Warming effects on marine microzooplankton and mixoplankton: Physiological responses, community reorganization, and implications for a warming ocean

Abstract

Microzooplankton and mixoplankton are key regulators of marine food webs, yet their responses to ocean warming and thermal extremes remain incompletely integrated across physiological, community, and ecosystem scales. This synthesis examines how temperature shapes the physiology, trophic strategies, and community dynamics of these protistan functional groups using experimental, observational, and modeling evidence from marine systems and relevant contrasts from freshwater and saline inland waters. Short‐term experiments with heterotrophic microzooplankton show strong temperature dependence of growth and grazing, but increased respiratory demand can reduce gross growth efficiency, accelerating nutrient recycling without necessarily increasing biomass transfer to higher trophic levels. Mixoplankton deviate from strict heterotrophic expectations because warming can alter the balance between phototrophy and phagotrophy, rather than uniformly accelerating all rates. At the community level, warming can tighten grazer–prey coupling in productive systems, whereas warm, stratified, nutrient‐poor systems more often favor communities dominated by smaller prey, trophic flexibility, and recycling‐dominated pathways. Thermal variability, including marine heatwaves, introduces additional nonlinearities, temporal mismatches, and legacy effects that remain poorly represented in ecosystem models.