Imagine an area the size of Massachusetts in the Gulf of Mexico that is completely devoid of marine life. This so-called "dead zone" develops each spring, driven by fertilizer runoff from the Mississippi river basin that empties into the Gulf. Given that ~50% of the nation's farmland empties into the Mississippi, it's easy to see that we're talking about a LOT of fertilizer.
Fertilizer contains high concentrations of nitrogen, an element that is normally "limiting" in a marine environment. Adding additional nitrogen to the water leads to explosive growth of algae. When the algae die, their decomposition uses up large amounts of oxygen. This depletes the water of dissolved oxygen, on which organisms like fish and shellfish depend. They must either move to different waters, or perish.
There is also the issue of harmful algal blooms, in which the algae produce neurotoxins that kill fish, dolphins, and other marine life. If the toxins build up in fish or shellfish, then other organisms such as birds and even humans may get sick.
The obvious solution is to drastically reduce fertilizer usage, but this is unlikely to happen anytime soon. Other tricks are being explored for "capturing" phosphorus and nitrogen, including building wastewater treatment plants that use algae to remove these elements (e.g. growing algae in a controlled environment). The algae would then be used as biomass to produce biofuels. But could this be done on a large enough scale?