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In the 1960s, American ecologist Robert T. Paine first introduced the concept of “keystone species.”

In fact, there are some organisms (animals, plants, or fungi) that help maintain an ecosystem in balance. Without them, the ecosystem would be completely different or would cease to exist altogether, as the survival of these species is essential for the existence of others.

Keystone species have low functional redundancy: if the species were to disappear, no other would be able to take its place in the ecological niche it occupied.

How did the concept of keystone species come about?

Paine conducted an experiment, not to be replicated at home. In a coastal area along the Pacific Northwest coast of the United States, the ecologist eliminated over a period of twenty-five years, the top predator of that ecosystem: the starfish Pisaster ochraceus, better known as the purple starfish.

Within months of the starfish disappearing, mussels took over the area. The increased number of mussels led to a decline in the number of other organisms, including benthic algae that supported communities of sea snails, limpets, and bivalves.

Overall, the area’s biodiversity had plummeted, with the number of species dropping from fifteen to eight.

In a 1966 paper, Paine explained what had happened and identified the purple starfish as a “keystone species,” which, by its presence or absence, influenced lower levels of the food chain, preventing certain species from monopolizing resources, including space and food. 

There is more than just the starfish

Keystone species were originally defined as consumers that significantly alter the composition and physical appearance of an ecological community. However, many studies have shown that not only predators can be classified as keystone species, but also ecosystem engineers, such as beavers, corals and even mangroves or mutualistic species such as bees and flowers. Let’s look at some of them together.

In the sea there are not only corals, but also other ecosystems that are fundamental to maintaining the health of the sea. An example are the Kelp forests. Kelps are brown algae that can reach up to 50 meters in height. Among the fronds of the Kelp forests of the North American west coast live the sea otters (Enhydra lutris). Sea otters are a keystone species, as they protect the Kelp forests from damage by the sea urchins they feed on. When the hunting of otters on the North American west coast for the commercial use of their fur began, the number of individuals dropped to such low levels that they could no longer contain the population of sea urchins. In fact, the sea urchins, in turn, grazed the algal meadows so heavily that they quickly disappeared, along with all the species that depended on them.

Among plants, important keystone species are mangroves that protect shorelines from coastal erosion, capture and store large amounts of carbon, and provide safe habitats for small fish and other organisms.

Among animals, in addition to otters and starfish, we find corals. These small animals grow as a colony of thousands and even millions of individual polyps. The rocky exoskeletons of these polyps create enormous structures, coral reefs.
Coral reefs support more species per unit area than any other marine environment, including about 4,000 species of fish, 800 species of hard corals, and hundreds of other species.

Coral reefs support more species per unit area than any other marine environment, including approximately 4,000 species of fish, 800 species of hard coral, and hundreds of other species.

And finally, sharks. As top predators, sharks play an important role in ocean ecosystems. As predators, they keep their prey populations healthy by catching slower, weaker fish.

Along the Atlantic coast of the United States, a decline in shark numbers has been observed, with a consequent increase in the population of the cownose ray Rhinoptera bonasus. This ray feeds on bivalves, clams, and scallops. The increase in its population has also had an impact on the economic activities of the bay. Scallops were in fact the pride of fishermen, but the stingrays were also fond of them. 

Difference between “umbrella species” and “keystone species”

An “umbrella species” is a species whose active conservation indirectly involves the conservation of many other species present in its range. Most umbrella species are migratory species, so they move even for thousands of kilometers during their life, having no direct impact on food webs, as keystone species do. Examples of umbrella species are: grizzly bear, tiger, wolf and giant panda.

Sea turtles are an umbrella species of marine ecosystems, as they play important roles in coastal and marine habitats contributing to the health and maintenance of coral reefs, seagrass beds, estuaries and sandy beaches.

The importance of conservation actions

Raising awareness about the importance of ecosystem restoration and vulnerable or endangered keystone species is a great way to engage both institutions and organizations and individuals.

