Addition of phosphate fertilizers to water bodies causes:

Phosphate fertilizers contain phosphates, which are essential nutrients for plants. When these fertilizers are added to water bodies, such as lakes or rivers, the phosphates dissolve in the water and become available to aquatic plants.

Algae are simple aquatic plants that thrive in nutrient-rich water. Phosphates act as a key nutrient for algae, promoting their rapid growth and reproduction. This process is known as eutrophication.

As algae multiply, they form dense blooms on the water surface. These blooms block sunlight from reaching deeper parts of the water body, preventing other aquatic plants from photosynthesizing. When these plants die, they sink and decompose.

Decomposition consumes dissolved oxygen in the water. This oxygen depletion harms fish and other aquatic life, leading to a decrease in biodiversity.

Now, evaluating the options:

A. Deposition of calcium phosphate: While phosphates can react with calcium to form insoluble calcium phosphate, this is not the primary or most significant effect of phosphate addition in most water bodies.

B. Enhanced growth of algae: Directly supported by the explanation above, as phosphates stimulate algal growth.

C. Increase in fish population: Contradicts the outcome, as oxygen depletion from decomposition reduces fish populations.

D. Increase in dissolved oxygen: Incorrect because decomposition processes decrease oxygen levels, and algal respiration at night further depletes oxygen.

Hence, the correct answer is Option B.