2013 was a grim year for dolphins in Florida. That year, bottlenose dolphins were dying en masse in the Indian River Lagoon and along the Atlantic coast. Once a haven of marine life, the region suddenly turned deadly — and no one knew why.

The National Oceanic and Atmospheric Administration (NOAA) declared it an Unusual Mortality Event (UME). Around 8% of the entire bottlenose dolphin population in the Indian River Lagoon — 77 dolphins — perished within just one year.

For over a decade, scientists searched for answers. Now, a new study finally points to the culprit: a perfect storm of ecological collapse fueled by human activity.

A series of unfortunate events

Scientists already know that the dolphin deaths were caused by starvation linked to a vast algae bloom that first appeared in 2011. This bloom reduced the amount of natural seagrass and seaweed in the Indian Lagoon region by a staggering 50 and 70 percent, respectively.

The sudden changes in natural vegetation affected many small organisms and propagated up the food chain, ultimately affecting dolphins. But why exactly this bloom happened in the first place is less clear.

To track the changes in their diet in detail, the study authors examined the teeth and muscle samples from dolphins that inhabited the lagoon region between 1993 and 2013. They analyzed special chemical markers in the dolphins’ body tissues, called stable isotopes, to find the nutrients the dolphins were consuming through their food. The analysis revealed that bottlenose dolphins went through a dramatic shift in their diet after 2011. 

Before this event, the dolphins primarily fed on ladyfish. After the algae bloom, ladyfish consumption steadily decreased. Instead, dolphins focused on eating sea bream (a type of ray-finned fish). However, sea bream isn’t as nutritious as ladyfish, so the dolphins were required to hunt and consume 15 percent more fish to meet their daily dietary requirement. This put increasing pressure on dolphins. Ultimately, it resulted in weakness and starvation, and eventually, many dolphins died. 

“Assessments of 337 free-swimming dolphins in the northern Indian River Lagoon and Banana River Lagoon indicated that 64% were underweight and 5% were emaciated,” the study authors note.

What did humans do?

Malnutrition accounted for just 17% of recorded bottlenose dolphin deaths from 2000 to 2020, but in 2013, that number surged to a staggering 61%. A natural algae bloom wouldn’t normally cause this level of destruction. In fact, natural blooms can even increase the amount of food available for small fish and other marine animals. Naturall blooms can support the food web in oceans, lakes, and rivers, researchers say.

“Blooms of phytoplankton are part of productive ecological systems. 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,” Charles Jacoby, one of the study authors, said.

But this wasn’t a natural algae bloom. It spread over a large area in a very short amount of time, and this is only possible when phytoplanktons receive excess nutrients. This means a lot of nitrogen and phosphorus made their way into the ocean. All the plausible sources come from humans: fertilizers, septic systems, and other types of human waste.

When this happens, the nutrient overloads phytoplankton and spreads greatly, blocking the sunlight and oxygen reaching the seagrass and fish. Invariably, this disturbs the food cycle and forces the dolphins to change their diet. 

“Managing our activities to keep nutrients at a safe level is key to preventing blooms that disrupt ecological systems,” Jacoby added.

These findings also suggest that human damage to nature doesn’t always unfold in obvious ways. Sometimes, it’s the quiet pollution from chemicals, sewage, and everyday waste that kicks off a deadly chain. The mass dolphin die-off could be just one of the many unfortunate events triggered by our actions, therefore, it is time to live more responsibly.

The study is published in the journal Frontiers in Marine Science.