QUALITY OF WATER: NEW CRITERIA

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Опубликовано в библиотеке: 2021-11-21
Источник: Science in Russia, №5, 2014, C.38-43

by Sergei OSTROUMOV, Dr. Sc. (Biol.), Department of Biology, Moscow State University

 

Life cycle of water ecosystems involves numerous physical, biological, and chemical processes. Up to a certain point, natural self-purification processes successfully cope with the problem of water purification. However, in the past decades, anthropogenic impact on the natural environment has increased insomuch that these natural mechanisms are sometimes unable to meet the challenge and even "break down". According to the results of multiyear studies, present-day priorities in the sphere of preservation of water quality and environmental security of water-supply sources are already insufficient and should be supplemented and reconsidered.

 

Planktonic wheel animalcules (Brachionus calyciflorus).

 

BIOLOGICAL FACTORS

 

Many physical and chemical processes of self-purification of water are regulated by biological factors or essentially depend on them*. For example, sorption of pollutants on sedimenting particles of suspended matter depends on the concentration of phytoplankton cells. Photochemical processes depend on water transparency, water transparency-on filtration capacity of aquatic organisms. Free radical processes of decomposition of pollutants (substances of anthropogenic origin polluting habitats of living organisms) depend on fixation of metal ions by dissolved ligands-organic molecules of biologi-

 

See: S. Ostroumov, "Biological Filters Are an Important Part of the Biosphere", Science in Russia, No. 2, 2009.-Ed.

 

cal origin. Consequently, biotic (biological) factors are the central point of the whole water self-purification system. Oxidation of organic substance and filtration of water by aquatic organisms, as the key factors, have been studied by scientists in every detail.

 

Oxidation of organic substance is typical of many hydrobionts (aquatic organisms), while bacteria play a special role in this process. It gets involved almost all representatives of main groups of bacteria. It is believed that about 60-70 percent of the overall heterotrophic decomposition (i.e. biological oxidation of organic substances) in the world ocean fall on bacteria. Eukaryotic microorganisms (single-cell organisms characterized by a higher cell organization compared with bacteria, in particular,

 
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diplomonads, ameboflagellates, infusorias, flagellates, etc.) are another important element in self-purification of water ecosystems. Moreover, these very single-cell plankton organisms produce a steady positive effect on stability of ecosystems since they also act as consumers* of bacteria. In such a way, they "rejuvenate" bacterial community and stimulate decomposition of organic matter by bacteria.

 

Studies of filtration capacity of separate groups of aquatic organisms (for instance, ascidia, barnacles, bryozoans, echinoderms, bivalve molluscs, gastropods, polychaetes, sponges) showed that they are able to filter 1-8.8 liters of

 

* Consumers-organisms consuming ready-to-eat organic substances-Ed.

 

water per hour for 1g of salt-free dry body weight of these invertebrates. According to the results of more detailed analysis, it was established that an overall filtration capacity of water by communities of macro-invertebrates (such relatively large organisms as molluscs, ascidia and polychaetes) is assessed in the range from 1 to 10 m3 per 1 m2 of the bottom area daily. Here are some specific examples. In the Uchinskoye reservoir (part of the Moscow water-supply system), molluscs filter at least two volumes of all water per annum. In the Volgograd reservoir, they filter 840 billion tons of water a year, i.e. the whole water body is filtered 24 times. In Lake Baikal filtering sponges purify coastal waters for 1.2 days. These figures show grandiose scale of the work of natural "laundry".

 
стр. 39

 

THEORY OF SELF-PURIFICATION

 

Our analysis of ecosystems resulted in establishing three key structural-functional blocks covering almost all hydrobiological mechanisms of self-purification of water bodies. First, the block in charge of filtration. It is composed of a number of groups of living organisms, in particular: invertebrate aquatic organisms-filter feeders; a group of littoral plants (macrophytes) catching a part of nutrients and pollutants getting to the ecosystem from the adjacent territory; benthos (organisms inhabiting the bottom of a water body) consuming a part of bio-genes (nitrogen, phosphorus) and pollutants migrating along the borderline of water bottom sediments; a group of microorganisms, attaching to fine particles suspended in water and extracting dissolved organic substances and biogenes from water. Secondly, the block of mechanisms of transmission, pumping over chemical substances from one section or part of the ecosystem (compartment) to another (from one medium to another), in other words, "pumps" as a part of self-purification mechanisms. In this block, some processes assist in transferring pollutants from the depth of water to bottom sediments, others from the depth of water to atmosphere through evaporation. Finally, the third block is responsible for disintegration of pollutant molecules-a kind of a "mill" grinding pollutants.

