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Marine phytoplankton depend on a combination of environmental resources to grow and photosynthesise. Among the most important are light and nutrients, yet these resources do not act independently. Changes in the physical environment, such as variations in mixing and water-column depth, can simultaneously affect the amount of light available to phytoplankton and their access to nutrients. Understanding these interactions is therefore essential for predicting how marine ecosystems respond to environmental change. In this study, we develop a theoretical framework based on marginal products and elasticities to quantify how changes in light and nutrient availability influence biological production. The approach provides a way to distinguish the relative importance of different environmental resources and to describe the trade-offs that arise when improving one resource comes at the expense of another.

A particular focus is placed on the interaction between light availability and nutrient supply in the water column. For example, deeper mixing can bring nutrients into the surface ocean but at the same time expose phytoplankton to lower average light levels. The resulting response of primary production therefore depends on the balance between these competing effects. The framework developed in the study provides a quantitative way of describing this balance and identifying the environmental conditions under which light or nutrients become the dominant limitation. The results contribute to a better understanding of the mechanisms controlling marine primary production and provide a theoretical basis for analysing how changes in ocean conditions may affect phytoplankton productivity. The work is particularly relevant in the context of climate change, where changes in stratification, mixing and nutrient supply are expected to modify the underwater light environment and the availability of nutrients to marine ecosystems. The study highlights the importance of considering multiple environmental drivers simultaneously, rather than treating light and nutrients as independent controls of marine photosynthesis. This perspective can help improve our understanding and modelling of primary production in a changing ocean.

The front page of the paper is shown below:

The paper can also be accessed via this link and dowloaded below.