Planktonic primary producers form the basis of aquatic food webs, and their community dynamics are tightly coupled to the prevailing physical and chemical conditions of aquatic systems. Ongoing climate change is altering key physical and chemical parameters—such as the timing and duration of summer stratification, average water temperature and nutrient cycling—with far‑reaching implications for plankton phenology and community composition. Although the fundamental relationships between physicochemical variables and phytoplankton growth are well established (e.g., as described in the conceptual Plankton Ecology Group (PEG) model), the accurate prediction of bloom initiation, intensity, and taxonomic composition remains elusive. Advancing our predictive capacity requires a mechanistic understanding of bloom formation that explicitly incorporates species diversity and associated functional traits. Trait‑based frameworks promise to clarify how particular trait combinations confer competitive advantages under shifting environmental conditions and, consequently, how they shape bloom dynamics. We therefore welcome contributions that address trait‑based plankton ecology linked to phenological responses of plankton communities to environmental change across marine, coastal, and freshwater systems based on experimental, empirical as well as theoretical approaches.
Lead Organizer: Sabine Wollrab, Leibniz Institute of Freshwater Ecology and Inland Fisheries (sabine.wollrab@igb-berlin.de)
Co-organizers:
Mia Stockenreiter, Ludwig-Maximilians-Universität München (stockenreiter@biologie.uni-muenchen.de)
Herwig Stibor, Ludwig-Maximilians-Universität München (stibor@zi.biologie.uni-muenchen.de)
Philippe Pondaven, Université de Bretagne Occidentale (philippe.pondaven@univ-brest.fr)
Presentations
09:00 AM
Transient Dynamics in the Vertical Competition Between Sinking and Buoyancy-Regulating Phytoplankton Species (10520)
Primary Presenter: Arthur ROSSIGNOL, AgroParisTech, Université Paris-Saclay (arthur.f.rossignol@gmail.com)
In the temperate zone of the Northern Hemisphere, lakes typically exhibit seasonal patterns of thermally stratified and fully mixed periods. Global warming is altering these cycles, particularly by prolonging summer stratification. Phytoplankton dynamics are closely linked to these patterns: spring bloom formation is typically associated with higher average light availability with the onset of summer stratification, yet stratification also increases the risk of cells sinking out of the euphotic layer. To avoid sinking, many phytoplankton species have evolved motility traits such as flagella or buoyancy regulation. Observations from deep, clear-water lakes in the Northern Hemisphere suggest that especially low-light-adapted cyanobacteria, such as Planktothrix rubescens, might benefit from extended stratification periods through their capacity to vertically migrate via active buoyancy regulation. This trait may confer a competitive advantage over sinking species. Using a 1D model, this study investigates competition between sinking and buoyancy-regulating phytoplankton species along the vertical water column. Species compete for light and nutrients (nitrogen and phosphorus) along heterogeneous depth gradients, making use of a modified Droop model. Specifically, we examine how seasonal stratification patterns influence phytoplankton dynamics and how physiological traits related to resource utilization and vertical movement strategies influence transient dynamics. Overall, this work provides new insights into the early development of phytoplankton blooms under changing lake stratification regimes.
09:15 AM
Phytoplankton intraspecific traits variation and trade-offs along environmental gradients (11562)
Primary Presenter: Arnaud Louchart, Netherlands Institute of Ecology (arnaud.louchart@gmail.com)
Phytoplankton traits vary substantially both across and within species. Interspecific trait variation plays a key role in shaping community interactions and underlies species sorting in response to environmental change. Beyond variation across species, research increasingly recognizes the importance of differences within species. This intraspecific variation can drive population structure and dynamics and underpins adaptive responses to shifting environments. Such variation arises not only among genotypes but also within genotypes through single-cell differences. Yet both levels of intraspecific trait variation remain poorly understood, particularly regarding the extent of trait plasticity, potential trade-offs, and their consequences for population dynamics in natural environments. Here, we synthesize recent advances on intraspecific variation, plasticity, and trade-offs in phytoplankton traits, and propose an approach for untangling genotypic and single-cell contributions to trait diversity to more accurately represent population responses to environmental change. We also highlight existing and developing techniques that jointly address both sources intraspecific trait variation.
09:30 AM
Long-term shifts in phenology of nearshore zooplankton in Lake Ontario (11778)
Primary Presenter: James Watkins, Cornell University (jmw237@cornell.edu)
Seasonal succession of nearshore zooplankton reflects the physical environment and biological competition and predation. Nearshore habitats are important spawning areas for fish, and thus recruitment success is largely determined by the alignment in the timing of zooplankton peaks with the larval fish emergence and growth at early life stages. We use a thirty-year (1995-2025) biweekly time series from May-October of seven nearshore (10 m depth) sites in Lake Ontario to track shifts in zooplankton community phenology. The seven sites are distributed along the US southern shore tracking a well-known west to east gradient in thermal structure. Copepod populations are first to peak in May with cyclopoid followed by calanoid species peaks. As the nearshore habitat warms we see bosminid and then daphniid grazers dominate. In late summer and fall, predatory cladocerans are a small component of overall biomass but have a major impact on zooplankton community structure. Sites were grouped by similarity in phenology that reflected geographic patterns. We evaluate whether zooplankton phenology has changed over this critical time period marked by climate change and the expansion of invasive species. This timing is then put in context of the life history of fish species known to utilize nearshore habitat.
