Discussion
Marine heatwaves are an increasing phenomenon with effects on plankton
organisms, food webs, biogeochemistry, and ecosystem services . Thus,
the development of reliable forecasting tools on heatwaves properties
(e.g., duration, intensity, depth) and ecosystem responses is crucial
for successful mitigation and adaptation actions for ocean
sustainability . Still, we lack a mechanistic understanding of how
temperature relates to the observed ecological alterations during and
after the heatwaves. This is mostly due to the strong connection of
temperature with various abiotic and biotic factors and the complex
temporal and spatial dynamics of marine communities.
In this study, we examine how the temperature anomaly of surface
seasonal heatwaves is affecting ecosystem dynamics in plankton
communities. We use a trait- and size-structured model that accounts for
protists and the life cycle of active and passive feeding copepods. We
highlight and discuss three key findings: Firstly, seasonal heatwaves
trigger contrasting, lasting effects on plankton communities (biomass,
size distribution, functional diversity), extending up to six years
post-heatwave onset. Secondly, it is difficult to separate temperature
as the key driver of the ecosystem changes we observe when we move from
individual processes to community dynamics. Lastly, temperature
anomalies can trigger functional groups differentially, with direct and
indirect effects varying across groups.
The model results show that the duration of community anomalies depends
on when the heatwave occurs. The system takes up to three years for the
winter and spring heatwaves and up to six years for the summer and
autumn heatwaves to reach the pre-heatwave state. The different mixed
layer depth of the seasons (shallow in summer/autumn, deeper in
winter/spring) could be a potential driver for this outcome, as it is
strongly related to the density of nutrients and plankton in the model
and the ocean . Though, since we kept the mixed layer depth fixed in our
experiments, we speculate that this outcome is driven by the temperature
differences among seasons caused by the heatwaves. The mean seasonal
temperature before the heatwaves varies between 15 ˚C and 16 ˚C for
winter and spring and 22 ˚C to 24 ˚C for autumn and summer, leading to a
temperature difference of up to 9 ˚C between seasons. The seasonal
heatwaves of 4 ˚C do not alter the mean annual temperature (20 ˚C).
Still, while winter and spring heatwaves maintain temperature
fluctuations within pre-heatwave seasonal ranges, summer and autumn
heatwaves lead to fluctuations exceeding the pre-heatwave ranges by 2 ˚C
(autumn) and 4 ˚C (summer). Thus, the model indicates that the
ecological disruption and recovery time are related to the temperature
anomaly compared to seasonal temperature fluctuation.
Our results show changes in biomass size bins, dominant groups, and
functional diversity index during and after heatwaves, differing across
protist and copepod functional groups. Starting with the temperature
environmental trait, the model includes functional groups of eight
temperature norms. For both protists and copepods, heatwaves lead to
changes in the relative biomass and order of the dominant groups during
and after the heatwave. Still, functional groups with temperature norms
of 20 ˚C and 24 ˚C dominate the plankton community before, during, and
after the heatwaves and between seasons.
The community size structure also reflects periodic variations caused by
marine heatwaves. However, no consistent pattern emerges across biomass
size bins for all heatwave scenarios, revealing the intricate synergy of
direct and indirect temperature effects. For years after the heatwaves,
the model shows changes in the order of dominant size groups,
highlighting that the effect of a seasonal heatwave on the community
properties can persist for a long period. However, the reposition of
some dominant size groups does not affect the core community size
structure. Our model output is supported by previous studies that show
that environmental factors beyond temperature likely contribute to size
structure variations we observe in nature . In-situ observations
have shown that surface heatwaves alter the properties of plankton
communities like diversity and size distribution, strongly connected
with other environmental conditions such as the passive entrance of
species via water masses, stratification, and changes in nutrient
concentrations . Field observations show a shorter recovery period than
our model projects ranging from a few months to three years depending on
the duration of the heatwave . In-vitro and mesocosm experiments
also indicate that warming can trigger alterations and different
recovery times on physiological rates, species density, and community
structure in the same direction as our model.
Our results are in a parallel direction with in-situ andin-vitro observations but are not directly comparative as
model-observation disparities stem from differences in design,
environment representation, and ecological realism. In-situobservations are snapshots of an ecosystem shaped by many physical,
chemical, and biological processes, most of them recorded with a limited
temporal and spatial resolution. In comparison to the Eulerian view ofin-situ observations, this study assumes a Lagrangian view and
allows us to focus on the theoretical community as moving through time.
Mesocosm and laboratory experiments also follow a Lagrangian approach,
but they run for shorter periods compared to our model experiments (days
to weeks). We also note that descriptive language in most published
studies (e.g., small vs big, warm vs cold species) lacks quantitative
data (e.g., body size, species temperature optima, and physiological
rates) necessary for direct model-observation comparisons. Given
plankton’s adaptive plasticity and morphological variations species can
manifest as ”cold” or ”warm” depending on regional context, contributing
uncertainty to model-observations comparisons.
We propose two more drivers of this model-observational mismatch other
than the differences between our model design andin-situ/in-vitro marine heatwaves on the Lagrangian vs Eulerian
approach and heatwave properties (e.g., temperature anomaly and heatwave
duration): (1) the lack of a 3-dimensional dynamic environment in our
model and (2) the need for enhanced ecological and plankton diversity
representation. In marine ecosystems, surface heatwaves do not occur in
isolation, as in our modelling set-up. They are strongly connected with
other abiotic drivers of ecosystem dynamics (e.g., mixed layer depth,
nutrient cycling, salinity) that can alter nutrient resources, prey
concentrations, and community dynamics during the heatwave . These
environmental drivers can also mitigate or aggravate the signal of
temperature effect through time. Additionally, even if our model design
has complex ecosystem dynamics and higher functional diversity than most
ecosystem models (Petrick et al., 2022), it does not consider phenotypic
plasticity, evolution, or behavioral decisions like vertical migration
and changes in foraging and predation avoidance that can maximize
fitness on an individual level and resilience on a community level .
These physiological and behavioral responses might allow species
persistence that could dampen the impact of the heatwave and accelerate
recovery periods.
Our study highlights the essential role of functional diversity in
population dynamics. In the model, protists are the most diverse
community with plasticity on energy uptake and short life cycles. They
show dynamic responses to environmental conditions and experience more
changes in community composition compared to copepods. Passive copepod
feeders are more vulnerable than active feeders and have the longest
recovery time in terms of biomass concentration. This is probably due to
their trade-off disadvantage on resource competition combined with
predation losses from active feeders. Traditionally, research has
focused on prey density and properties, but studies have shown that the
modes of energy uptake and foraging also have a strong impact on
ecosystem dynamics, biogeography patterns, and biogeochemistry . For
example, studies have shown that mixotrophy evolved as a survival
strategy against prolonged periods of darkness and that some copepod
species can actively switch between passive and active feeding depending
on the environmental conditions (Kiørboe et al., 2018a). Closer
attention to the feeding mode and organismal behavioral decisions that
lead to trait optimization and fitness can help us to better understand
the community status in different environmental conditions and extreme
events. A gradual increase of functional diversity in the model could
provide us with a new level of mechanistic understanding of ecosystem
dynamics but probably increase the difficulty of distinguishing drivers.
It could also highlight suggestions on data needs for advancing the
state-of-the-art of forecasting tools and our confidence in trustworthy
projections crucial for policy advice and actions. Our model is a useful
tool for mechanistically exploring the effect of abiotic parameters in
ecosystems from individuals to community levels. We wish to see future
studies using and adjusting the model design to explore the resilience
of plankton communities under different environmental conditions.