Phytoplankton and zooplankton counting and growth rate

Phytoplankton abundance in the experimental containers was analyzed daily to ensure that the nominal concentrations were maintained, using a Sedgewick-Rafter counting chamber in an inverted microscope (Olympus CKX41). Counting unit was 100 μm of filaments for filamentous Cyanophyceae and individual cells for the remaining species, according to standard techniques (Olenina et al., 2006). Phytoplankton growth rate was evaluated using 24-well plates and measuring optical density at 750 nm (OD750) daily with a plate reader (Floustar Omega, BMG Labtech). Growth rate was assessed in a separate growth experiment, performed in the 24-well plates (2 mL in well) using the same treatments used in the feeding experiment. Phytoplankton growth rate was calculated using the following equation, growth rate = ln(OD_b/OD_a)/t_b − t_a, in which OD_b and OD_a are the OD750 values at respective time, t_b or t_a. Zooplankton abundance was analyzed using a stereomicroscope (Olympus SX7).