The World in a Drop of Ocean
Ah, plankton- the basis of the ocean’s food web and the bane of the Krusty Krab.
In some ways, our irritable pal Plankton is appropriately named, but his title is also rather vague because plankton does not refer to a single species or even a single group of species. In reality, there are uncountable numbers of plankton in the sea belonging to numerous phyla and a tremendous variety of shapes, types, and sizes.
So grab your chum bucket and get your computer wife ready, because learning about plankton can get a little complicated.
Plankton are the drifters who are not able to swim against the currents and so travel as the tides and winds carry them. There are many types of plankton and they can be from any and every Domain and Kingdom of life (refer to taxonomy here). An easier way to separate them into smaller categories is to call those that photosynthesize phytoplankton, and those that eat zooplankton.
Unfortunately, these names aren’t that helpful in determining just what these plankton are. “Phyto” might make you think of plants, but in reality phytoplankton are actually either bacteria (the photosynthesizing cyanobacteria) or protists belonging to the diatoms, dinoflagellates, green algae, and coccolithophores.
The name zooplankton is just as unhelpful in describing its constituents as the name phytoplankton is. While “zoo” typically refers to animals, many of the smaller zooplankton are actually protists too!
These are usually separated from another group that we call nekton, which are animals that actually can swim by themselves, but there is a gray area between these two groups.
For example, jellyfish can swim, but some do not swim particularly well and so rely on currents to carry them most of the time. The Portuguese Man-of-War, which looks like and is related to jellyfish, is considered a type of plankton despite the fact that they can be 30 feet long from top to tentacle end! Their air-filled floating top acts as a sail to carry them across the sea and makes it unnecessary for them to swim. Thus, they are at the mercy of the wind, and so qualify as plankton. Their size, however, means that we lump them into the category of megaplankton. Plankton is such a broad group that we have to be more specific when we talk about size. The tiniest are the nanoplankton, which are usually single cells like amoeba that eat phytoplankton; followed by microplankton, which can be the larval form of just about any fish or invertebrate animal. What they lack in size, these organisms make up for in numbers. Phytoplankton blooms occur in nutrient rich waters in spring for the Northern and Southern hemispheres in spring when there is plenty of sunlight to fuel photosynthesis.
What we call a "red tide" or a harmful algal bloom (HAB) is a huge increase in the numbers of phytoplankton in a concentrated area. It is not always the same species of plankton that causes these, rather HAB is a collective term for any of the few species that produce toxins that can kill fish, shellfish, mammals, and birds. They can harm swimmers, too, but more often humans are affected by consuming the shellfish and fish that have built up the toxins in their bodies. Even some of the non-toxic species can be harmful on an ecosystem level. These huge masses of algae don't live forever, and when they die and decompose, it removes oxygen in the water. Fish and other sea creatures that do not surface to breathe face anoxic (without oxygen) conditions, and those that cannot swim out of the area are left without any option to escape.
Sometimes, a red tide can be a truly magical event. The waters have to be a unique balance of warm and nutrient-rich to trigger a bloom of one of only a few species of dinoflagellates, single-celled phytoplankton that contain the chemical luciferin and its associated enzyme luciferase (1). When these dinoflagellates are disturbed by waves, swimmers, or splashing animals, they combine these two chemicals and bioluminescence is the result:
Many of the larger zooplankton rely on these blooms for their own population spikes, and many of our favorite animals migrate across the world just to be there for these plankton. Be it a source of food or breath, every ocean animal is reliant on plankton. Plankton of all sizes are filtered and eaten as the primary source of food for the some of the largest species of fish on earth. The 30+ foot-long whale shark (Rhincodon typus), the 25+ foot-long basking shark (Cetorhinus maximus), and the giant manta ray (Mobula birostris) which can have a wingspan as wide as 29 feet are shown below. All are planktivores, animals that filter water through their gills to act as a sieve for trapping and eating plankton.
There is another plankton group that is, arguably, one of the most important food sources in the sea. They aren't algae, but they swarm in such high numbers that their red color resembles a red tide from an observer overhead. These creatures are crustaceans, related to shrimp, and their existence drives some of the greatest migrations known to science.
Say hello to krill, the animal that is as important to the world as its size is small.
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Krill are an important next step from the smaller phytoplankton and zooplankton in the food chain. They eat these smaller organisms, and then just about everything else eats them. In the cold, nutrient-rich polar seas, krill are in such high seasonal abundance that baleen whales the world over are willing to swim thousands of miles to find them. Their annual migration patterns are founded upon the presence or absence of krill. These shrimp-like crustaceans are a summer delicacy, and they are in highest abundance from January to March in the Antarctic and from May to August in the Arctic. Their name is Norwegian for "small fry of fish," and they are a staple in the diets of everything from squid to fish and penguins to seals. There is, in fact, an Antarctic seal named just for its dietary habits. The krill-eater seal (also known as the crabeater seal) eats them almost exclusively- krill make up as much as 95% of everything they eat(2). There are 82 different species of the animals that we collectively refer to as krill, and as most of them will get no bigger than about 3 inches long, the blue whale can eat as much as four tons of krill a day (3).
This means that plankton is not only the primary food source for the largest fish species in the world, it is the staple diet of the largest animal that has ever lived on Earth!
They may be exceptionally tiny, but phytoplankton are one of the most important groups of living things on Earth. At least half of the oxygen we breathe comes from phytoplankton, and some studies suggest that it is even as much as 85% (4, 5)!