Aquatic Ecosystems: Factors and Categories, Study notes of Biology

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Lesson Overview

4.5 Aquatic Ecosystems

Conditions Underwater

What factors affect life in aquatic ecosystems? Aquatic organisms are affected primarily by the water’s depth, temperature, flow, and amount of dissolved nutrients.

Water Depth

Photosynthetic algae, called phytoplankton, live in the photic zone. Zooplankton—tiny free-floating animals—eat phytoplankton. This is the first step in many aquatic food webs. Below the photic zone is the dark aphotic zone , where photosynthesis cannot occur.

Water Depth

Many aquatic organisms live on, or in, rocks and sediments on the bottoms of lakes, streams, and oceans. These organisms are called the benthos , and their habitat is the benthic zone. When the water is shallow enough for the benthic zone to be within the photic zone, algae and rooted aquatic plants can grow. When the benthic zone is below the photic zone, chemosynthetic autotrophs are the only primary producers.

Nutrient Availability

Organisms need certain substances to live, such as oxygen, nitrogen, potassium, and phosphorus. The type and availability of these dissolved substances vary within and between bodies of water, greatly affecting the types of organisms that can survive there.

Freshwater Ecosystems

What are the major categories of freshwater ecosystems? Freshwater ecosystems can be divided into three main categories: rivers and streams, lakes and ponds, and freshwater wetlands.

Lakes and Ponds

The food webs in lakes and ponds often are based on a combination of plankton and attached algae and plants. Plankton is a general term that includes both phytoplankton and zooplankton. Water flows in and out of lakes and ponds and circulates between the surface and the benthos, distributing heat, oxygen, and nutrients.

Freshwater Wetlands

A wetland is an ecosystem in which water either covers the soil or is present at or near the surface for at least part of the year. Water may flow through freshwater wetlands or stay in place. Wetlands are often nutrient-rich, highly productive, and serve as breeding grounds for many organisms. Freshwater wetlands purify water by filtering pollutants and help to prevent flooding by absorbing large amounts of water and slowly releasing it. Three main types of freshwater wetlands are freshwater bogs, freshwater marshes, and freshwater swamps. Saltwater wetlands are called estuaries.

Estuaries

Salt marshes are temperate estuaries that have salt-tolerant grasses above the low-tide line and seagrasses below water. One of the largest salt marshes in America surrounds the Chesapeake Bay in Maryland.

Estuaries

Mangrove swamps are tropical estuaries that have several species of salt-tolerant trees, collectively called mangroves. The largest mangrove area in America is in Florida’s Everglades National Park.

Intertidal Zone

Organisms in the intertidal zone are submerged in seawater at high tide and exposed to air and sunlight at low tide. These organisms are subjected to regular and extreme changes in temperature and are often battered by waves and currents. A typical rocky intertidal community exists in temperate regions where exposed rocks line the shore. There, barnacles and seaweed permanently attach themselves to the rocks.

Coastal Ocean

The coastal ocean extends from the low-tide mark to the outer edge of the continental shelf—the relatively shallow border that surrounds the continents. Water in the coastal ocean is brightly lit, and is often supplied with nutrients by freshwater runoff from land. As a result, coastal oceans tend to be highly productive. Kelp forests and coral reefs are two important coastal communities.

Open Ocean

The open ocean begins at the edge of the continental shelf and extends outward. More than 90 percent of the world’s ocean area is considered open ocean. Depth ranges from 500 m along continental slopes to more than 10,000 m in ocean trenches. The open ocean is divided into two zones based on light penetration— the photic and aphotic.

The Open Ocean Photic Zone

The open ocean typically has low nutrient levels and supports only the smallest species of phytoplankton. Still, because of its enormous area, most photosynthesis on Earth occurs in the sunlit top 100 meters of the open ocean.