Record amounts of sargassum seaweed are washing ashore in Florida, USA, adding a new and serious obstacle to sea turtle hatchlings as they make the perilous journey from their sandy nests to the Atlantic Ocean.
The turtles' route from nest to sea is already fraught with hazards, including risk from predators, artificial lights and beach debris. The additional challenge of navigating piles of seaweed reduces their chances of survival even further, according to a new study published in the Journal of Coastal Research.
What is sargassum seaweed?
Sargassum is a free-floating yellow-brown seaweed that forms mats on the ocean surface. It serves as an important resource in the marine world, providing food and a place to live for numerous sea animals.
It's also a valuable source of nutrients for deep-sea life: "When sargassum loses its buoyancy, it sinks to the seafloor, providing energy in the form of carbon to fishes and invertebrates in the deep sea, thus serving as a potentially important addition to deep-sea food webs," says NOAA Ocean Exploration (the US federal program dedicated to exploring the unknown ocean).
However, in large quantities, this fast-growing seaweed has the potential to cause problems, forming miles-long mats that can smother coral beds, prevent fish from getting oxygen, and entangle marine animals. When it washes ashore, it can be a nuisance to beachgoers as it rots and starts to smell.
Sargassum is also becoming increasingly problematic for sea turtle hatchlings, acting as an obstacle as they make the journey from their nests to the water. Yet little research has been done on exactly how it impacts these vulnerable reptiles.
How sargassum seaweed impacts sea turtle hatchlings
To find out more, researchers from Florida Atlantic University (FAU) conducted an experiment on three beaches on the Florida coast – Juno Beach, Jupiter and Boca Raton.
On each beach, they placed mats of sargassum up to 19cm high on the sand to simulate natural crawlways. They then measured how long it took leatherback, loggerhead and green turtle hatchings to reach the water.
The results showed that even low piles of seaweed (7-9cm) delayed hatchlings significantly, with some unable to climb the piles at all. Loggerheads found the obstacles particularly challenging; they slowed by 91% in the lightly spread sargassum and 175% in the deeper piles.
Hatchlings (of all species) often flipped onto their backs during these climbs, sometimes more than 20 times in a single trial, leaving them exposed to predators, and the sun, for longer.
“The longer a hatchling stays on the beach, the more at risk it becomes – not just from predators like birds and crabs, but also from overheating and dehydration, especially after sunrise,” says senior author Sarah Milton.
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The researchers also wanted to test how the seaweed impacted the turtles' energy levels. To do this, they measured the hatchlings' blood glucose levels after completing the crawl.
Interestingly, the results show that the turtles’ glucose levels remained relatively stable after crawling through the seaweed, suggesting that the added obstacle didn’t deplete their measurable energy stores as much as expected.
Despite this, Milton warns that time is the critical factor. The slower hatchlings move, the greater the danger. “When sargassum piles are higher – some can be over a metre high on South Florida beaches in the summer and extend for hundreds of metres down the beach – we can expect more failed attempts, particularly when hatchlings have to cross multiple bands of seaweed just to reach the ocean,” Milton says.
An abundance of sargassum seaweed can also reduce available nesting space and change incubation conditions, adding further pressure to already vulnerable sea turtle populations.
“For sea turtle hatchlings, reaching the ocean is already a race against time – and survival. Now, increasingly large mats of sargassum are adding new challenges to this critical journey,” says Milton.
The study highlights the need for more responsive beach management. Without intervention, researchers warn, this rising tide of seaweed could quietly undermine sea turtle conservation efforts for years to come.
Top image: Sargassum accumulations greater than 30 centimetres high are obstacles for sea turtle hatchlings crossing the beach. (Juno Beach, July 2021). Credit: Abbey M. Appelt, Florida Atlantic University
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