In 2013, dozens of dolphins living in Florida’s Indian River Lagoon mysteriously began to die. Their remains washed up, showing the animals had been emaciated. Now, over a decade later, ecologists believe they’ve figured out the cause of the bizarre die-off.

While the deaths have long been linked to gigantic algae blooms in the water, it took until now to determine exactly how the two events were connected, and it turns out, it’s mostly humanity’s fault. This might be hard to believe, but apparently dumping massive amounts of human waste and fertilizer into waterways can be bad.

As the ecologists note in the journal Frontiers in Marine Science, the long-lasting phytoplankton blooms began in 2011. The spread of the tiny plant-like organisms led to a widespread change in the Indian River Lagoon’s ecology. Their presence caused the amount of seagrass in the water to decrease by over 50%, and a 75% loss of macroalgae (better known as seaweed).

That alone wouldn’t have killed off the dolphins, but when the ecologists examined isotopic ratios in teeth samples taken from the carcasses, and compared them to teeth taken from 44 dolphins that hadn’t been part of the die-off, they realized their diets had been drastically altered. The dolphins had eaten 14% to 20% fewer ladyfish, a key dolphin prey animal, but had eaten up to 25% more sea bream, a less nutritious fish. In essence, the presence of such large amounts of phytoplankton had reduced the amount of food available for the dolphins’ usual prey. As the prey numbers dwindled, the dolphins had to catch more prey to consume the same amount of energy. The effects weren’t felt just by those dolphins that died, but by the area’s dolphin population as a whole. At the time, 64% of observed dolphins were underweight, while 5% were classified as emaciated.

“In combination, the shift in diets and the widespread presence of malnourishment suggest that dolphins were struggling to catch enough prey of any type,” said Wendy Noke Durden, a research scientist at the Hubbs-SeaWorld Research Institute, who worked on the research, in a statement. “The loss of key structural habitats may have reduced overall foraging success by causing changes in the abundance and distribution of prey.”

The historic record bears this out. According to records kept of the recorded causes of death for stranded dolphins, starvation was the cause of death in 17% of recorded dolphin deaths in the area between 2000 and 2020. That number spiked to 61% in 2013.

“Blooms of phytoplankton are part of productive ecological systems,” said Charles Jacoby, strategic program director at the University of South Florida, who also worked on the study. “Detrimental effects arise when the quantities of nutrients entering a system fuel unusually intense, widespread, or long-lasting blooms. In most cases, people’s activities drive these excess loads. Managing our activities to keep nutrients at a safe level is key to preventing blooms that disrupt ecological systems.”

There is a small silver lining to this grisly finding. As the researchers noted, waste and other crap dumped into Indian River Lagoon is being gradually reduced and is expected to hit safe levels in 2035.

Still, it’s no surprise that human activity can be harmful to ecosystems—from chopping down shocking amounts of rainforest, to the melting of polar ice, to the inadvertent introduction of thousands of invasive species to new territories. This latest example shows that beyond the obvious primary effects, our habit of dumping God-knows-what into natural environments can have all sorts of trickle-down results—including killing some of our most beloved wildlife.