The water bodies of the earth are being continuously polluted by a variety of sources since time immemorial as all the human habitation and activity developed along rivers, lakes sea coasts and other such water bodies. This problem is increasing rapidly today with growing industrialization and urbanization. Industrial and domestic wastes are indiscriminately allowed to enter water bodies. Even if such wastes are deposited on earth, pollutants are found to be washed away to water bodies. Further, pollutants gradually seep down into the soil and reach underground water bodies, from where they reach the water overground through pumped out water. The pollution is occurring in all types of water bodies; both freshwater bodies like ponds, lakes and rivers as well as marine bodies like coastal and deep-water seas. Major causes of water pollution are deposition of acid, organic sewage, detergents, agricultural chemicals, industrial effluents, silt, oil and heat into the water bodies. Though effects of water pollution on fauna are more visible and dramatic, effects on flora are no less important from the point of view of ecosystem.
Effects of acid deposition
Various acid gases, aerosols and other acidic substances released into the atmosphere from the industrial or domestic sources of combustion of fossil fuels eventually come down to the ground. These substances are deposited directly on the water bodies. In addition, these substances also reach the water bodies along with run-off rainwater from the polluted soil. Deposition of acidic substances causes acidification of water by lowering its pH below 6.0. The sulphates, nitrates and chlorides have been reported to make water bodies like lakes, rivers and ponds acidic in many countries.
- Nutrient deficiency in aquatic ecosystem: The decomposing bacteria and fungi decrease in acidified water. This reduces the rate of decomposition of organic matter and, therefore, the nutrient cycling in the aquatic ecosystem. Thus, low pH causes nutrient deficiency and consequent general reduction in abundance of aquatic plants in the affected water body.
- Decrease of species diversity: Critical pH for most of the aquatic species is 6.0. The number and variety of aquatic species in the water body generally decreases below this pH.
- Change in species composition: The number and abundance of acid tolerant species increases while that of sensitive species decreases.
In the initial phase of water acidification, filamentous algae grow very fast and form thick mats. However, most of the diatoms and green algae disappear below the pH 5.8. Diatoms and small siliceous phytoplankton populations are highly sensitive to pH changes and species composition of their communities shows highly specific changes with pH change of the water body. Among green algae, Cladophora is highly acid tolerant species and becomes abundant in highly acidic freshwater bodies. Euglena and some other unicellular algae are found up to pH of 1.6 while Chlamydomonas acidophila is found in water up to pH of 1.0.
Macrophytes are generally absent in extremely acidic water. Potamogeton pectinalis is only aquatic macrophyte found in heavily acdified water. At pH lower than 4.0, angiosperm species using dissolved carbon dioxide e.g. Juncus bulbosus, Juncus effusus, Sparganium emersum, Gyceria fluitans, Eleocharis acicularis, Typha latifolia and bryophytes like Polytrichum, Anisothecium, Fontanalis, Catharina become the only survivors. The roots of macrophytes are generally affected adversely in acidic water and result in poor plant growth. Plants with deep roots and rhizomes are less affected while plants with short root systems are severely affected. Yellowing of plants is common in polluted water.
Effects of organic matter deposition
Large amounts of dead and decaying animal and plant material, fecal material and other organic material is deposited directly from sewage discharges or is washed along with rainwater into the water bodies. Most important consequnces of such organic matter deposition are as following.
- Increase in decomposer microbes: Increased addition of organic matter into the water body results in rapid multiplication and increase in decomposer aerobic and anaerobic bacteria. The rapid decomposition of organic matter by these increases nutrient availability in the water.
- Eutrophication: Addition of organic matter and its rapid decomposition resulting in increased nutrient supply causes much nutrient enrichment (eutrophication) of water body. In such a condition, planktonic green and blue-green algae grow very rapidly causing water blooms. In addition to these many types of hydrophytes like Salvinia, Azolla, Eicchhornia etc. also become abundant. All this rapid growth of planktonic and free-floating hydrophytes reduces light penetration into deeper layers of water body and submerged flora gradually declines. Abundant flora after death further increases supply of organic matter in the water body.
- Oxygen depletion: Rapid decomposition of organic matter by aerobic bacteria during eutrophication phase consumes much water-dissolved oxygen. On the other hand, gradual decrease of submerged aquatic flora results in reduced oxygenation of water. Both these phenomena together result in increase in biological oxygen demand (B.O.D.) of the water. Biological oxygen demand (B.O.B.) of water is defined as the amount of oxygen needed by a unit volume (usually one litre) of water sample to completely decompose the organic matter present in it by microbial activity, measured at 20oC and tested at least five days after sampling. The B.O.D. value of fresh, unpolluted water is usually below 1 ppm while in organic matter polluted water, it may be more than 400 mg/litre.
Effects of detergent deposition
Various detergents from domestic or industrial use directly released or washed down into the water bodies cause serious effects of plants.
Most of the domestic and industrial detergents contain high (up to 40%) phosphate content. Addition of such detergents into water results in phosphate-enrichment of water.
Most of the detergents that are toxic, enter the plants through roots or surface absorption. Common effects of detergents on plants are as follows.
- Retardation of plant growth, root elongation, carbon dioxide fixation, photosynthesis, cation uptake, pollen germination and growth of pollen tubes.
- Destruction of the chlorophylls and cell membranes.
- Alteration of the absorption maxima of chlorophylls.
- Binding of membrane lipids and proteins.
- Denaturation of proteins and thus causing enzyme inhibition in various metabolic processes.
- Non-degradable alkyl benzene sulphonates and phosphate-rich detergents interfere with gaseous exchange even in very low concentrations.