With the IOC-UNESCO Ocean Literacy program and the regional program of the Decade of the Sea we want to raise awareness of the beauty and richness of marine ecosystems and the challenges they are facing so that each of us can better respect and love the wonderful creatures that inhabit the ocean.

Remember that during boat trips or excursions along the coasts you can help scientists monitor and protect the environment by sending reports of animals or plants that you happen to encounter. Together for the Blue Planet!

The article explores how salinity varies in different ocean basins, comparing them to one another, and finding out which is the saltiest and why.

The salinity of seawater (called salinity) varies greatly across different ocean basins. Scientists refer to salinity as parts per thousand (ppm), which is the total amount of salt dissolved in the water: grams of salt dissolved in one kilogram of water.

How is salinity measured?

The instruments used to measure salinity are different, but let’s remember the main ones:

• Refractometer: an optical measuring instrument that uses different wavelengths to determine the refractive index of a substance and measure the concentration of salt in water.
• Hydrometer: an instrument for directly measuring the density of the liquid and therefore understanding its salinity.
• Satellite images: satellites such as NASA’s Aquarius take images of the same area periodically, usually on a weekly basis. These satellites are able to calculate and represent salinity using a color scale. Each color represents a certain amount of organic and inorganic substances dissolved in the water at that precise moment.

What affects salinity?

Precipitation and evaporation determine the distribution of salinity, which is also controlled by water currents. But the salinity in a specific part of the ocean also depends on the runoff from rivers.
Near the equator, the tropics consistently receive the most rainfall. As a result, freshwater falling into the ocean helps to decrease the salinity of the surface water in that region. As you move toward the poles, rainfall decreases, and with less rain and more sun, evaporation of seawater at the surface increases.

Some lakes, such as Mono Lake in California and the Caspian Sea in Asia, are even saltier. Evaporation can cause isolated bodies of water to become extremely salty, or hypersaline. A good example is the Dead Sea. The high salt content of the Dead Sea dramatically increases the density of its water, allowing humans to float much more than in the ocean. Salts are left behind when water evaporates from these landlocked water sources. Salt levels continue to increase over time. Many of these salt lakes are located in arid places, with little rainfall and warm temperatures during the day.

A Fun Fact About the Dead Sea

The Dead Sea has a salinity of 280 ppm, about eight times saltier than average seawater (35 ppm). It is so salty that no fish or aquatic plants can live there, however, some colonies of bacteria and microalgae have managed to adapt and survive in this hyper-salty ecosystem.

Pacific Ocean Basin

The salinity of surface waters in the Pacific Ocean Basin is strongly influenced by winds, precipitation, and evaporation patterns. The waters in the calm belt near the Equator generally have lower salinities than those in the trade wind belt. This is because there is a lot of rain near the Equator and little evaporation; salinity can be as low as 34 ppm.
Salinity in the open areas of the southeast, however, can be as high as 37 ppm, while the lowest salinities, less than 32 ppm, are found in the far north of the Pacific.

Atlantic Ocean Basin

The surface waters of the North Atlantic have salinity levels exceeding 37 ppm, among the highest in the world. Salinity levels in the South Atlantic are lower, at about 34.5 ppm.
This discrepancy, for example, can be explained by the strong evaporation of the Mediterranean Sea and the discharge of high-salinity water, which helps maintain the salinity of the North Atlantic.

The Sargasso Sea, which covers about 2 million square miles and is located about 2,000 miles west of the Canary Islands, is the saltiest region of the North Atlantic. The floating brown seaweed “sargassum”, from which the sea is named, separates the Sargasso Sea from the open ocean.
The high water temperatures (up to 28.3°C) and the remoteness of the Sargasso Sea from the mainland cause its high salinity. This sea does not receive influxes of fresh water.

A fun fact about the Mediterranean Sea

The Mediterranean Sea, for example, is saltier than the rest of the Atlantic Ocean basin. By studying the salt of the Mediterranean Sea, scientists have discovered that 5.33 million years ago, the Mediterranean Sea dried up for a long period of time. This period is known as the Messinian Salinity Crisis (MSC).

