[Studies in Environmental Science] Chemistry and Biology of Water, Air and Soil - Environmental Aspects Volume 53 || 4 Biology of water
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4 Biology of water
4.1 Foundations of the ecology of aquatic organisms
The need for water is a common feature of all living organisms since water represents the only and indeed the essential environment in which the pro- cesses of life can take place. A cell as the fundamental and functional unit of living mass cannot grow and develop without a supply of water. In the case of actively growing cells, this amount is only rarely lower than 80% of live weight, regardless of whether it concerns microbes or multicellular macroorganisms. In this sense, water is a unifying component. However, there is a considerable versatility with respect to the relationship of living organisms to water as a component of the environment, particularly the na- ture and extent of the water in which the organisms live. Some organisms require water for their existence as the predominating environmental fac- tor, others are satisfied just with a short contact with water, and some are able to utilize water in a mostly dry environment. As well as purely aquatic or land organisms, species with various transition positions between these two extremes also exist in nature. Many live in water only in a certain zone of the transition from aqueous to land environment all their life. If a specialization to the aqueous environment appears, it is the more strict the more the organism is at a higher stage of development. And still, among microorganisms receiving nutrients only from water and aqueous solutions and therefore unable to reproduce on the land and to grow without a direct contact with water, there are also species which are in some ways adapted to the life on land. On the other hand, among the typical and specialized aquatic animals such as fish, primitive species occur sporadically, which are even able to survive temporary periods of drought in swamps.
The fact that many organisms are not explicitly aquatic or terrestrial corresponds to the present konwledge of the evolutionary changes in Nature.
Life originally existed in the sea, and only at a certain stage of evolution land started to be inhabited, including adaptation to the life in fresh waters. Afterwards, however the evolution did not continue only in the direction from water to land, but also to some extent vice versa. Some of the present fresh water species comes directly from the sea and others come from land organisms which again adapted to the life in water. It is assumed that the evolution of some species was continued by their return to the sea.
4.1.1 Characteristics of the aqueous environment
In common with all the habitats of living organisms, also the aqueous envi- ronment can be characterized by its physical and chemical properties; the main aspects are now discussed in some detail.
Aquatic organisms are mostly exposed to smaller temperature changes than the land organisms. The fact that water in Nature undergoes much smaller temperature deviations than the air is primarily due to specific physical properties of water (anomalies; see Section 3.2).
One crucially important factor for aquatic life in large enclosed areas of water such as lakes or reservoirs is that they do not freeze to the bottom, particularly because of the anomalous changes in the density of water when its temperature approaches zero.
126.96.36.199 Concentration of oxygen and carbon dioxide
Oxygen is required for breathing of aerobic organisms which eliminate CO, . Oxygen enters water via diffusion from the atmosphere, or is released from photosynthesizing aquatic organisms binding simultaneously CO, , and its reduction to organic compounds. Thus, both gases appear in two funda- mental processes of life. The concentration of dissolved oxygen, which is of special importance for aquatic organisms, depends on both physical and biological factors.
Physical factors are the water temperature and partial pressure of oxy- gen in the atmosphere. These change quite markedly due to the effect of climatic and physical conditions but their mutual dependence is well known and can be determined quite simply. Figure 4.1 illustrates the relation- ship between the concentration of dissolved oxygen and water temperature at normal atmospheric pressure; it is evident that with increasing water temperature the amount of dissolved oxygen decreases significantly. The
TEMPERATURE ( % I Fig. 4.1. Dependence of dissolved oxygen content on water temperature
TEMPERATURE ( % I Fig. 4.1. Dependence of dissolved oxygen content on water temperature
oxygen content decreases with a decrease in atmospheric pressure, which changes in dependence on the climatic conditions and altitude. If the con- tent of dissolved oxygen is in equilibrium with the atmospheric oxygen, then the water is saturated with oxygen. Under natural conditions the content of 0, in saturated water ranges from 7 to 15 mg 0, 1-' of water.
