Consider the following statements about Eutrophication.

1. It decreases the Biochemical Oxygen demand of water.
2. It increases the biomass of phytoplanktons.

Which of the above statements is/are correct?
a) 1 only
b) 2 only
c) Both 1 and 2
d) None of the above

Ans – [B] 

Explanation – Eutrophication occurs when large quantities of nutrients such as nitrates and phosphates enter an aquatic environment. Sources of these nutrients include animal wastes, agricultural runoff, and sewage. The ecosystem quickly experiences an increase in photosynthetic and blue-green algae, as these organisms thrive in the presence of the added nutrients. An algae bloom occurs as the algae accumulates into dense, visible patches near the surface of the water, prohibiting light from penetrating deeper areas of lake or stream. This can be a problem in marine habitats:

• This causes phytoplankton to grow and reproduce more rapidly, resulting in algal blooms. Hence statement 2 is correct. Biomass of phytoplanktons gets increased.
• This bloom of algae disrupts normal ecosystem functioning and causes many problems.
• The algae may use up all the oxygen in the water, leaving none for other marine life. This results in the death of many aquatic organisms such as fish, which need the oxygen in the water to live.
• The bloom of algae may also block sunlight from photosynthetic marine plants under the water surface.
• Some algae even produce toxins that are harmful to higher forms of life. This can cause problems along the food chain and affect any animal that feeds on them.

Some fish are unable to survive due to this, but for them an even more serious problem arises when the algae begin to die. At this point, oxygen-demanding bacteria take over the ecosystem, decomposing the algae and using up dissolved oxygen in the process. These bacteria increase the biological oxygen demand (BOD) of the ecosystem. BOD is the amount of oxygen required for the decomposition of organic compounds by microorganisms in a given amount of water.