Which of the following statements is not correct ?

The correct option (the statement that is not correct) is: Pyramid of biomass in sea is generally upright.

To understand why this statement is incorrect, let us analyze each of the ecological pyramids mentioned in the options:
1. Pyramid of Biomass in the Sea: Biomass refers to the total dry weight of living organic matter at a particular trophic level. In a marine ecosystem, the primary producers are phytoplankton. Although they are extremely numerous and highly productive, their individual size is microscopic, and their life span is very short. Consequently, their standing crop (biomass present at any given moment) is very small. In contrast, the consumers, such as zooplankton and small fish (herbivores) and large fish (carnivores), have longer life spans and accumulate a much larger biomass over time. This makes the pyramid of biomass in a sea inverted (with a smaller base of producers supporting a larger weight of consumers), rather than upright. Therefore, the statement claiming it is generally upright is false.
2. Pyramid of Energy: According to Lindeman's 10% law of energy transfer, only about 10% of the energy available at a lower trophic level is transferred to the next higher level. The rest is lost as heat during respiration and other metabolic processes. Because energy is continuously lost at each successive level, the pyramid of energy is always upright and can never be inverted.
3. Pyramid of Numbers in a Grassland Ecosystem: In a grassland, a very large number of grass plants (producers) support a smaller number of herbivores (such as grasshoppers and rabbits), which in turn support an even smaller number of carnivores (such as frogs and snakes), leading to a minimum number of top predators (like hawks). This results in a classic upright pyramid structure.