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.

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