Água do mar

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Água do mar é a água encontrada em mares e oceanos. A água do mar de todo o mundo tem uma salinidade próxima de 35 (3,5% em massa, se considermos apenas os sais dissolvido, mas a salinidade não tem unidades),[1] o que significa que, para cada litro de água do mar há 35 gramas de sais dissolvidos, a maior parte é cloreto de sódio (cuja fórmula é NaCl).

A água do mar não tem salinidade uniforme ao redor do globo. A água menos salina do planeta é a do Golfo da Finlândia, no Mar Báltico. O mar mais salino é o Mar Morto, no Médio Oriente, onde o calor aumenta a taxa de evaporação na superfície e há pouca descarga fluvial.

Í

A origem da salinidade do oceano

As teorias científicas para explicar as origens do sal marinho começaram com Edmond Halley, em 1715, que propôs que os sais e outros minerais foram transportados para o mar pelos rios, tendo sido sugado da terra por queda da chuva, lavando as rochas. Ao alcançar os oceanos estes sais seriam retidos e concentrados pelo processo de evaporação (veja Ciclo hidrológico) que removem a água.[2] Halley notou que do pequeno número de lagos no mundo que não têm saídas para o oceano (como o Mar Morto e o Mar Cáspio), a maioria tem alto teor de sais. Halley denominou este processo de "intemperismo continental".

A teoria de Halley estava correta em parte. Ou seja, o sódio foi sugado do fundo do oceano quando os oceanos se formaram. A presença dos outros elementos dominantes como cloreto, resultaram do escape de gases do interior da terra (na forma de ácido clorídrico), por vulcões e fontes hidrotermais. O sódio e o cloreto então se combinaram para formar o constituinte mais abundante da água do mar, o cloreto de sódio.

A salinidade do oceano tem-se mantido estável por milhões de anos, provavelmente como uma conseqüência de um sistema tectônico/químico que recicla o sal. Desde o surgimento do oceano, o sódio não é mais libertado pelo fundo do oceano, mas é capturado de camadas sedimentares que cobrem o leito do oceano. Uma teoria diz que a tectônica de placas faz com que o sal seja forçado para baixo das massas continentais, onde é lentamente trazido de volta à superfície. Outra fonte importante é o que chamamos de Água Juvenil, este material é proveniente do interior da Terra e sai por meio de fenômenos como o vulcanismo. Esta água nunca esteve na superfície da Terra, por isso leva o nome de água juvenil.

 

Condutividade elétrica

A água do mar apresenta uma elevada condutividade elétrica. Os sais na água se dissociam em íons. A condutividade varia sobre todo com a temperatura e a salinidade (a maior salinidade, maior condutividade), e sua medição permite, uma vez controlada a temperatura, conhecer a salinidade.[3]

 

Composição química

A ciência que estuda a composição química dos oceanos e as concentrações dos compostos na água do mar se chama oceanografia química. A água do mar tem composição química quase constante. Há um pouco mais de 70 elementos dissolvidos na água do mar, mas apenas seis desses constituem mais de 90% dos sais dissolvidos; todos ocorrem como íons.

Os cientistas estudam principalmente os macronutrientes na água do mar (nitrogênio, fósforo e enxofre), já que são os mais importantes para a vida marinha, principalmente para as plantas, que são a base da produção primária. Mas os micronutrientes também são largamente estudados, uma vez que, devido às suas baixas concentrações, podem tornar-se limitantes para vários tipos de organismos marinhos.

 

Principais íons salinos da água do mar

 

Gases dissolvidos na água do mar

A água do mar também contém pequenas quantidades de gases dissolvidos, principalmente nitrogênio, oxigênio e dióxido de carbono. A água a uma dada temperatura e salinidade está saturada com gás quando a quantidade de gás que se dissolve na água é igual à quantidade que sai ao mesmo tempo. A água do mar superficial está geralmente saturada com gases atmosféricos, como oxigênio e nitrogênio. A quantidade de gás que pode se dissolver na água do mar é determinada pela temperatura e salinidade da água. Aumentando-se a temperatura ou a salinidade reduz-se a quantidade de gás que pode ser dissolvido.

Uma vez que a água afunda para baixo da superfície oceânica (por exemplo, por se tornar mais densa pela evaporação), os gases dissolvidos não podem mais ser trocados com a atmosfera. A quantidade de gás num dado volume de água permanecerá inalterado, exceto pelo movimento das moléculas de gás através da água, por difusão (processo lento), ou pela mistura da água com outras massas de água que contêm diferentes teores de gases dissolvidos.

