Question: Around areas of increased fertilizer runoff, all of the heterotrophs within a lake food chain may show an increase in numbers. After the initial increase, the populations begin to decline. Which event most likely caused the populations to decline?

  Options: 
 A. More predators were introduced into the lake.
 B. Water was depleted of nutrients such as nitrogen.
 C. Breeding season for the organisms came to an end.
 D. Decomposing algae lowered the amount of dissolved oxygen. 
Answer: D