By Filipe Pereira
English edit by Carla Elliff
Illustration by Joana Ho.
We humans are terrestrial animals. It is therefore natural that we are more familiar with continental environments than with oceanic ones. Therefore, to show how the physics of the oceans can affect the lives of the organisms that live there, especially the primary producers, I will begin our discussion by drawing a parallel between the oceanic subtropical gyres and terrestrial forests.
Primary producers, such as plants and algae, also called photosynthetic organisms, are essential for the functioning of most food chains on the planet. They are the only living beings capable of transforming the energy of sunlight into the chemical energy that drives life on Earth by producing organic matter (food) through photosynthesis.
In order to carry out photosynthesis, they need light, water, carbon dioxide and nutrients. In terrestrial environments, the incidence of light is usually not a major problem, and the availability of carbon dioxide is much less so. What usually limits the growth of terrestrial plants is the availability of water and nutrients. Under ideal conditions, water and nutrients are within reach of the plants’ roots and they can thus thrive. In the marine environment, however, the main photosynthetic organisms are, for the most part, microscopic and transported by currents, known as phytoplankton. Of course, water is not a problem in the sea, nor is carbon dioxide either. The limiting factors for the growth of marine phytoplankton are, therefore, light and nutrients.
Seawater has the property of absorbing light very efficiently. At a depth of just a few meters, the ocean is already a dark environment and therefore incapable of sustaining photosynthetic organisms. In other words, the only habitable zone for phytoplankton is a thin surface layer that rarely exceeds 100 m in depth and is called the euphotic zone, where there is enough light to sustain the photosynthesis of these organisms.
Nutrients are rapidly consumed by phytoplankton in the euphotic zone and therefore have low concentrations at the ocean surface. On the other hand, part of the organic matter produced sinks and is remineralized (decomposed) and transformed into nutrients again at greater depths. Since nutrients are not greatly consumed below the euphotic zone, they are found in higher concentrations at greater depths.
Diagram showing the distribution of water, light, and nutrients on land and in the ocean. Photosynthesis occurs in well-lit regions on both land and in the ocean. The difference is that on land, vertical transport of nutrients is done by the plants themselves, while in the ocean, this depends on physical processes to bring nutrients to the euphotic zone. OM denotes organic matter, and H2O represents liquid water. License CC 4.0 BY-SA.
See, we have a problem here: in the large subtropical gyres, which correspond to most of the oceans and are illustrated in the figure below, the greatest concentration of nutrients is at depth, far from the reach of primary producers. In other words, surface concentrations of nutrients are low and therefore there is no great growth of phytoplankton communities. This makes the gyres less productive than coastal regions, for example.
Global map of satellite-measured surface chlorophyll (SeaWiFS), the areas circled in red show the major subtropical gyres with lower chlorophyll concentrations. Chlorophyll concentration is an indirect way to estimate the amount of phytoplankton present in the water. Adapted from https://earthobservatory.nasa.gov/images/4097/global-chlorophyll. NASA.
Since light is abundant at the surface and the highest concentrations of nutrients are below the euphotic zone, only physical phenomena occurring in these regions can alter these environmental conditions. There is no way to make light go deeper, so only when nutrient-rich waters are somehow brought to the surface can the growth rate of phytoplankton increase. These conditions, of increased nutrient concentration at the surface, allow the ecosystem to become more productive relative to the average state, as discussed above. This is where ocean physics is essential to better understand how these organisms can thrive in these ecosystems.
We can think that physical processes condition the environment where these organisms live. When we talk about physical processes, we are basically referring to water movements. These movements are horizontal: transporting phytoplankton to regions that are more or less favorable for their growth or trapping them in a limited area; and vertical: they can alter the concentrations of nutrients available to phytoplankton if the velocities are upward, or pushing these organisms out of the euphotic zone if the velocities are downward.
Several physical processes can affect the ecology of the surface layers of the ocean. One of the most studied mechanisms are mesoscale vortices. These are the phenomena responsible for oceanic weather (as in weather forecasting) and can be roughly understood as the marine version of hurricanes and atmospheric typhoons. These structures are hundreds of kilometers in size and are common in the ocean. They can be easily seen from satellite data on temperature, sea level and even chlorophyll concentration, the pigment responsible for photosynthesis in primary producers. Vortices are areas of high or low oceanic pressure and consist of circular movements, being in geostrophic balance, that is, they are large enough for their dynamics to be governed by the Earth's rotation. They are classified as anticyclones when they rotate in the opposite direction to the Earth's rotation, acting as high-pressure centers, or cyclones when they rotate in the same direction as the Earth, acting as low-pressure centers.
