Planktonic ecosystems are usually modeled in terms of autotrophic and heterotrophic compartments. However, the trophic strategy of unicellular organisms can take a range of mixotrophic strategies with both autotrophic and heterotrophic contributions. The dominant emerging strategy found in nature depends on the environment (both biotic and abiotic aspects) and the cell size and influences key ecosystem functions like trophic transfer and carbon export. Ecosystem models that faithfully represent this diversity of trophic strategies are lacking. Here we develop a trait‐based model of unicellular plankton with cell size as the master trait and three other traits that determine trophic strategies: investments in photosynthesis, nutrient uptake, and phagotrophy. This unicellular model spans the entire auto‐ mixo‐ hererotrophic strategy continuum and the entire size range of unicellular organisms. The model reproduces observed latitudinal patterns in biomass, primary productivity, vertical carbon export, and energy transfer efficiency; all increase with increasing latitude. The size range of mixotrophic cells is independent of the season at low latitudes. At high latitudes, the dominance of pure phototrophs during early spring restricts mixotrophic behavior to a narrower range of cell sizes and with the occurrence of relatively smaller mixotrophs during summer. The model's ability to adapt to different environmental conditions, combined with its simple conceptual structure and low number of parameters and state variables (10), makes it ideally suited for global simulation studies under changing environmental conditions.

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The competition-defense tradeoff is a significant source of functional diversity in ecological communities. Here, we present a theoretical framework to describe the competition-defense tradeoff and apply it to a size-based model of a unicellular plankton community. Specifically, we investigate how the emergent community structure depends on the shape of the tradeoff, and on whether the cost of defense is paid for by a lowered resource affinity or by an elevated metabolic rate. The inclusion of defense affects the size distribution and trophic strategies of the emerging community dependent on environmental conditions (eutrophic versus oligotrophic) and leads to increased diversity in size and trophic strategy under eutrophic conditions. Eutrophic conditions allow for better-defended organisms than oligotrophic conditions. In most scenarios, competition-defense tradeoffs dampen trophic cascades in the seasonal cycle simulations, and increase the abundance of mixotrophs. We further demonstrate that it matters how the cost of defense is manifest (decreased affinity versus increased metabolic rate), and that it has a significant effect on the resulting plankton community (overall biomass, size and feeding strategy diversity), particularly when the efficiency of the defense increases in direct proportion to the investment. Our results demonstrate that the structure of the ecosystem crucially depends on details of the defense tradeoff. This finding highlights the importance of a mechanistic understanding of defense tradeoffs, e.g. obtained through experimental measurements of specific defense mechanisms.

In some regions, tidal energy can be a key factor in the generation of variability in physical and biogeochemical properties throughout the water column. We use a numerical model resolving tidal cycles and simulating diversity in phytoplankton to assess the impact of tidal mixing on vertical stability and phytoplankton community (total biomass and diversity) in a macrotidal sea (Iroise Sea, France). Two different time scales have been considered: semidiurnal and spring/neap tidal cycles. Our results show that the latter is the one primarily influencing the phytoplankton growth conditions by modifying the vertical stratification. During spring tide, the growth is rather light limited, whereas neap tide conditions lead to vertical stabilization and better light conditions in the shallow surface layer. The transition from high to low tidal mixing conditions is thus associated with a total phytoplankton biomass increase (caused by the rapid development of fast‐growing diatoms) and reduced phytoplankton diversity.