Introduction
Marine heatwaves, periods characterized by temperatures well above the mean ranges for at least five days , are increasing in presence and duration with various impacts on marine communities and ecosystem services across spatial and temporal scales . Even if temperature has been identified as the major environmental driver, our mechanistic understanding of how temperature affects marine populations is still incomplete, which limits our ability to forecast changes in marine ecosystems.
Filling this knowledge gap is challenging, as temperature is strongly linked with multiple biogeochemical processes, and each one of them is a potential driver for changes at the individual level that can lead to alterations in population, community, and ecosystem dynamics . For example, temperature has a direct effect on an individual’s enzymatic kinetics resulting in changes in physiological rates, e.g., metabolism, cell division, growth, ingestion, and respiration that can alter oxygen and resource needs and impact species traits like size, mobility, and reproduction . Temperature also affects organisms indirectly by altering the environmental status of their habitat (e.g., stratification, oxygen concentration, abundance, distribution, resources, and predators). Thus, the fitness of an individual in its environment during a heatwave is determined by the combination of temperature effects on an organismal and environmental scale filtered through the organism’s morphological, behavioral, and life-history traits . Therefore, understanding and identifying ecosystem drivers requires a perspective that encompasses both individual-level traits and community dynamics.
Plankton regulates marine life and has been recognized as an Essential Ocean and Climate Variable linked with various ecosystem services such as climate stabilization, fisheries, water quality, or tourism . Hence, it is crucial to understand how climate variability and human activities affect plankton organisms and communities. Observations have shown that the duration of community change depends on the nature of the heatwave (e.g., seasonal appearance, temperature anomaly, duration), the environmental and community properties; some ecosystems respond and return to the pre-heatwave condition faster than others . For planktonic metazoans, the main observed in-situ pattern during and after a heatwave is the alteration in community structure and composition, either via indigenous or the appearance of invasive species that enter the ecosystems through new warm water masses in an area . Thein-situ observations reflect a mixed Eulerian perspective of advection and ecosystem processes, which complicates pinpointing the drivers of observed ecosystem changes before and after the heatwave. Are the alterations of the observed ecosystem dynamics driven by the impact of temperature on physiological processes (direct effects), the resulting environmental changes (indirect effects), or both?
Laboratory and modeling studies, even if they may lack realism, can scale down complexity and isolate drivers. Studies have revealed that temperature triggers different responses in physiological processes and rates of individuals . If these responses will be reflected on a population or community level depends on how individuals react to their entire environment, rather than just one environmental factor . Therefore, it is important to design studies that focus on causation and not only on the correlation between individual responses and environmental drivers.
The present study investigates the direct and indirect effects of temperature on plankton communities during seasonal heatwaves within a mechanistic framework. Our study aims to answer three key questions:
What impact do seasonal heatwaves have on functional diversity, biomass, and community structure during and after the heatwave?
What is the recovery time for a plankton community to regain its pre-heatwave composition and characteristics, such as biomass size distribution and functional diversity?
Which plankton community changes result from temperature itself versus indirect effects from food web dynamics?
We examine those questions using the Nutrients-Unicellular-Multicellular (NUM) model . NUM is a mechanistic size and trait-based model that simulates community structures and predator-prey interactions for protists (autotrophs, mixotrophs and heterotrophs) and the life cycle of active and passive feeding copepods. We focus on the direct temperature effects on individual rates and predator-prey interactions, without considering species adaptation and any temperature effect on other environmental factors like changes in the mixed layer depth and nutrient concentration. For our experiment, we assume a Lagrangian view and track a community as it experiences warming caused by a surface heat flux anomaly. Using this model, we analyze how seasonal heatwaves impact community biomass, diversity, and structure.