Dead pogies float in a fish kill in a pass near Bay Joe Wise on the Louisiana coast. (Patrick Semansky/AP)

It began forming in May, when heavy spring rains loaded the rivers and creeks with fertilizer washed from farms and suburban lawns. It grew rapidly over the summer, as a broth of chemicals, animal waste and microbes simmered in the warm, slow-moving waters of the Chesapeake Bay.

By early August, the “dead zone” was back: more than a cubic mile of oxygen-depleted water in which nothing — fish, crab nor shrimp — can survive.

The phenomenon has been recurring in the Chesapeake whenever hot summer weather and pollution combine to trigger algae blooms that suck life-giving oxygen from the water. But this year’s dead zone was bigger than most, making 2014 the eighth-worst year since record-keeping began in the 1980s, according to monitoring data compiled by Maryland’s Department of Natural Resources.

The grim news for the bay comes during a year when U.S. waterways have seen large dead zones and nuisance algae blooms, brought on in part by pollution from the land. In the Gulf of Mexico, this year’s swath of “hypoxic,” or oxygen-depleted, water covered 5,052 square miles, an area nearly the size of Connecticut.

Toxic algae in Lake Erie last month forced nearly a half-million people in Toledo to temporarily stop drinking tap water. About the same time, scientists in Florida observed one of the biggest “red tide” algae blooms ever recorded, a 100-foot-deep column of poisonous microbes stretching nearly 90 miles off the state’s southwestern coastline.

“We’ve been working on this for 30 years and we’re not seeing reductions,” said Nancy Rabalais, a marine ecologist who monitors water quality in the gulf near the mouth of the Mississippi River. Despite years of efforts to stem the flow of pollutants into the river basin, “some people haven’t gotten the message,” she said.

Dead zones are areas in which the amount of oxygen in the water drops below 2 milligrams per liter, a level too low to support life for most marine species. Crabs and other creatures that cannot move quickly enough usually suffocate.

While some dead zones are caused by natural phenomena — such as an upwelling of nutrients from deeper waters along the continental shelf — the ones in the Chesapeake Bay and Gulf of Mexico have been linked to pollution from farms, cities and suburbs in the watersheds farther upstream. The problem has persisted despite vast sums of money spent by state and federal officials to keep nitrogen and phosphorous from seeping into waterways.

Nature has played a role as well, especially in the Chesapeake. This year, heavy rainfall in the spring and late summer washed fertilizers and livestock waste into the bay at higher-than-normal volumes, said Donald Boesch, a professor of marine science and president of the University of Maryland’s Center for Environmental Science.

Hurricane Arthur, which passed just east of Maryland in early July, scrambled the bay’s waters and almost wiped out this year’s dead zone before late-summer storms dumped a fresh load of pollutants into the Chesapeake, he said. If not for the rains, the bay might have seen a smaller dead zone this year, as recent curbs on agricultural and urban runoff have begun to take effect.

“We still have high variability among years and even — as we see this year — within years, because of climatic variation,” Boesch said. “We still have a way to go and need reductions in nutrient sources.”

In the gulf, a slightly smaller dead zone this year was hardly a cause for celebration. A task force of officials from a dozen states and five federal agencies has been working for 15 years to shrink the gulf’s dead zone to a more manageable size. This year’s 5,052 square miles was 2 1 / 2 times larger than the group’s projected target for this year, said Rabalais, the marine ecologist.

Part of the problem, she said, is that the Mississippi basin is so awash in nitrogen from farms and cities that it will take years for the nutrients to flush out. River sediments are so saturated with fertilizers, animal waste and plant carbon that short-term reductions in new sources of pollution will barely make a dent in the problem, she said.

“Even if we turned off the spigot today, it would take a couple of years to get to a reasonable level,” Rabalais said. Until that happens, fishing boats will have to continue traveling deeper into the gulf to find waters with enough oxygen to sustain fish and shrimp, she said.

Rabalais worries that any gains achieved could be reversed as state governments push back on regulations intended to lower the volume of nutrients pouring into the gulf. Indeed, the only thing that is shrinking consistently is the money allocated for monitoring the health of the gulf. This year, Rabalais said, her team had enough money for only one expedition into the gulf to collect water samples.

As for next year, “we’re just not sure,” she said.