You’ve signed up to save the whales and buy dolphin-free canned tuna. But rescuing the entire ocean? That may be just a bit too titanic. From endangered sea turtles to escalating pollution, the issues are as varied as they are colossal. Having trouble wading through them all? Here are three top concerns—and what you can do.

1. Dead zones

The seafloor encompasses vast networks of life (think crabs, shrimp, clams) that support the ocean’s food chain. However, large swathes of these species’ habitats—particularly along coastlines and at river mouths worldwide—are dying off as bottom water becomes increasingly oxygen deprived, a result of excess fertilizer runoff. The most notable dead zone lies in the Gulf of Mexico; it’s more than 8,000 square miles.

What you can do:Always choose organic when buying fertilizer-intensive products such as corn, soy, or cotton. More info: oceanconservancy.org and oceanchampions.org.