Plankton community properties determined by nutrients and size-selective feeding

The potential impacts of climate change on marine planktonic ecosystems remain difficult to predict. Climate forcing can alter nutrient availability and preda- tor community composition, and here we show that these shifts may dramatically alter plankton trophic structure, size distributions and biomass. We modeled phytoplank- ton and zooplankton as a highly resolved size spectrum with size-dependent nutrient uptake and predation and analyzed the model both as a size spectrum and as a food web. Model results identified 2 distinct regimes defined by the average zooplankton feeding preferences. Regime I communities, where planktonic predators are specialists or large relative to prey, had low omnivory, many top predators, low connectance and relatively flat size spec- tra. Regime II communities, where predators are general- ists or small relative to prey, had a high degree of om- nivory, no top predators, high connectance and steep size spectra. Model ecosystems with generalist predators had lower size diversity, smaller plankton and gappier size distributions than ecosystems with specialist predators. Nutrient availability had little influence on trophic struc- ture but strongly impacted size structure and biomass. Most surprisingly, phytoplankton biomass sometimes de- creased with added nutrients if predators were small rela- tive to prey, implying that both predators and nutrients mediate shifts between bottom-up and top-down control. Based on our synthesized estimates of size-selective feed- ing parameters, we infer that size and trophic structure should be strongly affected by abundances of generalist, bloom-forming taxa such as salps and jellyfish, many of which are responsive to ocean temperature. Size-selective feeding fundamentally affects community structure and is a likely mechanism of change in planktonic ecosystems where community composition varies with temperature.