A little over a decade ago, scientists monitoring the Indian River Lagoon – a 156-mile-long, shallow water estuary along Florida’s east coast – noticed that the local dolphin population was struggling. Of the some 337 individual dolphins observed, 64% were underweight, 5% were emaciated, and 77 of them died. It was classified at the time as an unusual mortality event.

This was back in 2013, when a spike in local dolphin deaths that took out as much as 8% of the bottlenose dolphins living in the stretch of water. 

12 years on from the event and this week, researchers from the Florida Flood Hub for Applied Research and Innovation believe they have uncovered the root cause of the issue – and it’s all to do with a major bloom in phytoplankton which took place two years prior, in 2011.

But how did a sudden explosion in the prevalence of phytoplankton lead to the severe malnourishment of Florida’s bottlenose dolphin population?

The answer has been published this week in Frontiers in Marine Science, in which analysis of historic evidence has linked this major bloom to the loss of key habitats in the Indian River Lagoon and therefore changes in the prey available to its bottlenose dolphins. 

“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. 

In fact, what Dr Jacoby had linked was a far wider web of cause and effect; one that could be traced back an accumulation in the Indian River Lagoon of chemicals from fertiliser, effluent from septic tanks, and a list of other by-products of human activity rich in nutrients like nitrogen and phosphorus.

“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 macro-algae,” said Dr Jacoby. “These reductions were driven by shading from an intense, extensive, and long-lasting bloom of phytoplankton.”

Bottlenose dolphins are 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, the phytoplankton bloom shaded much of the bottom-dwelling seagrass and macro-algae across large swathes of the lagoon, essentially killing off these key habitats for dolphins’ own food source – namely the energy-rich ladyfish – and forcing them to shift their diet towards a less-nourishing source of sea bream.

For a long time, this was the theory held by research scientists within the region. But it was a hard one to prove – tracking just what dolphins are eating has never been an exact science. Until now… 

To get a fuller picture of this suspected diet-shift, researchers analysed the presence of carbon and nitrogen in muscle samples collected from dolphins between 1993 and 2013. The ratios of carbon and nitrogen in such samples are representative of a mixture of similar ratios in their prey. What this means is that by using reference values from their prey species, scientists were able to track dietary changes over time.

What they found is that over the two years between 2011 and 2013, these bottlenose dolphins were eating more sea bream and less ladyfish – a more energy-dense fish widely associated with seagrass. This correlated not only with changes recorded in the availability of the two species themselves but with the falling abundance of seagrass and macro-algae 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 on the study. 

“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%.

Blooms of phytoplankton are – of course – part of a productive ecological system. Issues arise, however, when the quantities of nutrients, as witnessed in this case, fuel what Dr Jacoby has described as “unusually intense, widespread, or long-lasting blooms”.

Findings from the study provide further evidence for the importance of controlling discharges of nutrient-rich by products of human activities. 

“In most cases, people’s activities drive excess loads of phytoplankton,” said Dr Jacoby. “Managing our activities to keep nutrients at a safe level is key to preventing blooms that disrupt ecological systems.”

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