 

What are the power-supply sources for biotic mechanisms of self-purification of aqueous ecosystems? The key sources are: photosynthesis, oxidation of autochthonous (i.e. produced within the ecosystem) and allochthonous (i.e. imported from outside) organic matter, as well as other oxidation-reduction reactions. It is worth pointing out that power supply is partially ensured by oxidation of the components (dissolved and suspended organic matter) the system is getting rid of. In otherwords, nature employs a kind of power-saving "technologies".

 

Traditionally, self-purification is mainly associated with oxidation of organic substances by aerobic (oxygen consuming) organisms. Anaerobic (without participation of oxygen) processes the efficiency of which is ensured by transmission of electrons to acceptor molecules different from oxygen are no less important. Anaerobic energy ensures metabolism in microorganisms of different communities: methanogenic community (decomposing organic matter "produces" methane), sulphidogenic community (decomposition of organic matter results in the formation of hydrogen sulphide, hydrogen and methane), anoxygenic phototrophic (photosynthesizing) community (producing anion SO2-4, hydrogen dioxide, hydrogen and methane). It is worth mentioning that products produced by organisms of the abovementioned communities are further used as oxidation substrates by other communi-

 
стр. 40

 

ties, including organisms from the "bacterial oxidizing filter" group. The latter is already functioning in aerobic conditions and oxidizes hydrogen (hydrogen bacteria), methane (methanotrophs), hydrogen sulphide (sulphur bacteria), etc. High efficiency of microbial communities of bottom sediments, producing and consuming gases, was proved in the course of studies of geochemical composition of sediments of the World Ocean. Thus, these blocks of the self-purification system operate according to the principle of waste-free technology.

 

What kind of taxons (systematic groups of organisms) participate in self-purification of water ecosystems? Microorganisms, taxons of phytoplankton, higher plants, invertebrate animals, fishes, and other groups of organisms are among basic groups. Each group plays its own role and is engaged in more than one or two processes.

 

The process of filtration important for the water self-purification process is implemented by representatives of numerous taxons. For example, speaking of halo-plankton, the finction of fine filter-feeders-nanophages is carried out by invertebrates such as Appendicularia, Doliolidae, Calanoida, meroplankton (larvae), and other animals; the function of coarse filter feeders-euryphages is effected by Oithona, Oncaea, etc., i.e. omnivorous microorganisms.

 

Quantitative parameters characterizing the role of concrete processes vary from one ecosystem to another. For example, this is how different groups of organisms behave while extracting hydrogen from the ecosystem in the course of breathing (in percents of total breathing) of the community inhabiting the central part of the Sea of Okhotsk for a minimal population of phytoplankton in summer: macrophytes (large aqueous plants)-0.3 percent, phytoplankton-8.9 percent, bacteria-55.6 percent, microzooplankton-7.7 percent, peaceful zooplankton-12.2 percent, predatory zooplankton-4.45 percent, zoobenthos-8.3 percent, fishes-2.5 percent, mammals and birds-0.05 percent. In other regions these figures may vary. Thus, from 5 to 30 percent of the total production and total breathing of the heterotrophic (non-photosynthesizing) plankton inhabiting fresh and sea water, including bacteria, falls on protozoans.

 

Thus, to ensure efficient functioning of the self-purification process, almost all groups of organisms (pertaining to prokaryotes and eukaryotes) are useful, which confirms the generalization proposed by Acad. Vladimir Vemadsky: practically no group of organisms separated from the community is able to exist for a long time.

