High (Toxic) Tide

Cod populations in the Baltic Sea aren't what they used to be. They're actually only 25 percent of what they used to be—leaving the sea vulnerable to "toxic tides," according to the BBC. These algal blooms, caused by increased levels of nutrients in the water, could be poisonous to humans, fish, and wildlife.

Nutrients, like nitrogen and phosphorus, wash into the seas from farms and factories, and turn into algae that turns into blooms. Tiny creatures eat the algae, little fish eat the tiny creatures, and cod—the top of the Baltic’s food chain—eat the little fish. They did anyway, until ten or so years ago when they were nearly all recruited to the neighborhood fish fry. So cod ate fewer little fish, and the little fish ate more tiny creatures leaving nobody to snack on the algae. Research suggests that such over-fishing could be leading to similar off-kilter, hazardous sea balances worldwide.