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Understanding how altered temperature regimes affect harmful cyanobacterial bloom formation is essential for managing aquatic ecosystems amidst ongoing climate warming. This is difficult because algal performance can depend on both current and past environments, as plastic physiological changes (acclimation) may lag behind environmental change. Here, we investigate how temperature variation on sub‐weekly timescales affects population growth and toxin production given acclimation. We studied four ecologically important freshwater cyanobacterial strains under low‐ and high‐nutrient conditions, measuring population growth rate after acclimation and new exposure to a range of temperatures. Cold‐acclimated populations (15.7°C) outperformed fully acclimated populations (held in constant conditions) across 65% of thermal environments, while hot‐acclimated populations (35.7–42.6°C) underperformed across 75% of thermal environments. Over a 5‐day period, cold‐acclimated Microcystis aeruginosa produced ~2.5‐fold more microcystin than hot‐acclimated populations experiencing the same temperature perturbation. Our results suggest that thermal variation and physiology interact in underappreciated ways to influence cyanobacterial growth, toxin production, and likely bloom formation.

Understanding the processes that influence range expansions during climate warming is paramount for predicting population extirpations and preparing for the arrival of non-native species. While climate warming occurs over a background of variation due to cyclical processes and irregular events, the temporal structure of the thermal environment is largely ignored when forecasting the dynamics of non-native species. Ecological theory predicts that high levels of temporal autocorrelation in the environment– relatedness between conditions occurring in close temporal proximity– will favor populations that would otherwise have an average negative growth rate by increasing the duration of favorable environmental periods. Here, we invoke such theory to explain the success of biological invasions and evaluate the hypothesis that sustained periods of high environmental temperature can act synergistically with increases in mean temperature to favor the establishment of non-native species. We conduct a 60-day field mesocosm experiment to measure the population dynamics of the non-native cladoceran zooplankter Daphnia lumholtzi and a native congener Daphnia pulex in ambient temperature environments (control), warmed with recurrent periods of high environmental temperatures (uncorrelated-warmed), or warmed with sustained periods of high environmental temperatures (autocorrelated-warmed), such that both warmed treatments exhibited the same mean temperature but exhibited different temporal structures of their thermal environments. Maximum D. lumholtzi densities in the warmed-autocorrelated treatment were three- and eight-fold higher relative to warmed-uncorrelated and control treatments, respectively. Yet, D. lumholtzi performed poorly across all experimental treatment relative to D. pulex and were undetectable the end of the experiment. Using mathematical models we show that this increase in performance can occur alongside increasing temporal autocorrelation, and should occur over a broad range of warming scenarios. These results provide both empirical and theoretical evidence that the temporal structure of the environment can influence the performance of species undergoing range expansions due to climate warming. This article is protected by copyright. All rights reserved.

Many phytoplankton species have evolved induced defenses against herbivore grazers, but the effectiveness of, and costs associated with, these defenses may depend on environmental conditions. For example, grazer chemical cues (kairomones) induce colony formation in many phytoplankton, but temperature, light and nutrient availability can also affect colony size. Little is know about how kairomones and abiotic variables interact to induce algal defenses. Here, we investigate this interaction in the freshwater chlorophyte Scenedesmus acutus, which responds to kairomones from zooplankton herbivores and changes in the abiotic environment by forming colonies (coenobia) of up to eight cells. We tested the effects of ambient phosphorus concentration (0.5, 1.0 and 5.0 mM) and exposure to kairomones from the zooplankton herbivore Daphnia magna on S. acutus growth and coenobia formation. Daphnia magna kairomones did not affect S. acutus population growth during the exponential phase. While kairomones induced coenobia formation in all three P treatments, more coenobia were formed in the absence of kairomones in high-P treatments than in low-P treatments. Thus, coenobia formation appears to be a primarily constitutive defense when P is abundant, but an induced defense when P is scarce, suggesting a trade-off between coenobia formation and phosphorus availability in S. acutus.

Summary 1. Warmer temperatures may increase cyanobacterial blooms in freshwater ecosystems, yet few ecological studies examine how increases in temperature and cyanobacterial blooms will alter the performance of non-native species. We evaluated how competitive interactions and interactions between these two drivers of ecological change influence the performance of non-native species using the native zooplankton Daphnia pulex and the non-native zooplankton Daphnia lumholtzi as a model system. Based on the literature, we hypothesised that D. lumholtzi would perform best in warmer temperatures and in the presence of cyanobacteria. 2. Laboratory competition experiments showed that in the absence of competitors, growth rates of D. pulex (but not D. lumholtzi) were reduced at higher temperatures and with the cyanobacterial foods Anabaena flos-aquae and Microcystis aeruginosa. In the presence of competitors, however, D. pulex emerged as the superior resource competitor at both temperatures with cyanobacterial food. We therefore predicted that, if competitive interactions are important to its establishment, D. lumholtzi would perform best in the absence of cyanobacteria in heated environments. 3. As predicted, when both species were introduced at low densities in field mesocosms, D. lumholtzi performed best at high temperatures without added cyanobacteria and worst at ambient temperatures with added cyanobacteria, indicating that competitive interactions are likely to be important for its establishment. 4. Taken together, these studies indicate that, while D. lumholtzi may benefit from increases in temperature, associated increased cyanobacterial blooms may hinder its performance. Thus, our findings underscore the importance of considering biotic interactions such as competition when predicting the future establishment of non-native species in response to climate warming.

