In 2013, Florida’s Indian River Lagoon witnessed a significant and alarming increase in bottlenose dolphin deaths, with 77 individual dolphins—approximately 8% of the local population—succumbing to what was classified as an Unusual Mortality Event (UME). The Indian River Lagoon is a shallow-water estuary located along Florida’s east coast, stretching approximately 156 miles from Ponce de Leon Inlet near New Smyrna Beach southward through Brevard County, and on to Jupiter Inlet in Palm Beach County.
A recent study published in Frontiers in Marine Science may finally shed some light on the underlying causes of this event, linking it to substantial ecological changes that disrupted the dolphins’ food sources. While the event may seem like the distant past, it has implications for today and a warning about the overall health of the Indian River Lagoon, the second-largest body of water of its kind in the world.
Ecological Disruptions and Dietary Shifts
The study, led by Dr. Charles Jacoby of the University of South Florida’s Florida Flood Hub for Applied Research and Innovation, identifies a severe phytoplankton bloom in 2011 as the catalyst for a chain reaction affecting the lagoon’s ecosystem. This bloom led to extensive losses of seagrass and drifting macroalgae, critical habitats for many fish species. Consequently, the availability of preferred prey for bottlenose dolphins, such as ladyfish (Elops saurus), declined sharply. In response, dolphins altered their diets, increasing consumption of less nutritious alternatives like sea bream (Archosargus rhomboidalis).
Courtesy: Florida Museum of Natural History
This dietary shift had significant implications for the dolphins’ health. Sea bream offers lower energy content compared to ladyfish, meaning dolphins had to expend more effort to meet their nutritional needs. Compounding this issue, below-average temperatures in 2010 and 2011 may have further stressed the dolphins, particularly those already weakened by inadequate nutrition.
Implications for Conservation and Management
The 2013 UME in the Indian River Lagoon is part of a broader pattern of marine mammal die-offs along the U.S. East Coast. Between July 2013 and March 2015, over 1,600 bottlenose dolphins stranded from New York to Brevard County, Florida. This event was attributed to a cetacean morbillivirus outbreak, highlighting the vulnerability of dolphin populations to both ecological disruptions and disease outbreaks.
The findings underscore the intricate connections between environmental changes and marine mammal health. Human activities, such as nutrient runoff leading to phytoplankton blooms, can have cascading effects on marine ecosystems, ultimately impacting top predators like dolphins. Dr. Jacoby emphasizes the importance of comprehensive ecosystem management to mitigate such events:
“We linked mortality and malnutrition to a decreased intake of energy following a shift in dolphins’ diets… These reductions were driven by shading from an intense, extensive, and long-lasting bloom of phytoplankton.”
Addressing nutrient pollution, protecting critical habitats, and monitoring prey populations are essential steps in safeguarding the health of marine mammal populations in the Indian River Lagoon and similar ecosystems.
To that end, Florida Governor Ron DeSantis recently announced that roughly $100 million dollars in state funding for Indian River Lagoon protection has been allocated. Those funds will be spent on sewage improvements as well as a baffle box for Indian Harbor Beach. Baffle boxes are underground structures designed to trap debris, trash, and pollutants from storm water runoff, particularly nitrogen and phosphorus.
