Zooplankton in a Spanish lake were studied to see if they move up and down in the water to find food. The researchers used a special tracer to track where the zooplankton were going. They found that the zooplankton did not move much up and down during the day and night. Instead, they mostly stayed in the same area. The zooplankton in the top layer of the lake increased at night, but there was no decrease in the deeper layers. This suggests that the zooplankton are not moving vertically to find food. Instead, they might be moving horizontally to gather in the central area of the lake where there is more food.