7-1 Aquatic Environments

What Are the Two Major Types of Aquatic Life Zones? Salty and Fresh

The aquatic equivalents of biomes are called aquatic life zones. The major types of organisms found in aquatic environments are determined by the water’s salinity. As a result, aquatic life zones are divided into two major types: saltwater (or marine) and freshwater.

What Kinds of Organisms Live in Aquatic Life Zones? Floaters, Swimmers, Crawlers, and Decomposers

Saltwater and freshwater life zones contain four major types of organisms:

  1. A weakly swimming, free-floating plankton carried by currents. There are 3 types of plankton:

    1. Phytoplankton (plant plankton)

    2. Zooplankton (animal plankton)

    3. Ultraplankton (photosynthetic bacteria responsible for 70% of the primary productivity near the ocean surface)

  2. Nekton, strongly swimming consumers such as fish, turtles, and whales.

  3. Benthos, dwells on the bottom. Examples are barnacles and oysters.

  4. Decomposers (mostly bacteria) that break down the organic compounds in the dead bodies and wastes of aquatic organisms.

How Do Aquatic Systems Differ from Terrestrial Systems? Living without Boundaries and Large Temperature Fluctuations

There are several differences between aquatic and terrestrial systems:

  • Aquatic systems have less pronounced and fixed physical boundaries than terrestrial ecosystem

  • Food chains and webs in aquatic systems are usually more complex and longer than those in terrestrial biomes.

This makes it difficult to count and manage populations of aquatic organisms.

What Factors Limit Life at Different Depths in Aquatic Life Zones? Living in Layered Zones

Most aquatic life zones can be divided into three layers: surface, middle, and bottom. A number of environmental factors determine the types and numbers of organisms found in these layers. Examples are temperature, access to sunlight for photosynthesis, dissolved oxygen content, and availability of nutrients.

  • Surface Layer (Euphotic Zone):

    • Sunlight penetrates, allowing photosynthesis to occur. This leads to high dissolved O2 and low dissolved CO2

    • Nutrients are typically abundant in shallow waters, but scarce in the open ocean.

    • Upwellings can bring nutrients from the deep ocean to the surface, increasing net primary productivity (NPP).

  • Mid-Depths:

    • No photosynthesis occurs, leading to a decline in dissolved O2, however CO2 is higher

  • Deep Ocean and Deep Lake Bottoms:

    • No light, so organisms rely on dead plankton sinking from surface layers as a food source.

    • Low food availability leads to slow reproduction rates in deep-dwelling fish.