“Eco-evolutionary dynamics of ecological stoichiometry in plankton communities”

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Pedro Branco, Martijn Egas, James J. Elser, and Jef Huisman (July 2018)

Daphnia spp. feeding on freshwater phytoplankton. Our model predicts that selective grazing by these phosphorus-demanding cladocerans will favor dominance of phytoplankton with high N:P ratios.
(Image © Jan van Arkel, IBED/University of Amsterdam)

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Nitrogen and phosphorus are two key nutrients that limit the growth of plants, macroalgae, and phytoplankton in many ecosystems across the globe. Nitrogen and phosphorus contents in phytoplankton cells of lakes and oceans are often quite variable, and largely appear to reflect nutrient availability in their environment. Yet, there are also some interesting patterns. In particular, the nitrogen-to-phosphorus ratio (N:P ratio) of freshwater phytoplankton is, on average, considerably higher than that of marine phytoplankton. Moreover, the N:P ratio of freshwater phytoplankton tends to be higher than the body N:P ratio of freshwater zooplankton, whereas the N:P ratio of marine phytoplankton tends to be lower than that of marine zooplankton. What might explain these intriguing large-scale patterns?

Pedro Branco, Martijn Egas, Jim Elser, and Jef Huisman investigate this problem with a mathematical model that describes eco-evolutionary dynamics of the N:P ratio of phytoplankton in plankton communities. The model assumes that phytoplankton evolve their physiological investment in the uptake of nitrogen and phosphorus. Moreover, the model takes into account that many zooplankton species graze selectively, as they prefer to feed on phytoplankton with a N:P ratio matching their own nutritional demands.

In the absence of zooplankton, the model predicts that phytoplankton optimize their investment in nitrogen and phosphorus uptake, such that their growth rate becomes co-limited by both nutrients. In the presence of zooplankton, however, evolution favors phytoplankton that are not only able to thrive at the prevailing nutrient conditions, but also suppress their grazing losses. In particular, selective grazing by nitrogen-demanding copepods favors evolution of phosphorus-rich phytoplankton (i.e., phytoplankton with low N:P ratios). Since copepods dominate the zooplankton communities of marine environments, they may contribute to the relatively low N:P ratios of phytoplankton in marine ecosystems. Conversely, selective grazing by phosphorus-demanding cladocerans like the water flea Daphnia favors evolution of nitrogen-rich phytoplankton (i.e., with high N:P ratios). Cladocerans are known to be particularly widespread in freshwater environments, and thus may contribute to the predominance of phytoplankton with relatively high N:P ratios in lakes. In total, these results show that selective grazing by zooplankton species can have a major impact on the N:P stoichiometry of phytoplankton in lakes and oceans.


Abstract

Nitrogen (N) and phosphorus (P) limit primary production in many aquatic ecosystems, with major implications for ecological interactions in plankton communities. Yet it remains unclear how evolution may affect the N:P stoichiometry of phytoplankton-zooplankton interactions. Here, we address this issue by analyzing an eco-evolutionary model of phytoplankton-zooplankton interactions with explicit nitrogen and phosphorus dynamics. In our model, investment of phytoplankton in nitrogen versus phosphorus uptake is an evolving trait, and zooplankton display selectivity for phytoplankton with N:P ratios matching their nutritional requirements. We use this model to explore implications of the contrasting N:P requirements of copepods versus cladocerans. The model predicts that selective zooplankton strongly affect the N:P ratio of phytoplankton, resulting in deviations from their optimum N:P ratio. Specifically, selective grazing by nitrogen-demanding copepods favors dominance of phytoplankton with low N:P ratios, whereas phosphorus-demanding cladocerans favor dominance of phytoplankton with high N:P ratios. Interestingly, selective grazing by nutritionally balanced zooplankton leads to the occurrence of alternative stable states (ASS), where phytoplankton may evolve either low, optimum or high N:P ratios depending on initial conditions. These results offer a new perspective on commonly observed differences in N:P stoichiometry between plankton of freshwater versus marine ecosystems, and indicate that selective grazing by zooplankton can have a major impact on the stoichiometric composition of phytoplankton.