34.4

 Freshwater ecosystems include water bodies with very little dissolved salt, such as most ponds, lakes, streams, and rivers.
 Ponds and lakes are standing (not flowing) bodies of water.

 Lakes and large ponds are divided into zones based on water depth and distance from shore.
 The shallow water close to shore and the upper zone of water away from shore make up the photic zone, so named because light is available for photosynthesis.

 Phytoplankton, microscopic algae and cyanobacteria that carry out photosynthesis, live in the photic zone, along with water plants.
 The deep, murky areas of a lake, where light levels are too low to support photosynthesis, are called the aphotic zone.

 The bottom of any aquatic ecosystem is called the benthic zone.
 The benthic zone consists of rock, sand, and sediment.
 The organisms of deep (aphotic) benthic areas feed on wastes that sink down from the photic zone.

 A body of flowing fresh water is known as a stream or a river.
 A river changes greatly between its source and the point at which it empties into a lake or the ocean.
 Near the source, the water is usually cold, low in nutrients, and clear.

 Few phytoplankton inhabit this part of a river.
 Instead, the major producers near a river's source are algae attached to rocks on the river bed.
 Many species of arthropods live in the benthic zone, feeding on algae, fallen leaves, and one another.

 Downstream from its source, a river generally becomes wider and deeper.
 Marshes and other wetlands are common in these downstream areas.
 The water is usually warmer and murkier than it is upstream.
 More phytoplankton live in this part of a river.

 Streams and rivers merge with ocean water in areas known as estuaries.
 Estuary organisms experience changes in salt concentration and temperature as the fresh water and salt water mix.
 Estuaries serve as breeding grounds for many invertebrate and fish species, and as nesting and feeding areas for a great diversity of birds.

 Along most of the east coast of the United
States, the major ecosystems found in estuaries are salt marshes.
 In tropical areas, the typical estuary ecosystems are mangrove swamps.
 These swamps are dominated by mangrove trees, which are anchored by tangled networks of arching roots.

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Zones of depth include the benthic zone, or ocean floor, and the pelagic zone, or open water above the ocean floor.
Also as in freshwater ecosystems, the ocean has a photic zone, which receives enough sunlight to support photosynthesis, and a dark aphotic zone.
Zones of distance from shore include the intertidal zone, neritic zone, and oceanic zone.

 The area of shore between the high-tide and low-tide lines is called the intertidal zone.
 Pounded by waves during high tide and exposed to the sun and drying winds during low tide, benthic organisms in this zone must be well-adapted to survive these harsh conditions

 The area of the ocean from the low-tide line out to the edge of the continental shelf is the neritic zone.
 Since the ocean here is fairly shallow, some sunlight reaches the bottom in most of the neritic zone.
 As a result, many organisms that require light for photosynthesis can live in this zone, including seaweeds and phytoplankton.

 The vast open ocean from the edge of the continental shelf outward is called the oceanic zone.
 Phytoplankton drifting in the photic layer are the major producers in this zone.
 Microscopic animals called zooplankton also inhabit the photic zone and in turn are a source of food for other animals.

 All the invertebrate phyla are found on coral reefs, including sponges, sea anemones, worms, sea stars, and mollusks.
 Vertebrates such as sea turtles and fishes also roam the reefs.
 Most reefs are formed by colonies of coral polyps, animals in the phylum Cnidaria that secrete hard external skeletons.

 Imagine the surprise of biologists when they first glimpsed giant tube worms living at depths of 2,500 meters.
 These unfamiliar organisms live around deep-sea hydrothermal vents, spots on the ocean floor where hot gases and minerals escape from Earth's interior into the water.

 No sunlight reaches this deep, dark zone.
 The vent communities use the chemical energy from Earth's interior as their energy source.
 The producers in these ecosystems are prokaryotes that, instead of carrying out photosynthesis, can extract energy from sulfur compounds spewing from the vents and use this energy to make carbohydrates.