Eco-Evolutionary Dynamics of Ecological Stoichiometry in Plankton Communities

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, where phytoplankton may evolve either low, optimum, or high N∶P ratios, depending on the initial conditions. These results offer a new perspective on commonly observed differences in N∶P stoichiometry between plankton of freshwater and those of marine ecosystems and indicate that selective grazing by zooplankton can have a major impact on the stoichiometric composition of phytoplankton.

Interplay between the paradox of enrichment and nutrient cycling in food webs

“…Another important step will be to include in models a true brown food web containing decomposers feeding on detritus in parallel to the green food webs relying on photosynthesis (Moore et al, 2004;Zou et al, 2016). The interactions between green and brown food webs are conditioned by stoichiometric constrains on primary producers and detritus that affect the competition/mutualist interactions between primary producers and decomposers (Daufresne and Loreau, 2001;Cherif and Loreau, 2013;Zou et al, 2016) To go further, the flexible stoichiometry of primary producers (and phytoplankton in particular) can also deeply affect food web dynamics and consumer persistence as it can limit herbivore assimilation efficiency (Loladze et al, 2000;Branco et al, 2018). In fact, Urabe and Sterner (1996) demonstrated experimentally that increasing light availability first increases phytoplankton and zooplankton biomass productions but then led to zooplankton extinction because of the low nutritional quality of phytoplankton biomass if the light to nutrient ratio was to high.…”

“…S3-8 in the supporting information), the links between food web structure and the degradability of detritus might strongly influence food web response to nutrient enrichment through their impact on nutrient availability. In addition, primary producer stoichiometry can be a flexible trait responding to nutrient limitation or herbivory, which can limit herbivore assimilation efficiency (Branco et al, 2018), thus affecting the energy transfer in the food chain. Including these mechanisms would thus need to be tested in new versions of our model.…”

From low to high gear: there has been a paradigm shift in our understanding of evolution

“…On the theoretical side, a model of stoichiometric ratios in a phytoplankton‐zooplankton interaction showed that allowing investment patterns in nitrogen and phosphorus to evolve on the same time scale as biomass dynamics produced qualitative changes in the nutrient fluxes when compared to the dynamics in the absence of evolution (Branco et al . ). This result is similar to the results from models of species interactions that show how contemporary evolution can produce qualitative changes in numerical dynamics and stability regimes (Abrams & Matsuda ; Doebeli ; Hiltunen et al .…”

Why Do Phytoplankton Evolve Large Size in Response to Grazing?

“…Second, zooplankton can graze selectively on nutritious phytoplankton (Cowles et al 1988;Buskey 1997;Schatz and McCauley 2007;Meunier et al 2016). Such selective grazing will favor dominance by phytoplankton of low nutritional quality (Grover 1995;Branco et al 2010Branco et al , 2018. Combining these two observations, large phytoplankton might evolve because their low nutritional quality reduces their palatability to selective zooplankton and thereby alleviates their grazing pressure.…”