Professor University of British Columbia Vancouver, British Columbia, Canada
Abstract: Phytoplankton resource allocation strategies connect demographic rates to cellular stoichiometry. Increases in temperature are thought to shift the resource allocation strategy that maximizes fitness – typically defined as the rate of cell division (i.e., birth rate) – from relatively high investment into Phosphorus (P) rich biosynthetic machinery (i.e., RNA) to relatively high investment in Nitrogen (N) and Carbon (C) rich resource uptake machinery (i.e., Protein, Pigments), thus producing an increase in the C:P and N:P ratio of phytoplankton biomass. However, phytoplankton resource allocation strategies may also impact their susceptibility to mortality by grazers – grazers may be able to selectively consume phytoplankton with stoichiometries that match their own requirements. How such mortality costs may impact the evolution of resource allocation strategies across temperatures is not well understood.
Here we examine the consequences of selective grazing for the evolution of phytoplankton resource allocation strategies, and hence stoichiometry, across a temperature gradient. To do so, we analyze a temperature-dependent Nutrient-Phytoplankton-Zooplankton (NPZ) model in which the Phytoplankton stoichiometry, birth rate and mortality rate – i.e., losses via zooplankton consumption – are functions of their resource allocation strategy. To study the evolution of allocation strategies, we use an adaptive dynamics approach: we examine whether residents with a given strategy at ecological equilibrium can be invaded by species with different strategies.
Consistent with previous work we find that, in the absence of grazing mortality, increasing temperatures yields modest decreases in investment to P-rich biosynthetic machinery, and hence modest increases in the C:P ratio of phytoplankton cells (e.g., from 100 molC:molP at 10 C to 180 molC:molP at 25 C). However, in the presence of zooplankton selectively consuming P-rich phytoplankton cells (e.g., 100-150 molC:molP), increasing temperatures typically shifts the system from one evolutionarily stable strategy (fast-growing low C:P ratio) to alternative stable strategies – one that maximizes phytoplankton birth rates (100-180 molC:molP) and one that sacrifices birth rate to reduce mortality rate ( >200 molC:molP) – to one evolutionarily stable strategy (slow-growing high C:P strategy). These results show that selective grazing can amplify the impacts of warming on phytoplankton stoichiometry, producing more C-rich cells than expected based on changes in the allocation strategy that maximizes rates of cell division. More generally, this work highlights the importance of integrating multiple demographic consequences of trait variation – including births and deaths – when investigating adaptation to environmental change.