Indian Ocean Basin

The salinity of the surface waters of the Indian Ocean Basin ranges from 32 to 37 ppm, with substantial regional variations. The subtropical zone of the Southern Hemisphere has high surface salinity, while the low-salinity zones range from Indonesia to Madagascar along 10°S. At 60°S, the salinity of the surface water is between 33 and 34 ppt.

The Arabian Sea has a high-salinity upper layer, reaching 37 ppm due to high evaporation rates.

Due to the drainage of fresh water from rivers, the salinity of the surface layer of the Bay of Bengal is significantly reduced, less than 32 ppm.

A Fun Fact about the Arabian Sea

The northern Indian Ocean basin has a dipolar sea surface salinity, unique even at the same latitude range. This is because the Arabian Sea is dominated by high and low evaporation regimes and is the main discharge region for high salinity waters: the Red Sea and the Persian Gulf.
In contrast, the Bay of Bengal is characterized by higher rainfall and freshwater outflow from the world’s largest rivers (Ganges and Brahmaputra).

However, the Arabian Sea and the Bay of Bengal exchange water around Sri Lanka, maintaining a balance of salt water.

Arctic and Southern Ocean Basins

Low salinities occur in polar seas, where salt water is diluted by melting ice and continuous precipitation. Seas with river mouths or coastal inlets that receive significant runoff from precipitation falling on land can also have low salinities.

Although the Arctic Ocean basin is generally cooler than other oceans, with salinity levels ranging from 30 to 34 ppm, salinity levels vary by region, and areas with strong river inflow can have even lower salinities.

On the other hand, the Southern Ocean basin is characterized by high sea surface salinity (SSS) north of the subtropical front, large salinity gradients across the major polar fronts, and low surface salinity in the Antarctic area south of the polar front.

Bibliography

https://oceanservice.noaa.gov/facts/whysalty.html

https://oceanservice.noaa.gov/facts/oceanwater.html#:~:text=The%20ocean%20covers%20more%20than,be%20found%20in%20our%20ocean.

https://www.nhm.ac.uk/discover/quick-questions/why-is-the-sea-salty.html

https://www.whoi.edu/know-your-ocean/did-you-know/what-makes-the-ocean-salty/

https://www.usgs.gov/faqs/why-ocean-salty

https://www.britannica.com/story/why-is-the-ocean-salty

https://www.americanoceans.org/facts/why-is-the-ocean-salty/

http://ponce.sdsu.edu/usgs_why_is_the_ocean_salty/usgs_why_is_the_ocean_salty.html

http://iprc.soest.hawaii.edu/users/jensen/jensenGRL01.pdf

https://www.google.com/search?q=arabic+and+bengal+sea+salinity&rlz=1C1GCEU_enIT992IT992&oq=arabic+and+bengal+sea+salinity&aqs=chrome..69i57j0i546.5409j0j7&sourceid=chrome&ie=UTF-8

https://www.nature.com/articles/23231

In this article, we will examine the sources of salt in the sea and answer the question: why is the ocean salty? But before we answer that question, let’s review some basic concepts:

1. About 70% of the Earth’s surface is covered by the ocean. Of the total water on Earth, 3% is fresh water and almost 97% of the water is salt water.

2. The six most abundant chemical elements in the ocean are chloride, sodium, potassium, sulfate, magnesium, and calcium, which make up 99% of sea salts.

3. The freezing point of salt water is -2 °C; the freezing point of fresh water is 0 °C.

4. Salt water also has economic importance. For example, the sea salt we use in cooking often comes from the evaporation of sea water, which is a natural source of sodium.

Where does all the salt in the ocean and its seas come from?

Salt in the ocean comes from two main sources:

1.Runoff from the land

Rain carries mineral ions from the ground into the water. Rainwater is slightly acidic because some of the carbon dioxide in the air dissolves in it.

