Phytoplankton, microzooplankton, copepod and dissolved nutrient data from a mesocosm experiment, which took place in summer 2016. A range of Si:N ratios and two levels of copepod grazing pressure were manipulated on a natural plankton community in Kiel Bay, Southern Baltic Sea, Germany.

Experimental manipulationsBefore the experiment, we pre-filtered Baltic seawater with 125 µm mesh size sieves. Twenty 20L plastic bags served as experimental units to fully cross five Si:N ratio treatments and two grazing regimes with two replicates for each treatment.Grazing treatmentsHigh copepods - adult Eurytemora affinis copepods were added to half of experimental units at a target concentration of 30 individuals per liter on day 5 of the experiment.Low copepods - remaining bags had lowered copepod numbers due to sieving before the start of the experiment.Nutrient treatmentsNitrate and silicate were added to experimental bags at the start of the experiment to reach the following Si:N ratios: 0.25:1, 0.4:1, 0.55:1, 0.7:1 and 0.85:1. Silicon concentration was manipulated and nitrogen was kept constant at 40 µmol/l. Additionally, phosphate was added to reach 2 µmol/l.