Phytoplankton cell size acts as a master trait as it correlates with e.g. grazing resistance- and nutrient uptake-related sub-traits and as such shapes phytoplankton’s ecological niches. Temperature increase is predicted to decline average phytoplankton cell size due to increased grazing and decreased nutrient availability. However, the functional consequences of decreased phytoplankton size at the basis of the pelagic food-webs remain to be studied. Here we experimentally tested whether (i) individual size determines the plankton’s vulnerability to grazing, and (ii) a community wide size shift affects phytoplankton’s response to nutrient enrichment and depletion. Using a natural plankton community from the Kiel Bight (Baltic Sea) we conducted an indoor mesocosm experiment (February-March 2016). Grazing phase: The plankton was first treated with a gradient of six copepod (Acartia tonsa) densities (0, 10, 20, 40, 80, 160 ind/L) for seven days in order to test for (i) and shift the community size structure. Nutrient manipulation phase: After the removal of grazers three of the pre-grazed (0, 40 and 160 ind/L) communities were treated with two different nutrient levels (ambient conditions, 16N/1P) to assess the community response (ii).
The results show that phytoplankton individual size can determine its vulnerability of being consumed by mesograzers. Grazing pressure was highest on medium (mainly nano-plankton) sized phytoplankton. The effect was weaker for smaller and bigger sized cells or even positive for pico-plankton (<10 μm3) due to competitive release. Grazing, however, also differently affected single species that belonged to the same size class. Nutrient addition increased phytoplankton growth. This effect was reduced for those communities that were pre-treated with grazers. Larger cells generally responded stronger to nutrient addition. Individual species response to nutrients, however, could deviate from the mean response of their size class. Among the size classes pico-plankton were the least nutrient limited. We conclude that with some deviations, cell size can be used as an indicator for phytoplankton responses to growth, i.e. nutrients, and loss, i.e. grazing, factors.