What compound, when present in excessive amounts, can stimulate growth in aquatic plants and contribute to blue baby syndrome?

The correct choice is nitrogen. In excessive amounts, nitrogen can indeed stimulate the growth of aquatic plants, a process known as eutrophication. This phenomenon occurs when nutrient runoff, often from fertilizers or waste, enters water bodies, leading to an overgrowth of algae and aquatic plants. This excessive growth can degrade water quality and disrupt aquatic ecosystems.

Additionally, high levels of nitrogen compounds, particularly nitrates, in drinking water sources can pose health risks. One notable concern is blue baby syndrome, or methemoglobinemia, which affects infants. This condition occurs when nitrates are converted to nitrites in the human body, which then interfere with the ability of blood to carry oxygen, resulting in a bluish discoloration of the skin.

Phosphorus is also a contributor to eutrophication, but it does not specifically relate to blue baby syndrome. Manganese and fluoride, while they can have various impacts on health and water quality, are not as directly linked to either excessive aquatic growth or the syndrome in question. Thus, nitrogen is the most relevant answer regarding both aquatic plant stimulation and the health issue mentioned.