Upper left: Mean vertical gradients of temperature (thin line) and algal concentration (closed circles) during the experiment. Thermal variation was too small to be depicted in this graph. Other panels: Vertical biomass distributions of the total population of D. pulicaria at increasing densities. Numbers in different panels indicate the mean dry biomass in g per tower in the five total biomass groups (both towers combined). Broken lines delimit the zone with the algal maximum. Error bars: ± 1 s.e.