Large diatoms don’t simply appear because upwelling delivers nutrients. New CBIOMES study finds their emergence depends on a precise ecological balance.
Reporting by Helen Hill for CBIOMES
A new study by Jann Paul Mattern, Stephanie Dutkiewicz, Jordyn E. Moscoso, and Christopher A. Edwards shows that it is not a lull in grazing, but rather the right kind of grazing pressure, combined with nutrient supply, that enables large diatoms to thrive. Published in Limnology and Oceanography, the study offers a fresh perspective on one of coastal oceanography’s enduring questions: what controls the sudden rise of large diatoms during upwelling events? Using a regional ecosystem model, the authors disentangle the interplay between nutrient supply (bottom‑up control) and zooplankton grazing (top‑down control), revealing a more nuanced mechanism behind these dramatic blooms.
In coastal upwelling systems, nutrient‑rich waters rise to the surface, fueling intense phytoplankton blooms that support productive fisheries and drive significant carbon export. Large diatoms are often the hallmark of these blooms, despite typically being outcompeted by smaller phytoplankton in terms of the nutrient uptake efficiency and growth rates. Mattern and colleagues set out to understand why, using a high‑resolution model tailored to the California Current System. Their simulations allowed them to manipulate grazing pressure and nutrient delivery independently, creating a controlled environment in which the drivers of diatom success could be isolated and examined.
Their results show that nutrient supply sets the stage by enabling rapid phytoplankton growth—but nutrients alone are not enough to determine which phytoplankton succeed. Instead, grazing plays a critical and somewhat counterintuitive role. High overall grazing pressure does not suppress large diatoms; rather, it can promote their emergence. This is because grazing is typically strongest on smaller, faster-growing phytoplankton, which are more readily consumed by microzooplankton. In an experiment in which grazing was artificially reduced in the model, overall phytoplankton biomass increased strongly, but that of large diatoms declined.
Beyond trophic interactions, the study also highlights the role of physical variability in shaping these dynamics. Upwelling systems are inherently patchy, with filaments, eddies, and fronts creating spatial heterogeneity in both nutrient fields and grazer distributions. The model captures how these physical structures can create localized refuges where diatoms temporarily escape grazing or encounter concentrated nutrient parcels. These micro‑environments, though transient, can seed broader blooms if conditions remain favorable. The authors argue that this coupling between physical transport and ecological interaction is essential for understanding bloom formation and should be incorporated more explicitly into regional and global ecosystem models.
The implications of this work are substantial. It reinforces the idea that predicting phytoplankton community structure requires more than tracking nutrient inventories; it demands a mechanistic understanding of trophic interactions and their sensitivity to environmental variability. By revealing how nutrient supply and selective grazing together control the timing and composition of blooms, this study advances our ability to understand and predict the behavior of coastal ecosystems in a changing ocean.
Publication:
Mattern, Jann Paul, Stephanie Dutkiewicz, Jordyn E. Moscoso, Christopher A. Edwards (2026), Zooplankton grazing and nutrient supply control the emergence of large diatoms in coastal upwelling systems: Insights from a regional ecosystem model, Limnology and Oceanography, doi: 10.1002/lno.70332