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.

A phytoplankton bloom damaged habitats, deprived bottlenose dolphins of nutritious prey, and led to a sharp rise in strandings and deaths.

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 fertiliser, effluent from septic tanks, and other by-products of human activity that are rich in nutrients like nitrogen and phosphorus.  

Corresponding author Dr Charles Jacoby of the Florida Flood Hub for Applied Research and Innovation, said: “We linked mortality and malnutrition to a decreased intake of energy following a shift in dolphins’ diets.

“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.

First author Megan Stolen, of the Blue World Research Institute, said: “An unusual mortality event is a stranding event that is unexpected and involves a significant die-off of any marine mammal.

“The 2013 event was characterised 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 fuelled 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. 

Co-author Wendy Noke Durden, of Hubbs-SeaWorld Research Institute, said: “In combination, the shift in diets and the widespread presence of malnourishment suggest that dolphins were struggling to catch enough prey of any type.

“The loss of key structural habitats may have reduced overall foraging success by causing changes in the abundance and distribution of prey.” 

Limitations

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%. 

Co-author Dr Graham Worthy, of the University of Central Florida, cautioned. “All studies have their limitations.

“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.” 

Jacoby added: “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.

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

The study is published in Frontiers in Marine Science.