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.