When rain falls on rocks, they release mineral salts, which separate into ions. These ions are then carried by the water and end up in the sea. Over 90% of all ions in salt water are sodium and chloride, the main ingredients of cooking salt.

2.Seafloor vents

Hydrothermal fluids are also a source of salts in the ocean. Water seeps through cracks in the seafloor, where it is heated by lava and magma from the Earth’s interior. The increase in temperature triggers a series of chemical reactions: the water tends to lose oxygen, magnesium and sulphates and to collect metals such as iron, zinc and copper from the surrounding rocks.

Some ocean salts originate from underwater volcanic eruptions, phenomena that release the minerals directly into the sea.

What is the effect of salt on water?

At the same temperature, seawater is denser than freshwater because of the salt it contains. This is because water molecules (H₂O) cluster around salt molecules, resulting in salt water having more molecules overall than freshwater, making it denser and causing it to sink below freshwater or less dense water.

Salts and minerals are also used extensively by marine life, for example by removing iron, zinc, and copper from the water.

Differences in salinity and temperature of seawater in ocean basins create what we know as ocean water masses. These masses of different salinities and temperatures make it possible for water to move and transport nutrients around the world. This phenomenon is called deep ocean circulation, and it plays a key role in regulating currents and transporting heat.

Because of the higher density of salt water in the ocean, people, animals and other objects float more in sea water than in fresh water. Each ocean basin and sea has certain characteristics in terms of salinity, think about what happens in the Dead Sea.

Bibliography

https://oceanservice.noaa.gov/facts/whysalty.html

https://oceanservice.noaa.gov/facts/oceanwater.html#:~:text=The%20ocean%20covers%20more%20than,be%20found%20in%20our%20ocean.

https://www.nhm.ac.uk/discover/quick-questions/why-is-the-sea-salty.html

https://www.whoi.edu/know-your-ocean/did-you-know/what-makes-the-ocean-salty/

https://www.usgs.gov/faqs/why-ocean-salty

https://www.britannica.com/story/why-is-the-ocean-salty

https://www.americanoceans.org/facts/why-is-the-ocean-salty/

http://ponce.sdsu.edu/usgs_why_is_the_ocean_salty/usgs_why_is_the_ocean_salty.html

http://iprc.soest.hawaii.edu/users/jensen/jensenGRL01.pdf

https://www.google.com/search?q=arabic+and+bengal+sea+salinity&rlz=1C1GCEU_enIT992IT992&oq=arabic+and+bengal+sea+salinity&aqs=chrome..69i57j0i546.5409j0j7&sourceid=chrome&ie=UTF-8

There is one thing about the ocean that we all know for sure, and that is that the ocean and the sea appear to be blue. Why is the ocean blue to our eyes?

The ocean contains more than 97% of the Earth’s water and supports 99% of the global biosphere. Therefore, the ocean is extremely important for life on our planet.

Due to its vastness, only 5% of the ocean has been explored and charted by humans. The rest, especially its deepest parts, remain unexplored.

In this article, we will explain why the ocean is blue, so you can finally understand how it works.

Is the ocean blue?

Of course, we all know that clean water is colorless, it is transparent. So, why does the ocean – and other bodies of water – appear blue?

For many years, we have believed that the ocean and other bodies of water are a shade of blue because they reflect the blue sky… but this is not entirely true! Sure, the surface of the water reflects the sky: from the coast, it may appear blue on a sunny day, gray when it is cloudy or stormy, or even show shades of pink during sunset. But if we go below the surface, the blue color remains, and it also remains when we look at the ocean from space. In these cases, the water does not reflect the sky.

The reason why the ocean appears blue is indeed a reflection, but it is not simply a mirror of the sky.
In fact, sunlight contains the entire spectrum of colors, from red to violet, as we see in the rainbow. Each color has a specific wavelength, the color red has longer wavelengths. The wavelengths of the other colors become progressively shorter, with blue and violet at the end of the range, having the shortest wavelengths of all.

