These tiny and mysterious creatures of the water are called Plankton; included in these are passively floating or weakly swimming animals and plants. The word plankton comes from the Greek word “planktos,” which means “to drift.” The plankton that drifts in the ocean currents are among the most abundant creatures of the planet. A bowl of water taken from an ocean consists of millions of these tiny organisms; they cannot be perceived by the naked eye. The only way to really become familiar with these creatures is to find them and observe them with the help of a video scope or a microscope. Many marine plants and animals go through a stage in their life cycle when they are plankton, but they ultimately outgrow this stage. These types of creatures are called meroplankton. Unlike meroplankton, holoplankton are tiny creatures which live their whole life as plankton.
Plankton are also classified as either plant plankton or animal plankton. Phytoplankton is the scientific name for plant plankton, whereas zooplankton is the term used for animal plankton. Phytoplankton are usually smaller than zooplankton and it is hard to observe them even under the microscope. Most of the food chains in the ocean begin with phytoplankton, which are eaten by tiny zooplankton. These tiny zooplankton in turn are eaten by larger animals living under the water, including sharks and blue whales.
Since plankton are at the bottom of most of the food chains in the ocean, anything that causes damage to their lives may effect many other organisms. Losing large numbers of plankton may affect the abundance of krill, which is the main diet of whales. Phytoplankton produce oxygen that people breathe; therefore a decrease in the number of phytoplankton may cause problems for human beings. Most plankton live near the surface of the oceans. Pollution, especially that caused by chemical pollutants, has a direct impact on the water at the surface. This is a great threat to plankton populations. Plankton are not only critical for their role in the food chain, but they are also important for their usage in the production of valuable minerals. Ancient deposits of plankton, which were buried under the seafloor and later mined, have become important sources of oil, shale, and many other valuable minerals.
Like land plants, phytoplankton fix carbon through photosynthesis, making it available for higher trophic levels.1 The major environmental factors influencing phytoplankton growth are temperature, light, and availability of nutrients. Phytoplankton can undergo rapid population growth or “algal blooms” when water temperatures rise in the presence of excess nutrients. While increased phytoplankton populations provide more food to organisms at higher trophic levels, too much phytoplankton can harm the overall health of the oceans. During these blooms, most of the phytoplankton die and sink to the bottom, where they decompose. This process depletes the bottom waters of dissolved oxygen, which is necessary for the survival of other organisms, including fish and crabs.
Major groups of phytoplankton include: diatoms, golden-brown algae, green algae, blue-green algae, dinoflagellates and crypto monads. Phytoplankton are being used as indicators of environmental conditions within the oceans because their populations are especially sensitive to changes in nutrient levels and other water quality conditions.
Zooplankton are planktonic animals that range in size from microscopic rotifers to macroscopic jellyfish. Their distribution within the oceans is governed by salinity, temperature and food availability. The zooplankton community is composed of both primary consumers, which eat phytoplankton, and secondary consumers, which feed on other zooplankton. Zooplankton can be classified into three size classes: Microzooplankton-protozoans and rotifers, Mesozooplankton-including copepods and invertebrate larvae, and Macrozooplankton-including amphipods, shrimp, fish larvae, and jelly fish. Zooplankton, like phytoplankton, are excellent indicators of environmental conditions within the oceans, because they are sensitive to changes in water quality.
The most common animal in the plankton group is the copepod. There are more than 7,500 species of copepods. Copepods are small shrimp-like animals.
Copepods have appendages that are used like paddles for movement. They eat diatoms and other plankton and in turn are eaten by other, larger, drifters. A single copepod can eat an average of 200,000 diatoms a day.
Amphipods, which are the main diet of the gray whale, look like a cross between a shrimp and an isopod. The amphipod typically ranges in size from 2 to 50 mm, although a few may be larger. Amphipods are common in aquatic ecosystems throughout many parts of the world, inhabiting marine, brackish, and freshwater environments. A few species also live in terrestrial ecosystems.
One of the most interesting plankton species is the barnacle. It lives in the upper zone of the oceans where the water only comes at high tide. The appearance of a barnacle is rather deceptive. At first glance it looks like a mollusk, but when you observe the larva of the barnacle the truth becomes clear. Barnacles cause serious problems on the hulls of ships and buoys.
Jellyfish, which are basically nothing more than a large stomach with long tentacles, are also plankton. Their tentacles have stingers on them which they use to catch and paralyze food and then bring it to their stomachs. They move through the water by pumping their stomachs. For the most part they move up and down in the water, letting the currents carry them from side to side.
Copepods, amphipods, jellyfish, and barnacles are some of the most abundant and well-known examples of plankton. The oceans of the world contain a world of tiny organisms; most of them invisible to the human eye and yet these are the basis for nearly all the life in the sea. Our awareness of these tiny creatures will help enhance our appreciation of God’s uniqueness and greatness. The more we learn about the mysteries of the deep, the more we believe in the existence of God.
1 Trophic levels: Producer, primary consumer, secondary consumer, tertiary consumer.