Plankton , Greek for "drifter", are small plants (phytoplankton) and animals (zooplankton) that drift with the ocean's currents. They form the bottom of the food chain in the sea and are very important in ocean food webs. Oceanographers use nets to catch these small creatures and study them. The nets are of a much finer mesh than fish nets, as the mesh openings must be small enough to concentrate the plankton while still allowing water through. Phytoplankton nets have a very small mesh opening (about 36/1000 of a mm) and zooplankton nets have larger meshes (about one 1/3 to 1/2 of a mm). The nets are attached to the hydrowire and towed behind the ship. Plankton tows can be done at any depth or time of day, and can be used with opening/ closing mechanisms to enable them to collect at a desired depth.
Zooplankton, small drifting animals in the sea, are the next step up in the marine food chain from phytoplankton. They are important because they consume plants (and each other), and are food for many fish, seabirds, and some marine mammals. Oceanographers can collect live zooplankton at sea and perform experiments with them on-board ship. This technician is picking through a newly collected sample to remove certain species for use in an experiment.