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Rapid and drastic anthropogenic impacts are affecting global biogeochemical processes and driving biodiversity loss across Earth's ecosystems. In aquatic ecosystems, species distributions are shifting, abundances of many species have declined dramatically, and many are threatened with extinction. In addition to loss of diversity, the ecosystem functions, processes and services on which humans depend are also being heavily impacted. Addressing these challenges not only requires direct action to mitigate environmental impacts but also innovative approaches to identify, quantify and treat their effects in the environment. Mesocosms are valuable tools for achieving these goals as they provide controlled environments for evaluating effects of stressors and testing novel mitigation measures at multiple levels of biological organisation. Here, we summarise discussions from a survey of marine and freshwater researchers who use mesocosm systems to synthesise their opportunities and limitations for advancing solutions to grand ecological challenges in aquatic ecosystems. While most research utilising mesocosm systems in aquatic ecology has focused on quantifying the effects of environmental threats, there is a largely unexplored potential for using them to test solutions. To overcome spatio-temporal constraints, there are opportunities to scale up the size and time-scales of mesocosm studies, or alternatively, test the outcomes of habitat-scale restoration at a smaller scale. Enhancing connectivity in future studies can help to overcome the limitation of isolation and test an important aspect of ecological recovery. Conducting 'metacosm' studies: coordinated, distributed mesocosm experiments spanning wide climatic and environmental gradients and utilising more regression-based experimental designs can help to tackle the challenge of context dependent results. Finally, collaboration of theoretical, experimental and applied ecologists and biogeochemists with environmental engineers and technological developers will be necessary to develop and test the tools required to advance solutions to the impacts of human activities on Earth's vulnerable aquatic ecosystems.

Ice and snow cover on lakes plays a fundamental role for under-ice ecology by reducing water column mixing and light availability. Previous studies have shown that such reductions can significantly influence the growth and reproduction of phytoplankton, primarily focusing on changes in ice-on and ice-off dates in a warming climate. This study goes beyond studying the effects of ice phenology on phytoplankton by addressing two fundamental questions: (1) how does a snow cover on ice influence below-ice phytoplankton chlorophyll-a and community composition and (2) how do variations in ice phenology influence spring phytoplankton chlorophyll-a and community composition after ice-off? To address these two questions, we assessed long-term monitoring data collected at least monthly on phytoplankton chlorophyll-a and community composition. We combined the phytoplankton data with annual ice phenology and nearby meteorological data on daily snow depth between 1997 and 2019 in a mesotrophic lake (Erken) in Sweden. Snow cover resulted in an exponential decrease of phytoplankton chlorophyll-a, with detectable effects during all 3 months studied (January-March). Deeper snow cover changed the community dominance from autotrophs to mixotrophs in two of the months studied (January and March), which we explain by reduced light availability caused by snow cover. In spring, phytoplankton chlorophyll-a increased with longer ice periods and delayed ice-off dates. A wide range of taxa in the spring community increased with delayed ice-off dates, while delayed ice-on dates mainly promoted diatoms. Convective mixing is important to keep non-motile taxa in the photic zone and could explain the increased phytoplankton growth with longer ice duration. Our results highlight seasonal ice and snow cover as key regulators for the timing of growth and reproduction of primary producers below ice, with effects of the ice cover period lasting after ice-off. Snow on ice causes light constraints, which commonly result in reduced under-ice primary production and a higher proportion of mixotrophs in the phytoplankton community. Losing high nutritional phytoplankton groups such as mixotrophs following changes in ice phenology and snow cover can have consequences for the trophic transfer and the biogeochemical cycling in lakes.

Ecosystems worldwide are experiencing a range of natural and anthropogenic disturbances, many of which are intensifying as global change accelerates. Ecological responses to those disturbances are determined by both the vulnerabilities of species and their interspecific interactions. Understanding how individual species contribute to the (in-)stability of an aggregated community property, or function, is fundamental to ecological management and conservation. Here, we present a framework to identify species contributions to stability based on their absolute and relative responses to disturbances. Using simulations, we show that these two dimensions enable identification of (de-)stabilizing species and reveal that competitive dominance determines the magnitude of both absolute and relative contributions to stability. Applying our framework to empirical data from a multi-site mesocosm experiment showed that species contributions varied among treatments, sites, and seasons. Despite this dependency on both biotic and abiotic contexts, species contributions were generally constrained by their relative dominance in undisturbed conditions. Rare species contributed positively to stability, while dominant species contributed negatively, indicating compensatory dynamics. Our framework offers an important step toward a more mechanistic understanding of ecological stability based on species performance.