09:45 AM
The effects of freshwater phytoplankton germination at population and community scales (11891)
Primary Presenter: Hannah Larson, University of Connecticut (hannah.larson@uconn.edu)
Dormancy, a process by which an organism enters a reversible state of low metabolic activity, is an important shared trait across phytoplankton taxonomic groups as both a bet hedging strategy and a method to survive unfavorable conditions. Phytoplankton use dormancy in response to many biotic and abiotic cues, including temperature and light. The ecological effects of germination from dormant individuals in benthic sediments are largely unknown. We conducted experiments to investigate: (1) how environmental conditions and sediment age affect the abundance and identity of germinating phytoplankton, and (2) how strongly contributions from sediment affect community assembly and dynamics. We created a series of microcosms by incubating alpine lake sediment samples from 0 to 86 years old under different temperature and light conditions. Age of sediment, incubation temperature, and photoperiod affected overall community growth and composition, with functional groups like diatoms and green algae responding favorably to different temperatures and photoperiods. To investigate how such germination affects the whole microalgal community, we also need to understand how germinants interact with the contemporary phytoplankton community. In a second microcosm experiment, we added sediment to unfiltered lake water, using sediment or lake water alone as controls. Over three weeks, we observed differences in the growth rate and composition of these communities, likely influenced by different assembly patterns.
10:00 AM
Thermal Limits and Decline of Synechococcus under Accelerated Warming and Marine Heatwaves (10678)
Primary Presenter: Luthfiyyah Azizah, King Abdullah University of Science and Technology (luthfiyyah.azizah@kaust.edu.sa)
Marine picophytoplankton contribute roughly 20% of global oceanic primary production, including thermally resilient taxa such as Synechococcus, which dominate warm oceans and are projected to benefit from future warming. Tropical populations exist near their upper thermal limits, making them highly vulnerable to further warming, a largely unexplored risk for Synechococcus. Here, we combine high-frequency in situ observations and laboratory experiments to examine the thermal tolerance of Synechococcus in one of the warmest marine ecosystems. Over a seven-year period (2018–2024), we monitored population dynamics alongside continuous sea surface temperatures, capturing the increasing frequency and duration of marine heatwaves (MHWs) in 2023–2024, the warmest years on record. Abundance of Synechococcus increased with temperature and peaked at ~30.2°C, but extreme temperatures recorded in 2023–2024 surpassed the thermal capacity of Synechococcus populations. Laboratory experiments of Synechococcus clades isolated from the warmest Red Sea, confirmed strain-specific optima ranging from ~25°C (clade IIIa) to ~33°C (clade IIa), with maximum thermal limit up to 35.2°C. During the unprecedented warming of 2023–2024, when sea surface temperatures exceeded 35°C and MHWs persisted for up to 55 days, Synechococcus blooms weakened by ~4.5-fold. While bloom timing and growth rates remained stable, lower initial abundances at bloom onset drove the decline, signaling a shift in population dynamics under intensified thermal stress. Comparison with published temperature–abundance models demonstrates prior datasets fail to capture responses to extreme warming. Our results provide direct evidence of ecological niche loss in tropical Synechococcus, challenging predictions of their future dominance and highlighting the vulnerability of even the most heat-tolerant primary producers to accelerated warming. These findings underscore the capacity of extreme warming events to rapidly destabilize plankton communities, reduce primary production, and alter ecosystem function, emphasizing increasing uncertainty in forecasting ocean productivity under accelerating climate change. Keywords: accelerated warming | Synechococcus | heatwaves | phenology | thermal limits | high-frequency sampling | picophytoplankton | tropical ocean | Red Sea |
10:15 AM
HOW BIODIVERSITY MODULATES BLOOM FORMATION: EVIDENCE FROM NATURAL PHYTOPLANKTON COMMUNITIES (10741)
Primary Presenter: Maria Stockenreiter, Ludwig-Maximilians-Universität München (stockenreiter@biologie.uni-muenchen.de)
Phytoplankton communities in freshwater systems are characterized by their capacity to form rapid, transient biomass increases, known as blooms, in response to shifting abiotic and biotic conditions. The ability of potential bloom species to monopolize resources depends on their life-history traits, their ecophysiological alignment with environmental conditions, and interactions shaped by community diversity. Diversity can influence bloom dynamics through two opposing mechanisms: selection effects, where high diversity increases the likelihood that a highly competitive species dominates, and complementarity effects, where diverse assemblages partition resources more efficiently, reducing the chance that any single species achieves bloom-level dominance. Grazing pressure further modulates these processes by altering resource monopolization and biomass accumulation. We examined potential bloom formations in natural phytoplankton communities spanning a broad diversity gradient and shaped by long-term environmental history. Communities of naturally different diversity were collected from multiple freshwater lakes and subjected to a mesocosm experiment manipulating grazing and nutrient supply. We quantified community- and species-level bloom metrics across the diversity gradient and compared these results to previous laboratory experiments using artificial communities.
SS092B Trait-Based Plankton Ecology and Environmental Change
Description
Time: 9:00 AM
Date: 14/5/2026
Room: 524C