Limnology (2+1)

Unit 5- Biological relations

5.2. Productive volume, flotation phenomena and body form adjustments
Productive volume
Form of basin determines the extent of productive volume. By productive volume is meant that portion of water in which virtually all biological production occurs. In a lake of the third order, total volume is productive at least during the open season. In lakes of the second order, productive volume is almost exclusively confined to the epilimnion and the thermocline during most of the summer stagnation period. During the overturns, the entire lake temporarily becomes productive volume but the duration of these periods may be too short to be or any great consequence. Lakes of the first order resemble those of the second order in that they maintain the productive zone in the upper stratum, usually limited by the presence of a thermocline. In those lakes having no complete overturn, the productive zone, during open season, merely varies in volume with those conditions which determine the depth to which circulation may extend. During prolonged ice cover, lakes of the second and third orders undergo gradual reduction of productive volume due to encroachment of the underlying stagnation zone, while lakes of the first order, under these conditions, may undergo less change in productive volume due to their size and to the presence of the permanent, deeper stagnation region. Complete ice cover may not occur in lakes of unusual size and depth, even though located in colder regions.
Both depth and area of a lake basin combine in innumerable ways to produce a great heterogeneity of lake forms. Since inland lakes seldom exceed certain area limit, depth is fundamentally of prime importance in determining productive volume.
The average depth is the factor which determines whether a lake is eutrophic or oligotrophic, computing average depth as the quotient of volume of the lake over area of the lake (V/A). In oligotrophic lakes, the volume of the hypolimnion is greater than the volume of the epilimnion, and that in eutrophic lakes the reverse occurs.
Flotation phenomena
i). Non-motile organisms
In calm water, non-motile plankton organisms depend entirely upon the relation between their own specific gravity, the density of the water, and the viscosity of the water in maintaining their vertical position. Non-motile, attached animals, particularly the colonial forms such as the larger colonies of fresh-water sponges and certain fresh water Bryozoa (Pectinatella), may develop forms and masses of body, which could not be maintained in the absence of the buoyant effect of water, and even the soft bodied Hydra would be helpless without it.
All sessile animals depend, to some extent at least, upon the buoyancy of the water. Many higher aquatic plants are dependent upon the supporting effect of the water in order to maintain their proper form and orientation.
ii) Motile Organisms
Those plankters which possess powers of locomotion vary greatly in the efficiency of their progression, but some change of position in space is possible due to their own activity. While such locomotion may be almost negligible when compared with the shifting and transporting effects of the water, it may nevertheless be vital to the organism in many ways, such as in the capture of food, and in the change of water in contact with respiratory surfaces.
Locomotion in water, consumes less energy to maintain their position above the bottom, due to buoyant effect of water. In fact, those organisms whose specific gravity is essentially the same as the surrounding water expend practically no energy in merely keeping up in the water. Certain aquatic animals, because of the possession of air stores or other special means, are distinctly lighter than water and must use a certain amount of energy to keep below the surface when they need to do so.
Many air breathing, aquatic insects have air stores so located that not only are they lighter than water but the posterior end is lighter than the anterior, enabling the insect to float at the surface in the proper respiratory position.
iii) Reduction of specific gravity
Protoplasm alone has a specific gravity which closely approaches that of water, but the various cell products which occur in animals and plants may combine to produce bodies which are either heavier or lighter than water. Products which tend to make the body heavier (such as chitinous exoskeletons of arthropods, bones, shells or various kinds) and those which tend to make it lighter are often present in the same body so that the specific gravity of the whole depends upon which of the contrasting materials predominate. The most effective and the most common of those cell products which reduce specific gravity seem to be the following:
1.Gases originate from various sources (metabolic products, external and internal air stores, and others) and remain, at least for a time, enclosed within or attached to the body. These gas accumulations may be of sufficient magnitude to make an otherwise heavy-bodied animal (as, for example, certain aquatic insects) much lighter than water.
2.Fats and oils are commonly produced and stored within aquatic organisms, notably in the plankton crustaceans and in the plankton ie Algae.
3.Gelatinous and mucilaginous secretions of varying amounts are common as matrices and external envelopes which helps in flotation.
iv) Relations of surface to volume
(a) In different Species.
Since in relatively compact bodies of organisms, the relations between volume and surface tend to conform roughly to the well-known mathematical principle that the surface varies as the square of the dimension while the volume varies as the cube of the dimension. The smaller the body, the greater will be the relative expanse of surface.
If the body has a specific gravity greater than water, it will sink, although resistance to sinking will be offered by the viscosity of the water. The greater the surface compared to the volume the greater will be the friction between water and body. Because of this relation, particles of very small size, even though composed of a substance having a specific gravity greater than 1, may not sink at all.
