Vertical distribution of juvenile (thin lines and circles) and adult (thick lines and squares) Daphnia at their different population densities in each of the four treatments (a–d) and the shape of the gradients for respective treatments (mean ± SD, (e–h), continuous lines for the gradient of food concentration and dashed lines for the gradient of light intensity): in the absence of any gradients (a, e), in the presence of the food gradient alone (b, f), in the presence of the gradient of predation risk alone (c, g) and in the presence of both gradients (d, h) in the first series of experiments conducted to verify the model predictions performed in the apparatus comprised of twin columns to study the vertical distribution of zooplankton (Maszczyk, 2016). The gradient of predation risk was shown only as the gradient of light intensity, but it also consisted of two other components, i.e. the presence of kairomones and alarm substances, which was assumed to be evenly distributed throughout the columns, and the presence of the light spectrum gradient, which was the same in all the experiments performed in the apparatus and is shown in Fig. 1 of the earlier study (Maszczyk, 2016).