Zooplankton are microscopic animals that live on or near the top of large bodies of water. They are an integral part of the biosphere and are extremely adept at developing strategies and physical characteristics that allow them to survive in drastically different environments.
Information on zooplankton varies based on type and regional location. There are literally hundreds of thousands of variations of zooplankton based on adaptations made to survive in particular microcosms. These variations are so pronounced that differences in the zooplankton content of the waters on either side of the Gulf Stream are vastly different. This difference is often used by scientists to determine the origin and movement of ocean currents.
What is Zooplankton
Plankton is the mass of microscopic plants and animals that float near the surface of large lakes and oceans. Phytoplankton is the plant matter while zooplankton is the animal component. Zooplankton feeds on phytoplankton and acts as a link in the food chain between plants and larger animals.
Without these tiny creatures to graze on the phytoplankton, there would be a catastrophic collapse of the entire marine ecosystem and bioweb. These little creatures are the transfer point between energy producers (photosynthesizing plants) and larger animals. Sometimes they are referred to as “secondary producers” because they bring the energy from the plant kingdom to the animal kingdom.
Not all zooplankton are tiny though; some reach sizes up to and over eight inches including the Portuguese Man o’ War. Size classifications help determine where on the food chain zooplankton rests.
- Picoplankton - less than 2 micrometers
- Nanoplankton - 2 to 20 micrometers
- Microplankton - 20 to 200 micrometers
- Mesoplankton - 200 micrometers to 20 millimeters
- Macroplankton - 20 to 200 millimeters
- Megaplankton - over 200 millimeters
Just as in the larger animal kingdom, larger zooplankton will eat smaller ones. Most megaplankton are carnivores and most picoplankton and nanoplankton are strictly herbivores. Microplankton, mesoplankton and macroplankton are generally omnivorous, feeding on both phytoplankton and smaller zooplankton.
Types of Zooplankton
Just as there are different sizes of zooplankton, there are also different types. They are divided into two major categories; Holoplankton and Meroplankton.
Holoplankton are animals that spend their entire lives as part of the plankton while Meroplankton are larval animals in early stages of life that use the plankton biome as a place to grow from larvae to adulthood.
Holoplankton consists of:
- Pteropods
- Chaetognaths
- Larvacea
- Siphonophore
- Copepods
Meroplankton consists of:
- Worms
- Mollusks
- Crustaceans
- Coral
- Echinoderms
- Fish
- Insects
A very interesting class of life forms also inhabit the plankton: Mixotrophs. These organisms straddle the line between plant and animal. They are capable of photosynthesis but also can envelope whole other cells in a process called phagocytosis. This is the same process that amoeba use. The entire cell of a mixotroph closes around another cell and breaks it down once it has been enveloped.
Day/Night Cycles
Zooplankton depend on ocean currents for the majority of their movement, but they also posses means of locomotion. Some have flagella, spines or even liquid expulsion propulsion systems.
Most of the time, these means of movement are used to avoid predators or to help in grazing. They also migrate to deeper areas of the water during the day using these systems. They then come to the surface at night to feed.
This “night swimming” technique is thought to keep the animals safe from visual predators. It is also believed that the cooler temperature of the surface water means there is less movement in the plankton sheet so the animals don’t have to use as much energy while grazing.
Indicator Species
Using specific strains of zooplankton as indicator species can help scientists recognize larger environmental changes. If there is a significantly lower number of zooplankton than is expected, the scientists can begin to focus on the possible causes.
A lower than expected phytoplankton bloom in the spring can result in a lower number of zooplankton but also a higher number of predators in the region in the summer or fall could cause the same drop in numbers. If the phytoplankton bloom is low, then the scientists can check for environmental changes including changes in water chemistry and weather patterns. If there are more predators in the area then it is more likely that a food source that was abundant in one area has dwindled causing the predators to migrate to the new region.
Understanding these changes in the ecosystem by using information on zooplankton as a starting point is just starting to come into its own in the marine biology community. Because most of the forces that affect zooplankton growth, reproduction and population are understood, they are the best starting point for any marine environmental inquiry.
References
" Zooplankton - MarineBio.org". MarineBio.org. 7 July 2011, https://marinebio.org/Oceans/Zooplankton.asp
National Oceanic and Atmospheric Administration, “Zooplankton”; Ecology of the Northeast U.S. Continental Shelf, https://www.nefsc.noaa.gov/ecosys/ecology/Zooplankton/
Hammer, William M. 1974. “Blue Water Plankton” National Geographic Magazine, October.
Gaskell, T.F. 1964. World Beneath the Oceans, The Story of Oceanography. The Natural History Press, Garden City, New York.
Image: hilmil1 @ FlickR, https://www.flickr.com/photos/hilmil/3990545446/sizes/m/in/photostream/
