Freshwater Biome: Types, Plants, and Wildlife

Of the two major aquatic environments, the freshwater biome is often overshadowed by its marine counterpart, Earth's largest and home to some of the weirdest, most wonderful wildlife on the planet. But don't count out the rivers and streams of the world just yet.

Although it represents only 2.5% of the water on Earth, freshwater is the main artery of all life. While it would be simple to define freshwater as "saltless," it is more specifically defined by the U.S. Geological Survey as "containing less than 1,000 milligrams per liter of dissolved solids." Why the complicated description? Because freshwater actually does contain a small amount of salt, which runs off the rocks and soil around it. The plants and animals that inhabit freshwater are accustomed to this conservative measure of salt and would not survive in the high salinity of seas or oceans.

Freshwater is where most of the water we drink comes from and plays countless other important roles in nature. Learn more about the biome and how climate change is affecting it, below.

Locations

Whereas land biomes are mostly geographically limited by climate, freshwater biomes occur just about anywhere and everywhere, from the rainforest to the Arctic. North America's Great Lakes are one of the most prominent examples of this biome. These five interconnected bodies of water—Erie, Ontario, Michigan, Huron, and Superior—span nearly 100,000 square miles and contain a fifth of the world's total surface freshwater. Lake Superior itself is the largest freshwater lake based on surface area.

Abroad, making up an even larger portion of total surface water are the African Great Lakes and Russia's Lake Baikal, which is the largest lake by volume.

Types

When most people think of freshwater, they probably imagine those familiar lakes and other obvious bodies of water. These are certainly the most well-known examples, but they aren't the only or even the most abundant types.  

Surface Water

The freshwater category covers lakes, rivers, ponds, creeks, streams, and some swamps, marshes, and bogs. Surface water is any water that collects on land, known in the agricultural sector as blue water. Despite how incredibly vast these bodies of water can be, surface water in general makes up only a tiny sliver of Earth's total freshwater, about 1.2%.

Groundwater

Groundwater is 25 times more plentiful than surface water. Groundwater systems and aquifers are created when water fills the cracks and pores of the earth, below land. These subterranean reservoirs can occur in a range of environments—under mountains, beneath deserts, underlying plains, and more. They are home to a unique set of invertebrates called stygofauna. They also feed surface water systems and therefore play an essential role in the freshwater biome.

Glaciers and Ice

Technically, the bulk of freshwater on Earth (68.7%, precisely) is stowed away in glaciers and ice caps. Interestingly, Antarctica—an entirely solid continent fixed in salty seas—claims 70% of the world's total volume of freshwater. Despite the inhospitable conditions that ice provides, these frigid environments are teeming with life. As far as biome distinctions go, though, they are typically classed as terrestrial tundra biomes, not freshwater, because they're frozen.

Plants

Aquatic plants are also called macrophytes or hydrophytes. The term "aquatic" can describe either freshwater or saltwater plants even though the two types are very different.

Types of Freshwater Plants

The vast category that is aquatic plants can be broken down into three simple distinctions: emergent plants, floating plants, and submergent plants. Even these classifications are so huge they branch out into many subcategories—for example, floating plants encompasses "true" plants and microscopic algae. Algae alone can be broken down further into five classes containing anywhere from 30,000 to more than a million species. "True" floating plants include duckweed, water hyacinths, water lettuce, and water lilies (aka lily pads).

Emergent plants jut from shallow water but are rooted to lake bottoms, like cattail, honeysuckle, milkweed, and American lotus. Submergent plants are also rooted to the bottom, but most of their vegetation is sunken. Examples of submergent plants include several types of pondweeds, wild celery, and bladderwort.

Shoreline plants—like bergamot, river bulrush, and bog birch—are not technically aquatic plants but require close proximity to freshwater to survive.

Where Freshwater Plants Thrive

Plants can be found anywhere there is freshwater. Even if you can't see them cropping up on the surface of the water, it could be teeming with microscopic algae or suppressing an underwater world of submerged flora below. One type of ecosystem that is particularly favorable to plants is the wetland. Sometimes called marshes or swamps, these saturated lands occur on every continent except Antarctica. It's been estimated that wetlands cover about 6% of the earth's surface and provide habitat for 40% of all plant and animal species.

What They Need to Survive

The four components needed for freshwater plants to survive are water (of course), light, carbon dioxide, and nutrients. Plants consume CO2 and turn it into oxygen, therefore providing an essential service to the water itself and its wildlife. This means that some plant species can survive on very few nutrients, namely nitrogen and phosphorus. Contrary to low-nutrient systems, though, there are also systems that are too high in nutrients because of human pollution. These systems are great for algae, which use the nutrients to grow and spread, but bad for the ecosystem, as a surplus of algae can lead to a shortage of oxygen in the water.

