What Happens to Manatees When Seagrass Dies Off?

With their main food source dying off, manatees in Florida’s Indian River Lagoon have turned to eating large amounts of algae to supplement their diets. Researchers are concerned about what this change will do to the animals’ well-being.

About a decade ago, a harmful algal bloom (HAB) significantly reduced the amount of seagrass in the lagoon. These blooms happen when algae grows out of control and can have toxic effects on living things in the ecosystem or use up all the oxygen in the water. Around that time is when Florida manatees (Trichechus manatus latirostris) went from eating primarily seagrass to eating primarily algae, a team of researchers found.

Several years ago, scientists from the Florida Fish and Wildlife Conservation Commission (FWC) noticed that many manatees were dying in the Indian River Lagoon (IRL) for an unknown reason. This unusual mortality event sparked an investigation.

Study author Aarin-Conrad Allen, a Ph.D. student at Florida International University, Institute of Environment, had just finished work on manatee diets in Belize and began investigating the Florida situation.

“There was a noticeable trend with a significantly larger proportion of algae in these manatee stomach samples. This presented an excellent opportunity for comparison of manatee diet over time,” Allen tells Treehugger.

Manatees primarily eat seagrass, but will sometimes supplement their diets with other foods like algae.

Researchers collaborated with the U.S. Geological Survey, which had collected and archived more than 100 stomach samples from manatees that had died in the lagoon between 2013 and 2015. They found that their diet consisted of about 50% algae and 34% seagrass.

They compared those samples to some that had been archived from the late 1970s through late 1980s when the lagoon was not so unhealthy. Then, the manatees’ diets were almost 62% seagrass and only 28% algae. The rest of their diets included other aquatic plants, small fish, and invertebrates.

“We compared the two time frames to look back at what the manatees in the IRL consumed when the estuary was healthy to what manatees were consuming now after a notable decline in their primary food source. The lack of seagrass within the lagoon is a grave concern for the well-being of manatees, so we were eager to present this data documenting the diet of manatees before and after periods of seagrass decline,” Allen says.

“Now we are working to investigate the nutritional value of these items consumed to compare with the seagrass which manatees typically feed on, as well as working with the St. Johns River Water Management District to use biomass estimates to study the carrying capacity of manatees within the waters of the IRL.”

The results were published in the journal Estuarine, Coastal and Shelf Science.

What Does Diet Do?

Harmful algal blooms lower the oxygen levels and limit how much light is able to permeate lagoon waters. This is what ultimately caused the seagrass die-off. As the seagrass disappeared, it was replaced by algae and the manatees turned to algae for a larger part of their diet.

Researchers aren’t sure what exact consequences this dietary shift from seagrasses to algae is having on manatees.

“Clearly, there is a greater issue with the overall health of manatees in the IRL and the visible declines in body condition of these animals, plus the record number of mortalities which we are seeing (1,101 in 2021). Future research is necessary to investigate how resilient or susceptible manatees are to resource loss, and determine what health and ecological implications are associated with a diet consisting mostly of algae,” Allen says.

Researchers are curious whether the manatees’ decline may be due to switching to a diet that’s not as nutritious. It could be like eating too much fast food, which makes you feel full but doesn’t give you enough nutrients to keep you healthy.

“Manatees might not be getting the nutritional value they need from algae, although it may give them the sensation of being full in the short term,” Allen says. “Consuming food of lower or malnutritional content (known as the junk food hypothesis) has been shown to reduce the fitness of other species, including marine mammals; we're presently looking further into this.”

Although algal blooms happen naturally, human activities disturbing the ecosystem often determine their frequency and intensity. Manatees will have to learn to manage somehow. Researchers say it could take a decade or more for the seagrass to fully recover in the lagoon. In the meantime, manatees will have to rely more heavily on vascular plants and algae or they may have to travel farther to find seagrass to consume.

“It may be some time before we adequately understand what impact this dietary change has on the survival of manatees inhabiting the IRL, but clearly it has become much more dire in recent years,” Allen says.

He points out that FWC biologists are investigating manatee mortalities and rescuing the animals when they can, which is helping their survival and providing information for researchers.

“There is a need to address the health of the IRL at the ecosystem level in order to restore the habitat and ensure the survival of manatees and other wildlife in this area,” Allen says. “Without turning the tide and implementing changes to the nutrients entering the lagoon, addressing the water quality and conditions for healthy seagrass beds, manatees ultimately may not persist in the region.”