Em geral, o nitrogênio e gases raros inertes (argônio, hélio e outros) comportam-se dessa maneira: suas concentrações são conservativas e somente afetadas por processos físicos. Em contraste, alguns gases dissolvidos são não-conservativos e participam ativamente em processos químicos e biológicos que modificam suas concentrações. Exemplos são o oxigênio e o dióxido de carbono que podem ser liberados e usados a variadas taxas nos oceanos, especialmente pelos organismos.

 

Ciclo do carbono

Os oceanos (pela sua dimensão, mas também as massas de água continentais) têm um papel muito importante no equilíbrio do dióxido de carbono na atmosfera terrestre. Este gás têm a propriedade de reagir com os íons presentes na água para formar íons bicarbonato. Dessa maneira, quando há excesso de dióxido de carbono na atmosfera, ele é "absorvido" pela água que se torna um reservatório de carbono. Quando a biomassa vegetal na água aumenta (por exemplo, por aumento da temperatura ou dos nutrientes), aumenta também a necessidade de dióxido de carbono para essas plantas realizarem a fotossíntese - nessa altura, o bicarbonato pode "transformar-se" de novo em dióxido de carbono para repôr o equilíbrio.

 

Aspectos culturais

Mesmo num navio ou ilha no meio do oceano pode haver falta de água, isto é, água doce. É um paradoxo, já que uma pessoa cercada de água pode morrer de sede. É que por ser salgada a água do mar não é potável. Muitas nações na África e no Oriente Médio com problemas hídricos aplicam hoje um processo caro, chamado dessalinização, para obterem água potável a partir da água do mar. No futuro este processo pode se tornar muito utilizado, dada a presente poluição intensa dos corpos d'água continentais.



By Jane J. Lee, National Geographic

PUBLISHED

 

Yes, this sea turtle is glowing neon green and red. No, it's not radioactive.

 

The critically endangered hawksbill sea turtle is the first reptile scientists have seen exhibiting biofluorescence—the ability to reflect the blue light hitting a surface and re-emit it as a different color. The most common colors are green, red, and orange.

Biofluorescence is different from bioluminescence, in which animals either produce their own light through a series of chemical reactions, or host bacteria that give off light.

Corals fluoresce, and recent research has found the ability in a number of fish, sharks, rays, tiny crustaceans called copepods, and mantis shrimp. But researchers never expected to find it in a marine reptile. (See pictures of other animals that glow.)

"I've been [studying turtles] for a long time and I don't think anyone's ever seen this," says Alexander Gaos, director of the Eastern Pacific Hawksbill Initiative, who was not involved in the find. "This is really quite amazing."

On Guard

 

Watch: National Geographic Emerging Explorer David Gruber discovers a biofluorescent sea turtle near the Solomon Islands.

Marine biologist David Gruber, of City University of New York, was in the Solomon Islands in late July to film biofluorescence in small sharks and coral reefs.  

During one night dive, his team was on guard for crocodiles that frequent the area, "and there came out of nowhere this fluorescent turtle," says Gruber.

It looked like a big spaceship gliding into view, he recalls: An alien craft with a patchwork of neon green and red all over its head and body.

The marine biologist captured the turtle sighting on a video camera system, whose only artificial illumination was a blue light that matched the blue light of the surrounding ocean. A yellow filter on the camera allowed the scientists to pick up fluorescing organisms.

Gruber followed the turtle for a short while, but "after a few moments I let it go because I didn't want to harass it." The hawksbill proceeded to dive down into the pitch-black ocean.

 

Those stolen moments were the only ones Gruber could capture on his trip. But when he spoke with locals, the marine biologist discovered a nearby community that kept several captive young  hawksbills.

When Gruber examined these animals for a biofluorescent ability, he found that they all glowed red.

A Neon Universe Expands

Gaos and Gruber think it's too early to say for sure why these hawksbill sea turtles have the ability to fluoresce, or whether populations in other places do as well.

"[Biofluorescence is] usually used for finding and attracting prey or defense or some kind of communication," says Gaos. In this instance, it could be a kind of camouflage for the sea turtle. (See pictures of insects that are masters of camouflage.)

The hawksbill's shell is very good at concealing the animal in a rocky reef habitat during the day, Gaos explains. "When we go out to catch them, sometimes they're really hard to spot."

The same could be true for a habitat rife with biofluorescing animals—like a coral reef.

In fact, Gruber pointed out that some of the red on the hawksbill he saw could have been because of algae on the shell that was fluorescing. The green is definitely from the turtle though, he says.

This find has opened up a whole universe of questions that Gruber is eager to explore. They include whether these turtles can see the biofluorescence, where they get the ability—do they take in compounds from their food that let them fluoresce, or do they make their own compounds—how they're using it, and whether other sea turtle species possess a similar ability.

"It'd be fairly difficult to study this turtle because there are so few left and they're so protected," says Gruber. Worldwide, their population numbers have declined by nearly 90 percent in recent decades.

