The rise and fall of these factors very frequently affect the fauna, altering their number and diversity. Some of the important factors of fresh-water environment are following:
Pressure, Density and Buoyancy:
The pressure imposed on a lake-dwelling organism is the weight of the column of water above it plus the weight of the atmosphere. In all fresh-water environments maximum pressure as much less as in the ocean, and organisms appear to adjust to them readily. The absence of animal life from deep water ordinarily a consequence of low oxygen supply, or low temperature, rather than pressure.
The density of water varies inversely with temperature and directly with the concentration of dissolved substances. Water is most dense at approximately 4°C and becomes progressively less dense as it cooled below + 4°C. Ice also expands markedly the colder it gets.
It is because the coldest water is at the surface in winter that ice forms there, rather than at the bottom of a lake. In summer, the coldest waters of deep lakes are at the bottom. Dissolved salts increase the density of water; the density of most inland water-bodies is much less than that of the ocean.
However, when great evaporation occurs in a lake having no outlet, the lake may come to contain a higher percentage of salts (i.e., hyper salinity) than the ocean. The few species capable of living in these very salty lakes (e.g., Great Basin of USA) include some algae and Protozoa, the brine shrimp Artemia gracilis and the immature stages of two brine flies, Ephydra gracilis and E. hians.
According to the law of Archimedes the buoyancy of an object is equal to the weight of the water it displaces. Buoyancy varies with the density of water, and is influenced by the factors that affect density. Viscosity, the measure of the internal friction of water, varies inversely with temperature and also influence buoyancy.
Most aquatic organisms keep stations by swimming movements or have special adaptations to decrease the specific gravity of the body and take advantage of any turbulence in the water.
For this purpose fresh-water aquatic organisms have some swimming adaptations, clinging organs (in case of animals of lotic habitat) or following adaptations—absorption of large amounts of water to form jelly-like tissues; storage of gas or air bubbles within the body; formation of light-weight fat deposits within the body or oil droplets within the cell; increase of surface area in proportion to body mass, which increase frictional resistance (David, 1955). When an organism so equipped dies, the special mechanisms quickly cease to function, and it sinks to the bottom.
The unique thermal properties of water are best demonstrated by fresh-water environments. Diurnal and seasonal variations of temperatures are very much evident in these environments than in marine environments.
A diurnal variation range of 4.8—5.0°C has been recorded by Sreenivasan (1964) in a tropical pond, with an average depth of 3.0 meters. In shallow water habitats, difference between day and night temperatures remains more conspicuous.
For example, in a polluted moat with an average depth of 1.5 meters, the lowest night time temperature was 26.6°C, the highest day time temperature was 32.0°C with a variation of 5.4°C (Sreenivasan, 1964). However, the Kodaikanal Lake in South India showed a diurnal variation of only 2.8°C (Sreenivasan, 1964). Flowing lotic waters of streams and rivers lack such wide fluctuations in temperature.
Further, the lento water of lakes and ponds undergo thermal stratification phenomenon according to the seasons. Thermal stratification has been reported most frequently in the lakes of tropical countries like Java, Sumatra, etc.
In fact, according to their temperature relations, lakes have been classified into three types: (1) Tropical lakes in which surface temperatures are always maintained above 4°C ; (2) Temperate lakes in which surface temperature vary above and below 4°C and (3) Polar lakes in which surface temperatures never go above 4°C.
The seasonally regulated thermal stratification of lentic habitats has a significant influence on their inhabitant biotic communities. Decreasing temperatures often cause a fall in metabolism resulting in a lower rate of food consumption.
The extremes of lower and higher temperature have lethal effects on the aquatic organisms. So fluctuations in temperature of aquatic media regulate the breeding periods, initiate hibernation, conidial activitation and a number of other biological phenomena such as thermally oriented migration, etc., of fresh water biota on the basis of their ability to tolerate thermal variations, most fresh water organisms are stenothermic with a narrow range temperature tolerance, but sonic euthermic with a wide range of temperature tolerance.
For example, the stenothermic oligochaets includes stenominimotherma forms (narrow range of temperature, e.g., Aeoiosoma, Megascolex mauritii), stenomaximo- thermal (e.g., Dero limosa) or steno-optimothermal forms (e.g., Branchiodrilus seniperi and B. menoni) (Sitaramiah, 1966).
Light influences fresh-water ecosystems greatly. The fresh- waters often have a lot of suspended material. While affording protection to the light sensitive species, these substances more often obstruct the light that normally reaches the water the degree of such obstruction of light influence the productivity of the freshwater ecosystems.
A shallow lake receives light to its very bottom resulting in an abundant growth of vegetation both phytoplankton and rooted vascular plants. These plants in living or dead states form nice food for consumers of grazing food chain or organisms of detritus food chain, respectively.
The running water contains little plant or animal plankton not due to the lack of sunlight but because of the action of the currents in washing it away. Further, light controls the orientation and changes in position of attached species and their nature of growth and it also caused into diurnal migration of plank tonic species of fresh-water.
Chemically pure water is biologically uninhabitable and all ‘fresh-water contains an array of chemical substances. The oxygen, which is a most essential chemical component of life processes, regain dissolved in fresh-waters.
