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Aquatic Ecosystems: Characteristics & Definition

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Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

Aquatic ecosystems are fascinating, complex, and filled with interconnected parts. Read this lesson to learn some of the secrets hiding beneath the surface of your favorite lake, river, pond, or sea. I promise it's worth the read!

What Is an Aquatic Ecosystem?

An ecosystem is the complete set of living and nonliving components within a region of interest. The term aquatic refers to water, so an aquatic ecosystem refers to living and nonliving parts of a waterbody and the interactions that take place among them.

Aquatic Ecosystems Types

Broadly speaking, a body of water can be classified as being freshwater, marine, or estuarine. A freshwater body of water has fewer dissolved compounds, or salts, present, while a marine body of water has various salts dissolved in it, hence the term 'salt water'. The average salinity of salt water is around 35 parts per thousand. Estuarine areas are those that experience a flux of both fresh and salt water, depending on the tides and water currents. For example, the area where the Mississippi River meets the Gulf of Mexico is considered estuarine because there is a constant mix of fresh and salt water.

We can categorize aquatic systems even further if we look at patterns of water movement. Lentic waterbodies have very slow-moving or stagnant water. These include lakes and ponds. Lotic waterbodies have faster-moving water, like rivers and streams. You can remember the difference by thinking about how lotic waters flow, the 'o' in each is pronounced the same. Finally, we have wetlands, which are exactly what they sound like. The soil of a wetland is saturated or inundated with water at least part of the time. Wetlands often occur in the zones connecting land and large waterbodies.

We can break down an aquatic ecosystem even more by looking at the structure of an individual waterbody. Aren't you amazed at how much thought has gone in to studying what goes on beneath the surface? I know I was!

For example, in a lake (a lentic system, remember), the area along the shoreline where rooted plants grow is called the littoral zone. Plants need light to survive, so the littoral zone is the region where sunlight reaches all the way to the bottom of the lake. These plants provide food and shelter to various organisms that live in the waterbody (we'll talk about these next). The depth that light penetrates varies within each waterbody based on how many floating particles are suspended in the water column and the chemical composition of the water. So, we know the zone closest to land is called the littoral zone, but what about the area that's far away from land, the area we call open water? This area is called the pelagic zone, and it's broken down into an area that light penetrates and an area that light doesn't reach. The top layer, where light penetrates, is called the photic zone. The area on the bottom where light does not reach is called the aphotic zone.

You're probably wondering why we care about how a waterbody is partitioned. That's a great question! We classify waterbodies based on their structure and chemical makeup because it allows us to understand the species that live there.

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