why biomass increases as the level of trophic level increases ?

As we go up the trophic level, the biomass decreases.

For example - The biomass of each tertiary consumer will usually be larger than those of the organisms in the lower trophic levels, since they have to feed on their prey. But since their (tertiary consumers) number is much less than the organisms in the lower trophic levels, their combined biomass is smaller than those of the trophic levels below it. This is because only 10% of the energy is obtained from the previous trophic level.

(A census of the population, multiplied by the weight of an average individual in it, gives an estimate of the weight of the population. This is called the biomass)