Question

Organic matter is considered as a major source of water pollution caused by wastes of food, animal and human excreta, garbage etc. The excess of organic matter in water causes a threat to aquatic life because (a) the space available to aquatic life decreases (b) microorganisms consume dissolved oxygen to decompose organic matter (c) organic matter is swallowed by small animals (d) decomposition of organic matter increases the temperature of water.

Short answer

The excess of organic matter in water causes a threat to aquatic life because (b) microorganisms consume dissolved oxygen to decompose organic matter.

Step-by-step solution

Identify the correct statement

In a water body, the presence of excess organic matter leads to an increase in the populations of microorganisms that decompose organic matter. These microorganisms require dissolved oxygen for their metabolic processes, hence they consume the oxygen available in the water.

Understand the effect on aquatic life

When microorganisms consume more dissolved oxygen to decompose the excess organic matter, less oxygen is available for aquatic life. This can stress or even kill fish and other aquatic organisms which require oxygen from the water to survive.

Eliminate incorrect options

Options (a), (c), and (d) present scenarios that are either unlikely or less directly related to the specific impact of organic matter on dissolved oxygen levels and thus on aquatic life. (a) suggests a physical space limitation which is not directly related to organic pollution. (c) presents a scenario of ingestion that is not universally applicable for all aquatic animals with respect to organic pollution. (d) suggests a thermal effect, however, the primary issue with organic matter is related to oxygen depletion, not temperature increase.

Key concepts

Dissolved Oxygen Depletion

Understanding the role of dissolved oxygen in aquatic ecosystems is pivotal when discussing water pollution by organic matter. Dissolved oxygen is just like it sounds - oxygen that is dissolved in water, an essential requirement for the survival of fish, invertebrates, and other aquatic organisms. When a body of water receives an overabundance of organic matter, like food waste or excreta, it becomes a feast for bacteria and other microorganisms. These decomposers break down the organic material, a biochemical process requiring oxygen.

The crux of the problem lies in the fact that these microorganisms consume oxygen at a rate that can surpass the water's natural replenishment capacity. This can lead to a significant reduction in dissolved oxygen levels, which scientists refer to as dissolved oxygen depletion. The knock-on effect on aquatic life can be severe; oxygen levels can drop to a point where it becomes insufficient to support the resident organisms, leading to a condition known as hypoxia or, in extreme cases, anoxia where no oxygen is present at all.

To depict the severity, imagine a crowded room where everyone is trying to breathe but there's not enough air; a similar scenario unfolds under water leading to distress among aquatic creatures. Hence, dissolved oxygen depletion is a key indicator of water quality and the health of aquatic ecosystems.

Aquatic Life Impact

The impact of water pollution on aquatic life is a cascade of detrimental effects triggered by factors such as dissolved oxygen depletion. Aquatic organisms, from the smallest invertebrate to the largest fish, depend on a delicate balance within their ecosystem. Oxygen is the cornerstone of this balance.

When the dissolved oxygen levels fall, aquatic animals experience stress, which can manifest in various ways. Behavioral changes, stunted growth, decreased immunity, and in extreme situations, mass die-offs are all possible outcomes of this environmental stressor. When the oxygen level becomes critically low, the environment can no longer sustain life, leading to the creation of 'dead zones' where aquatic life is virtually nonexistent.

Conserving Biodiversity

Conserving the variety and variability of life, known as biodiversity, is pivotal for the resilience of aquatic ecosystems. With the reduction in oxygen levels, species that are not tolerant of low oxygen conditions disappear, leaving behind a less diverse and often less resilient ecosystem.

Ripple Effect on Food Web

The reduction in aquatic species not only affects the organisms themselves but also has a ripple effect on the entire food web. Predatory species lose their food sources, and any species that rely on the aquatic environment for breeding or part of their life cycle may suffer. It is clear that maintaining adequate dissolved oxygen levels is essential for preserving the intricate web of aquatic life.

Organic Waste Decomposition

Organic waste decomposition in water bodies is part of a natural and essential process. Microorganisms, such as bacteria and fungi, play a beneficial role in breaking down dead material and fecal waste, converting it into simpler substances that can be utilized as nutrients by aquatic plants, promoting the cycle of life in these ecosystems.

However, when an excessive amount of organic waste enters a body of water—be it from agricultural runoff, untreated sewage, or industrial discharges—the balance is tipped. The decomposition process becomes accelerated and the demand for dissolved oxygen increases. This is where the beneficial process turns into a pollutant.

Nutrient Overload

Excessive nutrients, particularly nitrogen and phosphorus from decomposing organic material, can fuel the growth of algae in a phenomenon known as eutrophication. Large algae blooms can block sunlight from reaching underwater vegetation, further depleting oxygen levels as these plants are unable to photosynthesize and produce oxygen.

By-Products of Decomposition

During decomposition, apart from nutrient release, other by-products such as ammonia or sulphide can be harmful to aquatic life. These compounds can directly poison the habitat, making it even more unfriendly for the ecosystem's inhabitants.

It is important to manage organic waste inputs in the aquatic environment to prevent such dire environmental consequences. Proper sewage treatment, agricultural practices that reduce runoff, and regulations on industrial discharges can play pivotal roles in controlling organic waste decomposition rates, preserving water quality and protecting aquatic biota.