7-2 Saltwater Life Zones

What Is the Coastal Zone? Abundant Life Near the Shore

Oceans have two major life zones: the coastal zone and the open sea. The coastal zone is the warm, nutrient-rich, shallow water that extends from the high-tide mark on land to the gently sloping, shallow edge of the continental shelf.

Although it makes up less than a tenth of the world’s ocean area, the coastal zone contains 90% of all marine species. Most ecosystems found in the coastal zone have a high net primary productivity per unit of area due to the ample supplies of sunlight.

What Are Estuaries, Coastal Wetlands, and Mangrove Swamps? Stressed Centres of Biological Productivity

One highly productive area in the coastal zone is an estuary, a partially enclosed area of coastal water where sea water mixes with fresh water and nutrients from rivers, streams, and runoff from land.

Estuaries and their associated coastal wetlands include river mouths, inlets, bays, sounds, mangrove forest swamps in sheltered regions along tropical coasts, and salt marshes in temperate zones.

The constant water movement in estuaries and their associated coastal wetlands stirs up the nutrient-rich silt, making it available to producers. These systems filter toxic pollutants, excess plant nutrients, sediments, and other pollutants. And they provide food, habitats, and nursery sites for a wide variety of aquatic species.

What Niches Do Rocky and Sandy Shores Provide? Hold On, Dig In, or Hang Out in a Shell

The area of shoreline between low and high tides is called the intertidal zone. It is a challenging place to live as the waves may crush or sweep away organisms. They must also survive changing levels of salinity when heavy rains dilute salt water. Due to this, many of these organisms hide in protective shells.

What Are Coral Reefs? Aquatic Oases of Biodiversity

Coral reefs form in clear, warm coastal waters of the tropics and subtropics. These reefs are vulnerable to damage because they grow slowly and are disrupted easily. They also thrive only in clear, warm, and fairly shallow water of constant high salinity.

The biodiversity of coral reefs can be reduced by natural disturbances such as severe storms, fresh-water floods, and invasions of predatory fish. However, historically they have been able to adapt to these natural environment changes.

Today the biggest threats to the survival and biodiversity of many of the world’s coral reefs come from sediment runoff and other human activities.

What Biological Zones Are Found in the Open Sea? Where Is the Light?

The euphoric zone is the lighted upper zone where floating drifting phytoplankton carry out photosynthesis. Nutrient levels are low (except around upwellings), and levels of dissolved oxygen are high. Large, fast-swimming predatory fish such as swordfish, sharks, and bluefin tuna populate this zone.

The bathyal zone is the dimly lit middle zone that does not contain photosynthesizing producers because of a lack of sunlight. Various types of zooplankton and smaller fish populate this zone.

The lowest zones, called the abyssal and hadal zones, are dark, very cold, and under immense pressure. Relatively few species live here. Some examples are sea cucumbers, tube worms, and deep-sea anglerfish.

Most organisms of the deep waters and ocean floor get their food from showers of dead and decaying organisms (detritus) drifting down from upper lighted levels of the ocean. Some of these organisms are deposit feeders, which take mud into their guts and extract nutrients from it. Others such as oysters, clams, and sponges are filter feeders, which pass water through or over their bodies and extract nutrients from it

Average primary productivity and NPP per unit of area are quite low in the open sea except at an occasional equatorial upwelling, where currents bring up nutrients from the ocean bottom.