in the ocean, zooplankton feed on diatoms. why does the diatom mass increase in april and may? why does the zooplankton mass take until july and august to reach its max point?

During April and May, the inflow of the nutrients allows diatoms to grow, multiply and flourish. Their population increases rapidly during this time.
In June-July, strong upwelling currents take diatoms to the shore and their population starts declining. Further, other organisms start eating diatoms and their population further declines.