Some zooplankton are of microscopic size and they are even able to feed on bacteria. They are unicellular organisms also known as protozoans. Although the largest zooplankton, such as jellyfish, can reach several meters in diameter, the average size of this group is only about 0.25 mm and the main group, the crustaceans make up about 70% of zooplankton species.

MIGRATION

For animals that small, moving in seawater is not as easy as it seems. Seawater is far more dense and viscous (thick) than air so as soon as a zooplankton stops moving his appendages, he comes to a standstill. It is sort of like humans trying to make their way through molasses.  During the 1940's very dense layers were detected in the water column using echo sounding (guess what they were really looking for) and it was assumed to be the bottom of the sea. Surprisingly, those layers moved up to the surface every night and descended again in the morning. These layers, called Deep Scattering Layers (DSL), are formed by the bodies of countless zooplankton whose swimming bladders or fat droplets reflect the sound waves from the sonar.  Although you already know that zooplankton cannot move against the fast, horizontal currents, they are able to swim to avoid predators, capture prey or in this case to migrate up and down in the water.

How far and how fast can zooplankton migrate? They travel 10,000 to 50,000 times their body length in each direction at speeds of 100-200 m per hour!  In the case of some crustaceans, it was found that the distance can be more than 800 m. For a human being, this would mean walking 25 miles to and from breakfast every 24 hours.

So why do they migrate? The surface is where food is, as phytoplankton depend on light for photosynthesis. So zooplankton migrate to the upper water layers to feed on the algae during night. The reason for descending at dawn is avoidance of visual predators like fish. Another explanation might be energy conservation. Metabolic rates like respiration are lower at lower temperatures so they might conserve energy by migrating to cooler layers to digest and wait for the next night. This would permit them to use a higher portion of ingested energy for growth instead for respiration. It has also been suggested that zooplankton might avoid damage by ultraviolet radiation, which penetrates down to about 10 meters. Last but not least, animals might use migration to stay essentially in the same location when surface and bottom currents go in opposite directions. A combination of these factors may be decisive for different species.

Why are scientists interested in vertical migration? Between the ocean surface and the atmosphere there is an ongoing exchange of carbon dioxide. Algae, being primary producers, convert CO₂ to body carbohydrates and the resulting plant biomass is grazed by zooplankton. Fish prey on zooplankton and both produce fecal pellets. Bacteria convert the fecal pellets back into inorganic nutrients for use in primary production again unless the particles fall out of the surface to deeper layers where plants cannot reuse the nutrients and they accumulate. When such organic material leaves the surface layers, CO₂ is effectively removed from the atmosphere where it is an important gas causing global warming. How much carbon is removed from the atmosphere by zooplankton grazing is an important part of understanding global warming! The daily migration of zooplankton may substantially increase flux of organic matter to depth by actively transporting food items in their guts and releasing it at depth during the night. Of course their nighttime respiration also moves CO₂ to the ocean depths.

The major objective of my study then is to find out how long it takes for a zooplankton with a full stomach to clear out the undigested matter. To be effective at removing carbon from the surface, the time must be long enough to allow the migrators to return to depth before defecating most of their gut contents. I am conducting gut evacuation experiments with the most common migrating and non-migrating species from the Sargasso Sea and I am analyzing their gut contents to learn about feeding habits.

Animals are captured by using a big plankton net with a mesh size of 0.5 mm. We tow at night and during daytime in depths between 80-200 m. I pick dominant species and transfer them to incubation bottles where they are kept in the dark at constant temperature. Every 15 minutes time points are taken and the crustaceans are frozen for later analyses in the laboratory. Plant pigments like chlorophyll absorb light. When chlorophyll molecules are hit by a certain wavelength, they fluoresce which means they give off light at another wavelength. This fluorescence is measured to determine the amount of plant biomass that was ingested by the crustaceans.