The salinity of a brackish habitat________

Analyze the question: A brackish habitat (such as an estuary or lagoon, e.g., the Lagos Lagoon or the Niger Delta creeks) is where fresh water from rivers mixes with salty sea water. The question asks what happens to its salinity (salt concentration).

Concept: Salinity depends on how much fresh water versus sea water is present, and on how much water evaporates.

• Rainy season: Rain and river runoff add large amounts of fresh water, which dilutes the salts, so salinity decreases.
• Dry season: There is little rainfall and river flow is low. Evaporation is high, and sea water pushes further into the habitat. Salts become more concentrated, so salinity increases.

Step-by-step evaluation of the options:

• A. Increases immediately after rainfall: Wrong. Rain is fresh water and dilutes the salts, so salinity falls.
• B. Increases at the end of the rain: Not reliable. Salinity only starts to rise gradually as the dry season sets in, and the option does not describe the real cause.
• C. Decreases with an increase in the number of organisms: Wrong. The number of organisms does not control salinity. Salinity is a physical factor, and it affects the organisms instead.
• D. Increases during the dry season: Correct. Evaporation is high and there is less fresh water input, so salts become concentrated.

Answer: D. The salinity of a brackish habitat increases during the dry season.

Exam tip: Remember "Rain dilutes, sun concentrates." Rainy season means low salinity, dry season means high salinity. Whenever you see a question on salinity, think of water input and evaporation first.