Interactions between cyanobacteria and zooplankton influence both the production of upper trophic level biomass and the occurrence of cyanobacterial blooms in lakes. Gloeotrichia echinulata, a large colonial diazotrophic cyanobacterium, is causing nuisance blooms in oligotrophic and mesotrophic lakes in northern New England (USA). We sought to determine whether crustacean zooplankton are capable of grazing G. echinulata, and if so, which species could be responsible for the observed grazing and what the life history consequences of G. echinulata consumption might be. In three in situ mesocosm experiments, there was a positive relationship between crustacean zooplankton density and the number of damaged G. echinulata colonies. In laboratory incubations, Daphnia pulex, Holopedium gibberum, Ceriodaphnia quadrangula and Bosmina longirostris increased the proportion of damaged G. echinulata colonies; C. quadrangula damaged the greatest proportion of colonies. Additionally, survivorship of Daphnia pulex feeding exclusively on G. echinulata was higher than that of D. pulex fed no food, but lower than that of D. pulex fed nutrient-rich Cryptomonas erosa; however, only one D. pulex fed G. echinulata reproduced. Our studies therefore demonstrate that G. echinulata is damaged by exposure to crustacean zooplankton; we believe this damage may be evidence of active grazing because G. echinulata can be used as a supplemental food source, at least by D. pulex. Further study is required to determine the full life history consequences and food web implications of G. echinulata consumption by zooplankton.

Environmental variability is ubiquitous, but its effects on populations are not fully understood or predictable. Recent attention has focused on how rapid evolution can impact ecological dynamics via adaptive trait change. However, the impact of trait change arising from plastic responses has received less attention, and is often assumed to optimize performance and unfold on a separate, faster timescale than ecological dynamics. Challenging these assumptions, we propose that gradual plasticity is important for ecological dynamics, and present a study of the plastic responses of the freshwater green algae Chlamydomonas reinhardtii as it acclimates to temperature changes. First, we show that C. reinhardtii 's gradual acclimation responses can both enhance and suppress its performance after a perturbation, depending on its prior thermal history. Second, we demonstrate that where conventional approaches fail to predict the population dynamics of C. reinhardtii exposed to temperature fluctuations, a new model of gradual acclimation succeeds. Finally, using high-resolution data, we show that phytoplankton in lake ecosystems can experience thermal variation sufficient to make acclimation relevant. These results challenge prevailing assumptions about plasticity's interactions with ecological dynamics. Amidst the current emphasis on rapid evolution, it is critical that we also develop predictive methods accounting for plasticity.

"Enemy release" occurs when invading species suffer from interactions with pathogens, parasites, herbivores, or predators to a lesser degree than native species due to a lack of shared evolutionary history. Here we provide strong support for the hypothesis that variable thermal sensitivities between a consumer and its resources can generate temperature-dependent enemy release using both a mathematical model and a field experiment. We identify three common scenarios where changes in temperature should alter enemy release based on asymmetric responses among enemies and their resources to changes in temperature: (1) the vital rates of a shared enemy are more sensitive to changes in temperature than its resources, (2) the enemy's thermal maximum for consumption is higher than the resources' maxima for growth, and (3) the invading resource has a higher thermal maximum for growth than its native competitor. Mathematical representations indicated that warming is capable of altering enemy release in each of these three scenarios. We also tested our hypothesis using a mesocosm warming experiment in a system that exhibits variable thermal sensitivities between a predator and its native and nonnative prey. We conducted a six-week experiment manipulating the presence of Lepomis sunfish (present, absent) and water temperature (ambient, heated) using the nonnative crustacean zooplankter, Daphnia lumholtzi, whose morphological defenses reduce predation from juvenile sunfish relative to native Daphnia pulex. Our results indicate that D. lumholtzi benefited to a greater extent from the presence of Lepomis predators as temperatures increase. Taken together, our model and experiment indicate that changes in environmental temperature may directly influence the success of nonnative species and may assist with forecasting the community consequences of biological invasions in a warming world.

While climate change is likely to modify biological interactions between species, it is not clear how altered biotic interactions will influence specific processes such as community assembly. We show that small increases in water temperature can alter the establishment success of the nonnative, tropical zooplankton species, Daphnia lumholtzi, and suggest a general framework for understanding species establishment in the context of climate change. We compared the establishment success of D. lumholtzi and the native congener D. pulex in a mesocosm experiment manipulating temperature, food conditions, and the identity of the resident vs. establishing species. To understand if our mesocosm results could have been predicted by thermal physiology, we characterized the thermal sensitivity of each species' population growth rate and estimated the temperatures at which each species would outperform the other. As predicted by the thermal sensitivities, invading D. lumholtzi were able to establish regardless of temperature and food resources, and established more rapidly in heated mesocosms. Invading D. pulex reached higher initial abundances in ambient-temperature mesocosms but failed to establish in any heated mesocosms. These findings suggest that thermal sensitivity may predict how altered interactions between species can influence community assembly, and that higher lake temperatures will likely aid the future establishment of nonnative D. lumholtzi in North America.