**5.2 Causes of eutrophication**

The term "eutrophic" has been derived from the Greek words eu meaning "well" and trophe meaning "nourishment." Eutrophication refers to the abundant growth of phytoplanktons causing imbalanced primary as well as secondary productivity with a high rate of succession from the existing seral stage to a higher seral stage as a result of nutrient enrichment from fertilizer runoff and humans waste. It takes place at the point when a water body moves toward becoming enriched in key-limiting nutrients, such as nitrates, phosphates, and initiating symptomatic changes, including the expanded production of algae (**Figure 1**). Nitrogen (N) and phosphorus (P) are present in all aquatic ecosystems in some limited amount and are considered as an essential nutrient for the biological growth of organisms. Phosphorus being a macronutrient is essential for all living cells as it is an important constituent of adenosine diphosphate, adenosine triphosphate, nicotinamide adenosine dinucleotide phosphate, nucleic acids as well as phospholipids in the cell wall. Phosphorus is stored as polyphosphates in intracellular volutin granules in prokaryotes as well as eukaryotes. Both N and P are essential nutrients that are required by plants and animals for maintaining their growth and metabolism. However, in wastewater, these essential nutrients are available in abundant as phosphates, combined organic nitrogen, nitrates, and ammonia. On discharge into some receiving water body, their increased concentration can initiate

**Figure 1.** *Eutrophication process [62].*

eutrophication with several adverse consequences on the ecological health of the water body [63, 64]. Eutrophication is a natural phenomenon that takes thousands of years to occur in water bodies such as lakes, rivers, and reservoirs. However, an increased rate of nutrient input as a result of anthropogenic activities initiates the process of completing it within a short time period, which is referred to as artificial or cultural eutrophication [65]. Natural eutrophication pushes the succession from open water lake to the marsh to the meadow to the forest, which may take place anywhere within a time period of 500–10,000 years or more depending on the initial condition of that area. Human activities accelerate the rate at which the influx of nutrients into the ecosystems takes place. Runoff resulting from agriculture, urban, and industrial development, mainly from septic systems, sewers, and other human-related actions, increases the rate of entry of both inorganic nutrients and organic substances into aquatic ecosystems.

## **5.3 Nutrients in aquatic ecosystems**

The minimum acceptable concentration of total inorganic phosphate in water is 0.03–0.04 mg l−1 and in many lakes, streams, and rivers where the problem of eutrophication is found to occur and its value has been found to increase by 20–25 times during the past 10–15 years especially in cities and industries. Around 60% of the phosphate present in the waterways of the US is contributed from domestic sewage. Phosphate is also contributed from mines, fertilizer runoff, and domestic sewage containing a high concentration of phosphate with about 50% resulting from human waste and 20–30% from detergents. Animal wastes are also rich in nitrate as well as phosphates [66]. Phosphorus resulting from agriculture runoff is the major source of phosphorus loading in riverine sediments, which is being utilized by benthic algae and rooted plants. Eutrophication has become a major concern in many developed as well as developing countries, especially in highly populated countries such as India, China, Bangladesh, Indonesia, and Pakistan. Lakes as well as reservoirs of several industrialized countries of Europe and North America including the Great Lakes of USA and Canada are facing severe threats due to eutrophication. Several lakes of Asia (54%), Europe (53%), North America (48%), South America (41%), and Africa (28%) are eutrophic. As compared to point source pollution, management of diffuse sources is far more challenging due to the difficulty in controlling nutrients contributing from runoff arising from agricultural and urban areas. Most of the phosphorus enters to water body *via* runoff and erosion taking during winter storm events. Thus, phosphorus influx from diffuse sources may be of little significance in the eutrophication of rivers due to the fact that the timing of the transfers does not usually overlap with the period of maximum biological demand. On the other hand, phosphorus being a significant element in the process of eutrophication needs to be identified and quantified from various sources during periods of low flow. Symptoms of eutrophication mostly take place during the plant growing season, that is, spring and summer, when there is a low flow, high water residence times, abundant sunlight light levels, and water temperature is on the higher side, which cause fast algal growth. During the growing period, phosphorus originating from point discharge in rivers is a source of high concentrations of dissolved, bioavailable phosphorus fractions into the water body. According to Meybeck [67], streams and rivers around the world have nearly doubled their concentration of nutrients that is, nitrogen and phosphorus, with local increases of about 50 times. Overall, cultural eutrophication of river ecosystems is a global phenomenon that has, during the past few years, gained much less attention than lake eutrophication. This may be partly due to the effects of increased nutrient concentrations in rivers that are least affected because some factors apart from the

*Implications of Sewage Discharge on Freshwater Ecosystems DOI: http://dx.doi.org/10.5772/intechopen.100770*

nutrients limit algal growth. Although some progress has been made, still there is a less conceptual understanding of eutrophication in rivers and streams. Hydraulic flushing of nutrients, water velocity, and light limitation are indeed significant in regulatory algal growth interacting in several ways. Moreover, short residence time in rivers (<3 days) will have different effects in comparison with longer residence time in impounded rivers or riverine lakes (>3 days). In comparison with lakes (>30 days retention time) and considering some of the factors mentioned above, Hilton [68] devised a conceptual model of how the process of eutrophication takes place in rivers. Since natural streams are net heterotrophic, Dodds [69] formulated the trophic state of rivers into autotrophic, nutrient controlled, and heterotrophic, external carbon-regulated state. The autotrophic state in lotic water bodies is mostly dependent on phosphorus and nitrogen values. Algal biomass is positively correlated with gross primary production in streams and rivers. Eutrophication is a problem that is persistent worldwide. In Spain for example, 80% of the lakes, 70% of the reservoirs, and 60% of the river sites were eutrophic in the 1990s with hypertrophy increasing downstream [70]. There may be several deleterious effects of eutrophication on the environment, which have adverse consequences on the health of the exposed animal population apart from humans through several pathways. Certain health risks appear when extracted freshwater from eutrophic water bodies is supplied for drinking purposes. A severe impact can also occur during animal watering from eutrophic waters.
