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Global change is rapidly and fundamentally altering many of the processes regulating the flux of energy throughout ecosystems, and although researchers now understand the effect of temperature on key rates (such as aquatic primary productivity), the theoretical foundation needed to generate forecasts of biomass dynamics and extinction risk remains underdeveloped. We develop new theory that describes the interconnected effects of nutrients and temperature on phytoplankton populations and show that the thermal response of equilibrium biomass (i.e. carrying capacity) always peaks at a lower temperature than for productivity (i.e. growth rate). This mismatch is driven by differences in the thermal responses of growth, death, and per‐capita impact on the nutrient pool, making our results highly general and applicable to widely used population models beyond phytoplankton. We further show that non‐equilibrium dynamics depend on the pace of environmental change relative to underlying vital rates and that populations respond to variable environments differently at high versus low temperatures due to thermal asymmetries. [ABSTRACT FROM AUTHOR]

Intraspecific trait variation (ITV) is a widespread feature of life, but it is an open question how ITV affects between‐species coexistence. Recent theoretical studies have produced contradictory results, with ITV promoting coexistence in some models and undermining coexistence in others. Here we review recent work and propose a new conceptual framework to explain how ITV affects coexistence between two species. We propose that all traits belong to one of two categories: niche traits and hierarchical traits. Niche traits determine an individual's location on a niche axis or trade‐off axis, such that changing an individual's trait makes it perform better in some circumstances and worse in others. Hierarchical traits represent cases where conspecifics with different traits have the same niche, but one performs better under all circumstances, such that there are winners and losers. Our framework makes predictions for how intraspecific variation in each type of trait affects coexistence by altering stabilizing mechanisms and fitness differences. For example, ITV in niche traits generally weakens the stabilizing mechanism, except when it generates a generalist–specialist trade‐off. On the other hand, hierarchical traits tend to impact competitors differently, such that ITV in one species will strengthen the stabilizing mechanism while ITV in the other species will weaken the mechanism. We re‐examine 10 studies on ITV and coexistence, along with four novel models, and show that our framework can explain why ITV promotes coexistence in some models and undermines coexistence in others. Overall, our framework reconciles what were previously considered to be contrasting results and provides both theoretical and empirical directions to study the effect of ITV on species coexistence. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]