A more complicated situation is encountered in water containing living organisms; the 0, concentration can be remarkable increased or decreased due to the biological activity. The amount of oxygen from photosynthesis depends particularly on the amount of photosynthesizing organisms, inten- sity and duration of allumination, and water temperature. Biologically, oxygen is consumed primarily during breathing and microbial degradation of organic matter. The rate of consumption of oxygen depends primarily on the content of well-degradable organic matter in water.
The CO, content in water is influenced by the same microorganisms as in the case of 0, but in an opposite direction. Free CO, is present in water mostly as a dissolved gas. Only a small fraction (0.7%) reacts with water to form carbonic acid which dissociates to ions H+ and HCO,. Carbonic acid converts difficultly soluble carbonates of calcium and magnesium into readily soluble bicarbonates. Thus, so-called bonded (carbonate) CO, is transformed into a semi-bonded (bicarbonate) form which is utilizable for
aquatic organisms via photosynthesis. Free carbon dioxide can be present in water in markedly varying amounts: 1-100 mg CO, 1-' depending on whether its biological production or its consumption prevail. Usually, it ranges from a few to 20 mg 1-'. (In mineral waters the content of non- biological origin CO, can exceed 1000 mg 1-'.)
188.8.131.52 Light radiation
Adequate intensity of light radiation at suitable wavelengths together with sufficient quantity of CO, is an essential condition of photosynthesis; the intensity, duration and wavelength of illumination are also important. If illumination gradually decreases, it is obvious that the intensity of photo- synthesis decreases its well.. This results in lower CO, bonding and less intensive oxygen release. At a certain level of illumination the quantity of CO, consumed by photosynthesis equals the quantity of CO, released by respiration. This is the so-called light compensation point and its value also depends on the water temperature [l, 21. If the intensity of illumination con- tinues to decrease, the CO, release ever more outweighs the bonding until photosynthesis stops. The relationship between the intensity of photosyn- thesis and the intensity of illumination is not linear, it is more logarithmic in character (Fig. 4.2). Too high an intensity of light can also reduce the photosynthetic activity . The overall relationship corresponds to a more generally understood law of minimum , formulated by Liebig for the rela- tionship between the growth and concentration of nutrients. The inhibition effect on photosynthesis of too intensive illumination is, for example, that on bright sunny days the maximum photosynthesis is achieved a t a certain depth below the water surface. On account of this, some algae move from the surface layer into the deeper layers.
An important, sometimes even limiting, factor is the duration of illumi- nation. Some plant species require for their proper development a certain number of hours of sunshine. For example, increasing the light intensity un- der the conditions of a short day cannot satisfy the requirements of plants requiring a long day. Other species require a certain minimum period of dark. However, many plants develop well under the conditions of both short and long days. This phenomenon of dependence on light duration, termed photoperiodicity, has nothing to do with photosynthesis but is con- cerned with the production of hormones controlling the development of an individual [ 5 , 6 ] . It is quite well studied in the case of land plants, but cer- tainly it is important for the development of aquatic species (for example,
fish farms can regulate their stocks of juvenile fish by imposing an artificial photoperiod).
Along with its quantitative properties, the quality of light is of great im- portance. Of the total energy of sunshine impinging on the earths surface, approximately 10% is UV radiation, 45% visible light and 45% infrared light. These components have different effects on living organisms. For photosynthesis particularly the visible light of the spectrum is used, but not evenly in its whole width, and also the individual species of photo- synthesizing organisms may differ in the utilization of various parts of the spectrum. This depends on their photosynthetic, and to a certain degree on other colour pigments. The most important photosynthetic pigment is chlorophyll, present in plants and algae. In cyanobacteria (blue-green al- gae) it is phycocyan. It is typical of aquatic autotrophic organisms that they often contain other dyes which complete and extend their capability to utilize the spectrum of the sun. In plant, green and brown algae one can find carrotenoids playing this role, in cyanobacteria and red a