Countless living things float and drift in the world’s ocean and other bodies of water. These living things, or organisms, are known as plankton. They include animals, protists, bacteria, plant-like and other kinds of aquatic organisms. Plankton range in size from bacteria up to large jellyfish. They are the wanderers and drifters of the sea, moving wherever the oceans currents take them. Plankton have a critical place in the food chain that supports fish and other sea creatures—and the people who eat them. They are the unseen heroes of many ecosystems providing food to a wide variety of species from tiny bivalves to whales.
The word plankton comes from the Greek word planktos, which means “drifter.” Their name fits, because plankton, while they may swim, are unable to swim against the current. They drift about in the water, moving with winds, waves, currents, and other factors which determine where they go.
Despite this limitation on horizontal movement, plankton often can undertake remarkable vertical migrations, helping them find food and avoid predators. Zooplankton vertical migrations can be 10,000 to 50,000 body lengths each day. If plankton were a human, it is equivalent to a human walking a roundtrip distance of 50 miles each evening to get dinner or to avoid becoming dinner!
Types Of Plankton
Two classifications of plankton are
- Phytoplankton (plant-like)
- Zooplankton (animal)
Phytoplankton includes not only algae but also archea, bacteria and fungi that also float in the world’s waters. These living things may also be considered plankton.
Plankton are comprised of two main groups, permanent members of the plankton, called holoplankton (such as diatoms, radiolarians, dinoflagellates, foraminifera, amphipods, krill, copepods, salps, etc.), and temporary members (such as most larval forms of sea urchins, sea stars, crustaceans, marine worms, some marine snails, most fish, etc.), which are called meroplankton.
Plankton diversity and abundance are affected by many factors such a water temperature, pH, nutrient density, water turbidity and clarity.
Plankton that is made up of algae, bacteria, archea, protist or plantlike organisms is called phytoplankton. These organisms are often no larger than a single cell. For example, a single-celled type of algae, called a diatom, is a common form of phytoplankton found in the coastal ocean. Phytoplankton floats near the surface of the water. Like plants it uses sunlight to produce energy and then releases oxygen in the process of photosynthesis.
They form the base of the photosynthetic marine food web and are grazed on by many ocean creatures, from sardines to whales. Viewed under a mircoscope, a mere drop of water teems with an astonishing diversity of phytoplankton. These microscopic organisms, largely invisible to the naked eye, are crucial to the survival of life on earth.
These tiny one-celled organisms make up what might be considered the grasslands of the ocean. While most plankton are microscopic, some can reach 33m in length.
For every breath of air you take, half of the oxygen is courtesy of plants from the land, and the other half from phytoplankton and other photosynthesizing aquatic organisms. Phytoplankton produce oxygen and contribute close to half of the total global oxygen supply. On the other side of the equation, phytoplankton absorb one third of the carbon dioxide (CO2) produced globally, whether from our exhaled breath or by the combustion of fossil fuels. Phytoplankton play a pivotal role in the removal of CO2 from the atmosphere to the oceans and thus are key mediators of the global climate.
Plankton that is made up of animals or animal-like organisms is called zooplankton. Some of these organisms, such as larval crustaceans and protozoans, are very small. Others, such as jellyfish, can grow larger. Some fishes and most invertebrates begin their lives as eggs or tiny larvae. These eggs and larvae are often also part of the zooplankton.
Zooplankton are also sensitive to their environment and like phytoplankton—a change in zooplankton concentration can indicate a subtle environmental change. Zooplankton are highly responsive to nutrient levels, temperatures, pollution, levels of light, and increases in predation. As well as providing an essential link in the marine food chain (which is an understatement), the diversity of species, amount of biomass and abundance of zooplankton communities can be used to determine the health of an ecosystem.
Choose A Net
The type of net you use will dictate the type of plankton you collect. Do you want to look at big things, little things, in between? Do you want to sample a particular part of the water column? Keep the following in mind:
Net opening: The mouth of the net can be different sizes. As you probably guessed, the larger the net opening, the more water the net samples. But be careful, particularly in shallow applications - you do not want your net to drag on obstructions or the bottom.
Net mesh: Nets have different mesh sizes (a measurement of the sides of the mesh squares in the fabric of the net). They are usually measured in microns, which is 1/1000 of a millimeter. The smaller the mesh size, the smaller the animals you’ll collect, and vice versa. Plankton nets range anywhere from 150 microns to 1000 microns, depending on what the researcher hopes to collect.
Cod End: The cod end is the bottom of the net where the fabric tapers and all the organisms are caught. Some cod ends are just bottles screwed into the bottom, but others have mesh windows to filter water as the net samples.
Collect A Sample
The key to effective net sampling is to a pass sufficient amount of water through the net. We typically do this by taking turns towing the net back and forth. Nets can also be lowered vertically and back to the surface to collect mixed samples from depth and the surface.
While we are doing our tow, we are also collecting other data such as the wind speed, pH of the water and temperature.
Created with images by Masrur Rahman - "Love deeper than the blue that you see now. " • Sarah Lee - "A flurry of air bubbles from scuba divers, 20m below the surface." • Jeremy Bishop - "untitled image"