How do dissolved oxygen and pH relate to aquatic life health?

• A. Adequate dissolved oxygen supports respiration; most aquatic life thrives near neutral pH
• B. Only salinity determines aquatic life health
• C. High pH alone guarantees healthy ecosystems
• D. DO and pH have no effect on aquatic life

Answer: (A)

Dissolved oxygen and pH are the water quality factors that most directly influence how well aquatic life can breathe, grow, and carry out normal bodily processes. Dissolved oxygen is needed for aerobic respiration in fish, invertebrates, and many microorganisms. When oxygen levels are adequate, cells can efficiently generate energy, activities proceed smoothly, and health remains good. When oxygen is depleted, organisms experience stress, reduced growth, and, at very low levels, death. Oxygen solubility in water decreases as temperature rises and as water becomes more polluted with organic matter that uses up oxygen during decomposition, so warm or dirty waters are especially risky.

pH affects how chemicals behave in water and how enzymes work inside organisms. Most aquatic life functions best around a neutral pH because enzyme systems operate optimally and chemical forms of nutrients and toxins are balanced. Extreme pH levels can disrupt metabolism, damage gill membranes, and change the toxicity of substances like ammonia, which becomes more harmful at higher pH. Metals also become more toxic at certain pH ranges, further stressing organisms.

So the best choice links respiration needs to adequate DO and recognizes that near-neutral pH supports most aquatic life, while the other options ignore these critical interactions or overstate a single factor.