- Cation-active compounds hinder algal growth between 0.1 and 10.0 ppm while non-ionic compounds hinder algal growth between <1.0 and around 10,000 ppm concentrations depending upon the species and the compound.
- Macrophytes are most sensitive to damage by anionic surfactants.
Effects of agricultural chemical deposition
Many chemical fertilizers, pesticides, insecticides, herbicides etc. are applied to crops far in excess. These excess chemicals are washed away with rainwater, first into the soil then finally into the water bodies.
Chemical fertilizers entering the water bodies result in eutrophication by enriching the water with major plant nutrients. Many of these fertilizers are acidic in nature e.g. ammonium. These cause acidification of water.
Pesticides, herbicides and insecticides also cause pH changes in the water bodies. The effects of these plants on aquatic plants are similar to those of their overdose in foliar application. The herbicides act directly on aquatic flora but insecticides act indirectly by allowing algal blooms to develop in the water body. Different substances have different patterns of their toxic action, decomposition pathways and environmental persistance. Most common effect of these substances is reduced photosynthesis. Some may uncouple oxidative phosphorylation or inhibit nitrate reductase enzyme. The uptake and bioaccumulation of these substances in aquatic plants is great due to their low solubility in water.
Effects of industrial effluent deposition
Various inorganic and organic waste products from industries and mining activities are directly deposited into the water bodies. A large amount of these substances deposited on the soil, comes into water bodies indirectly along with surface run-off. Fly-ash, various organic/liquid effluents and heavy metals e.g. Hg, Cu, Cd, Pb, Zn, Ni, Ti, Se etc. are important industrial pollutants of water.
- Fly ash forms thick, floating covering over the water surface. This reduces the penetration of light into deeper layers of water body. Fly-ash increases the alkalinity of water and thus, causes reduced uptake of essential bases. All these phenomena cause death of aquatic plants.
- Organic/liquid effluents disturb the pH of water and depending upon their chemical composition, cause specific toxicity effects on the aquatic plants. Change in the floristic composition of the water body is most obvious and direct effect of pollution by such effluents.
- Heavy metals usually occur together and with many other types of pollutants so the effects of single metal are usually difficult to interpret. There may be synergistic, additive or antagonistic interactions between metals regarding their effects on plants. Impact of metals is reduced in hard, well-buffered freshwater systems. For example, Cd-uptake by Nitella and Elodea is less in hard water, Zn-toxicity is less with high Ca for Stigeoclonium and Hormidium while is less with high pH for Hormidium. Bioaccumulation of metals is more in mosses than in angiosperms and is usually more in lower plant parts. Chelators decrease while methylated forms increase the metal toxicity to aquatic plants. Reduced oxygen and low temperature also increase metal toxicity. While bryophytes appear to be highly resistant to heavy metal toxicity, in all classes of algae, strains vary in tolerance to metals. Photosynthesis and growth in most of the algae is inhibited at 1-2 ppm of Cu++. Cholrella is more sensitive to Cu++ than Scendensmus. Cholrella is retarded more by Ag than by Cd, Hg or Ni while cell division in the genus is reduced more by Cd than by Cu or Hg at 0.32 ppm. Lemanea is quite resistant to Zn and Pb.
Effects of silt deposition
The top soil removed due to erosion is carried with rainwater or flood water and deposited into the water bodies causing silt deposition in them.
- The deposition of silt increases the turbidity of water and reduces light-penetration deep into the water, causing decline in submerged flora.
- Silt deposition, in general, inhibits growth of aquatic plants. Phytoplankton is particularly affected by silt deposition due to reduction in surface exchange of gases and nutrients.
- Species tolerant to turbidity (e.g. Ceratophyllum demersum, Lemna minor agg., Nuphar lutea, Polygonum amphibium, Sagittaria sagittifolia, Scirpus lacustris) becomes highest followed by species of intermediate tolerance (e.g. Callitriche spp., Myriophyllum spicatum, Potamogeton natans, P. pectinatus, Sparganium emersum, S. erectum) while least tolerant species (e.g. Elodea canadensis, Potamogeton perfoliatus, Ranunculus spp., Mosses) are much reduced.
Effects of oil deposition
Washing of oil tankers and storage containers in many rivers, large lakes and particularly near sea coasts causes deposition of oil slicks on water. Oil spills in water are also common during normal transport operations or during accidents involving oil tankers.
- Oil pollution of water body prevents oxygenation of water.
- Oil depletes oxygen of the water body by consuming dissolved oxygen in oil degradation.
- Oil inhibits planktonic growth and photosynthesis in aquatic macrophytes.
- Oil may even cause destruction of aquatic flora if it catches fire.
Effects of waste heat deposition
Many industries, particularly thermal power plants, take water from rivers, lakes or sea to cool the heat-producing boilers and equipment. The heated water is then returned to the water body. The deposition of waste heat into the water body has many consequences for the plants in it.
- The solubility of oxygen in water is reduced at higher temperature. Thus, the reduced oxygenation of water adversely affects the aquatic flora.
- Reduced oxygenation and high temperature of water causes reduction in the activity of aerobic decomposers. The reduced decomposers result in decreased organic matter decomposition and consequently, reduced nutrient availability in the water body.
- In high temperature, aquatic plants show increased respiration, reduced photosynthesis and general inhibition of enzyme activity with increasing temperature.
- The aquatic flora and primary productivity of the aquatic ecosystem declines with increasing temperature. Green algae are mostly replaced by blue-green algae, which have comparatively less primary productivity.
- With increase in temperature, species diversity of the water body declines and heat-tolerant species gradually become dominant.