Aquatic photic zone. The area on the bottom

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.

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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.We already know that rooted plants are found within the littoral zone, but what else lives there? This area is also the warmest part of the lake because the sun is warming it during the day, so it makes for a good home for aquatic critters. Algae, or floating plant matter, is abundant in the littoral zone, and this provides food for zooplankton, tiny floating animals, and benthic macroinvertebrates, small insects that live in the bottom substrate. This, in turn, provides food for certain fish species, who can also take cover in the plants to avoid getting eaten by other fish species. It’s an interconnected web, and there’s a lot more going on in a waterbody than you might think at first glance.If we move out the photic zone, within the pelagic zone, we also find algae and zooplankton, as well as the occasional fish.

However, there are no rooted plants, so there isn’t any refuge for critters to take cover. Usually, fish species move in and out of the photic zone to feed. Even zooplankton will move up to the photic zone to feed during certain times of day and then retreat to the aphotic zone to hide from predators the rest of the time. Organisms found within the aphotic zone are those that eat dead, decomposing material or the benthic macroinvertebrates that live in the dark. Some fish species hide out here during the day because it’s cooler, and then they move up to the photic zone or littoral zone at night to feed.

Lesson Summary

Whoa, there are a lot of cool things happening in the water, which is why people dedicate their entire lives to studying aquatic ecosystems, which are living and nonliving parts of a waterbody and the interactions that take place among them.

And remember that an ecosystem is the complete set of living and nonliving components within a region of interest. Bodies of water can be categorized as freshwater, which has fewer dissolved compounds present; marine, which has various salts dissolved in it, on average about 35 parts per thousand; or estuarine, which are those that experience a flux of both fresh and salt water, depending on the tides and water currants.And we can further break them down based on patterns of water movement (i.e. lentic zones, which are slow moving water bodies like lakes; lotic zones, which are fast moving like rivers; and wetlands, which are where soil is saturated with water at least part of the time).

Finally, each individual waterbody can be split up into zones based on what lives there and how much light the zone receives. These include the litoral zone, which is the area along the shoreline where rooted plants grow; the pelagic zone, or the area we call open water; the photic zone, where sunlight reaches underwater; and the aphotic zone, where sunlight doesn’t reach. There is an interconnected web of organisms, and they interact with each other and with their external environment.

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