Introduction

Because of differences between the physical characteristics of air and water, the physical environments facing organisms living in aquatic and terrestrial habitats are very different. Thus, the species that inhabit them face distinct problems that are unique to the two different habitats. Not surprisingly, species that inhabit aquatic and terrestrial environments have quite different characteristics.

Density

Water is 800 times denser than air. Buoyancy, the upward force exerted on an object immersed in a fluid, is determined by the mass of fluid displaced by an object. Because water is denser than air, objects of the same volume are more buoyant in water than they are in air. Therefore, many aquatic organisms can “float” relatively effortlessly in the water (i.e., they are neutrally buoyant). Others, however, tend to be be denser than the water and need flotation devices. Many fishes have a swim bladder that they can use adjust their buoyancy in the water. Planktonic organisms may have fat droplets or flattened shapes to increase their buoyancy. 

However, the greater density of water makes it more difficult for organisms to move through water because there is more drag. Thus, aquatic organisms that actively move through the environment should be selected to have streamlined shapes in order to reduce drag.The fastest swimming fishes like tunas have the most streamlined shapes of all fishes.

The Effect of Gravity

Although gravity affects all organisms living in air and water, the buoyant effect of water tends to offset the effect of gravity in aquatic environments. Thus, animals living in air must produce structures to fight the effects of gravity that are not necessary in aquatic organisms. For example, terrestrial plants (e.g., a pine tree) invest large amounts of resources in support structures (wood) whereas aquatic plants (e.g., kelp) invest fewer resources to support. Similarly, a much larger proportion of the body mass of large terrestrial animals (e.g., an elephant) is made up of bones than in large aquatic animals (e.g., a whale). In addition, organisms living in air spend a significant amount of energy maintaining support and on locomotion because they must fight the effects of gravity. The largest animals on earth (whales) are aquatic and do not have to deal with effects of gravity.

Compressibility

Water is much less compressible than air (water is virtually incompressible). It is easier for organisms to move through air because the air is compressed only slightly by their bodies as they move through it. Thus, air flows smoothly past organisms moving through air. Organisms moving through water must completely displace the water as they move. The resulting turbulence increases the drag experienced by organisms traveling through water. Fast swimmers have mechanisms to reduce drag, such as folding down fins so they do not protrude into the water. Not surprisingly, the maximum speed for organisms flying through air is much greater than the maximum speed of organisms swimming through water.

Oxygen Content

Oxygen makes up about 21% of Earth’s atmosphere and is spread evenly across the Earth’s surface. Thus, oxygen is readily available to most organisms living in terrestrial environments and so oxygen availability is not an important limiting factor in terrestrial environments. However, oxygen is dissolved in relatively low concentrations in water and the content of oxygen is not equal in all aquatic environments. For example, oxygen contents are higher in riffles of streams than in pools, so riffle communities tend to be more diverse and contain a greater biomass. In some aquatic environments with low oxygen, some fish have evolved mechanisms to breathe air.

Terrestrial organisms take up oxygen from the air by diffusion or by ventilating their lungs. Water is so dense that ventilation is not an effective strategy for oxygen uptake in aquatic habitats. Most large, active aquatic organisms with high oxygen requirements have gills with special adaptations to allow them to extract oxygen from the water as efficiently as possible. Other aquatic animals, such as sea turtles and aquatic mammals come to the surface to breathe air.

Light Availability

Light availability has a much greater influence on the distribution of life in the water than it does on land. Light is easily transmitted through air, so sufficient sunlight for photosynthesis reaches all terrestrial ecosystems (although the rate of photosynthesis may be limited by other factors such as moisture availability, temperature, etc.). However, water affects the penetration and spectral characteristics of light. Light from the red (long wavelength) end of the spectrum is filtered out sooner than light from the violet (short wavelength) end of the spectrum. Sunlight can only penetrate to a depth of about 183 meters (m) (the photic zone). Because photosynthesis is only possible in the photic zone and because the average depth of the oceans is about 3,800 m, most of the ocean is too deep to for photosynthesis to occur. Photosynthesizing plants and algae provide most of the energy used in marine food webs, so most life in the oceans is limited to a fairly narrow ring of shallow depths surrounding the continents. Deep ocean ecosystems rely either on the input of detritus (whose energy originally comes from the photic zone) or deep-sea vents. Many species of deep sea fishes are bioluminescent to make up for the absence of light in their environment.

Plate Coral (Fungia sp.) This picture was taken in Papua New Guinea, 2004. (Source: Brocken Inaglory, Wikimedia Commons).

Although limits to the depth of light penetration have important implications for life in the oceans (e.g., most life is limited to relatively shallow depths and the characteristics of coral and algae change with depth) many freshwater lakes, rivers, and springs are so shallow that light availability is not an issue.

Temperature Fluctuation

Compared to air, water has a very high specific heat (the energy required to raise the temperature of a gram of water by 1 degree centigrade). Thus, it requires more heat to change the temperature of water than it does to change temperatures in the air. Thus, terrestrial environments have much greater daily and seasonal changes in temperature than aquatic environments, so terrestrial organisms must be able to tolerate larger temperature ranges.

Further Reading

• PADI Project Aware. 2000. A.W.A.R.E. Our World, Our Water. PADI. Rancho Santa Margarita, CA. ISBN: 1878663267