THE FOUR SEASONS OF PLANKTON

Vivaldi’s Four Seasons, composed in 1721, are among the most famous classical music works of all time. By listening to each of the concerts, we instinctively move into nature in all its splendor and severity. In the same way, plankton live and vibrates in response to the sound of the seasons in a harmonious way that is repeated year after year. Here, I will give you a short summary of what are these seasons for a standard temperate ecosystem. I hope that the next time you look at the sea, either in summer, fall, winter, or spring, you can imagine how the little creatures that live there interact, relate and struggle to survive. You will notice that in the sea, biology, physics, and chemistry always go hand in hand and that without one we cannot understand the others. I hope that these few lines serve to share with me the need to take care of the fragile and invisible plankton ecosystem, which nourishes us by being the base of the marine food web and gives us half of the oxygen we breathe.

Before I begin, I recommend you to read my first post A teaspoon of seawater, a tiny ecosystem because there, I explain the different components of plankton and their trophic interactions. If you don’t want to, it is also OK, I give you a summary next: 

The basis of the trophic food web in the sea is phytoplankton, a multitude of unicellular algae that by photosynthesis incorporate CO2into their living tissues and produce O2. These tiny, but vital, beings are the food of microzooplankton (unicellular predators), and mesozooplankton (larger multicellular predators), such as copepods. Copepods are the basic food of many adult and fish larvae. 

Having said all that, let’s start with the cycle of the seasons. In order to facilitate comprehension, I have made a small drawing with a summary of the most outstanding facts of each season. We will start with winter when the water is cold and mixed by the wind action, and the light of a sun, which does not rise much above the horizon, penetrates with little intensity the surface of the ocean. The few phytoplankton we find, despite being full of nutrients because the intense mixing of surficial and deep waters, cannot grow much because it is limited by light availability and low temperatures. Towards the end of winter and the beginning of spring, the light is more intense and the temperature begins to rise. The water heats up slowly and a thin thermocline (a layer that separates two bodies of water with different temperatures and densities) develops, which will confine the mixing layer to more superficial areas. These conditions favor the phytoplankton bloom, which will be accompanied by a growth of the populations of microzooplankton first and larger zooplankton (for example, copepods) later. Time passes, the temperature rises, and summer enters. At this time, the already well-formed thermocline clearly separates two areas, like oil on a pan full of water: a shallow one (about 50 m deep in open waters), hot and poor in nutrients because de consumption of those by phytoplankton in spring and beginning of summer, and one deep, cold and full of nutrients. Early in the summer, nutrient-limited phytoplankton depleted by zooplankton consumption give way to a particular, and less numerous, community of algae adapted to these conditions. Summer algae are either small in size, with a high surface-to-volume ratio that facilitates the use of the few nutrients available, or they are large but motile (e.g. dinoflagellates) able explore the micro-patches of nutrients that may remain. These latter algae, the dinoflagellates, under favorable conditions (for example, within confined areas such as harbors and similar) can begin to grow to form harmful algal blooms, formerly misnamed “red tides” (they are neither tides nor often red). Consumers in these summer communities are either microzooplankton or a combination of small particle filter zooplankton, such as some gelatinous organisms or marine cladocerans (water fleas); we also find some carnivorous predators, such as some species of copepods, and some detritivores. When the first autumn storms arrive, and the wind intensity rises, the thermocline breaks down and allows nutrient-rich waters to reach the surface. Sometimes, if the weather conditions of the year allow it, there may be another small bloom, but often low irradiances and temperatures make the phytoplankton unable to take advantage of the fresh nutrient input. Winter is back and the cycle starts again. 

As you can see, life cycles are repeated year after year in plankton as well, and this has led many marine organisms to reach a kind of internal clock that tells them when to lay eggs, for example. With climate change, unfortunately, many of these clocks get out of adjustment, which affects the natural functioning of the ecosystem.  

Zooplankton’s most feared enemy

They are beautiful, they may be smart, we want to protect them, but from the perspective of our tinny friends they are terrifying. The whales!

I hope you enjoy my weekend’s drawing 🙂

An average-sized humpback whale can eat 2000-2500 kg of krill each day during the feeding season in cold waters (about 120 days). And they eat twice a day!

Help me! Kill the whales!

