in microbiomes from high-microplastic ponds. In the in vitro experiment, the bacterioplankton inoculum source significantly influenced Daphnia survival and microbiome composition. Network analysis identified specific taxa associated with MP within the Daphnia microbiome. Our findings highlight that urbanization leads to higher microplastic and antibiotic resistance gene burdens, influencing host-associated microbiomes through taxonomic shifts, functional enrichment, and survival outcomes, with potential implications for the resilience of aquatic ecosystems.

Species co-occurrence can lead to competitive interactions that influence fitness. Competition is typically assumed to be modulated by species niche, especially food-acquisition related traits. The influence of interspecific interactions on host microbiome communities has rarely been considered, and yet may provide an alternative mechanism regarding the effect of host species co-occurrence on their fitness. Here, we investigated whether the composition of the gut microbial community differs between two Daphnia species (D. magna and D. pulex), and whether the gut microbiome of one species depends on the presence of the other. We hypothesized the stronger filter-feeder D. magna to have a larger effect on the gut microbiome of the weaker filter-feeder D. pulex than vice versa. To this purpose, three D. magna and three D. pulex genotypes were first made axenic and then grown in monocultures or in co-cultures in natural environmental bacterioplankton-enriched water, before assessing the community composition of the gut microbiomes and bacterioplankton. We found that the composition of the gut microbiome of the two Daphnia species did not significantly differ overall. However, subtle differences (i.e. the relative abundance of certain bacteria) between mono- and co-cultures were found at the Daphnia genotype level. For most genotype combinations (six out of nine), the microbiome of D. pulex changed more (i.e. distance in microbiome composition was more sensitive to culture type in D. pulex than in D. magna) when grown in co-cultures with D. magna than in monocultures. This provides limited support for our hypothesis that the stronger filter-feeder has a larger effect on the gut microbiome of the weaker one than vice versa, and that this effect is possibly mediated via the bacterioplankton community.

Host–bacterial communities (microbiomes) are influenced by a wide range of factors including host genotype and parasite exposure. However, few studies disentangle temporal and host-genotype-specific variation in microbiome response to infection across several host tissues. We experimentally exposed the freshwater crustacean Daphnia magna to its fungal parasite Metschnikowia bicuspidata and characterized changes in host–bacterial communities associated with the parasite's development within the host. We used 16S rRNA gene sequencing to assess bacterial communities of the host (a) 24 h (‘initial parasite exposure’) and (b) 10 days (‘successful infection’) after exposure to a standard dose of M. bicuspidata spores, in host guts, body tissue (excluding guts) and whole individuals. We also investigated whether bacterial community responses to parasite exposure varied by host genotype.Parasite exposure did not immediately alter host gut bacterial communities, but drove host-genotype-specific changes in the bacterial community composition of whole individuals. We validated that these changes were not driven by shifts in bacterial communities of the culturing medium, due to the addition of the parasite spore solution. Successful infection (i.e. the proliferation of M. bicuspidata spores in the host body) reduced alpha diversity and shifted abundance of dominant bacterial orders in the gut. Moreover, it induced a host-genotype-specific changes in body bacterial community composition. Overall, bacterial community responses to parasite exposure and subsequent infection are complex: they occur in a host-genotype-dependent manner, differentially at distinct timepoints after parasite exposure, and in specific host tissue.

The concept of eco-evolutionary (eco-evo) dynamics, stating that ecological and evolutionary processes occur at similar time scales and influence each other, has contributed to our understanding of responses of populations, communities, and ecosystems to environmental change. Phenotypes, central to these eco-evo processes, can be strongly impacted by the gut microbiome. The gut microbiome shapes eco-evo dynamics in the host community through its effects on the host phenotype. Complex eco-evo feedback loops between the gut microbiome and the host communities might thus be common. Bottom-up dynamics occur when eco-evo interactions shaping the gut microbiome affect host phenotypes with consequences at population, community, and ecosystem levels. Top-down dynamics occur when eco-evo dynamics shaping the host community structure the gut microbiome.

Submerged macrophytes promote water clarity in shallow lakes in temperate regions via zooplankton refuge, allelopathy, and nutrient competition with phytoplankton, thereby increasing zooplankton grazing. However, in high-altitude Andean ecosystems, these interactions in shallow lakes have received far less attention. To understand the role of submerged plants in a relatively cold ecosystem (typical for the Andean region), two 100 L experiments were conducted in Yahuarcocha Lake, which has a permanent cyanobacterial bloom. In our first experiment, we evaluated the response of the cyanobacteria bloom to different concentrations of Egeria densa (15%, 35%, and 45% PVI). In the second experiment, we investigated the interactions between E. densa (35% PVI), zooplankton, and the small-sized fish Poecilia reticulata as well as their impacts on phytoplankton. We found a strong reduction in cyanobacteria in the presence of E. densa, whereas P. reticulata promoted cyanobacteria dominance and zooplankton had a null effect on phytoplankton. Remarkably, the combination of E. densa, fish, and zooplankton substantially reduced the algae. Our findings showed that the cyanobacteria bloom decreased in the presence of E. densa, thereby increasing the water clarity in the high-elevation eutrophic ecosystem in the Andes. This effect depended on the plant volume inhabited and the small-sized fish biomass.