When light hits the ocean, the water absorbs the longer wavelengths first, reflecting the colors with the shorter wavelengths back to our eyes. Then, like a filter, the water molecules absorb the red parts of the light spectrum and leave behind the colors of the blue spectrum, which our eyes see.

As soon as we reach a depth of a few meters, most of the red and orange light disappears completely, absorbed by the water molecules. Shortly after, the yellow and green wavelengths are absorbed, leaving only blue and purple, which are able to penetrate deeper.

However, this phenomenon only occurs up to a certain depth. In fact, most of the ocean is completely dark, as almost no wavelengths penetrate deeper than 200 meters.

Do all bodies of water appear blue?

Everything absorbs light at a different wavelength, and then reflects the remaining colors back to the observer.

When light penetrates the water completely, like in a glass or a very shallow body of water, we see the water as colorless, because not enough photons (the molecules made from light radiation) are absorbed. The light simply shines through.

This is mainly why different bodies of water can have different shades of blue. The deeper the body of water, the darker and deeper the color, because the more water there is, the more light is absorbed.

You may have noticed that, at times, water can also appear to be colors other than blue. This is often a result of the characteristics of the water or the soil.

Some rivers or ponds, for example, can appear a muddy brown rather than blue, which is often due to the presence of sediment in the water, especially after the water has been agitated.

The crystal clear waters of the Caribbean, which are usually closer to shades of light green than blue, you might have thought that it is because the water is cleaner. Sure, this has an impact, but often the colors are given by the plant life that exists in many Caribbean waters, helping to reflect more green light. Often, the most beautiful Caribbean waters also tend to be shallow, and the composition of the ocean floor, given mainly by coral, can be responsible for a different way of reflecting light.

Why do scientists study the color of the ocean?

Some types of particles (for example, phytoplankton cells, also called microalgae) also contain substances that absorb different wavelengths of light, changing the color reflected by the water. There are many substances in the water that absorb light and therefore change the colors reflected. Usually, these substances are composed of organic carbon, and scientists generally refer to them as CDOM, short for colored dissolved organic matter.

One essential substance that absorbs light in ocean water is chlorophyll, which phytoplankton use during the process of photosynthesis. Chlorophyll is a green pigment, and so phytoplankton preferentially absorb the red and blue portions of the light spectrum for photosynthesis, thus reflecting green light.

Thus, regions of the ocean with high concentrations of phytoplankton appear with shades other than the usual blue: they range from teal to green, depending on the density and type of phytoplankton population they contain.

The principle behind studying the color of the oceans from space is simple: the more phytoplankton there is in the water, the greener it appears; the less phytoplankton, the bluer it will be.

So, scientists are studying the color of the ocean to better understand phytoplankton and how they affect the ocean and the Earth.

It turns out that these tiny organisms can have a big impact on a large-scale system, such as climate change. For example, phytoplankton use carbon dioxide for photosynthesis, providing nearly half of the oxygen we breathe on the planet. So, a large and thriving global population of phytoplankton means that more carbon dioxide is being pulled out of the atmosphere, mitigating the effects of pollution and lowering the effects of global warming and climate change.

Scientists have found that a given population of phytoplankton can double in number about once a day, meaning they are able to respond very quickly to changes in their environment.

Examining the color of the ocean helps researchers understand and monitor phytoplankton, which can be a step toward predicting environmental changes.

Changes in any phytoplankton population, such as changes in its density, distribution, and rate of population growth or decline, will alert scientists to changing environmental conditions.

Bibliography

https://oceanservice.noaa.gov/facts/oceanblue.html

https://www.scientificamerican.com/article/why-does-the-ocean-appear/

https://science.nasa.gov/earth-science/oceanography/living-ocean/ocean-color

https://www.wonderopolis.org/wonder/why-is-the-ocean-blue

https://www.mcgill.ca/oss/article/environment-general-science-you-asked/why-sky-blue-or-better-yet-why-ocean-blue