Aquatic Life Zones

Fresh Water Habitats

Water has more properties beneficial to living things than any other substance. It is 770 times heavier than air. This greater density of water gives it more supporting power than air. Not only does it buoy up parts of the body, but it may also support the entire body of some organisms, allowing them to float at various depths. Water possesses a surface film on which some organisms can walk. This film is stronger than that of any other fluid except mercury. Because its temperature does not fluctuate as widely as air, water is a safer place in which to exist. It is a poor conductor of heat and has a relatively high freezing point. Since water freezes at 0°C (32°F), the temperature of the fresh water environment can fall only to 0°C. The biological activity of many plants and animals can continue at the freezing point.

Like air, warm water rises to the top and cold water falls to the bottom, up to a certain limit. As the surface water of the pond cools, it tends to drop. Since water is at its heaviest (most dense) when cooled to 4°C (39.2°F), it tends to form a bottom layer. If the surface water becomes colder than 4°C, it becomes lighter and less dense and remains at the top where it freezes. This blanket of ice prevents the water below from radiating its heat. This is the reason that few ponds freeze to the bottom.

The ultimate source of food of all water inhabitants is the raw materials in the water: carbon dioxide, oxygen, carbonic acid, and salts of phosphates, silica, calcium carbonate, and nitrates. Through the process of photosynthesis, aquatic plants -the algae, diatoms, pondweeds, and water lilies - are able to convert these raw materials into chemical bond energy stored in carbohydrate molecules. In turn, these carbohydrate molecules are utilized by a chain of aquatic animal life from copepods to predacious fish. In addition, water serves as a transport for food materials dropped into the water from land sources.

Fresh water habitats are often divided into two broad categories. These are the standing water or LOUTIC habitats and the running water or LOTIC habitats. Each possesses its own distinctive problems for existence and each contains forms of life admirably adapted to meet the various environmental challenges presented. The following contain examples of each type of habitat.


1. Zonation
a. LITTORAL zone - the shallow water region with light penetration to the bottom; occupied by rooted plants
b. LIMNETIC zone - open water zone to a depth of effective light penetration; occupied only by plankton, fish and other free swimming organisms that rest or swim on the surface.
c. PROFUNDAL zone - bottom and deep water area, beyond the depth of effective light penetration; often absent in ponds; bacteria and fungi, bloodworms or hemoglobin containing chironomid larvae and worms (annelids), small clams (Sphaeridae); oxygen and light are important limiting factors.
2. In lakes, the open water, bottom and deep water are much larger than that of ponds. The open water zone is the chief producing area of a pond.
3. Temperature and Oxygen Stratification
a. Circulation of water in ponds and lakes
1) limited in ponds
2) seasonal in lakes
b. Circulation of lake waters
1) upper warm circulating water is called EPILIMNION
2) colder, non-circulating water is called HYPOLIMNION
3) zone between the two layers is called THERMOCLINE
4. Water is at its maximum density at 4°C, thus any water that is warmer or cooler will float on top of water at this temperature. When the underlying water has a temperature lower than 4°C, as in early spring, or higher than 4°C, as in the fall, a top heavy condition will develop.
During the summer, only the top layer will circulate and this does not mix with lower, colder water. As the temperature rises in the summer, the difference between the temperature of the top and the temperature of the bottom increases, creating the zone known as the THERMOCLINE. If this zone is below the range of effective light penetration, the oxygen supply of the lower waters becomes depleted. This is the period of summer stagnation. With cooler weather, the temperature of the surface waters drops until it is the same as the bottom. Water then begins to circulate and oxygen is returned to the depths. This is the "fall overturn." As surface water cools below 4°C, it becomes lighter, remains on the surface and freezes (if the climate is cold). Stratification again occurs since ice and colder surface waters prevent circulation. In winter the oxygen usually is not reduced because of:
1) slower bacterial decomposition and respiration, and
2) cold water holds more oxygen than warm water.
Winter stratification is usually not so severe, but if the ice completely covers the surface, oxygen depletion may result. In spring, ice melts, surface waters warm and sink to the bottom. When the temperature of the surface water rises to 4°C, the lake undergoes the "spring overturn.


Limnologists often classify lakes into two major categories according to the amount of plant and animal life which the waters sustain. (Not all agree on these classifications).


Lakes which do not produce abundant organisms in relation to their volume and which always possess abundant oxygen in their lower regions are called OLIGOTROPHIC (little-producing) lakes. They are usually deep lakes. Like other deep lakes, during the summer they often have, at the bottom, a region of cold water (the hypolimnion) which is of greater volume than that of the upper region of warm water (the epilimnion). The hypolimnion in oligotrophic lakes contains considerable oxygen at all times. Separating the epilimnion and the hypolimnion is the metalimnion, a layer in which the temperature of the water falls very rapidly with increasing depth. Within the metalimnion is a zone of maximum rate of decrease in temperature called the thermocline. In this zone, the rate of decrease in temperature must be at least 1° Celsius per meter of increase in depth.

