Study warns weed fed by human activity killing off dolphins

By Stephen Beech

An aquatic weed fed by human activity is killing off dolphins, warns new research.

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

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 the phytoplankton bloom.

Scientists say the 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 such as nitrogen and phosphorus.

Study 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.”

In 2013, scientists monitoring the Indian River Lagoon realized that the dolphin population was struggling with almost two-thirds of the 337 dolphins they observed underweight (64%), 5% were emaciated, and 77 died: a toll classified as an unusual mortality event.

Study 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 characterized by a marked increase in mortality and widespread evidence of malnourishment.”

Bottlenose dolphins are usually 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 key habitats for dolphins’ prey and potentially compromising the dolphins’ ability to hunt.

But the research team said it’s difficult to prove what dolphins are eating as observations of them feeding at the water’s surface don’t yield a full picture of their diets, and stranded dolphins often have empty stomachs.

The researchers focused instead 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.

The findings, published in the journal Frontiers in Marine Science, showed a shift in the dolphins’ diets.

In the period from 2011 to 2013, they ate more sea bream and less ladyfish - a more energy-dense fish associated with seagrass.

That 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, according to the research team.

Study 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.”

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, the figure rose to 61%.

Co-author Dr. Graham Worthy, of the University of Central Florida, said: "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.”

Dr. 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.”