In 2013, the Indian River Lagoon in Florida witnessed a tragic event: approximately 8% of the bottlenose dolphin population perished. A recent peer-reviewed study published in Frontiers in Marine Science has now traced these dolphin deaths to widespread ecological disruption caused by a massive phytoplankton bloom.
This harmful bloom was fueled by nutrient pollution—including fertilizer runoff, septic tank leakage, and other human-related nutrient sources rich in nitrogen and phosphorus.
“We linked dolphin mortality and signs of malnutrition to a significant decrease in energy intake, which was the result of dietary changes caused by loss of prey habitats,” said Dr. Charles Jacoby of the Florida Flood Hub for Applied Research and Innovation, and lead author of the study.
The Ecological Chain Reaction: From Pollution to Dolphin Deaths
The research team found a clear ecological domino effect:
- Nutrient runoff entered the lagoon.
- This fueled an intense, long-lasting phytoplankton bloom.
- The bloom blocked sunlight, killing large areas of seagrass and macroalgae.
- As these habitats vanished, the availability of high-energy prey species for dolphins declined.
- Dolphins had to adapt their diet, leading to nutritional stress and increased mortality.
Warning Signs: Malnourished Dolphins and an Unusual Mortality Event
In 2013, dolphin monitoring in the lagoon revealed alarming statistics:
- 64% of the 337 dolphins were underweight.
- 5% were severely emaciated.
- 77 dolphins died, triggering what experts called an “unusual mortality event” (UME).
“A UME is defined as a significant, unexpected die-off of marine mammals,” explained Megan Stolen of the Blue World Research Institute, first author of the study. “The 2013 event was marked by widespread malnourishment and a sudden spike in dolphin deaths.”
Tracking Dolphin Diets Through Isotope Analysis
Studying dolphin diets can be challenging due to limitations in direct observation and the frequent absence of stomach contents in stranded dolphins. Instead, researchers used stable isotope analysis of dolphin muscle tissue samples collected between 1993 and 2013.
This approach allowed scientists to trace dietary shifts over time by comparing carbon and nitrogen isotope ratios in dolphin muscle to those found in potential prey species.
Key Findings: Dietary Shifts, Habitat Decline, and Increased Energy Demands
The results revealed that from 2011 to 2013, dolphins consumed:
- More sea bream
- Less ladyfish—a species rich in energy and typically associated with seagrass habitats
This shift aligned with:
- Fisheries data showing declines in ladyfish and seagrass
- A 15% increase in the energy requirement for dolphins to meet their nutritional needs using sea bream instead of ladyfish
“The loss of critical structural habitats likely reduced overall foraging success,” said Wendy Noke Durden, co-author and researcher at HSWRI.
Malnutrition as a Major Cause of Death in 2013
Between 2000 and 2020, malnutrition accounted for 17% of dolphin deaths in the region. However, during the 2013 mortality event, this number skyrocketed to 61%, underscoring the link between ecosystem disruption and dolphin health.
“While our study had limitations, such as incomplete historical isotope data from all prey species, the evidence strongly suggests a link between dietary changes and malnourishment,” noted co-author Dr. Graham Worthy of the University of Central Florida.
Preventing Future Marine Mammal Die-Offs Through Nutrient Management
Phytoplankton blooms are naturally occurring, but when excess nutrients from human activity cause them to become unusually large or persistent, they can trigger severe ecological imbalances.
“Managing nutrient levels from agricultural runoff, wastewater, and urban development is critical to maintaining healthy coastal ecosystems,” emphasized Dr. Jacoby.
Source & Citation
Study Title: An unusual mortality event for bottlenose dolphins links to altered diets resulting from ecological changes
Journal: Frontiers in Marine Science (2025)
DOI: 10.3389/fmars.2025.1531742