Limnology (2+1)

Unit 5- Biological relations

5.4. Relations of Dissolved Oxygen, Relations of Carbondioxide
Relations of Dissolved Oxygen
Oxygen supply in air and in natural waters affords a striking contrast. Normally, air contains oxygen to the extent of approximately 21 per cent, which is an abundant supply for the respiration of air breathing organisms.
One liter of water will contain only about 9 cc of oxygen when saturated with this gas, whereas a liter of air will have 210 cc. In view of the active interplay of oxygen producing and oxygen consuming processes in inland water sets the stage for serious limits in aquatic respiration.
Excess of oxygen
Moderate supersaturations of dissolved oxygen occur in natural waters from time to time, usually owing to the photosynthetic activities of large masses of green plants in very calm water. Under special and still rare circumstances, large accumulations of excess oxygen appear in the upper part of the thermocline or in deeper strata of a lake.
Normal dissolved oxygen requirements
In the dynamics of natural waters, oxygen supplying and oxygen consuming processes are in constant action, the limits of an adequate supply of dissolved oxygen for organisms become an important matter.
The minimal oxygen requirement may be affected some what by other environmental features, e.g., temperature, CO₂ and certain conditions existing within the organism itself such as age or life history stage. In general “dissolved oxygen at levels of 3ppm or lower should be regarded as hazardous to lethal under average stream and lake conditions; and that 5ppm or more of dissolved oxygen should be present in waters, if conditions are to be favorable for freshwater fishes”. This statement assumes, of course, that other vital requirements are maintained within their proper limits. It also applies primarily to warm water fishes. It has been claimed that cold water fishes require a higher dissolved oxygen content.
The respiration of aquatic organisms depends not only on the dissolved oxygen content but also in a significant measure upon the temperature of the surrounding water; that the oxygen consumption is almost doubled by a rise of 10°C; that the same amount of dissolved oxygen has about twice as great a supply value at 5 as at 15°C.
Source of oxygen supply
1.Storage of oxygen
The hemoglobin of the blood may act as a storehouse for oxygen that such storage at times of abundant free oxygen may furnish the supply during oxygen deficiency.
2.Internal chemical transformation
The idea of chemical transformations taking place within the animal, such as occur in the utilization of foodstuffs in which oxygen is released and made available for recombination.
3.Catalysts facilitating oxygen absorption
Many profundal bottom animals have manganese in their tissues. This element may serve as a catalyst, facilitating oxygen absorption at low tensions.
4.Atomic oxygen from decaying plant tissues
The decomposing plant tissues in the profundal mud, even under anaerobic conditions, gradually liberate small amounts of an oxidizing substance could be utilized by the animals living in such close relationship to the decaying plant debris.
Temporary anaerobiosis
Animals living in the muddy bottoms of shallow water or other similar conditions in which the oxygen exhaustion occurs quickly and for limited times, may be forced to meet these temporarily unusual conditions.
Certain representatives of Protozoa, nematodes, earthworms, leeches, and immature stages of insects, mollusks, fishes, and others exhibit this ability.
During oxygen lack a certain amount of energy may be released by the splitting of carbohydrates into reduced substances, thus building up an “oxygen debt,” this debt being repaid by the increased rate of oxygen consumption when the organism is returned to aerobic conditions.
Some effects of insufficient dissolved oxygen
1.Attempts to migrate
In lakes, it is usually an upward migration into overlying, better oxygenated waters.
2.Onset of diseases
A close relation between insufficient dissolved oxygen to diseases of fishes, parasitic and bacterial and serious epidemics in the fish Leucichthys artedi, which seems to occupy the cooler water below the thermocline during summer.
3.Suffocation beneath ice cover
Shallow waters with bottoms containing large amounts of putrescible matter and occurring in regions where prolonged ice cover in winter is common may, at times, almost or completely exhaust the dissolved oxygen of the unfrozen water with resulting mortality (winter kill) among the organisms.
4.Summer kill
Critically low dissolved oxygen and unfavorable temperatures have been suspected as causal agents, but the case is not clear since other conditions were probably in a simultaneous state of flux. Warm waters reduced solubility of DO and release of oxygen by warm waters (less holding capacity).
Relations of Carbon Dioxide
General effects on organisms
Carbon dioxide is one of the most important substances in the life of organisms.
Small quantities
Usually, the quantities in the air are very small but yet sufficient for the photosynthetic activities of chlorophyll-bearing plants. Likewise, in natural waters, the amounts may be very small in the upper circulating waters.
Large quantities
Large quantities of carbon dioxide usually have a detrimental effect. Ordinarily, accumulations in unpolluted, natural waters do not reach such lethal amounts, owing to the ease with which they are released into the air or combine chemically. Increasing amounts of free carbon dioxide in association with other decomposition products may gradually render the hypolimnion untenable by all organisms save the resistant anaerobic animals in the bottom.
Such accumulations may render bottom waters acid in reaction and thus affect organisms sensitive to acid waters. High carbon dioxide content seems to be more toxic in the presence of low oxygen content.
Since an excess of dissolved carbon dioxide is usually accompanied by a much reduced dissolved oxygen content and other important conditions, it has been proposed that the carbon dioxide content of the water is probably the best single index of the suitability of water for fishes.
Carbon dioxide has a very definite effect upon the affinity of blood for oxygen in fishes and certain other animals.
Fishes may tolerate wide, but not sudden, ranges of carbon dioxide tension of the water by “increasing the alkali reserve of their blood in high carbon dioxide tension water and by lowering the alkali reserve of their blood in low carbon dioxide water”.
Carbon dioxide tension
While the carbon dioxide tension within natural waters and within the atmosphere constantly tends toward equilibrium, circumstances prevailing in the water (slow diffusion, rapid production of carbon dioxide, insufficient agitation of the water) may be such that at some depths the carbon dioxide tension is greater than in the air.
Relations of other Dissolved Gases : Methane, Hydrogen Sulfide
Methane
Some have claimed that it is nontoxic; others, that its effects on organisms are minor or that, at most, it may be occasionally toxic to animals or that it may cause them to migrate from particular situations. Possibly methane accumulations in bottom waters may have something to do with the increasingly severe conditions which develop with the progress of stagnation periods.
Hydrogen sulfide
Inherently, hydrogen sulfide is very poisonous. Certain marine fishes are said to be very sensitive to this gas and to avoid water containing it.