 

"REPARATION" MECHANISMS

 

How does nature ensure reliability of the self-purification system? As you know, regular operation of sophisticated mechanisms is provided by way of duplicating many of its elements. The same principle has been identified in the course of functional analysis of water ecosystems. For instance, the filtration activity of hydrobionts (aqueous organisms) is duplicated in such a way that it is carried out by two big groups of organisms-plankton (dwelling in the water column) and benthos (dwelling in the soil and in the soil of the bottom of reservoirs). Both groups filter water at a high speed. Besides, benthos additionally duplicates functions of planktonic organisms permanently living in the pelagic zone (a sea or ocean zone located somewhere not in close vicinity to the bottom) due to the fact that larvae of many benthic filter feeders lead a planktonic way of life. The plankton consists of two big groups of multicellular invertebrate filter feeders-crustaceans and rotifers-duplicating each other. But, in this case, nature has provided for one more additional "actor": this role has been assigned to a big group of organisms (Pro-

 
стр. 41

 

 

Surface-active substance dodecylsodium sulfate.

 

Surface-active substance "Triton X-100".

 

 

tozoa) characterized by somewhat different type of nutrition, as compared with crustaceans and rotifers.

 

The processes of fermentative decomposition of pollutants is partially duplicated by bacteria and fungi. The function of oxidation of dissolved organic matter is duplicated by almost all hydrobionts, more or less adapted to absorption and oxidation of the dissolved organic matter.

 

It should be noted that stability of any ecosystem is ensured by its self-regulation through interaction of relatively independent structural components. That is why we have reason to consider that self-regulating biota is one of the essential elements of reliability of the self-purification mechanism. Almost all organisms engaged in these processes are under a double control of the organisms of the preceding and subsequent trophic links of the nutrition chain. Different forms of signal systems, including chemical substances-carriers of information or regulatory signals-play an important role too. The author of this article proposed to call such substances natural chemoregulators and ecological chemomediators.

 

So, we can conclude that there are two important factors necessary to ensure reliability and stability of the water self-purification system in the ecosystem: plurality of constituent processes implemented to some exent at the same time and well-developed system of regulation and self-regulation of biota. In addition, by itself purification of water and steady regeneration of its quality are an important element of self-maintenance of stability of the whole water ecosystem. This contributes to continuous regeneration of normal conditions of natural habitats of plant and animal species. It is necessary to bear in mind that contamination of water is not only due to organic pollutants. For example, the amount of precipitating phosphorus is estimated to a considerable extent per 1 unit of water surface a year. That is why self-purification of water has the same importance for the ecosystem as DNA reparation (current repair of molecules of genetic material by cell enzymes) for heredity systems.

 

UNSAFE CHEMISTRY

 

In this context, it is expedient to point out that the reliability margin of self-purification mechanisms is limited. Based on the results of our experiments, we revealed a significant element of lability (instability) of one of them-filtration of water by invertebrate hydrobionts (molluscs, rotifers). It was established that these processes were inhibited under influence of sublethal concentrations of such anthropogenic pollutants as surfactants, contained in many detergents. For example, we established deterioration in the filtering capacity of Crassostrea gigas oysters under action of cationic surfactant tetradecyltrimethyl-ammonium bromide in concentrations of 0.5 mg/l and higher and anionic surfactant sodium dodecyl-sulphate in the same concentrations. In our other works we showed that nonionogenic surfactant Triton X-100 also (in concentrations of 0.5 mg/1 and higher) reduced filtration capacity of Mytilus eclulis mussels. It should be noted that anionic and nonionogenic surfactants form part of most popular household detergents-washing powders, liquid detergents, and shampoos.

 

It was established that surfactants and surfactant-containing mixed preparations can inhibit activity not only of filter feeders but also of invertebrates with different type of nutrition. Thus, Lymnaea stagnalis molluscs play a significant role in ecosystems, since they eat hydrophytes and excrete considerable quantity of non-assimilated organic matter. It rapidly sinks to the bottom, which accelerates the process of transfer of organic matter from upper layers of water to bottom sediments. It turned out that under the action of tetradecyltrimeth-

 
стр. 42

 

ylammonium bromide (2 mg/1), trophic activity (nutrition rate) of molluscs was inhibited by 27.9-70.9 percent. We also registered inhibition of mollusc activity by other pollutants, including, inter alia, not only powder detergents, dishwashing liquids, and shampoos, but also toxic chemicals, oil components, and heavy metals.

