In an aquatic food chain:

• Phytoplanktons are primary producers ($T_1$).
• Zooplanktons are primary consumers ($T_2$).
• Small fishes feeding on zooplankton are secondary consumers ($T_3$).
• human consuming fish acts as a tertiary consumer, occupying the fourth trophic level ($T_4$).

The pyramid of biomass in sea and ocean ecosystems is generally inverted because the biomass of primary consumers (zooplankton and fish) at any given time far exceeds the standing biomass of primary producers (phytoplankton with very rapid turnover rates). Note that the pyramid of energy is always strictly upright.

Humification leads to the accumulation of a dark-coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Humus serves as a rich reservoir of nutrients in the soil.