But he thinks he might be able to study the slightly more common—although still endangered—green sea turtle, which is closely related to the hawksbill. (See pictures of millions of sea turtles that might have been killed accidentally.)

Hawksbill sea turtles are one of the rarest species on our planet, Gruber says, yet for all their conservation importance, the animals remain a mystery.

Follow Jane J. Lee on Twitter.

This chain catshark dwells in the dark night of the deep sea. But without a yellow filter to block out blue light—which some biofluorescent fish have—these neon colors would be invisible.

 

 

 

 

 

 

Two-Toned

This flatfish flashes a fiery orange-red on its back (pictured), but shows off a green fluorescent pattern on its belly.

 

Surprise!

Marine biologist David Gruber first noticed biofluorescence in fish when a green eel (similar to the one pictured) photobombed him and colleagues as they took pictures of biofluorescent coral.

 

Useful Glow

Biofluorescence could be employed for a number of reasons, says Alexander Gaos, director of the Eastern Pacific Hawksbill Initiative. They include finding or attracting prey, defense, or some kind of communication.

 

Hidden

This bright red scorpionfish (center, left) employs biofluorescence to blend into the neon green and red coral reef the fish calls home.


Highlights

Like the hawksbill sea turtle, this seahorse gives off more than one color. The fish is mostly red while sporting bright green highlights around its eyes. The green also appears in speckles on other parts of the seahorse's body.

 

Normal

Under white lighting conditions, the stripes on this bream appear yellow. But turn on some blue light and attach a yellow filter to a camera to catch fluorescence, and you get the next photo.

 

Neon

This bream is the same one from the previous picture—it's just been photographed so that the fluorescence is visible to us. Scientists are just starting to figure out why biofluorescence is so widespread in the ocean.

 

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Chile Creates Largest Marine Reserve in the Americas

The newly protected waters around the Desventuradas Islands contain many marine species found nowhere else on Earth.

Picture of sea lions at Desventuradas

Two Juan Fernandez fur seals (Arctocephalus philippii) slide through the water off the Desventuradas Islands about 559 miles (900 kilometers) west of Chile. Divers snapped this picture during a 2013 expedition to an area that is now the largest no-take marine reserve in the Americas.


By Jane J. Lee, National Geographic

PUBLISHED

This article has been updated.

Valparaiso, CHILE—The Chilean government on Monday announced that it has created the largest marine reserve in the Americas by protecting an area hundreds of miles off its coast roughly the size of Italy.

 

The new area, called the Nazca-Desventuradas Marine Park, constitutes about eight percent of the ocean areas worldwide that have been declared off-limits to fishing and governed by no-take protections, says Russell Moffitt, a conservation analyst with the Marine Conservation Institute in Seattle, Washington. (Read about the world's largest marine reserve in the Pacific Ocean.)

The Pac-Man-shaped marine protected area (MPA) encompasses roughly 115,000 square miles (297,000 square kilometers) of ocean around San Ambrosio and San Felix islands. Together, they're known as the Desventuradas (or Unfortunate in Spanish) Islands, which are part of the underwater Nazca Ridge, which runs southwest from Peru to Easter Island. 

These islands had been subject to a modest amount of fishing, mainly for swordfish, before the creation of the new park, says Alan Friedlander, chief scientist for National Geographic Society’s Pristine Seas project. The project partnered with Oceana to promote designation of the new MPA. (Learn about what makes a successful MPA.)

 

The swordfish catch around the Desventuradas Islands amounted to about 0.5 percent of Chile's total swordfish haul.

 

WATCH: See the rich and diverse ocean habitat surrounding Desventuradas Islands, which is now a marine protected area.

Fishing will be allowed to continue in an unprotected wedge-shaped area that gives the new MPA its distinctive shape, says Alex Muñoz, vice president of Oceana in Chile. In addition, a small lobster fishery, which has been certified as sustainable by the Marine Stewardship Council, an international organization, will continue in a small area outside of the reserve.

The main reason groups pushed for the new MPA was to protect an intact ecosystem, Friedlander explains. That's important because studying pristine ecosystems gives scientists a good idea of how marine communities are supposed to function.

Protecting a Wealth of Life

Desventuradas sits in a unique oceanic environment, harboring a mix of tropical and temperate species.

Due to its isolation from the mainland—it takes a two-day boat ride from Chile’s coast to get there—much of Desventuradas’ marine life is endemic, or found nowhere else in the world, says marine ecologist Enric Sala. Endemic species include Juan Fernández fur seals, the Chilean sandpaper fish, and Juan Fernández trevally.

