Scientists link a phytoplankton bloom to starving dolphins in Florida
A phytoplankton bloom damaged habitats, deprived bottlenose dolphins of nutritious prey, and led to a sharp rise in strandings and deaths
Frontiers
image:
A deceased stranded dolphin photographed by researchers. Photograph supplied by Hubbs-SeaWorld Research Institute. Stranding response conducted under a Stranding Agreement between HSWRI and NOAA Fisheries.
view moreCredit: Hubbs-Seaworld Research Institute
In 2013, 8% of the bottlenose dolphins living in Florida’s Indian River Lagoon died. Investigations have now revealed that the dolphins may have starved because key habitats for nutritious prey were destroyed by a phytoplankton bloom. This bloom was driven by the accumulation in the lagoon of fertilizer, effluent from septic tanks, and other by-products of human activity that are rich in nutrients like nitrogen and phosphorus.
“We linked mortality and malnutrition to a decreased intake of energy following a shift in dolphins’ diets,” said Dr Charles Jacoby of the Florida Flood Hub for Applied Research and Innovation, corresponding author of the article in Frontiers in Marine Science. “We linked the dietary shifts to changes in prey availability, and we connected changes in prey to system-wide reductions in the abundance of seagrass and drifting macroalgae. These reductions were driven by shading from an intense, extensive, and long-lasting bloom of phytoplankton.”
Signs of trouble
In 2013, scientists monitoring the Indian River Lagoon noticed that the dolphin population was struggling. 64% of the 337 dolphins they observed were underweight, 5% were emaciated, and 77 died: a toll classified as an unusual mortality event.
“An unusual mortality event is a stranding event that is unexpected and involves a significant die-off of any marine mammal,” explained Megan Stolen of the Blue World Research Institute, first author of the article. “The 2013 event was characterized by a marked increase in mortality and widespread evidence of malnourishment.”
Bottlenose dolphins are large, long-lived animals that eat relatively large amounts of many different types of prey, which means that any disruption to the local ecosystem can affect them. In this case, researchers suspected that critical changes were caused by a 2011 phytoplankton bloom that was fueled by nutrient-rich by-products of human activity flowing into, and accumulating in, the lagoon. The bloom shaded bottom-dwelling seagrass and macroalgae in large parts of the lagoon, killing off these key habitats for dolphins’ prey and potentially compromising the dolphins’ ability to hunt. But it’s difficult to prove what dolphins are eating: observations of dolphins feeding at the water’s surface don’t yield a full picture of their diets, and stranded dolphins often have empty stomachs.
Instead, the researchers focused on isotopic analysis of muscle biopsies collected from stranded dolphins between 1993 and 2013. The ratios of stable isotopes of carbon and nitrogen in muscle from dolphins represent a mixture of similar ratios in their prey, so — using reference values from prey species — the scientists could track dietary changes over time and compare them to contemporary fisheries monitoring and the presence of seagrass and macroalgae.
Ripple effect
The scientists found a shift in the dolphins’ diets: during 2011-2013, they ate more sea bream and less ladyfish — a more energy-dense fish associated with seagrass. This agreed with the fisheries monitoring, which recorded changes in the availability of the two species: less ladyfish and more sea bream. It also matched the falling abundance of seagrass and macroalgae habitat over the same period. The shift from ladyfish to sea bream meant that dolphins would need to eat about 15% more prey to acquire the same amount of energy.
“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 of Hubbs-SeaWorld Research Institute, a co-author. “The loss of key structural habitats may have reduced overall foraging success by causing changes in the abundance and distribution of prey.”
The data also tallied with causes of death recorded for stranded dolphins. Between 2000 and 2020, malnutrition caused 17% of all recorded deaths, but in 2013, this figure rose to 61%.
“All studies have their limitations,” cautioned co-author Dr Graham Worthy of the University of Central Florida. “We did not have data on ratios of stable isotopes in all the prey that dolphins were eating from 1993 to 1999, so we could not fully explain the shift in diets observed from that early period to 2000–2010. Additionally, the link between malnourishment and a change in diet would have been enhanced by stable isotope data from the muscle of surviving dolphins.”
“Blooms of phytoplankton are part of productive ecological systems,” said Jacoby. “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.”
A researcher lifts a deceased dolphin from the water. Photograph supplied by Hubbs-SeaWorld Research Institute. Stranding response conducted under a Stranding Agreement between HSWRI and NOAA Fisheries.
Credit
Hubbs-SeaWorld Research Institute.
Journal
Frontiers in Marine Science
Method of Research
Observational study
Subject of Research
Animals
Article Title
An unusual mortality event for bottlenose dolphins links to altered diets resulting from ecological changes
Article Publication Date
7-Apr-2025
Research to tackle Prymnesium algal blooms which affect fish populations
University of East Anglia
A scientific initiative to combat harmful Prymnesium algal blooms in the Broads is gaining fresh momentum, thanks to a renewed collaboration between the University of East Anglia (UEA), the Environment Agency, the Broads Authority, and the Norfolk pike angling community.
Prymnesium algal blooms can occur on the Broads and produce toxins that threaten fish stocks, local biodiversity, and East Anglia’s thriving angling industry, which contributes more than £100 million annually to the local economy.
The golden algae Prymnesium parvum is therefore of significant research interest to prevent environmental incidents and associated fish mortalities from occurring.
Researchers at UEA are working to better understand the environmental, chemical and biological conditions that trigger these Prymnesium blooms and find solutions to prevent their harmful impact.
While this long-term research project has been active for nearly a decade, with the return of researcher Prof Rob Field to UEA, this work is regaining momentum and support.
The initiative has received strong support from Norfolk’s angling community, which has donated nearly £4,000 to the research effort and is actively assisting in water sampling across the Broads National Park.
This grassroots involvement underscores the urgency of tackling this environmental challenge and the strong public interest in finding sustainable solutions.
Prof Field, Pro-Vice Chancellor for UEA’s Faculty of Science, said: “This research is crucial for the future of the Broads’ ecosystem and economy.
“With the support of local anglers and environmental agencies, we are making significant strides in understanding and heading off harmful algal blooms.”
The research will involve laboratory work and field studies to identify specific locations at risk of Prymnesium blooms, how it is impacted by nutrient levels and salinity and how these factors can influence blooms.
John Currie, Chairman of the Norwich and District Pike Club, said: “Local anglers well know the devastating effects caused by a toxic Prymnesium bloom and the negative effects on the whole ecosystem.
“My experience over many years in communicating with scientists from around the globe who specialise on the subject qualifies me to say how gratified I am that we now have on our doorstep scientists who are world leaders on all aspects of Prymnesium.
“I look forward to what Rob and his team come up with next as their past work has been inspirational.”
As the research progresses, stakeholders hope to develop strategies to safeguard Norfolk’s waterways and preserve its rich biodiversity.
Broads Authority Environment Policy Adviser Andrea Kelly said: “We are delighted that this research to better understand what triggers algal blooms of Prymnesium parvum is taking place in the Broads.
“Preventing Prymnesium algal blooms will help protect populations of fish, Broads angling and improve our management of the navigation.”
Recent breakthroughs from the UEA and John Innes Centre have revealed crucial insights into how and when these algae produce toxins, and how the toxins are released into the environment.