Other major predators are fish, jellyfish and other zooplankton

The most dangerous predators of the ocean, the protists

Mr. Vaughn, what we are dealing with here is a perfect engine, an eating machine. It’s really a miracle of evolution. All this machine does is swim and eat and make little sharks, and that’s all

This is how oceanographer Matt Hooper (Richard Dreyfuss) described the white shark in Steven Spielberg’s 1975 film Jaws. Certainly, when we think of an efficient and terrible marine predator, the shark always comes to our mind, and more specifically the white shark, which is the worst famous. However, if Mr. Spielberg had known of the terrible ways in which marine protists kill and devour their prey, perhaps the film would have been called “The Protist”. Jokes aside, if there are any creepy and dangerous beasts for their congeners in the ocean these are the protists.

Protists are eukaryotic (i.e., with nucleus) unicellular organisms ubiquitous in seas and oceans. We can classify them in various fashions, for example, by their way of obtaining energy: those that do photosynthesis (autotrophs or algae), those that eat other organisms (heterotrophs, also called protozoa) or those that combine both strategies (the mixotrophs). Here we will focus only on those that eat live prey, i.e. protozoa and mixotrophs.

Of the approximately 50 gigatons (50 billion tons) of carbon that algae produce annually in the seas and oceans, protozoa (perhaps with the help of mixotrophs) consume about 30- 60%. We think that the next relevant consumers, the copepods, only eat about 6 gigatons. Protozoa and mixotrophs do not eat only algae, they also feed on bacteria, other protozoa and even some animals much larger than them. But how do these tiny, mouthless unicellular beings eat them? The truth is that they have different prey capture and feeding strategies depending on the group, and they are all very curious.

The main feeding strategies in protists

Filtration: Many microorganisms use whale-like feeding systems, either by attracting prey into the oral orifice or by swimming and collecting prey. This feeding strategy is usually used for very small prey, such as bacteria or flagellates.


Engulfment: When prey begins to gain considerable size, many protists can catch and ingest them whole in a process resembling that of a boa eating a goat. In fact, some protists, such as the dinoflagellate Gyrodinium dominans have a very flexible body (cell) and can ingest chains of diatoms much larger than theirs (Fig. 1). Some foraminifera, distant relatives of amoebas and provided with an outer cover formed by calcium carbonate, can swallow even large copepods. The process is slow but effective (Fig. 2). Many protists use venom-laden stingrays or release toxins into the water to immobilize prey. Some toxins are so efficient that they can kill fish and other organisms, and even once accumulated by filters, such as mussels, they can lead to serious poisoning in humans.

Figure 1. Process of swallowing a chain of diatoms by the dinoflagellate Gyrodinium dominans. The red arrow indicates a G. dominans with a chain of diatoms inside. The blue arrow shows the size of the same species without prey inside. Photo by Albert Calbet

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Figure 2. The image shows a foraminifer that has just captured two copepods. Photo by Albert Calbet

Tube or peduncle: Certain dinoflagellates have a retractable tubular structure that they insert into the prey to suck its contents, as if it were a straw on a Margarita Cocktel (Fig. 3). They use this mechanism to eat prey similar in size to theirs, but also to kill and devour, like tiny leeches, animals much larger than themselves, such as copepods, worms, and so on.

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Figure 3. Peduncle feeding of a Dinophysis sp. on a myxotrophic ciliate (Mesodinium rubrum). Drawing Albert Calbet

Pallium or veil: This is perhaps the most curious and complex mechanism. Like sea urchins and starfish, they evaginate their stomachs (well, not an actual stomach indeed, but a membrane with digestive characteristics) in order to slowly digest their prey, such as large diatom chains (Fig. 4). Gradually, the trapped cells are consumed and the predator incorporates the dissolved nutrients into the membrane. Once finished, only a siliceous skeleton will remain.

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Figure 4. Protoperidinium sp. dragging a chain of diatoms into its pallium. Photo by Albert Calbet

Piston: Not long ago, a very odd dinoflagellate was discovered and named also with a very odd name, Erythropsidinium. It has a small piston that can be expanded and hided very fast. It seems Erythropsidinium uses it to detect and, by a suction mechanism, catch prey that will be eventually swallowed (Fig. 5). The most interesting thing about this unicellular creature is that it also has a kind of primitive eye (ocelloid), with its lens and all. The function of this ocelloid is still subject of debate among the scientific community, but it could be used, in a very rudimentary way, to locate prey. Remember, we’re talking about a single-celled organism!