 

 

The Animals of the Desventuradas Islands

1 / 8 

Hidden Huddle

Chilean sandpaper fish (Paratrachichthys fernandezianus) shelter in a coral cave. This species is endemic to the Desventuradas Islands—now a new marine reserve—and the Juan Fernandez archipelago, located 466 miles (750 kilometers) to the south.


Grazers

Divers survey a towering wall covered in urchins off the Desventuradas Islands. Urchins have eaten the algae, stripping the formation clean. Schools of fish hover nearby.


Taking Cover

Photographer Manu San Felix hides in a clump of kelp as he attempts to film fish off the coast of San Ambrosio Island. The island is one of two that make up the Desventuradas Islands.


In The Open

Yellowtail amberjack (Seriola lalandi) school against a background of deep blue. A 2013 survey by scientists estimated this species comprised about 13 percent of the living organisms, by weight, around San Ambrosia Island.


Festive Fish

Colorful Gay's wrasses (Pseudolabrus gayi) swirl in front of a stand of kelp.


A Sunfish Stare

An ocean sunfish (Mola) swims off the coast of Chile's Desventuradas Islands.


Giant Predator

The bristling spines of a sea urchin are no deterrent to this sea star. Sea stars are the primary predators of urchins around San Ambrosio Island, one of two islands that make up the Desventuradas chain.


Curious

Several Juan Fernandez trevally (Pseudocaranx chilensis) approach a camera during a 2013 expedition to the Desventuradas Islands. This species is endemic to this area, meaning they aren't found anywhere else.


About 72 percent of the species found around Desventuradas and an island chain known as the Juan Fernández archipelago—about 466 miles (750 kilometers) to the south—is endemic, says Sala, an Explorer-in-Residence with National Geographic who heads the Pristine Seas project, which aims to protect the last wild places in the oceans.

Monday's announcement triples the amount of Chile's offshore waters that are under the strongest protections.

"For many years, Chile has been one of the most important fishing countries in the world," says Muñoz.

"Unfortunately, that led to depletion of our marine resources," he says. "With the creation of this marine park around Desventuradas, we're also becoming a leader in marine conservation."

One Step at a Time

Moffitt, who was not involved in the Desventuradas project, says it may be more urgent to protect near-shore waters that are heavily impacted by fishing or pollution than to close off these waters so far offshore.

"The prevailing trend has been to protect large MPAs and large reserves far from shore and far from large population centers, usually with low fishing interest," he says. These isolated MPAs are low-hanging fruit in a way, he says. (See pictures from Gabon's newest marine protected area.)

"We really need to be creating a more diverse portfolio of marine reserves,” he says.

Both Sala and Muñoz agree that nearshore waters need protection, too. But isolated areas like the Desventuradas are threatened by long-distance fishing fleets and bottom-trawling.

 

And protecting them before they become degraded is not only scientifically valuable, but also cost effective, says Friedlander

The Chilean navy has a small presence on San Felix Island and will be helping enforce the no-fishing rules, says Muñoz.

"[The navy] will be ensuring the integrity of this park," says Sala. "They are a very, very important player."

 
 
MATTHEW W. CHWASTYK, NG STAFF
SOURCE: UNDERSECRETARY OF FISHERIES AND AQUACULTURE, CHILE
 

Countries around the world still have a long way to go to meet the United Nations’ goal of protecting 10 percent of the world's oceans by 2020. But Moffitt says the Desventuradas park is a step in the right direction, and Muñoz says he has already set his sights on protecting another area in the Juan Fernández archipelago.

Also on Monday the Chilean government and the Rapa Nui community committed to begin negotiations to create a marine protected area around Easter Island of roughly 278,000 square miles (720,000 square kilometers).

In addition, the United States announced two new marine sanctuaries. One is 875 square miles (2,300 square kilometers) on the western side of Lake Michigan off Wisconsin, stretching from Port Washington to Two Rivers. It contains 39 shipwrecks, 15 of which are listed on the National Register of Historic Places. The second sanctuary is a 14-square-mile (36-square-kilometer) patch of the Mallows Bay-Potomac River (map) in Maryland. It is largely undeveloped and contains almost 200 shipwrecks, including a fleet of World War I wooden steamships.

Cuba also announced negotiations between its government and the U.S. to establish sister marine protected areas. The U.S. side would include the Florida Keys National Marine Sanctuary, the Flower Garden Banks in the Gulf of Mexico, and the Dry Tortugas and Biscayne National Parks. The Cuban side includes the Guanahacabibes National Park (map) on the western tip of the country. The agreement would help coordinate management, research, and education efforts between the two areas.

And last week, New Zealand announced formation of a 239,000 square mile (620,000 square kilometer) reserve in the Kermadec region. It includes a chain of islands and underwater volcanoes 620 miles (1,000 kilometers) northeast of New Zealand's North Island.

 

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