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.