Question

The excessive use of fertilizers in the field leads to the deficiency of an substance in water of a nearby lake.name the substance

Answer 1

$\textbf{First reason}$; Unwanted plants leads to a growth of Weeds forming wonderful mauve-colored Flowers appearing very appealing to ones eye and look like floating green flowers, in water bodies. Introduced for incorporating exotic flowers into a water body, these plants are generally more efficiently grown on water bodies, and, usually have faster capabilites to grow, further forming multiple species of itself, difficult to remove them and hinders the ability to use chemicals (affects marine and aquatic life), the clog the waterways and wreck havoc by depleting the oxygen content, that is, the substance which gets decreased, is $\textbf{Dissolved Oxygen}$ or $\textbf{Oxygen in Dissolved form}$.

One peculiar example would be of the world's more dangerous yet problematic weed $\textbf{Eichhornia crassipes}$ or in common language $\textbf{Hyacinth}$ this has been named (especially in India), as far as I remember, $\textbf{"Terror of Bengal"}$ (correct me if incorrect). Ability to grow in abundance in eutropic or eutrophic affected water bodies, this leads to a improper balancing in the ecosystem, and hence, changes the statistics of the food chain, Web, Dynamics, etc. of a water body.

$\textbf{Second Reason}$, The excessive use of fertilisers in the field will eventually run-off by the media of surface run-off, where all the nutrients including high amounts of fertilisers settle into the water surfaces and makes a breeding ground which are regarded as one of the most basic reproductions by planktonic or free-floating algaes, that include Planktons in major parts. Agrochemicals when added to crop fields, particularly they seep and find a water body, the Planktons cause increased nitrification, demand for oxygen supply, ammonia radicals that are unionised which is definitely toxic to aquatic species and production of excess in Planktons form Red-tides.

Named as $\textbf{Eutrophication}$ the natural aging of lakes are common, but, Pollution by nutrient-enriched components lead to $\textbf{Cultural Eutrophication or Accelerated Eutrophication}$ causes attributed to highly and severely eutrophied bodies by increase in agricultural and industrial wastes. Eutrophication on other hand is a slow process converting cold clean and clear water into warm water bodies which is supportive of a number of organisms by causing accumulation silt and organic matter formed at bottom of the water bodies, floating plants on surface of water and amphibious plants on the usual edges of water.

Now, this too leads to oxygen depletion and deficiency in the substance called $\textbf{Dissolved Oxygen}$. In this oxygen deficiencies promote the production of Hydrosulfide, toxicity is high in natural form when found in shallow waters, further leading to prevention of proper development and nourishment of eggs given by a fish at a spawn or fish spawn by which a organism releases multiple unfertilised eggs in deep waters and by which several or many males contribute by releasing many sperms at one time which can fertilise some of the unfertilised eggs. Accumulation of Macro-Sized Algaes in shallow waters cut-off the efficient reaching of a source (or sunlight) from submerged plants, henceforth, decreasing the supply of oxygen for replenishment inside the water.

At most, it leads to, $\textbf{Organic Loading}$ or simply stated, occurrence of excess in organic matter petrifaction (or the process of decaying substances or a rotting of a specific body or bodies of another organic matters or components present in the body or water bodies itself), this depletes even more Dissolved oxygen. Decomposition of fishes further depletes oxygen content and Rob remaining oxygen which is merely even present in minorities.

Decreasement and depletion of Dissolved Oxygen and increasing toxicity levels kills aquatic and marine life, exceptions would be eels and Hardy worms (adaptations adopted). In such an odd fashion, a lake is enough to choke every species to its ultimate death.

$\textbf{Anaerobic Putrefaction}$ are the main applicable thieves to produce pollutants, such as, methane, hydrogen sulphide, carbin dioxide, nitrogen and ammonia, they are effective in clearing out Bloom forming plants, but, it accompanies of strong unpleasant odours or foul smell, brown colouration, bad taste and formation of oils layer by layer. Cellular metabolism in a organism (at the stages when it's alive) and aerobic microbes contribute further decay. The body appears to be bloated and organism starts to degenerate by tissue liquefaction and total disintegration, prime reason of strong odours.