Why are krill numbers dropping?

Krill feed on microscopic phytoplankton ("phyto" - plant) that are extremely abundant in Antarctic waters due to the great upwellings of deep waters at the Antarctic convergence. These upwellings bring with them great amounts of dissolved nutrients, especially nitrate and phosphate that fertilize the microscopic but hugely abundant phytoplankton in the same way that a farmer puts fertiliser on the fields. Add to this the 24 hour intense sunlight of the summer months and the scene is set for a super-abundance of lifeType your answer here...