The aquatic environments which remain in close proximity with atmosphere contain an abundance of oxygen that reaches the water either by direct diffusion or by movements of water like wave action or water circulation. Lotic (moving) water of streams and rivers often has a high percentage of oxygen.
Aquatic plants and phytoplankton’s supply water with oxygen that is formed as a product of photosynthesis. The amount of photo synthetically produced oxygen remains high at warm temperatures and at greater light intensities.
The oxygen level in a tropical pond exhibits diurnal variation- it remains at peak between 14.00 and 17.00 hours of day (Sree- nivasan, 1964). Oxygen contents of a fresh-water body are depleted in numerous ways. Primarily oxygen is utilized in the respiration of organisms and decomposition or’ dead organisms in the aquatic environment.
While photosynthesis remains restricted to the surface layer of water containing phytoplankton and exposed regions of rooted vascular plants, respiration and decomposition occur at all levels. In stagnant pools with a lot of decaying vegetation oxygen content often reaches a stage of complete depletion.
The reduction in dissolved oxygen is magnified by the release of many gases as end products of decomposition or by the mixing up of waters of low oxygen content reaching the habitat as an inflow.
Aquatic animals with very few exceptions, i.e., those that breathe air, utilize the oxygen dissolved in water. Certain fresh-water inhabitants such as many anaerobic bacteria and insect larvae of chironomids perform anaerobiosis and require no oxygen.
Aquatic vegetation and phytoplankton require carbon dioxide for photosynthetic activity. The carbon dioxide of fresh-water environments is produced as the end product of respiration and of decomposition. Carbon dioxide also diffuses directly from the atmosphere and is readily dissolved in water to result in carbonic acid (H2CO3) which affects the pH of water.
It is also present in the fresh-water as carbonates and bicarbonate of calcium, magnesium and other minerals. The growing plants and lime-depositing bacteria and other animals may cause depletion in carbon dioxide resources. Photosynthesis is the major cause for its drain. The high saturation levels of O2 and CO2 have been found to have toxic effects on aquatic biota.
Streams and lakes contaminated by sewage and stagnant pools with decaying vegetation show an abundance of the gas hydrogen supplied which is a decomposition product. This gas is highly toxic to living organisms and results into complete denudation of bottom fauna. Methane and carbon monoxide are other toxic gases which are the products of decomposition. Nitrogen, hydrogen, sulphur dioxide and ammonia are some of the other gases which are found dissolved in fresh-waters.
Dissolved Salts and Salinity:
Fresh-water being efficient solvent contains many solutes in solution, but even then its salt contents remain under 1/5%0 than marine water which contains about thirty-five parts per thousand (%0) dissolved salts (see Clapham, Jr., 1973). Different dissolved salts reach the water by erosion inflow and decay of aquatic forms.
Dissolved substances have peculiar significance for floating aquatic vegetation and phytoplankton; since these organisms do not depend on the substratum for the supply of nutrients. Compounds of nitrogen, phosphorus and silicon are most important substances found dissolved in fresh-water.
Nitrates, nitrites and ammonium salts are essential for the food of aquatic vegetation such as algae and water weeds. Nitrates always remain available due to nitrogen cycles occurring between nitrogen fixing bacteria and nitrogen consuming plants.
Ammonium salts in excess have a lethal effect on the fauna. Dissolved silicates of fresh-water are readily utilized by diatoms and sponges in constructing their body structures such as shell in case of diatoms and spicules in case of sponges.
All fresh-water environments also contain small amounts of phosphorus which more often acts as a limiting factor. Utilization of phosphorus by plankton during the periods of abundance may result in a total elimination of other plants that require the element (phosphorus).
Many other elements like calcium, magnesium, manganese, iron, sodium, potassium, sulphur, and zinc are found dissolved in water and influence the fauna variously. Iron being a growth promoting element for plants exists as the compound of oxygen (ferrous oxide) or sulphur (ferrous sulphide) in different fresh-water bodies. Its influence is often modified by the pH of water. Calcium is an essential element for plants.
The abundance and scarcity of carbonate of calcium determine the faunal composition. Deposition of calcium carbonate in water called marl is produced by the activity of plants. External coverings of arthropods and the shell of mollusks and tubes of some worms need calcium carbonate.
Snails are found to develop a heavy shell if the water in which they lived contained excess of calcium. Bryozoans, sponges, and cladocerans prefer increased calcium content.
Due to low salinity of fresh-water animals face the problem of osmoregulation. Because the salt concentration of body fluids of animals remains higher than the fresh-water hence the water continuously tend to enter the body which should be readily removed.
Most aquatic animals (e.g., Protoza and fishes) have the means to excrete extra amount of water of body by osmoregulation. For this purpose Protozoa employ contractile vacuoles and other multicellular invertebrates and chordates use excretory organs, such as nephridia, kidney, etc.
pH or Hydrogen Ion Concentration:
In fresh-water environments pH is a determining factor for the biota by becoming a limiting factor. The pH value of different fresh-water bodies may fluctuate seasonally and annually. The pH of surface waters and deeper waters exhibit marked differences. Sreenivasan (1968) has reported a marked pH variation of 2.2 units between surface water and deepest water in the Sandynulla reservoir in the Nilgiris (India).
Though pH range is species specific, yet lower aquatic forms in general showed little reaction to alterations in pH, while higher aquatic organisms (e.g., fishes) responded quickly to little pH variations.