Snail-borne diseases affect more than a quarter of a billion people worldwide and pose a high burden in the livestock industry. A fundamental understanding of the drivers of the epidemiology of these diseases is crucial for the development of sustainable control measures. The microbiome is increasingly being recognized as an important player in the tripartite interaction between parasitic flatworms, snail intermediate hosts and the snail microbiome. In order to better understand these interactions, transplant experiments are needed, which rely on the development of a reliable and reproducible protocol to obtain microbiome-disturbed snails. Here we report on the first successful snail microbiome transplants, which indicate that Biomphalaria glabrata can accrue novel bacterial assemblies depending on the available environmental bacteria obtained from donor snails. Moreover, the phylogenetic relatedness to the donor significantly affected the survival probability of the recipients, corroborating the phylosymbiosis pattern in freshwater snails. The transplant technique described here, complemented by field-based studies, could facilitate future research endeavors to investigate the role of specific bacteria or bacterial communities in parasitic flatworm resistance of B. glabrata and might ultimately pave the way for microbiome-mediated control of snail-borne diseases.

The fitness of an organism is often impacted by the composition and biological activity of its associated bacterial community. Many factors, including host genetics, diet, and temperature can influence the bacterial community composition. Furthermore, these factors can differ strongly between natural and laboratory environments. Consequently, several studies have highlighted results from laboratory experiments investigating host-associated bacterial communities to be conflicting with those obtained under field conditions. Here, we compared the Daphnia magna gut bacterial communities in natural host populations with those of laboratory cultured hosts. We further analyzed changes in the gut bacterial communities after transferring hosts from natural populations to the laboratory on the short- and long-term. Results show that, in general, the gut bacterial communities from natural populations differ from those of laboratory cultures and that their composition and diversity changed one hour after being transferred to the laboratory. Over the following 14 days, the composition and diversity changed gradually. On the longer term (after two years of rearing hosts in the laboratory) the composition and diversity of the gut bacterial communities was strongly altered compared to the initial state. Our findings indicate that the gut bacterial communities of Daphnia magna in laboratory experiments is not representative for natural field conditions, and that caution should be taken when interpreting results from laboratory experiments for natural settings.

Organisms are increasingly facing multiple, potentially interacting stressors in natural populations. The ability of populations coping with combined stressors depends on their tolerance to individual stressors and how stressors interact, which may not be correctly captured in controlled laboratory settings. One largely unexplored reason for this is that the microbial communities in laboratory settings often differ from the natural environment, which could result in different stressor responses and interaction patterns. In this study, we investigated the impact of single and combined exposure to a toxic cyanobacterium and an oomycete-like parasite on the performance of three Daphnia magna genotypes. Daphnia individuals were first sterilized and then experimentally given a natural or a laboratory-derived microbial inoculum. Survival, reproduction and body size were monitored for three weeks and gut microbiomes were sampled and characterized at the end of the experiment. Our study confirmed that natural and laboratory microbial inocula and gut microbiomes are differently structured with natural microbiomes being more diverse than laboratory microbiomes. Our results showed that exposure to the stressors reduced D. magna performance compared to the control. An antagonistic interaction between the two biotic stressors was revealed with respect to D. magna survival, when Daphnia individuals were exposed to the laboratory microbial inoculum. This effect was consistent across all three genotypes. In Daphnia exposed to a natural microbial inoculum this antagonistic interaction could not be detected and the genotype x exposure interaction was genotype dependent. Our results indicate that host-stressor interactions depend on the microbial inoculum and that the gut microbiome has potentially a strong role in this, thereby providing an unexplored dimension to multiple-stressor research.

The high-elevation plateaus of the inter-Andean valleys are home to shallow lakes that have become eutrophic. These lakes share similarities with shallow lakes in temperate and subtropical areas. Because native species diversity is low, invasive species dominate the fish and macrophytes communities. The study aimed to investigate the behavioral response of the local Daphnia pulex from the Andean shallow Lake Yahuarcocha to the exotic submerged macrophyte Egeria densa and the exotic fish Poecilia reticulata . Laboratory habitat choice experiments revealed that D. pulex from Lake Yahuarcocha strongly avoid E. densa , irrespective of the presence of the fish P. reticulata or chemical cues indicating fish predation on D. pulex . This observation could be explained by the fact that P. reticulata displayed a strong attraction to E. densa during the daytime, probably to avoid bird predation. D. pulex from the nearby Lake San Pablo where P. reticulata is absent but where the fish community is dominated by Oncorhynchus mykiss displayed the same avoidance behavior to submerged macrophytes as D. pulex from Lake Yahuarcocha. These results indicate that macrophytes in these high-elevation shallow lakes may not facilitate top-down control of phytoplankton, since plants do not offer refuge to D. pulex from fish predation.