Abstract
Unicellular eukaryotes make up the base of the ocean food web and exist as a continuum in trophic strategy from pure heterotrophy (phagotrophic zooplankton) to pure photoautotrophy (‘phytoplankton’), with a dominance of mixotrophic organisms combining both strategies. Here we formulate a trait-based model for mixotrophy with three key resource-harvesting traits: photosynthesis, phagotrophy and inorganic nutrient uptake, which predicts the trophic strategy of species throughout the seasonal cycle. Assuming that simple carbohydrates from photosynthesis fuel respiration, and feeding primarily provides building blocks for growth, the model reproduces the observed light-dependent ingestion rates and species-specific growth rates with and without prey from the laboratory. The combination of traits yielding the highest growth rate suggests high investments in photosynthesis, and inorganic nutrient uptake in the spring and increased phagotrophy during the summer, reflecting general seasonal succession patterns of temperate waters. Our trait-based model presents a simple and general approach for the inclusion of mixotrophy, succession and evolution in ecosystem models.
Highlights
Photoautotrophic plankton combines photosynthesis with uptake of dissolved nutrients to convert CO2 and minerals into the biomass that fuels higher trophic levels in ocean food webs
Despite several observations of mixotrophy since the 1980s, it has only recently been represented in plankton modeling studies (Thingstad et al, 1996; Stickney et al, 2000; Bruggeman, 2009; Flynn and Mitra, 2009; Ward et al, 2011; Våge et al, 2013; Mitra et al, 2014)
This is partly due to the reliance on functional-group type of modeling paradigms, where organisms are pre-described as ‘phytoplankton’ and ‘zooplankton’. Representing mixotrophy in such models leads to increased complexity and computational costs
Summary
Photoautotrophic plankton combines photosynthesis with uptake of dissolved nutrients to convert CO2 and minerals into the biomass that fuels higher trophic levels in ocean food webs. Driven by recent observations of the Despite several observations of mixotrophy since the 1980s, it has only recently been represented in plankton modeling studies (Thingstad et al, 1996; Stickney et al, 2000; Bruggeman, 2009; Flynn and Mitra, 2009; Ward et al, 2011; Våge et al, 2013; Mitra et al, 2014). This is partly due to the reliance on functional-group type of modeling paradigms, where organisms are pre-described as ‘phytoplankton’ and ‘zooplankton’. By disposing of functional groups and species altogether, and focusing on the distribution of continuous trait values, trait-based approaches have the potential to represent the full spectrum of trophic strategies and partly overcome this complexity problem (Norberg et al, 2001; Bruggeman, 2009; Andersen et al, 2015)
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