Aquatic Ecosystems: Characteristics & Definition

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.