What could explain the curve in this population growth graph ?

"d.) carbon dioxide and hydrogen sulfide"

you can have just about every answer you can think of

bal is blood alcohol level. bac will normally be higher in a person with more body fat. so the answer to your question would be, as body fat decreases, the increasing rate of bal decreases.  

it is a limiting nutrient limited by a single nutrient in the ecosystem, & the single goes very slow.