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Aquatic Ecosystems

Aquatic Ecosystems

No worldwide expedition could be complete without exploring Earth’s waters. Since almost three quarters of Earth’s surface is covered with water, don’t be surprised at how much there is to see. Many living things make their homes in and near the water. Your travels will take you to two types of aquatic, or water-based, ecosystems: freshwater ecosystems and marine (or saltwater) ecosystems.

All aquatic ecosystems are affected by the same abiotic factors: sunlight, temperature, oxygen, and salt content. Sunlight is an especially important factor in aquatic ecosystems. Sunlight is necessary for photosynthesis in the water just as it is on land. However, because water absorbs sunlight, there is only enough light for photosynthesis near the surface or in shallow water. The most common producers in aquatic ecosystems are algae rather than plants.

Ponds and Lakes
Your next stop is a pond. Ponds and lakes are bodies of standing, or still, fresh water. Lakes are generally larger and deeper than ponds. Ponds are often shallow enough that sunlight can reach the bottom even in the center of the pond, allowing plants to grow there. In large ponds and most lakes, however, algae floating at the surface are the major producers.

Many animals are adapted for life in the still water. Along the shore of the pond, you observe dragonflies, turtles, snails, and frogs. Sunfish live in the open water, feeding on insects and algae from the surface. Scavengers such as catfish live near the pond bottom. Bacteria and other decomposers also feed on the remains of other organisms.

Freshwater Ecosystems

Even though most of Earth’s surface is covered with water, only a tiny fraction is fresh water. Freshwater ecosystems include streams, rivers, ponds, and lakes. On this part of your expedition, you’ll find that freshwaterecosystems provide habitats for an amazing variety of organisms, from microscopic algae to huge bears.

Streams and Rivers
Your first stop is a mountain stream. Where the stream begins, the cold, clear water flows rapidly. Animals that live here are adapted to the strong current. For example, insects and other small animals have hooks or suckers that help them cling to rocks. Trout have streamlined bodies that allow them to swim despite the rushing water. Few plants or algae can grow in this fast-moving water. Instead, first-level consumers rely on leaves and seeds that fall into the stream.

As the stream flows along, other streams join it. The current slows, and the water becomes cloudy with soil. The slower-moving water is warmer and contains less oxygen. This larger stream might now be called a river. Different organisms are adapted to life in a river. Plants take root among the pebbles on the river bottom. These producers provide food for young insects and homes for frogs and their tadpoles. These consumers, in turn, provide food for many larger consumers.

Marine Ecosytems

The expedition now heads to the coast to explore some marine ecosystems. On your way, you’ll pass through an estuary. An estuary (es choo ehr ee), is found where the fresh water of a river meets the salt water of the ocean. Algae and plants such as marsh grasses provide food and shelter for numerous animals, including crabs, worms, clams, and fish. Many animals use the calm waters of estuaries for breeding grounds.

Intertidal Zone
Next, you take a walk along the rocky shoreline. Here, between the highest high-tide line and the lowest low-tide line, is the intertidal zone. Organisms here must be able to survive pounding waves and the sudden changes in water levels and temperature that occur with high and low tides. You observe animals such as barnacles and sea stars clinging to the rocks. Others, such as clams and crabs, burrow in the sand.

Neritic Zone
Now it’s time to set out to sea. The edge of a continent extends into the ocean for a short distance, like a shelf. Below the low-tide line is a region of shallow water called the neritic zone (nuh rit ik), which extends over the continental shelf.

Because sunlight passes through the shallow water of the neritic zone, photosynthesis can occur. As a result, this zone is particularly rich in living things. Many large schools of fish, such as sardines, feed on algae. In warm ocean waters, coral reefs may form. Coral reefs provide living homes to a wide variety of other organisms.

The Open Ocean
Out in the open ocean, light penetrates only to a depth of a few hundred meters. Algae carry out photosynthesis in this region of the open ocean, which is known as the surface zone. Marine animals, such as tuna, swordfish, and some whales, depend on the algae for food.

The deep zone is located below the surface zone. The deep zone is almost totally dark. Most animals in this zone feed on the remains of organisms that sink down from the surface zone. The deepest parts of the deep zone are home to bizarre-looking animals, such as giant squid whose eyes glow in the dark.

Section Assessment 2.5

(a) Reviewing Name two major types of aquatic ecosystems.
(b) Explaining Why is sunlight an important abiotic factor in all aquatic ecosystems?
(c) Predicting Would you expect to find many organisms living at the bottom of a deep lake? Explain.
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