Limnology (2+1)

5.3.2. Relations of organisms to Temperature relations, Light relations

Unit 5- Biological relations

5.3.2. Relations of organisms to Temperature relations, Light relations
Temperature relations
With the exception of the aquatic birds and mammals, all aquatic animals are cold blooded (poikilothermous), i.e., their internal temperatures follow, usually within close limits, the temperatures of the surrounding medium. It must be understood, however, (1) that exceptions in the form of unusual deviations from surrounding temperatures may occur, as, for example, the claim that certain fishes may have an internal temperature of as much as 10°C. higher than that of the surrounding water; and (2) that the degree of agreement between body temperature and external temperature may differ with the temperature level of the latter.
Some aquatic animals live in surroundings the temperature of which is below freezing (glacier worms and others), but it has usually been supposed that the freezing point of their body fluids is depressed by substances in solution. Even if this is true, there remains to be explained the fact that under those very low temperatures they are not only active but grow, develop, and reproduce.
Influence on metabolism
Within the ordinary temperature limits for a given cold blooded animal, decreasing temperatures diminish metabolism, and vice versa, a relation which is opposite that for warm blooded animals. This means that metabolic rate is, to a large extent, governed by the external temperatures. It also means that the falling temperatures of increasing depths in water or of increasingly northern latitudes inflict lower rates of metabolism. A general rule for this change in metabolism in cold blooded aquatic animals can be stated as follows: a rise of 1°C increases the rate of metabolism about 10 per cent. This means that the rate of oxygen consumption and carbon dioxide output doubles with a temperature increase of 10°C.
Influence on development and other biological processes
Rising temperature increases the rate of (1) development of animals, (2) respiratory movements, (3) heart beat and circulatory rhythms, (4) enzyme action, and (5) other physiological process, although the operative limits in each process may differ. A cold blooded aquatic animal may be expected to complete its life cycle more slowly and to produce fewer generations per unit of time in the northern than in the southern part of its range; like wise, the normal individual life span may be longer. Onset of hibernation, breeding season, changes in reproductive activity, germination of asexual reproductive bodies, and a host of other biological activities are profoundly influenced by surrounding temperatures.
Temperature toleration
Each organism has a maximum and a minimum environmental temperature between which life is possible but beyond which conditions are lethal. Even for individual species, these temperature limits are not absolutely fixed, since they may vary with different individuals, with the different sexes, with different life history stages, with different physiological states, and in different parts of the geographic range. In spite of this variation, it is possible roughly to divide animals into two groups: (1) those which are restricted to a narrow range of temperature change (stenothermic animals) and (2) those which tolerate a wide range of temperature change (euthermic animals). Also, there are integrades between these two groups. It is a well known fact that acclimatization can shift temperature restrictions as well as those of other environmental factors. Somewhere between the maximum and minimum limits, an optimum region occurs, the position and extent of which vary with different animals. It is sometimes stated that the optimum is usually closer to the maximum than to the minimum, but in some instances the reverse condition prevails. Acclimatization may also affect the position of the optimum.
In temperate lakes of the first and second orders, only the non-migrating, profundal bottom organisms live under approximately a fairly even temperature throughout the year. On the contrary, those surface water forms which remain active throughout the year must endure the complete range of temperature. Those not active in all seasons have developed various forms of hibernation, and aestivation, as a means of passing over the more rigorous conditions. Many aquatic animals remain active thorough wide ranges of temperatures, the active period ending only just before the extremes are reached.
Effects of extremes of temperature
The specific effect of extremely low temperature is usually considered as being mainly mechanical, while that of extremely high temperature is principally chemical, affecting the protoplasm. The chemical effect of excessively high temperature is more severe than the mechanical effect of correspondingly low temperatures. It is true that even in the temperate latitudes, certain aquatic animals (mosquito larvae and others) may be frozen into surface ice and recover on release. This phenomenon seems to be more common in the arctic and subarctic regions.
The occasional rise of surface water temperature to unusual heights (although only a few degrees above the usual summer maximum) in protected bays in times of clear, hot weather and dead calm water promptly leads to a dying off of surface plankton and certain other shallow water organisms.
Recognition of temperature differences
Some aquatic animals have a well developed recognition of changing temperature and may respond with considerable precision. Under experimental conditions, certain fresh water animals have been found to recognize temperature differences of 0.2°C and react to them.
In thermally stratified lakes, it is very difficult to determine the presence or absence of a limiting effect to downward distribution by the steep thermal gradient in the thermocline, since other varying conditions are simultaneously present, such as light, chemical stratification, and viscosity changes.
Light relations
