In an aquatic system, the presence of noxious gases such as H₂S and CH₄ is associated with

Correct option is B

Introduction
· In aquatic ecosystems, the decomposition of organic matter is a primary process that dictates the chemical environment of the water column and sediments.
· When dissolved oxygen is plentiful, aerobic bacteria break down organic waste into harmless by-products like carbon dioxide and water.
· However, when the demand for oxygen exceeds its supply, the system shifts from an aerobic (oxygen-rich) state to an anaerobic (oxygen-depleted) state.

Information Booster

Oxygen depletion is the correct answer because noxious gases like hydrogen sulfide (H_2S) and methane (CH_4) are the signature products of anaerobic respiration. In the absence of free oxygen (O_2), specialized anaerobic bacteria use alternative electron acceptors such as sulfates ($$SO_4^{2-}$$​) and carbon dioxide.

This biochemical pathway results in "septic" conditions, where the water develops a foul odor (rotten eggs due to H_2S) and bubbles of "marsh gas" (CH_4) rise to the surface, signaling a severe lack of dissolved oxygen.

· Methane (CH_4) is produced by methanogenic archaea during the final stages of anaerobic decay in the deep, oxygen-free sediment layers of lakes and wetlands.
· Hydrogen Sulfide (H_2S) is highly toxic to fish and other aquatic life; its presence indicates that the Redox potential of the water has dropped significantly.
· Oxygen depletion often occurs in the "Hypolimnion" (bottom layer) of stratified lakes during summer or in water bodies receiving heavy organic pollution (high BOD).
· These noxious gases contribute to the "internal loading" of nutrients, which can further accelerate the degradation of the water body.
· The transition from aerobic to anaerobic conditions is a critical indicator used in wastewater treatment and limnology to assess the health of an aquatic system.
Additional Information
· Abundant algal growth, or eutrophication, is often the cause of oxygen depletion (as dead algae decompose), but the presence of H_2S and CH_4 is the direct result of the subsequent lack of oxygen, not the growth itself.
· Absence of macrophytic vegetation might be a consequence of poor water quality or high turbidity, but it does not chemically trigger the production of methane or hydrogen sulfide.
· Excess of dissolved oxygen is the polar opposite of the required condition; in an oxygen-rich environment, these noxious gases would be oxidized (e.g.,$$CH_4to CO_2 and H_2S to SO_4^{2-}$$​ and would not persist.