*3.2.3 Consequences of eutrophication*

The primary consequence of eutrophication is a decrease in the amount of dissolved oxygen in water. During photosynthesis, plants release oxygen into the water using sunlight. Conversely, in dark conditions, organisms, plants, and aerobic microorganisms consume dissolved oxygen during respiration. The balance between photosynthesis and respiration depends on the growth and population of the biomass. As a result of the high accumulation of biomass and the presence of an oxidation mechanism, underwater sediments form from the living mass, leading to a depletion of dissolved oxygen. Additionally, some decomposing bacteria consume oxygen from sulfate, resulting in the release of sulfur and the trapping of oxygen in the upper layers of the water. This reduction in oxygen levels endangers aquatic life. In the absence of oxygen, certain types of food can become toxic compounds, such as nitrate turning into ammonium, sulfate turning into hydrogen sulfide, and carbon dioxide turning into methane. These compounds are harmful to aquatic organisms.

**Figure 4.** *The role of phosphorus in eutrophication of water.*


#### **3.3 Evidences on the importance of phosphorus in aquatic ecosystems**

Experiments have shown that in lake systems, an increase in phosphorus relative to nitrogen leads to a significant increase in the number of cells of certain algae. This suggests that phosphorus is more limiting than nitrogen in this type of lake [67]. Additionally, evidence has shown that in cases of phosphorus deficiency in aquatic ecosystems, there is a direct correlation between the growth rate of algae (the rate of cell division) and the amount of phosphorus available within each cell [68, 69].

In 1972, Powers et al., conducted an experiment in which they placed 320 liters of water from Lake Organ and Minnesota in a closed environment and enriched it with various nutrients. By adding phosphorus, they observed a positive response from the system and concluded that phosphorus is the primary limiting factor. An experiment conducted by Schindler between 1974 and 1977 in the Ontario experimental lake clearly demonstrated the limitation of phosphorus. In this experiment, the lake water was saturated with phosphorus for several years, and the lake used atmospheric carbon and nitrogen to grow algae. The result of this work was an increase in primary nutrients, creating a eutrophic state in the lake water. An excess of phosphorus in the water acted as the trigger for the excessive bloom of cyanobacteria. When phosphorus, carbon, and nitrogen were added in deficiency, the effects were minimal [70].

In 1976, Vollenweider Weider created a model that could predict the eutrophic state in lakes and water reservoirs, with the only input element being phosphorus. This model was used worldwide and predicted the creation of the eutrophic state with very high accuracy. This model is in the form of Eq. 1:

$$Cl\_a = \frac{\left(L\_p/Q\_s\right)}{\left[1 + \sqrt{\frac{x}{Q\_s}}\right]}\tag{1}$$

These are: Cla is the algal biomass in units (mg/m<sup>3</sup> ), Lp is the input amount of phosphorus (g/m2 .d1 ) and Qs is the output amount of the lake per unit of lake area (m/Acre).

A group of researchers has discovered a sigmoidal relationship between the logarithm of the total phosphorus concentration in the summer and the logarithm of chlorophyll levels. As the total phosphorus concentration increases, the amount of

chlorophyll reaches a constant state, beyond which the amount of chlorophyll remains almost constant with further increases in phosphorus. This finding contrasts with the previously assumed linear relationship. In the case where the phosphorus concentration reaches its maximum value, adding more phosphorus to the water does not increase the chlorophyll concentration, while adding nitrogen intensifies the enrichment phenomenon [71]. It should be noted that as we move from freshwaters to coastal waters and oceans, the limiting element changes from phosphorus to nitrogen [72]. However, some researchers have challenged this idea [73]. Due to the high persistence of phosphorus in lakes, it is still considered the most limiting factor [70].

### **3.4 Different parts of phosphorus in water**

There are three main terms used to describe phosphorus in water: soluble reactive phosphorus (DRP), total phosphorus (TP), and phosphorus bound to suspended solids. DRP refers to the portion of water phosphorus that can pass through a filter with a pore size of less than or equal to 0.45 μm and is analyzed using a colorimetric method. This method only measures the readily available portion of phosphorus, known as soluble orthophosphate, which directly contributes to water enrichment for aquatic plants. However, it should be noted that the filter with a pore size of less than 0.45 micrometers sometimes fails to effectively separate phosphorus.

To obtain the total phosphorus content of water, unfiltered water is digested with a strong acid, and the phosphorus content is measured using a colorimetric method. During the digestion stage, polyphosphates and phosphates attached to organic materials, which cannot be measured using the colorimetric method, are converted to orthophosphate and measured. Subtracting DRP from TP yields the phosphorus bound to suspended solids in the water. It is important to note that suspended solids play a crucial role in the phosphorus cycle in water. Therefore, water analysis measures these three parts of phosphorus: DRP, phosphorus bound to suspended solids, and TP (which is the sum of the previous two parts).
