Resolving the Community Size Structure of Arctic Protist Plankton – Are We Missing Important Details?

Cell size affects physiological processes such as nutrient uptake, photosynthesis and growth. Furthermore, predator-prey interactions are known to be size dependent. Thus, size-spectra of marine protist plankton give important insight to their respective community structure, and allow us to unravel and distinguish between bottom-up and top-down controlled mechanisms. Variations in size-spectra of unicellular plankton may also provide useful quantitative and qualitative biogeochemical information, e.g. explaining changes in organic carbon export from the surface to the deep ocean.

Samples of the unicellular planktonic community were collected from 2016 to 2018 during three summertime cruises and one autumn cruise in the Fram Strait (Arctic Ocean). Respective microscopic measurements resolve details on taxonomic composition and cell size. Based on these microscopic data we have derived continuous size-spectra, using the Kernel Density Estimation method. Applying an elaborate resampling (bootstrap) approach we inferred confidence intervals for the respective size-spectra, providing robust size-density estimates of cell concentration and biovolume.

Opposing traditional size-spectral analyses that report only the slope and y-intercept of a linear log-log relationship of cell size vs cell abundance, we can resolve the size structure in greater detail. We find considerable deviations from the linear log-log relationship in the size ranges close to 4, 9, 40, and 70 μm Equivalent Spherical Diameter, indicating size-selective predation. These features prevail amongst samples that were taken at different times and locations. Between the respective features, size-spectra reveal pronounced variability. The results of our analyses can be used to refine predator-prey relationships of size-based plankton ecosystem models.