Diagram of cyclonic and anticyclonic vortices in the southern hemisphere. F is the force generated by the pressure difference between the center and the edge of the vortex (always points to lower pressure), this force is balanced by the force C generated by the Earth's rotation (Coriolis force). This is the geostrophic balance, and generates velocities V that rotate around the centers of high (anticyclone) and low (cyclone) pressure. The pressure at the center of the vortex implies vertical velocities (w) downward (red arrow) in the anticyclone, and upward (blue arrow) in the cyclone. The vertical scale of the surface elevation is exaggerated for better visualization. License CC 4.0 BY-SA.
Okay, but how do these phenomena influence the growth rate of phytoplankton? Get this, a high-pressure center implies an “accumulation” of water in the vortex. This weight pushes the water downwards, causing the nutrient-rich water to be carried even further to the bottom; inhibiting the growth of phytoplankton. In the opposite situation, in a cyclonic vortex, the low pressure generates upward vertical velocities, bringing waters that are richer in nutrients to more superficial and more illuminated regions, which can favor the growth of these organisms.
The phenomena I presented earlier would be the ideal situations shown in oceanography books, but it is not uncommon to find cyclonic vortices with low productivity and anticyclonic ones with high… 😅😅😅. The interaction of the vortex with the wind, for example, can invert the sign of the vertical velocities in the first meters of the water column, generating an effect opposite to that expected: cyclonic vortices with downward vertical velocities and anticyclonic ones with upward vertical velocities!
Other smaller phenomena that commonly occur at the edges of vortices, called sub-mesoscale phenomena, can greatly intensify the vertical velocities at the edges of vortices, and these can present higher concentrations at the edges than at their centers. What we want to show with this “flood” of information is that the ocean is a complex and chaotic system. Several processes are happening at the same time, and the sum of all these phenomena results in what we observe in nature.
Okay, that's all really cool, right? But why should we understand it? Remember that these organisms are the basis of marine food webs. Imagine subtropical gyres as large deserts; mesoscale vortices would be oases where there is high primary production. They end up attracting other larger organisms, such as fish, due to the greater availability of food. Understanding the dynamics of these processes can be important for fisheries management in some regions, for example. In addition, the photosynthesis of marine primary producers is an important way of sequestering carbon from the atmosphere (as are forests), and is a key factor in understanding the carbon cycle on our planet and, therefore, has significant effects on the climate. Ultimately, understanding the distribution and ecological dynamics of marine phytoplankton is directly affected by ocean movements, and has important implications for human activities and certainly affects our way of life.
*the ocean has an average depth of 4 km.
Suggested reading:
McGillicuddy Jr, D.J., 2016. Mechanisms of physical-biological-biogeochemical interaction at the oceanic mesoscale. Annual Review of Marine Science, 8, pp.125-159. (doi:10.1146/annurev-marine-010814-015606)
Mahadevan, A., 2016. The impact of submesoscale physics on primary productivity of plankton. Annual Review of Marine Science, 8, pp.161-184. (doi:10.1146/annurev-marine-010814-015912)
About Filipe Pereira:
Filipe is from Bahia, originally from Alagoinhas. He has always been curious about how the Earth works and decided to be a scientist since the third grade of elementary school. He began his studies in biology at the State University of Feira de Santana (UEFS), but soon realized that he was looking for a more interdisciplinary education. He went to the Federal University of Bahia (UFBA) to study oceanography, where he was encouraged to go to the University of São Paulo (USP), where the course would have a stronger foundation in physics. He graduated in oceanography in 2017 from USP, and is currently studying for a PhD in Physical Oceanography in the Dual Degree Program in Marine Sciences between USP and the University of Massachusetts Dartmouth. He mainly studies the dynamics of oceanic and coastal fronts at the mesoscale and sub-mesoscale, and their effects on plankton ecology. In addition to being passionate about the Earth, he loves aquariums and music, and is a singer in the USP Todo Canto Choir as a baritone.
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