Freshwater salinization is an increasing threat to lakes worldwide, but despite being a widespread issue, little is known about its impact on biological communities at the base of the food chain. Here we used a mesocosm set-up coupled with modern high-frequency sensor technology to identify short- and longer-term responses of phytoplankton to salinization in an oligotrophic lake. We tested the effects of salinization over a gradient of increasing salt concentrations that can be found in natural lakes exposed to road salt contamination (added salt range: from 0 to 1500 mg Cl- L-1). The high-frequency chlorophyll-a (chl-a) fluorescence measurements showed an increasing divergence of chl-a concentrations along the salinization gradient over time, with substantially lower concentrations at higher salt levels. At the sub-daily scale, we found a profound suppression of day-night signal cycles with increasing salinity, which could be related to physiological stress due to the impairment of photosynthesis via effects on the photosystem II or potential changes in the active migration of phytoplankton. Community analyses revealed a similar decline pattern for the total phytoplankton biomass and a collapse of the total zooplankton biomass. Interestingly, we found a loss of phytoplankton diversity coupled with a compositional re-organization involving the loss of dominant green algae but increased biomass of salt-tolerant cyanobacteria. Altogether, these results suggest that specific cyanobacterial taxa can benefit from freshwater salinization following the collapse of dominant phytoplankton competitors and zooplankton herbivores. The results also highlight the value of autonomous sensor technology to capture novel, small-scale ecological responses to freshwater salinization, and thereby to track fast changes in primary producer communities.

Changes in rainfall patterns driven by climate change affect the transport of dissolved organic matter (DOM) and nutrients through runoff to freshwater systems. This presents challenges for drinking water providers. DOM, which is a heterogeneous mix of organic molecules, serves as a critical precursor for disinfection by-products (DBPs) which are associated with adverse health effects. Predicting DBP formation is complex due to changes in DOM concentration and composition in source waters, intensified by altered rainfall frequency and intensity. We employed a novel mesocosm approach to investigate the response of DBP precursors to variability in DOM composition and inorganic nutrients, such as nitrogen and phosphorus, export to lakes. Three distinct pulse event scenarios, mimicking extreme, intermittent, and continuous runoff were studied. Simultaneous experiments were conducted at two boreal lakes with distinct DOM composition, as reflected in their color (brown and clear lakes), and bromide content, using standardized methods. Results showed primarily site-specific changes in DBP precursors, some heavily influenced by runoff variability. Intermittent and daily pulse events in the clear-water mesocosms exhibited higher haloacetonitriles (HANs) formation potential linked to freshly produced protein- like DOM enhanced by light availability. In contrast, trihalomethanes (THMs), associated with humic-like DOM, showed no significant differences between pulse events in the brown-water mesocosms. Elevated bromide concentration in the clear mesocosms critically influenced THMs speciation and concentrations. These findings contribute to understanding how changing precipitation patterns impact the dynamics of DBP formation, thereby offering insights for monitoring the mobilization and alterations of DBP precursors within catchment areas and lake ecosystems.

Telonemia are one of the oldest identified marine protists that for most part of their history have been recognized as a distinct incertae sedis lineage. Today, their evolutionary proximity to the SAR supergroup (Stramenopiles, Alveolates, and Rhizaria) is firmly established. However, their ecological distribution and importance as a natural predatory flagellate, especially in freshwater food webs, still remain unclear. To unravel the distribution and diversity of the phylum Telonemia in freshwater habitats, we examined over a thousand freshwater metagenomes from all over the world. In addition, to directly quantify absolute abundances, we analyzed 407 samples from 97 lakes and reservoirs using Catalyzed Reporter Deposition-Fluorescence in situ Hybridization (CARD-FISH). We recovered Telonemia 18S rRNA gene sequences from hundreds of metagenomic samples from a wide variety of habitats, indicating a global distribution of this phylum. However, even after this extensive sampling, our phylogenetic analysis did not reveal any new major clades, suggesting current molecular surveys are near to capturing the full diversity within this group. We observed excellent concordance between CARD-FISH analyses and estimates of abundances from metagenomes. Both approaches suggest that Telonemia are largely absent from shallow lakes and prefer to inhabit the colder hypolimnion of lakes and reservoirs in the Northern Hemisphere, where they frequently bloom, reaching 10%-20% of the total heterotrophic flagellate population, making them important predatory flagellates in the freshwater food web.