Animals

The freshwater biome is home to 10% of the world's animal species and 40% of fish species, according to the World Wildlife Fund. Here's more on the types of animals you'll find in freshwater, where they live, and what they need to survive.

Types of Freshwater Animals

Everything from mollusks, worms, reptiles, amphibians, mammals, and birds thrive in the freshwater biome, but let's start with the obvious: fish. There are more than 18,000 species of freshwater fish. More fish live in freshwater than saltwater (though only by 1%). Freshwater fish make up a quarter of the world's vertebrates.

In the U.S., freshwater fish range from tiny guppies to massive catfish, snakelike American eels, and thousands of diverse critters in between. They are joined in their aquatic biome by mollusks like snails, some mussels, and clams; worms including parasitic leeches; reptiles like alligators and crocodiles, turtles and tortoises, and water snakes; mammals like the ever-majestic manatee; and frogs.

Many birds—cormorants, herons, egrets, spoonbills, ducks, storks, swans, and loon, for example—also live in freshwater environments where they feed on fish. Bears and other terrestrial mammals do not live in the water, per se, but rely on it for food nonetheless.

Where Freshwater Animals Thrive

Depending on the species, freshwater animals may prefer still or moving water. Alligators, frogs, and most aquatic snakes will be found in still water, including lakes, ponds, and slow-moving rivers. The salamander, similarly, prefers a calm wetland environment. But fish with high oxygen requirements can only survive in fast-moving water because the movement traps air in the water.

Fast-flowing water allows freshwater species to migrate, too. One of the most well-known examples is the salmon run, when salmon swim (upstream!) from the ocean to the upper reaches of rivers to spawn. This occurs primarily during the fall, and it's the only time you'll see salmon in freshwater.

What Freshwater Animals Need to Survive

Freshwater animals need two main things to survive in water: oxygen and food. Fish can breathe in water, of course, using their gills to pull out oxygen molecules, but other animals like gators and turtles must surface for air. These animals can hold their breath for hours when necessary. Many enter a stage of brumation—like hibernation—to survive the winter. In this state, they become almost totally inactive and survive on stored fats while food is scarce.

Food can be either aquatic plants, other animals, or both. Aquatic herbivores include turtles and manatees. Carnivores include alligators, some fish, and water snakes. Many aquatic birds are omnivorous, balancing their fish diet with seeds, grains, and grasses.

Threats

If it seems like parts of the world are in a never-ending drought and the water around you is dwindling before your eyes, you would be right. The United Nations has declared a global water crisis, noting that declining access to clean water perpetuates poverty and disease in the world's poorest countries.

Threats to the freshwater biome are wide-ranging but mostly human-driven. As a result, almost a third of freshwater fish face extinction. Learn about the worst culprits ahead.

Pollution

When you think water pollution, you probably envision plastic floating in the ocean. Freshwater systems are susceptible to this kind of pollution, too, but an even bigger threat is pollution from wastewater and agrochemicals, which can cause nitrogen and phosphorous levels to rise to a level lethal to plant and animal life.

"Up to 80% of global wastewater is estimated to enter water bodies untreated with adverse impacts on human and ecosystem health," according to the U.N. Drinking contaminated water can cause cholera, dysentery, hepatitis A, polio, typhoid, and more in people.

Invasive Species

"Aquatic invasives" can wreak havoc on freshwater species and water quality overall. While introduced animals have the power to wipe out native species through feeding, invasive plants disrupt the nutrient balance of the water, eventually compromising the availability of dissolved oxygen. A good example of this in the U.S. is water hyacinth, native to South America's Amazon River Basin and introduced to the States in the late 1800s. The plant is now abundant and widespread, growing in 25 states.

Climate Change

The impacts of climate change are ever-present here. In some areas, temperature spikes are causing freshwater systems to dry up at an alarming rate, as is the case with the U.S. Southwest's Lake Mead. In other areas, melting glaciers are feeding freshwater systems, inevitably causing ocean levels to rise.

And the amount of water—whether too much or too little—isn't the only problem stemming from the climate crisis; the quality of the water is at stake, too. A hotter climate leads to more and worse algal blooms, many of which are toxic.

Human Development

As the global population grows (and grows and grows, by about 1% per year), more freshwater is needed to sustain our race. Groundwater reservoirs are being depleted for agriculture—look at California's Central Valley, which has physically sunk almost 30 feet in some sections—and dams or other obstructions are built to store water, control flooding, and generate electricity.

These disturbances aren't without consequence: The National Oceanic and Atmospheric Administration says 2 million dams in the U.S. block fish from migrating, therefore preventing them from reproducing. "As a result," it says, "many fish populations have declined."