mon océan & moi

in winter

During winter, wind blows (there are sometimes storms), day length is short and the sun remains hidden behind clouds, water is cold. The ocean mixes vertically, sometimes over depths of several hundred meters. Vertical mixing replenishes the ocean’s surface with nutrients coming from the deep ocean and thus makes them available to phytoplankton, which live in surface waters. However, mixing also transports phytoplankton from surface to depth and from depth to surface. As a consequence, phytoplankton do not receive, on average, enough sunlight for achieving photosynthesis and growth.

in spring

One has to wait until the end of winter or until spring to see phytoplankton grow. The sun shines again, the wind blows less and less, and the ocean surface gets warmer. In this ocean that has become calmer (the water column is said to be "stratified"), sunlit, and nutrient-rich, phytoplankton find optimal growth conditions. One may observe what oceanographers call a "phytoplankton bloom". Most often, the bloom is dominated by diatoms. Phytoplankton proliferate so much that the ocean turns green! This color is caused by high concentrations of chlorophyll a, a molecule that is found in all plants (among which phytoplankton) and enables them to capture sunlight.

in summer and autumn

 As the season progresses, nutrients available in surface waters are depleted by phytoplankton growth. The concentration of nutrients becomes so low that it is insufficient to support the growth of new phytoplankton. The phytoplankton population is senescent and decaying. The largest phytoplankton cells or aggregates of cells, which are also the heaviest (often diatoms), sink towards the deep ocean. Other cells are consumed by zooplankton organisms, which could serve as food for fishes. The phytoplankton organisms that stay in surface waters mostly comprise small cells such as nano-flagellates or cyanobacteria. Then comes the end of summer, and autumn. After winter, a new seasonal cycle will begin.

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unmarked seasons

In subtropical gyres, the water is warm and the sunlight is intense almost year round. Because the water column is stratified, no deep water can reach the surface layer which is consequently nutrient-poor most of the time. In these “oligotrophic” waters, phytoplankton are primarily composed of tiny cells belonging to the group of  cyanobacteria.

observing oceanic deserts

Because they are so immense and so remote, oceanic deserts have long been poorly studied. Thanks to satellites, we know a little more about them today. For example, we know that the waters located in the Subtropical South Pacific Gyre (east of Polynesia) are the poorest in phytoplankton. During the austral summer there, the concentration of chlorophyll a is so low that the water turns deep blue, almost purple. Profiling floats are extremely relevant for the observation of oceanic deserts. Will these floats enable oceanographers to discover the existence of a seasonal cycle in oceanic deserts?

responses of the ocean to climate changes

Researchers perform calculations that account for the predicted increase in the concentration of atmospheric carbon dioxide (CO₂) and in ocean temperature over the next 50 to 100 years. The first results indicate that oligotrophic regions could become more and more oligotrophic, and that at high latitudes, the favorable season for phytoplankton growth could become longer. The combined effects of these modifications could globally lead to a decrease in the primary production of the world’s oceans. The relative contribution to phytoplankton of large species may also diminish. However, these are only first results and a lot more work is needed to obtain robust predictions.

consequences on the biological carbon pump

Phytoplankton utilize CO₂ from the atmosphere to build organic matter, and thus contribute to the biological carbon pump which sequesters CO₂ in the deep ocean. If as predicted by researchers, phytoplankton production is modified over the next decades, the capacity of the ocean to biologically sequester CO₂ could decline. Hence long-term changes in ocean’s seasons may have a strong impact on our planet.