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Figure 5. Representation of an Erythropsidinium showing its ocular lens and piston. Drawing Albert Calbet

Luckily for us, all these creatures, which could be taken from the most terrifying of Stephen King’s books, are no more than a few tens of thousandths of a millimeter. Imagine what would happen if they were our size!

The biological pump and the role of ocean plankton in mitigating global warming

The biological pump is a process by which the ocean, with the help of marine organisms, captures CO2 from the atmosphere and buries it in sediments, where it will remain for hundreds or thousands of years. Through this process, the ocean helps to mitigate the effects of global warming, as it captures and integrates into the living matter the same CO2 as all plants on the planet’s surface. And all this is done mostly by tiny unicellular beings called phytoplankton. Phytoplankton consists of microalgae of a few thousandths of a millimeter, but of great relevance because they are responsible for photosynthesis in marine planktonic trophic food webs.

Schematic representation of the biological pump. One of the many possible pathways has been exemplified. Drawing Albert Calbet

To understand how the biological pump works, imagine for a moment that we are a carbon atom that, together with two oxygen atoms, forms a molecule of CO2, the dreaded byproduct of burning fossil fuels. Maybe we came straight from a car’s exhaust pipe, we came out of the chimney of an industry, or just out of the lungs of our neighbor, no matter what. We, in the form of carbon, fly happily by the proximity of the sea enjoying the view, but one day, we enter into the water through a process called diffusion. In the water we are quickly trapped by a small algae that turns us into living matter with the help of the sun’s energy and some or other inorganic nutrient. Although we feel proud to be part of something bigger and more organized than a simple molecule (we are now part of a sugar chain), our joy does not last long, because a small mixotrophic dinoflagellate swallows us. Within the digestive vacuoles the complex thing we had become disintegrates again into small fractions and is used to create other complex structures. Well, not so bad, now we are part of something even bigger and that makes us happy. However, a ciliate that passed by makes us part of his diet and the digestion process starts over again. But the odds decided that, this time, we did not finish the digestion process because a copepod chews us and we end up in the beast’s stomach. With time and patience (i.e., by catabolic and anabolic processes), we move to a lipid chain that goes to the cephalothorax of the copepod in the form of a drop of fat. Our host migrates to s deeper zone during the day to avoid being consumed by fish, which as we all know are visual predators. At sunset, we ascend to shallower layers, where there are algae and other prey, but along the way, a euphausiacea (krill) attacks us and the copepod of which we were part ends up split in two. The part where we are is not ingested and we are slowly settling to the depths of the ocean — if what had attacked our guest had been a fish or a jellyfish we might still be wandering the food web and our history would be different. On the way to the abyss, bacteria and other microorganisms begin to decompose the remains of the copepod and each time we find ourselves into a smaller piece. Suddenly, there is a adrupt change of speed. Several decomposing particles have been added together and now we fall more quickly stuck to a piece of sea snow. Upon reaching the bottom, after what have seemed days, we still have some chance of being part of the benthic trophic network again by the action of crabs, worms, or other critters. However, whether by chance or because we were in a difficult-to-chew bite, our carbon atom is respected and little by little we go deeper into the sediment, where the lower pH will keep us for years, maybe decades, centuries or even millennia. By this way, that carbon atom that was part of a CO2 molecule has been trapped in the depths of the ocean.

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Mixotrophic dinoflagellate. Ceratium sp. Photo by Albert Calbet.
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Copepod. Calanus minor. Photo by Albert Calbet

The pathways by which a carbon atom passes from the atmosphere to the ocean floor are unlimited and of very different durations, from a few days, like the one I have represented here, to hundreds or thousands of years, if it never gets there. Of course, in the process, this carbon atom will surely contribute its grain of sand for life to continue.

Microzooplankton, terrible predators of the oceans: The movies!

The two previous videos show different species (Protoperidinium spp.) of dinflagellates pallium feeding on diatom chains. Similar to the seastars, the dinoflagellates evaginate a sort of “stomach ” to slowly digest the poor prey.

Heliozoan feeding movie
Tube feeding of a Lessardia sp.

This snake-like thing that you see swimming all over the place is a Gyrodinium dominans that has engulfed a diatom chain.

Phoraminiphera can look peaceful unicellular guys. However, they are voracious predators. The one on the movie is trying to eat two copepods at the same time!

