Which chemical is beneficial in correcting the problem that causes a facultative pond to change from deep bright green to dull gray?

• A. Ferric chloride
• B. Ferric nitrate
• C. Sodium nitrate
• D. Sodium chloride

Answer: (C)

A key idea here is that the color of a facultative pond reflects the balance of nutrients available for algae. A deep bright green indicates healthy algal growth, while a dull gray suggests the algae are nutrient-limited, particularly lacking nitrogen, so their activity wanes and the color fades.

Sodium nitrate provides an immediate source of nitrate, a form of nitrogen that algae need for growth and photosynthesis. By supplying nitrate, you remove the nutrient limitation, allowing algae to proliferate again and restore the green color. The other options don’t help in this way: ferric compounds tend to bind phosphorus and remove nutrients, reducing algal growth; sodium chloride mainly changes salinity without adding usable nitrogen; thus sodium nitrate best addresses the nutrient deficiency causing the color change.