 

All these and other numerous data prove that chemical pollution of reservoirs and streams deteriorates self-purification mechanisms of water.

 

Our works have demonstrated that present-day priorities focused on preservation of water quality and ecological safety of water-supply sources are insufficient and already inadequate for the current situation. They should be revised and supplemented. What are disadvantages of the present approaches?

 

INADEQUACY OF PRESENT-DAY PRIORITIES

 

The main challenge declared today is to prevent discharge of pollutants to water in excess of the limits established by the list of maximum permissible concentrations (MPC). They were determined according to the methods worked out earlier and approved by relevant legal acts (including, for example, in the EU countries). If the discharge is within set limits, the target is hit, and water quality is supposed to be good. In fact, such priorities create a false sense of wishful thinking. Let me explain why.

 

The current danger criteria are based on three types of assessment: toxicity of a pollutant (heavy metal, toxic chemical, etc.) for planktonic algae, water fleas, fish; bio-accumulation capacity (accumulation of pollutants in the organism of water dwellers); biodecomposition capacity.

 

What do we mean by inadequacy of these goals? First, toxicity is assessed by the mortality rate of three groups of organisms (above-mentioned algae, water fleas, and fish). The threat of sublethal impacts (when no deaths are registered) is underestimated. Underestimated or ignored is also a negative impact on other organisms not related to these three groups.

 

The second problem is associated with bioaccumulation. It is stated that if a chemical substance is not accumulated in a body, it is practically not dangerous or relatively dangerous. But this approach fails to rightly assess a negative impact of such substance on cell receptors. It is possible even without accumulation in the body.

 

The third problem is biodecomposition of pollutants. It is assumed that if a pollutant is subject to rapid biodecomposition, it is not dangerous. But let's see how experts assess this capacity. Tests are carried out in laboratory conditions using retorts filled with microorganisms on the so called rockingchairs enabling cells to get more oxygen from water. In practice, biodecomposition is a result of oxidation of the pollutant by oxygen inside bacteria cells. In case of rapid oxidation (this seems to be good) the oxygen contained in water is also rapidly consumed, which is accompanied by an accelerated reduction of its concentration in water (which is already bad). This produces a negative impact on all aquatic organisms. Under such conditions, some aquatic bacteria start producing hydrogen sulphide. Accelerated decomposition of the pollutant leads to rottening of water. That means that finally all three estimates are inadequate.

 

Quality of water depends not only on getting of pollutants into the reservoir, but also on the efficiency of self-purification. As has been mentioned above, many anthropogenic pollutants have a negative impact on the capacity of organisms to filter water thus causing lowering of the quality of water. It is widely accepted that low concentrations of surfactants in water are admissible as they do not kill test organisms. But, according to our experiments, ecological peril still exists.

 

NEW DANGER CRITERIA

 

What measures should be taken to ensure in practice purity of water resources? To our point of view, in order to assess the danger level of this or that substance, it is necessary to assess not only three test groups (algae, water fleas, and fish), but also benthic organisms-filter feeders (molluscs). It is necessary to register the death rate of test organisms and decrease of activity of organisms to filter water. As for the criteria of assessment of the state of aquatic ecosystems, in addition to concentration of pollutants it is necessary to assess one more important factor-availability and biological activity of organisms in charge of water filtering. Clean water is a result of joint work of all aquatic organisms.

 

Present-day priorities focused on safety of water reserves is comparable with the attempts to treat a HIV patent from pneumonia with no regard to his immune system. It is an attempt doomed to failure. This is what we see-water quality is worsening everywhere irrespective of all attempts to reduce discharge of pollutants. If we want to keep water clean, we should take care of aquatic organisms.


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© Sergei OSTROUMOV () Источник: Science in Russia, №5, 2014, C.38-43

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