The plankton and the marine food web

Here, I will post few drawings of plankton and their role in the marine food web. All drawings and pictures belong to me, Albert Calbet. If you want to use any for teaching or non-comercial use let me know. In future blogs I will add some pictures and videos. Enjoy.

This is my representation of the main trophic interactions within a planktonic food web. There are many different ways to represent it and you may add as many compartments as you wish. But I think it is a nice one 😉
Planktonic food webs cannot be understood without nutrient and organic matter recycling. I opted to present it in a separate drawing to avoid excessive complexity.
Pelagia jellyfish
Calanus hyperboreus
Euphausiid
Mesodinium rubrum
Lithoptera sp
Oithona davisae
Aurelia aurita
This one is a joke. However, given all pelagic organisms depend on plankton…

A teaspoon of seawater, a tiny ecosystem

When I was studying ecology in college I remember that in one exam, we were asked the question: how many whales are there in the Mediterranean? To complete the exercise, if memory serves me right, they gave us data on primary production (what phytoplankton produces) and the whales’ average weight.  We had to apply a model of transfer of matter and energy through the trophic web with an efficiency of 10% in each trophic step. The calculation was very simple; the problem was knowing how many trophic steps to consider. For those times, in which we all exhibited the false security that gives ignorance and in which we believe that things happen as the books say, we had enough of a trophic network model of two (maximum 3) trophic steps between algae and whales. Now, after nearly thirty years devoted to the study of marine planktonic trophic food webs, I would surely fail the exam. The problem is that nature is much more complex than we would expect and that generalizations are often very difficult, if not impossible.

Tintinnind ciliate. Photo Albert Calbet

Usually, the smallest organisms are the most numerous, and often the most important. This happens with plankton, mostly invisible to the naked eye, but crucial to the functioning of marine trophic food webs. Plankton are responsible for life on earth, they provide us with half of the oxygen we breathe, and without them, we would certainly not eat fish; unfortunately, they are also the precursors of fossil fuels, such as oil. What are we going to do? Nobody is perfect.

Main components of plankton and their function

Before starting any story, we always need a small introduction to the main characters. To have an idea, in a teaspoon of sea water (about five milliliters), which you can take on the beach, we can find about 50 million viruses, five million bacteria, a few hundred thousand of small unicellular flagella, whether photosynthetic (autotrophs), consumers (heterotrophs), or a combination of both (mixotrophs), thousands of microscopic algae, about five heterotrophic cilia or dinoflagellates, and being lucky, some small crustaceans, such as copepods. The plant part of the plankton is called phytoplankton, and the animal part zooplankton. Although the term zooplankton includes both unicellular and multicellular, we typically separate these groups by size. Therefore, we have microzooplankton (mostly unicellular) and mesozooplankton (multicellular, or proper animals).

The copepod Calanus minor Photo Albert Calbet.

Each of the members of the plankton has its function. Viruses (do not suffer, they are not dangerous to humans) control the populations of bacteria and other microorganisms so that they do not proliferate excessively; bacteria break down dead matter and help recycle nutrients; autotrophs (plants) provide oxygen and new living matter that will be consumed by a multitude of different organisms, depending on their position in the food web. Among these consumers we can find both unicellular organisms (flagellate, ciliated, dinoflagellate, foraminifera, etc.) and pluricellular ones, such as larvae of worms, mollusks, starfish and fish, crustaceans, or jellyfish. The most abundant, however, of the multicellular ones are a group of crustaceans called copepods. Copepods are not much more than a millimeter in size and inhabit all seas and oceans, and are probably the most abundant animals on the planet (even more so than insects). To give you an idea, a typical Mediterranean copepod takes a couple of weeks to become an adult, and once adult, it can live a few weeks. The copepods inhabiting Arctic and Antarctic oceans, however, can live up to several years and even undergo hibernation during extreme cold and dark seasons. Copepods are the main food for fish (and sometimes whales, although they prefer the larger Krill), but before reaching this point in the food web, copepods have had to feed on ciliates, algae, etc. The ciliates, in turn, have also feed on algae or flagellates or bacteria; flagellates can too eat bacteria or other flagellates. In this way, a loop of interactions called trophic food web is created. By eating each other, the food web members also release the nutrients accumulated in the living matter, making them available for algae; this process is called nutrient recycling. Well, yes, we believe that we had invented recycling, and it turns out that it has existed for millions and millions of years. In fact, in the sea almost everything is used and very little is wasted.