A relatively shallow (though still deep) lake may be oligotrophic because there are so few organisms produced that little oxygen is expended in their decay at the bottom. A deeper lake may be more productive of organisms but remain oligotrophic because the hypolimnion is so large that there is ample oxygen to decompose the organic matter without undue depletion of the oxygen.

There may be other factors involved in creating oligotrophic conditions, such as the shape of the lake basin, direction of the prevailing winds with respect to the orientation of the lake, and the amount and nature of chemicals and organic materials washed into the lake. However, the classification is still based on productivity as indicated by the resulting condition: oxygen is never depleted in the hypolimnion.


Good-producing, or eutrophic, lakes sustain a rich plant and animal life. Such lakes are usually shallow and have a hypolimnion smaller than the epilimnion. During the summer, all the oxygen in the hypolimnion is used by oxidation of dead organic matter on the bottom. Organic materials are abundant both in the water and on the bottom.


In addition to the two major types of lakes, there are several kinds of lakes that have unusual characteristics. These include volcanic lakes, salt lakes, alkali lakes, and rift lakes. In these lakes, chemical and physical conditions are so unusual that it is difficult to classify them as either oligotrophic or eutrophic.

One kind of special lake is the acid-bog lake (sometimes called dystrophic). Acid-bog lakes may support plants and animals of certain types, but their growth is slow. Dissolved organic matter is high and imparts a brown stain to the water which limits the depth to which light may penetrate (often less than one meter), thus limiting productivity. Decomposition of some of the abundant organic matter may completely use up the oxygen during some seasons and thereby affect the productivity of such lakes. Much of this organic matter is not produced in the lake itself but in surrounding areas. There is so much organic substance that it cannot all decompose and, instead, accumulates as peat on the bottom. Lack of calcium, characteristic of acid-bog lakes, may also be involved in the incomplete decomposition.


A. Characteristics

1. Current a controlling factor; governs differences in various parts of a given stream, creates chief differences between pond and stream life.
a. Velocity of current varies throughout stream, both vertically and longitudinally and from time to time
b. Velocity determined by: 1) steepness or gradient of stream; 2) roughness of stream bed; 3) volume of water
2. Land-water interchange more extensive
a. Streams more intimately associated with surrounding land; they depend on land areas and connected ponds and lakes for food materials
b. Greater fluctuation in temperature than in ponds or still water
3. Oxygen tension higher and more uniform with little or no chemical stratification. Oxygen is not a limiting factor. Oxygen is abundant because:
a. small water depth
b. large exposed surface area
c. constant motion
B. Nature of the running water community

1. Three communities present: the rapid or riffles, the pool, and the waterfall. Influenced by the type of bottom: sand, clay, or rubble.
2. Current is major limiting factor; but hard bottom, especially if stone, offers favorable surface for organisms to attach or cling to
3. Composition of rapids community 100% different from that of a pond; pool community contains some organisms which also occur in ponds
4. Sand is least favorable bottom type, clay bottoms are better, stone and rubble bottom are most favorable
5. Organisms of the rapids communities show adaptations for maintaining position in swift water
a. Permanent attachment to firm substrate such as a log or stone. Examples: attached green algae, encrusted diatoms, aquatic mosses (Fontinalis), fresh water sponges, Caddis fly larvae
b. Hooks and Suckers. These enable animals to grip even seemingly smooth surfaces. Examples: Dipteran larvae (Simulium), sucker at posterior end of body and silken threads. If dislodged it is kept from being swept away by its safety rope and works its way back up the thread to a favorable attachment place. Caddis fly (Hydropsyche) cements net around itself; this acts as a shelter and as a trap for animal and plant material.
c. Sticky undersurfaces as possessed by snails and flatworms.
d. Streamlined bodies: possessed by nearly all stream animals. Broadly rounded in front, tapering posteriorly to offer minimum resistance to flowing water.
e. Flattened bodies. To enable animals to find refuge beneath stones, in crevices, etc. Examples: May flies and stonefly nymphs.
f. Positive rheotaxis. Orientation of animals upstream. If capable of movement the animals continually move upstream.
g. Positive thigmotaxis (touch or contact). Inherent behavior pattern to cling close to surface.
C. Chemical Relationships

Carbon dioxide, calcium, and the pH of a stream are closely interrelated. Carbon dioxide exists in water in five forms: as free gas, as carbonic acid, both undissociated and dissociated, as bound, like in calcium carbonate, and as half bound calcium bicarbonate.
The relative proportions of these forms varies with the chemical content of the water, the demand by plants, and the waste products of animals. Its constant liberation, its high absorbency coefficient, its formation of a weak acid in water, and its unstable condition in the bicarbonate form, all tend to make carbon dioxide and its various components all interdependent.

Marine Environments