I’m going to break this topic up into multiple parts because this is a pretty big subject. But I should note that I will be dealing mainly with plankton-eating animals such as clams, sea cucumbers, anemones and other non- or slow-moving invertebrates, and not fish and whales.
It’s true that although some fish (mullets, basking sharks) and baleen whales (blues and humpbacks) do "filter" their food from the surrounding water, the animals they feed on, such as krill, menhaden and sand lance are not plankton. These are free-swimming animals so they do not qualify as "drifters". And as far as I’m concerned it makes these larger fish and mammals predators and not filter feeders. This diary will start not with specific examples of these, but with a short study of what, exactly, plankton is.
Plankton, by definition, are any forms of life that drift through the water and are unable to determine where they go. Many types can move vertically through the water column, but the important point is they cannot control their horizontal movement. They are at the mercy of the winds and the currents. In contrast, nekton are animals that can control their horizontal movement.
Usually when one pictures plankton we think of them as tiny, microscopic creatures, which isn’t always the case. Jellyfish are planktonic and some, like the Arctic lion’s-mane jellyfish can reach a bell size of eight feet across and a tentacle length of over 200 feet, which makes them technically the longest creatures that have ever lived. How cool is that? One of the largest animals ever is a plankter. But generally, yes, most types of plankton are tiny.
Plankton are catagorized into several subgroups, the most obvious being those that are plants and those that are animals. Animal plankton are called zooplankton and plant plankton are known as phytoplankton (note that most of these are not true plants but algae, however all photosynthesize so we’ll call them all "plants" for convenience sake).
Phytoplankton may be microscopic one-celled organisms or made up of clumps or chains of multiple cells. Most are very small in either case. Because they rely on the sun for their energy, they are mostly limited to the ocean’s surface. I’ll ignore extreme cases, such as deep-sea plankton that are nourished by sulphur from hydrothermal vents.
Zooplankton are more diverse and need to be further subdivided into those that are permanent members of the planktonic community and those, like fish fry and other larval stages of marine animals, that are only temporarily planktonic. The later group are referred to as meroplankton and the former are haloplankton. Meroplankton will develop into nektons, like fish, or settle to the bottom (clams, sea urchins, crabs) and become benthic animals for the rest of their lives. Also, many species of marine animals have floating eggs and these too are considered to be members of the meroplanktonic community, both before and after they hatch.
This larval sea star would be called a
meroplanktonic zooplankter.
Furthermore, plankton can also be catagorized based on size. For example, drifting bacteria are picoplankton, slightly larger protists are called nanoplankton and so on all the way up to the largest jellyfish and comb jellies which make up the group called megaplankton. I generally don’t like this sort of annoying distinction based on size, however it will be important later when discussing feeding methods since the type of strategy a filter feeding animal uses will determine the size, rather than type, of plankton they can feed on.
So, here’s a list, in ascending order, of planktonic size groups (the first group is limited mainly to marine viruses):
- femtoplankton
- picoplankton
- nanoplankton
- microplankton
- mesoplankton
- macroplankton
- megaplankton
Copepods are medium sized planktonic
animals that fall under the mesoplankton
category.
Plankton are usually thought of as being at the bottom of the oceanic food chain. But if that’s the case, what do plankton feed on? For the most part plankton feed on other plankton. The best thing to do is to think of a food pyramid. Phytoplankton are at the base of this pyramid, and being photosynthetic their numbers are theoretically limitless. Like land plants they get their energy from sunlight. The smallest zooplankton feed on the phytoplankton and larger zooplankton feed on the smaller zooplankton and so on.
As far as distribution goes, pretty much all plankton are limited to sunlit areas of the ocean; phytoplankton for photosynthesis and zooplankton because that is where the food is. Ironically, the depth that light can travel through the water is in large part determined by the density of plankton. So in tropical areas, with very little plankton, the habitat available to these plants and animals is much greater than in the plankton-rich temperate areas. In deep waters, where light cannot penetrate, both filter feeders and zooplankton that live there feed on planktonic snow. This term refers to the steady flow of dead plankton that drifts down through the water column. As mentioned above, most types of plankton can control their vertical position in the water column. That is, until they’re dead.
Tropical seas are so clear because of the small
planktonic populations found in warm water.
Which brings us to our final topic, which is why are tropical areas so plankton-deficient compared to temperate and polar regions? The main reason has to do with seasonal upwelling. My friend Wayne has a very in-depth take on upwelling at his blog Niches, but I’ll give a very basic summary here.
In addition to light, phytoplankton require nutrients such as usable nitrogen, phosphorus, (and in the case of encasement-forming species, such as diatoms, silica) in the water to thrive. These dissolved nutrients are used up very quickly in the water column, most of it being bound up in sea life and then trapped at the bottom when the organisms die.
Diatoms, an encasement-forming species of
phytoplankton, require water-borne silica
molecules to form their shells.
Tropical areas have a pretty constant warm temperature year-round. Temperate areas on the other hand can be quite warm during the summer and then much colder in the winter. Here we get into a bit of physics. Water gets denser as it cools. In tropical areas the surface water stays warmer than the lower strata. However, in temperate areas the surface water cools in the winter until it is actually colder than the lower levels. When this happens the surface water sinks to the bottom, which then forces the bottom water to rise to the surface. This inversion pulls nutrients in the sediment up to the surface and makes the trapped minerals once again available to the phytoplankton. And remember, whenever phytoplankton populations increase, zooplankton numbers soon follow.
Upwelling can also be wind-driven, as what happens on the Pacific South American coasts during el Nino years and results in an explosion of the anchovy fishery.
This inversion in temperate areas happens twice a year. Once in the fall and once in the spring. As I mentioned (without explanation) in my bivalve dendrochronology diary, this upwelling produces a huge burst of planktonic life known as a plankton bloom. As we will see in future diaries, these blooms are extremely important to the life cycles of many, many animals.
The importance of this thriving planktonic community cannot be overstated. Basically, life in the sea would not exist without them. They are absolutely vital to the early life stages of nearly every species of marine animal, neccessary food sources for the myriad forms of sessile (non-moving) marine animals, play a huge role as carbon sinks, trapping greenhouse gasses and preventing them from being released into the atmosphere (in fact, some scientists have even proposed seeding the ocean with nutrients to encourage phytoplankton production to offset climate change), and in total produce more of the earth’s oxygen than all land plants combined.
Fun Fact: Like the word "planet", plankton comes from the Greek word "planktos", which means "wanderer".
Other diaries in this series can be found here.