Marine Biome: Types, Plants, and Wildlife

Oceans cover 70% of Earth's surface and contain 97% of its water. They are home to an estimated 2.21 million known eukaryote species (eukaryote encompassing all plants, animals, fungi, protists, and most algae), but that's only a glimpse of what's living in the sea. Experts predict that an astounding 91% of ocean life remains undiscovered still.

Even with a small fraction of our world's liquidy expanse having been explored, we know enough about oceans to declare the marine biome the world's largest and most biodiverse.

The marine biome describes any saltwater environment. These bodies of water are widespread and not geographically limited by climate. In fact, the marine biome often dictates climate, creating rain and wind and influencing terrestrial temperatures.

In addition to the water that separates the world's continents and the seas that separate countries and so forth, there are inland examples of the marine biome in saltwater lakes like Utah's Great Lake, the Caspian Sea, Lake Urmia in Iran, and the Dead Sea—all some of the largest.

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Oceans make up the most widespread and well-known type of marine biome. There are five on Earth: the Atlantic, Pacific, Indian, Arctic, and Southern (Antarctic). Smaller are the planet's approximately 50 seas, those bodies of water that connect land to oceans. Then, there are smaller gulfs and bays, a range of saltwater lakes, plus marshes, swamps, bogs, coastal wetlands, and more.

Some saltwater environments get their salinity from the ocean, but others, like saltwater lakes, are completely isolated and get their high salt content from the surrounding land.

Salt-tolerant plants are called halophytes. They take all shapes and forms, from stringy seaweed to microscopic algae to actual floating trees. They can be annuals or perennials, grasses, flowers, shrubs, and more.

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Ocean plants alone are so diverse they can be classed into three groups based on sunlight needs and five groups based on their biology. Light-loving marine plants are called euphotic, twilight-loving plants disphotic, and dark-loving plants aphotic. The five biological distinctions are seagrass, kelp, sargassum, phytoplankton, and red algae.

Even more types—including floating and emergent plants, not just fully submerged—occur in the shallow waters of saline lakes, bogs, marshes, swamps, and wetlands. Examples of these include mangroves (comprising floating and emergent species), glassworts, and saltgrass.

Like terrestrial vegetation, marine vegetation is varied in the geography, climate, and—much different than land plants—depth it thrives in. Euphotic plants like marine algae and plankton are close to or on the surface, where they can soak in plenty of sun. Aphotic plants are those very rare organisms that do not need photosynthesis and instead feed on other organisms' nutrients (one example is albino plants).

Besides the depth at which you'll find them, saltwater plants vary by their root systems. Some root into soil, meaning they only survive in the shallow waters of the shoreline or marshes and wetlands. Others float, their roots suspended in the water.

Despite the diversity you see in saltwater plants, only a minute portion of all plants can survive in these harsh, salty conditions. Studies have proven the marine biome to be lethal to 99% of plants.

The hardy and resilient remaining 1% is adapted to the saltwater environment. They may absorb nutrients from the water instead or in addition to soil, processing the salt into chlorine and sodium ions or disposing of it through a respiratory process (look closely at marsh grass and you will see the salt in the form of white crystals).

Salt aside, many halophytes have special roots that cling to rocks and prevent them being swept away by strong currents.

The marine biome is home to some of the wildest, most mysterious, fascinating, and outright perplexing wildlife on the planet, from blue whales the size of a Boeing 737 jet to much smaller, near-transparent "sea pigs" that promptly disintegrate if brought too close to the surface. Learn about the incredible animals that endure marine environments, below.

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Animals from almost all the six basic animal classes thrive in the marine biome. The one exception is perhaps amphibians, some of which are salt-tolerant but occur only in brackish water not as salty as saltwater.

As far as invertebrates go, the most celebrated in oceans is coral. Coral—indeed an animal and not a plant, as sometimes thought—aid in the survival of a quarter of all ocean animals despite covering less than .1% of the ocean floor. Other marine invertebrates include sponges, mollusks, arthropods, worms, jellyfish, and anemones.

Reptiles like sea turtles, sea snakes, and saltwater crocodiles live in the marine biome, as do more than 20,000 species of fish. Mammals are an obvious category, including whales, seals, sea otters, and polar bears, while birds—specifically 350 species of seabirds—are not-so-obvious marine animals.

Like saltwater vegetation, marine wildlife live in even the deepest, darkest, coldest corners of oceans, seas, and lakes. They surprisingly thrive as far down as the bottom of the ocean—dumbo octopuses, vampire squids, anglerfish, and zombie worms, for example, are some of the most surreal, nightmarish animal species in the marine biome.

Others—namely mammals like polar bears, dolphins, whales, seals, and sea lions—linger contently at the top where they can breathe oxygen from the air instead of in the water. Some, like seabirds, are considered marine animals even though they spend very little time touching the water at all (that is, to keep from being preyed upon).

The ocean is one of the harshest environments imaginable, yet it boasts the most life of any other biome on the planet—how so? Animals that thrive in saltwater are well-adapted to the light, temperature, food supply, and pressure it provides.

Deep-sea dwellers are equipped with lungs or swim bladders that compress to handle the intense pressure of the water. Many use bioluminescence to deter, hide from, or confuse predators in the dark water. They've evolved to glide without much effort, allowing them to travel long distances in search of food. Some can even go months without a meal where nourishment is scarce.

A 2018 study of human impacts on oceans drew a grim conclusion: that only about 13% of the world's oceans met the definition of "marine wilderness" and the rest had been damaged by human activity. Humans are responsible for many of the threats marine environments face, from plastic pollution to overfishing to global warming.

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Climate change is without a doubt the largest threat to marine ecosystems. The more carbon dioxide there is in the atmosphere, the more that is constantly dissolving into the ocean, causing the water to become more acidic. Ocean acidification is the reason why coral reefs are experiencing bleaching around the world. What's more, the rising temperatures that result from more CO2 in the atmosphere cause glaciers to melt at an alarming rate. With that, the amount of ice available to polar species is dwindling and ocean levels are rising.

The ocean is full of plastic—everything from large fishing nets that trap all sorts of marine life to near-invisible microplastic particles that accumulate in their bellies. The garbage doesn't just get swept offshore, either; it's often put there on purpose. The Great Pacific Garbage Patch and four other trash islands have long served as global dumping grounds for plastic waste.

In addition to plastic, there's also chemical pollution. Pesticides and nutrients used for agriculture run off into oceans and other marine ecosystems when it rains or when they're leached into groundwater.

Another, less tangible type of pollution threatening the marine biome is noise. Noise from ships, naval sonar, and underwater explosives prove lethal to aquatic life. Not only does it obscure animals' ability to hear natural sounds—an ability essential to their survival—but it can also directly injure them "by causing hearing loss, hemorrhages, and other kinds of tissue trauma," says the Center for Biological Diversity. The most well-known victims of noise pollution are whales.

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Many wildlife species of the marine biome have been fished to near extinction. Some of the most overfished fish, according to Greenpeace's famous Red List, include albacore tuna, Atlantic cod, halibut, salmon, and sea scallops. Overfishing creates a dent in the food chain that impacts the whole system, from apex predators to coral reefs. With 800 million people depending on fish for food and income globally, depleting fish species that are the most available is sure to also cause a human food crisis separate from the environmental effects.