The various relations of sunlight to aquatic organisms may be classified into two sets: (1) direct influences upon the organisms as a whole and (2) photosynthetic relations.
Direct influences
Lethal effects
Many aquatic organisms are sensitive to the higher intensities of sunlight and, in fact, must avoid them by occupying deeper levels in the water.
Plankton organisms occur in surface waters, exposed to the maximum light intensity. Many are phytoplankton and have photosynthetic relations, but it is a well established fact that in most natural waters the maximum populations of plankton occur at some lower depth, one of the important reasons being the more favorable light effects.
Both sunlight and ultraviolet light seemed to devitalize the diatoms (excepting Synedra) but stimulated the Chlorophyceae and the Myxophyceae, the ultra violet light providing a slightly greater stimulus. Since the ultraviolet light is quickly absorbed in the surface waters, its effects are very restricted.
Many aquatic organisms, especially bottom inhabiting forms, live in conditions of almost if not complete darkness and quickly succumb in direct sunlight. Light is often a powerful factor, sometimes the determining one, in the distribution of organisms in aquatic environments.
Behavior and orientation
Responses of aquatic organisms are often due to, or conditioned by, light. One of the most striking results of the alternation of day and night is the migration of certain plankton organisms from deep water to the surface at night and their return to the depths near dawn.
Light is the principal motivating influence of this migration. For some organisms, day is the period of general activity, night the period of quiescence; for other forms, the reverse is true.
In many aquatic animals, the light responses differ markedly with physiological state, age, life history stage, season and other conditions.
Other influences
Direct influences of light effects upon pigments and pigment production, upon growth, upon development, and in fact, upon many of the conditions involved in the general success of organisms.
Photosynthesis
One of the most profound influences of sunlight (and of moonlight to a limited extent) in water is its intimate role in the photosynthetic processes of all chlorophyll bearing, aquatic plants.
These plants furnish, directly or indirectly, the carbohydrate and the protein supply for the aquatic world. They occupy that strategic position between the inorganic and the higher organic components which makes the latter their complete dependents. The phytoplankton has been called the green pasture of the sea, and it plays a similar role in fresh waters too.
It becomes increasingly clear that the process commonly referred to as photosynthesis is a very complex phenomenon. Light, temperature, solutes, and carbon dioxide affect simultaneously the photosynthesis. However, it seems certain that light of the appropriate kind and intensity is the supreme factor.
Light requirements
The light supply has two important aspects: (1) light intensity and (2) effective wave lengths.
Intensity
The rate of photosynthesis increases with the intensity of light and infact, if certain conditions of temperature and carbon dioxide are met, the rate of photosynthesis is proportional to the intensity of the incident light. However, the rates of photosynthesis differ in different plants.
1.Ultraviolet rays are of little or no consequence in photosynthesis. This has been demonstrated experimentally with terrestrial plants. Considering the fact that ultraviolet waves are completely absorbed in the uppermost, thin layer of the water and that various aquatic plants thrive far below the level of disappearance of these wave lengths, the aquatic situation seems to offer confirmation of the statement.
2.Experimental evidence appears to show that with equal intensity of incident light, photosynthesis is affected by different wave lengths, being greatest in the red and least in the blue violet. Certain investigators claim that the rate of photosynthesis diminishes with decreasing wave length.
Effective light penetration
The normal existence of healthy chlorophyll bearing plants at various depth levels in water may be taken as evidence that some of the effective light is present in sufficient intensity to enable these plants to perform photosynthesis.
Algae have been found in certain mountain lakes below a depth of 400m and at greater depths in the ocean, but it remains to be conclusively demonstrated that these plants are performing photosynthesis.
It seems certain that light is a very influential factor in determining the occurrence and distribution of chlorophyll in a lake. Therefore it may be expected that since light conditions differ in different waters the quantity and activity of chlorophyll will be influenced correspondingly. Nevertheless the processes of light penetration and photosynthesis in natural waters are so complex.
The maximum rate of photosynthesis in lakes in full sunlight usually occurs somewhere below the surface layer.
Plants inhabiting situations having moderately reduced light intensity usually have more chlorophyll than do those living in full sunlight.
That light intensity at which oxygen production in photosynthesis and oxygen consumption by respiration of the plants concerned are equal is known as the compensation point, and the depth at which the compensation point occurs is called the compensation depth. For a given body of water this depth varies with several conditions, such as season, time of day, degree of cloudiness of sky, condition of the water, and taxonomic composition of the flora involved. As commonly used the compensation point refers to that intensity of light which is such that the plant’s oxygen production during the day will be sufficient to balance the oxygen consumption during the whole 24hr period.
Photochemical nitrification
An indirect effect of sunlight is through a possible photochemical nitrification. A portion of the nitrification which goes on in the sea is photochemically activated. Some chemical nitrification in soil is activated by sunlight in the absence of the biological agencies.