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    An aquatic ecosystemis an ecosystem located in a body of water. Communities of organisms that are dependent on each other and on their environment live in aquatic ecosystems. Aquatic ecosystems can be divided into two general types: marine ecosystems and freshwater ecosystems.

Freshwater ecosystem s are a subset of Earth's aquatic ecosystems. They include lakes and ponds, rivers, reservoirs, streams, springs, bogs, and wetlands. Estuaries, which are places where a river meets the sea, such as San Francisco Bay, are part freshwater and part marine in their makeup.   Every living thing on Earth needs water to survive, but more than one hundred thousand species, including our own, need a special kind of water that can only be found in certain places and is in very rare supply: fresh water. Less than three percent of our planet’s water is fresh water, and less than half of that is available as a liquid; the rest is locked away as ice in polar caps and glaciers. For these reasons, freshwater ecosystems are a precious resource.

 

    Fresh water starts out as water vapor that has evaporated from the surface of oceans, lakes, and other bodies of water. When this vapor rises, it leaves salts and other contaminants behind and becomes “fresh.” The water vapor collects in drifting clouds that eventually release the water back to Earth in the form of rain or snow.

After fresh water reaches the ground through precipitation, it flows downhill across a landscape called the watershed to lakes, ponds, rivers, streams, and wetlands. But fresh water can be found in less-obvious places too. More than half of all freshwater on our planet seeps through soil and between rocks to form aquifers that are filled with groundwater. The top surface of an aquifer is called the water table, and this is the depth where wells are drilled to bring fresh water into cities and homes.

Scientists who study freshwater ecosystems are called limnologists. Limnologists want to learn what creatures live in an ecosystem and how they interact with each other through the ecosystem’s food web, as well as how they interact with their environment. This knowledge can help the researchers know when a freshwater ecosystem is healthy and when it may be in danger.

    Freshwater ecosystems naturally share resources between habitats. The ecosystems in rivers and streams, for example, bring salts and nutrients from the mountains to lakes, ponds, and wetlands at lower elevations, and eventually they bring those nutrients to the ocean. These waterways also enable migrating species, like salmon, to bring nutrients from the ocean to upstream freshwater ecosystems.

  Lakes and ponds, on the other hand, can exchange nutrients in a seasonal cycle. Cold water is denser than warm water, so it sinks to the bottom, where a fairly steady temperature is maintained. However, as the air temperature drops with the arrival of winter, the water that is closest to the surface may drop below the temperature of the water at the bottom of the lake, causing it to sink and the warmer bottom water to rise. The same process happens as floating surface ice melts into very cold water in the spring. During these periods, nutrients are churned from the floor and brought to the surface.

    It is normal for ecosystems to encounter change. Temperatures may fluctuate, populations may rise and fall, and rain may bring an abundance of water, then tapering during drought. The plants, animals, and microbes in healthy freshwater ecosystems are resilient and have adaptations that allow them to adjust appropriately until ideal conditions resume. However, if any element of the ecosystem varies too far outside of the norm, the balance of the whole system can start to fail.

    The diversity of a freshwater ecosystem depends upon temperature, availability of light, nutrients, oxygen, and salinity.

A wide range of plants, animals, and microbes are found in freshwater ecosystems. The smallest are the microscopic plants and animals known as phytoplankton and zooplankton, which form the bottom layer of freshwater food chains. There are also many freshwater invertebrates including worms and insects. Among the freshwater vertebrates, amphibians, such as frogs, live on land and water, while fish are purely water-dwelling. Many species of birds, such as kingfishers and ducks, live on or near freshwater.