The dichotomy between day and night drives most rhythms in nature. The sun activates photosynthesis and with it, thousands of biochemical, physiological, and ethological processes. Humans sleep mostly at night, but many beasts thrive in the dark. During the night we see the moon, which also has its rhythms around 28-29 days. The duration between day and night in the different latitudes of the planet is directed by the seasons, which with greater or lesser accuracy are repeated year after year. In short, we are conditioned by rhythms. From the daily TV news to the winter of the Yogi bear, or from the appearances of the werewolf and other lunatics at the World Mobile in Barcelona (although this year the pace has been broken by the COVID). All of them are rhythms that are repeated with a periodicity. Because plankton could not be less, it is also deeply marked by rhythms of different amplitude and intensity. Here, are a few examples, but I do not intend to be exhaustive, nor do I want to go into too much detail about the mechanisms that trigger them, simply because we do not know many of them. In most cases, either it is an external factor that adjusts the rhythm every day (for example the hours of light) or, due to its periodicity in the evolution of the species, an internal clock has been created that works disregarding the presence or absence of light.
Phytoplankton (unicellular planktonic algae) actively photosynthesize during the day and breathe at night; this causes many species to take advantage to divide at night. Darkness is also when large zooplankton organisms, such as copepods and krill, migrate from deep, dark areas of the ocean to the surface, to feed on phytoplankton. These movements of organisms are considered the largest migrations on the planet; and they happen every day! By migrating for food at night, zooplankton members prevent their predators, fish, from seeing and attacking them. Copepods also consume microzooplankton (unicellular animals) that are about the same depth as algae. Microzooplankton, to minimize predation by copepods, mostly feeds on algae during the day, when copepods are not present.
As you can see, everything is in order and balance, mostly because of the millions of years of the joint evolution of predators and prey.
In the Arctic night, and supposedly in the Antarctic Ocean as well, the depth at which the zooplankton are located is marked by the illumination of moonlight; the brighter the moon the deeper zooplankton are. This behavior of zooplankton occurs to avoid being consumed by fish adapted to very dim lights. Moon effects similar to those described in the poles, also occur in other oceans, where the depth at which the zooplankton are found at night is modulated by the moonlight. Even eclipses, whether lunar or solar, disrupt the migratory patterns of zooplankton.
The four seasons are another example of periodicity that is more or less accurately repeated every year. As expected, the different weather conditions associated with the length of day and night in each of the seasons mark the dynamics of the marine ecosystem. Synchronous spawning of corals and polychaetes, or whale migrations, are some examples of the thousands of cases we find at sea. However, perhaps the most notable one for its global relevance is the succession of plankton organisms throughout the seasons. This succession, together with the physicochemical characteristics of the water associated with each season, is responsible for the spring phytoplankton bloom. This bloom at its turn will support a flourishing zooplankton community that will serve as food for fish and other marine creatures.
We may not be able to strictly call them rhythms, but there are great climatic phenomena that are repeated every few years. The best known is probably “El Niño”, which, although it mainly affects the Pacific Ocean, also has its consequences worldwide. “El Niño”, and its opposite phenomenon “La Niña”, are cyclical variations in temperature that normally occur every 4 years (year up and down) in the central and eastern tropical regions of the Pacific Ocean. Its consequences go beyond changes in temperature and rainfall, as they have a direct effect on the upwelling of the west coast of the America continent. Under “El Niño” conditions, trade winds that normally favor the upwelling of nutrient-rich deep waters reverse their direction and weaken the upwelling. This has consequences for the phytoplankton that feed on these nutrients and climb up the food web to reach fish. All in all, it has very serious socio-economic implications for the area.
Another similar phenomenon is the North Atlantic Oscillation, which is characterized by a change in pressure between the Azores and the subpolar zone of the North Atlantic. Positive oscillations involve high temperatures in northern Europe and usually the opposite in southern Europe. The Gulf Stream is affected and, with it, a whole set of planktonic species and fish.
There are many rhythms or cyclical processes as you see in the ocean, each with its idiosyncrasies and characteristics. We may not be aware, but nature always moves at the rhythm of its rhythms.