Question

what can happen if fertilizer runoff causes algae to grow too fast in water?
algae die, bacteria break it down, bacteria use up oxygen, fish die
water becomes safer for drinking
groundwater recharge improves
fish population increases rapidly

Explanation:

When fertilizer runoff causes algae to grow too fast (algal bloom), algae eventually die. Bacteria break down the dead algae. During decomposition, bacteria use up oxygen (in a process called eutrophication). Lack of oxygen (hypoxia) leads to fish death. Water with algal bloom is not safer for drinking (as algae can produce toxins in some cases). Algal bloom doesn't improve groundwater recharge. And fish population doesn't increase (due to oxygen depletion).

Answer:

Algae die, bacteria break it down, bacteria use up oxygen, fish die