Few aquatic organisms, irrespective of size, are spherical in form. Any departure from the spherical form results in relatively increased body surface. Relative increase of surface is accomplished in so many different ways by :
1.General body form. Main portion of body may present:
a.Various degrees of attenuation.
b.Various degrees of compression or depression or general flattening.
c. Miscellaneous forms of asymmetry.
2. Body surface sculpturing: ridges, furrows, striae, impressed or raised patterns.
3.Extensions and modifications of antennae, tentacles, gills, legs, cerci and others.
4.Development of special peripheral processes: hairs, setae, spines, bristles, filaments, radial axes, tubercles, cilia, pseudopodia, crests.
5.Formation of colonies: linear, dendritic, radial, lamellate, irregular.
Combinations of several of these structural features in the same organism may occur, sometimes with remarkable flotation results.
(b) In the same species
In certain plankton organisms, a striking seasonal change of body form of a very definite sort occurs. This change of body form is a response to changes in the viscosity of the water due to seasonal changes in temperatures.
(c) Accessory provisions of flotation
Cases and coverings of various kinds composed, of foreign materials are constructed by some aquatic organisms. While such cases often serve several other purposes. In certain cases that they either increase the tendency of the whole organism to be in suspension or completely support it at the surface. Instances of this sort are not uncommon among the aquatic insects (certain caddis-fly larvae, certain aquatic caterpillars, and others.
(d) Hydrofuge structures
Hydrofuge structures, such as hydrofuge pubescences, hydrofuge caudal filaments, and hydrofuge smooth surfaces, often play an important and sometimes a vital part in the flotation of organisms. Once at the surface they may provide (1) for the proper orientation of the body into the breathing position and (2) for the ability to remain at the surface in this position without much effort during the breathing period. Examples are not uncommon among aquatic insects. Certain hydrofuge structures are related directly or indirectly to respiration in some air-breathing aquatic insects.
(e) Precision in flotation adjustment
Among the plankton, flotation at the proper level is sometimes a matter of precise adjustment. A very small discrepancy in adjustment may result in the following chamges.(1) too great buoyancy, causing the organism to rise to an unfavorable level or even to rise to the surface. Owing to various hazards such as excess light, entanglement with the surface film, and evaporation; or (2) sinking to an unfavourable depth at which such features as reduction of effective light, critical reduction or absence of oxygen, and absence of proper nutritive materials may result seriously. Such adjustment is thus of vital importance to certain nonmotile plankters. This adjustment follows the seasonal changes of viscosity and density of water in such a way that organisms may continue to thrive is one of the marvels of aquatic life.
Water fleas swim intermittently but unceasingly, and certain microorganisms occupy proper levels by constant vibrations of flagella or cilia.
Body form adjustments
a) Streamline form
When a body is either in motion through quiet water or stationary in moving water, the water imposes some resistance over the body. Hence, the organism has to overcome this force to maintain its stationary position.
A body must occupy space by complete displacement of the water and that when either the body or the water moves; the body continues the process of displacement. Also, under these conditions, not only must the great weight (density) of the water be overcome in pushing it aside, but the internal friction (viscosity) of the water plus the friction of the water against the surfaces of the object must also be overcome. Thus, energy is expended by the object in overcoming these resistances, and, as will be shown later, the amount of this energy depends upon the form of the body when other conditions are the same.
Viscosity and density of water vary with certain conditions, particularly temperature; the resistance met by a moving organism or by an organism maintaining its position in moving water is considerable under all circumstances. Organisms vary greatly in their ability to overcome this resistance, owing to inherent differences, especially the form of the body. Obviously, certain forms of body are more effective for locomotion in water than others.
Among other things, it was found that the body consists of two principal parts: (1) the entrance or fore body, that part from the tip of the snout to the maximum transverse section; and (2) the run, or after body, that portion from the maximum transverse section to the tip of the caudal fin. It was also found that, in all specimens, the average position of greatest transverse section occurred at a distance of about 36 per cent.
The replacement of water following maximum displacement had something to do with the function of the after body.
Principle of streamline form
A body with streamline form moving through standing water and the same body maintaining a fixed position in running water present the same essential conditions.
The position of maximum transverse section represents the maximum displacement of water. it likewise determines not only the maximum energy expended in displacement but also the termination of virtually all energy expended in displacement but also the termination of virtually all energy expenditure so far as body surface is concerned.
(b) Other forms of adjustment
Some animals living in the strongest currents possess spines on the exposed surface (Blepharoceridae). These projecting structures would be detrimental by increasing the resistance. These roughnesses actually help to decrease resistance. In bodies such as spheres and cylinders, the nature of the resistance may change markedly with relatively small changes in the conditions involved; at certain velocities, the resistance of a sphere may be reduced by roughening its surface. In some of the species, spines will be developed as a means of diminishing the resistance to the fierce currents in which they live. The spines on such a body would increase the resistance at some water velocities but would decrease the resistance at certain higher rates of flow.