A blooming desert in the middle of the sea

Half of the amount of oxygen we breathe is produced by single-celled creatures that sail the ocean and serve as the basis for the marine ecosystem. These are the phytoplankton, which have become a booming research topic in the last two decades. In an era where the discourse on climate change and global warming is expanding and increasing, it is only expected that the importance of the oxygen producers from the ocean will continue to increase. However, against the current, the Weizmann Institute of Science researchers chose to go specifically to the research "desert", and turn their gaze not to those prosperous areas of blooms that can also be seen from space, but to the driest marine areas, also known as "oceanic deserts", and to explain the processes The biological and physical ones that sometimes allow for confusing blooms even in the heart of the desert. the study Recently published in the journal Nature Communications..

First, for phytoplankton blooms to be possible, a combination of sunlight, nutrients and supporting water currents is required. Although the oceanic deserts are poor in food, from time to time a vertical water current may inject an accumulation of nutrients from the deep sea into the photic layer - the area where the sun's rays can penetrate and allow photosynthesis (it is located between the surface of the water and about 100 meters below sea level). These injection events are known in the research literature, but the research led by Dr. Yoav Lehn, from the group of Prof. Ilan Koren From the Department of Earth and Planetary Sciences, focused not on those vertical currents coming from the deep sea, but on horizontal currents that exist in a very narrow layer, 20-30 meters below the surface of the water, known as the "mixing layer" and making up less than 1% of the ocean's depth. The existence of these currents makes it possible to dilute the "food supply", and to mix the contents of the rich system that originates in the depths of the sea with that of the poor "desert" system found near the surface of the sea.

"You get a one-time injection of nutrients, but it is not a continuous feeding tube that constantly brings up food from the depths of the sea. From the stage when this concentrated package arrived, a process of dilution is constantly taking place. And yet, after one 'shipment', one event of food injection, you see this rash over three months and thousands of kilometers."

In the research, it was found that the horizontal thinning process is what enables the desert bloom, and even intensifies it, by helping, on the one hand, the phytoplankton to utilize the nutrients effectively, and on the other hand, limiting the predation efficiency of the zooplankton, those marine creatures that feed on the phytoplankton. Prof. Koren explains: "You receive a one-time injection of food materials, but it is not a continuous feeding tube that constantly brings up food from the depths of the sea. From the stage when this concentrated package arrived, a process of dilution is constantly taking place. And yet, after one 'shipment', one incident of food injection, you see this rash over three months and thousands of kilometers."

For the purpose of the research, which is at the seam between the physics of the ocean and the ecology of the phytoplankton blooms, Dr. Lehan developed a new methodological approach, which combines remote sensing data - satellite observations of the phytoplankton blooms and the flow fields in the ocean - with a theoretical model that enables the transformation of the satellite images frozen in time and in space for moving images, and to connect the biological time constants of the blooms with the physical time constants of the marine currents. "Satellite images are by their very nature 'snapshots,' but in the sea the ecosystem is in constant motion," says Dr. Lehan, "if we return to the same point in space in a week, there will already be other water there. That's why we developed methods for tracking the body of water as it moves through the sea, based on the satellite data, so that the model takes something still and makes it dynamic." And Prof. Koren adds: "The most simplistic and correct metaphor in this context is: take a single 'frame' and turn it into a movie - a cinema." Research student Shlomit Sharoni also participated in the research. Prof. Assaf Verdi From the Department of Plant and Environmental Sciences, Dr. Francesco D'Ovidio from the University of Paris VII and Dr. Emmanuel Boss from the University of Maine.

The research showed that the horizontal thinning currents that enable the bloom affect not only those 'deserts', but also the existence of the entire marine ecosystem. "There are areas rich in food where we expect blooms, and there are areas that are considered deserts. A large part of the 'audacity' of this research lies in the fact that, contrary to the norm, that when researchers want to study outbreaks they investigate at the location of the outbreak, we went to a place where everything is limited and arid, and precisely there we received insights that can be used in research in more prosperous areas", explains Prof. Koren.

Will the insights that emerge from the research also enable the oceanic wilderness to blossom proactively in the future, and perhaps even have some influence on the global warming processes? Prof. Koren believes that, although this is not what the research is about, the processes described in it could also play a role in climate engineering - an entire branch that tries, among other things, to understand how to make the phytoplankton bloom more, in order to reduce the amount of carbon in the atmosphere.

"The most simplistic and correct metaphor in this context is: taking a single 'frame' and turning it into a movie - a cinema"

Streams of Bloom: Each frame in the video consists of eight consecutive photographs originating from three different NASA satellites, showing the concentrations of chlorophyll at the surface of the sea. The arrows show the sea surface velocity vector and the colorimeter shows the logarithmic progression of chlorophyll concentrations.