Lakes located in the boreal region are generally supersaturated with carbon dioxide (CO2), which emerges from inflowing inorganic carbon from the surrounding watershed and from mineralization of allochthonous organic carbon. While these CO2 sources gained a lot of attention, processes that reduce the amount of CO2 have been less studied. We therefore examined the CO2 reduction capacity during times of phytoplankton blooms. We investigated partial pressure of CO2 (pCO(2)) in two lakes at times of blooms dominated by the cyanobacterium Gloeotrichia echinulata (Erken, Sweden) or by the nuisance alga Gonyostomum semen (Erssjon, Sweden) during two years. Our results showed that pCO(2) and phytoplankton densities remained unrelated in the two lakes even during blooms. We suggest that physical factors, such as wind-induced water column mixing and import of inorganic carbon via inflowing waters suppressed the phytoplankton signal on pCO(2). These results advance our understanding of carbon cycling in lakes and highlight the importance of detailed lake studies for more precise estimates of local, regional and global carbon budgets.

The relevance of considering environmental variability for understanding and predicting biological responses to environmental changes has resulted in a recent surge in variability-focused ecological research. However, integration of findings that emerge across studies and identification of remaining knowledge gaps in aquatic ecosystems remain critical. Here, we address these aspects by: (1) summarizing relevant terms of variability research including the components (characteristics) of variability and key interactions when considering multiple environmental factors; (2) identifying conceptual frameworks for understanding the consequences of environmental variability in single and multifactorial scenarios; (3) highlighting challenges for bridging theoretical and experimental studies involving transitioning from simple to more complex scenarios; (4) proposing improved approaches to overcome current mismatches between theoretical predictions and experimental observations; and (5) providing a guide for designing integrated experiments across multiple scales, degrees of control, and complexity in light of their specific strengths and limitations.

Diatom blooms are often accompanied by an increase in parasitic chytrids that kill the host cells, which they are infecting, and can contribute to the decline of the bloom. However, host specificity and range of these chytrids are currently poorly understood. Low host specificity would enable the parasites to opportunistically infect any diatom species, while specialisation on infecting specific high-biomass species could result in high prevalence and rapid spread of infection. We investigated such host-parasite interactions by monitoring the diverse diatom spring bloom in lake Erken using amplicon sequencing. We also performed infection experiments with two different, newly isolated chytrid species and several diatom cultures from the bloom. Chytridiomycota displayed the highest relative abundance of all parasitic lineages and were probably physically attached to larger organisms. Since the chytrids reached maximum abundance shortly after a peak in diatom reads, they were probably infecting these important primary producers. Phylogenetic analyses of the isolated chytrid strains identified them as members of the classes Rhizophydiales and Lobulomycetales. The infection experiments revealed high host specificity in these two chytrids targeting different diatom species. The experimental results supported statistical analyses of the environmental sequencing data, which suggested the presence of two different infection strategies: the most abundant chytrid species were specialised on infecting dominant diatom genera (i.e. Stephanodiscus, Aulacoseira, Asterionella), while rarer chytid species infected a range of less abundant diatoms.

Human-induced salinization increasingly threatens inland waters; yet we know little about the multifaceted response of lake communities to salt contamination. By conducting a coordinated mesocosm experiment of lake salinization across 16 sites in North America and Europe, we quantified the response of zooplankton abundance and (taxonomic and functional) community structure to a broad gradient of environmentally relevant chloride concentrations, ranging from 4 to ca. 1400 mg Cl− L−1. We found that crustaceans were distinctly more sensitive to elevated chloride than rotifers; yet, rotifers did not show compensatory abundance increases in response to crustacean declines. For crustaceans, our among-site comparisons indicate: (1) highly consistent decreases in abundance and taxon richness with salinity; (2) widespread chloride sensitivity across major taxonomic groups (Cladocera, Cyclopoida, and Calanoida); and (3) weaker loss of functional than taxonomic diversity. Overall, our study demonstrates that aggregate properties of zooplankton communities can be adversely affected at chloride concentrations relevant to anthropogenic salinization in lakes.