Ocean productivity depends on a delicate balance of competing forces. New research reveals how feedbacks, thresholds, and resource trade-offs combine to shape the growth of phytoplankton, the microscopic plants that sustain life in the sea.
Reporting by Helen Hill for CBIOMES
What controls the growth of phytoplankton in the ocean? It is a deceptively simple question that has occupied oceanographers for decades. Two recent studies, Kovač and Sathyendranath (2025) and Mance, Mance, Kovač, Sathyendranath, and Kovač (2026), revisit some of the most fundamental ideas in biological oceanography and offer new ways to understand how light, nutrients, and physical mixing shape marine primary production.
At first glance, the papers approach the problem from different directions. One revisits Sverdrup’s classic Critical Depth Hypothesis, while the other borrows concepts from economic production theory to analyze primary-production models. Together, however, they point toward a common conclusion: ocean productivity is governed by trade-offs and feedbacks, not by any single environmental factor.
The Critical Depth Hypothesis, first proposed in the 1950s, remains one of the cornerstones of marine ecology. The theory argues that phytoplankton blooms occur when the ocean’s mixed layer becomes shallow enough that total photosynthetic production exceeds losses from respiration, grazing, and mortality. Below a critical depth, losses dominate; above it, biomass can accumulate and blooms emerge.
In Critical Times for the Critical Depth Theory, Kovač and Sathyendranath revisit this familiar framework and uncover a richer set of dynamics than previously appreciated. Their analysis incorporates the fact that phytoplankton are not merely influenced by light, but actively modify their light environment by absorbing it. This creates a bio-optical feedback in which growing biomass changes the conditions that support future growth.
Using analytical methods, the authors derive new equations describing how compensation depth and critical depth evolve through time. They show that certain light levels associated with these depths behave as constants of motion, revealing hidden structure within the classical theory. The study also identifies a bio-optical bifurcation, where relatively small changes in mixed-layer depth can shift the system between different biomass states.
If the Kovač and Sathyendranath paper revisits a classic ecological threshold, the study by Mance and colleagues asks a complementary question: how can we quantify the relative importance of light and nutrient supply in determining productivity?
In The Production-Function Structure of Marine Primary-Production Models, the authors treat marine productivity models as input-output systems. Light, nutrients, and mixed-layer depth are viewed as inputs, while primary production becomes the output. This perspective allows them to apply analytical tools such as marginal products, elasticities, and isoquants, concepts more commonly associated with economics than oceanography.
One of the paper’s central contributions is the development of a light-nutrient compensation threshold. This metric quantifies how much additional nutrient supply is required to offset the loss of productivity associated with reduced light exposure when mixing depths increase. In effect, it provides a direct measure of the trade-off between two resources that frequently change together in the ocean.
Taken together, the two studies reveal different aspects of the same underlying problem. The critical-depth framework emphasizes thresholds, feedbacks, and ecosystem stability. The production-theory approach focuses on sensitivities and trade-offs among environmental drivers. Both recognize that changes in mixed-layer depth affect productivity through multiple pathways simultaneously. Deeper mixing may supply nutrients to surface waters while also exposing phytoplankton to lower average light levels. The resulting biological response depends on the balance between those competing effects.
Image source: thelogocraft_ai from Pixabay
Publications
Žarko Kovač, Shubha Sathyendranath (2025), Critical Times for the Critical Depth Theory, JGR Oceans, doi: 10.1029/2024JC021415
Davor Mance, Diana Mance, Žarko Kovač, Shubha Sathyendranath, Anja Kovač (2026), The production-function structure of marine primary-production models: Marginal products, elasticities, and light–nutrient trade-offs, Journal of Marine Systems, doi: 10.1016/j.jmarsys.2026.104275