Featured Image Caption: Sargassum algae have been swamping tropical Atlantic ecosystems since 2011, raising awareness of the increasing and alarming climate crisis. (Image Source: “Sargassum Fish” by Clinton & Charles Robertson, licensed under CC BY 2.0).
Reference: Maurer, A., Gross, K., & Stapleton, S. (2021). Beached Sargassum alters sand thermal environments: Implications for incubating sea turtle eggs. Journal of Experimental Marine Biology and Ecology, 546, 151650. https://doi.org/10.1016/j.jembe.2021.151650
For the past decade, Sargassum algal blooms have been in the spotlight, as they cover beautiful Caribbean beaches with tons of (technically) harmless algae. Tourists see them as a nuisance because they occupy most of the beach area and make swimming and relaxing on the beach extremely uncomfortable, but these algal blooms, frequently referred to as “golden tides”, usually end up on the shoreline, having to be physically removed and altering the conditions on the coastline more extensively than can be seen by the eyes.
Sargassum is a type of free-floating brown macroalgae or seaweed that inhabits shallow waters in both temperate and tropical oceans. They serve various ecological purposes, like being the hatching grounds for migrating eels like the European and American eels. They also serve as cover for some species of sea turtles. helping them avoid predators as they grow as juveniles. As all species of Sargassum are free-floating at some point of their life cycle, they can end up on shores due to wind and water currents. Once beached, the algae could affect an even earlier life stage of the sea turtle that could be detrimental rather than helpful.
The Basics on Turtle Sex Determination
Sea turtles have a very distinct mechanism by which their sex is determined, and it is governed by temperature. In mammals, sex is determined as soon as fertilization takes place, but this is not the case for turtles. Sea turtle sex is determined by a temperature difference as small as 1° Celsius (1.8° Fahrenheit). The exact transitional range of temperatures varies by species, but the mean is 29° Celsius (84.2° Fahrenheit). In other words, if the nest is being incubated below 28° Celsius (82.4° Fahrenheit), then about 95% of the nest will be male, or if the nest is being incubated above 30° Celsius (86° Fahrenheit), then about 95% of the nest will be female. This means that even small changes in temperature could drastically alter the wild population of these animals. No sea turtle species have yet been irreparably affected by algal blooms and the overall global warming, but recent studies show that this could actually be the case in the future.
But How Will Sargassum Affect Sea Turtle Populations?
Sea turtle eggs face around two months of fluctuating temperatures that can ultimately affect their survival rates, traits, and sex ratio. Sex, specifically, is determined in the middle-third of this period of time and can be greatly influenced in the week prior to determination. That means that even subtle changes could cause drastic fluctuations in the sex distribution in this area.
Between June and November 2015 in Jumby Bay, Antigua, the researchers Andrew Maurer, Kevin Gross, and Seth Stapleton tracked the temperature under different Sargassum and shade conditions for four to five days at a depth of 25 cm, imitating the depth of an average hawksbill turtle nest, to better understand how these factors could affect nest incubation temperatures. They found that during the summer months, Sargassum cover led to a 0.17° Celsius (0.30° Fahrenheit) decrease in nest temperature, while during the autumn months, Sargassum cover led to a 0.21° Celsius (0.38° Fahrenheit) increase in nest temperature
These average temperature variations may not seem like a big deal, but the researchers found considerable variability in recorded temperatures. This trend can potentially sway the sex distribution of sea turtles to a point that finding a mate could begin to be more and more complicated.
Maurer and his fellow authors detail in their paper that even though they’ve identified how Sargassum can affect temperature, it would also be interesting to see if the algae affects other factors as well. For example, it would be interesting to determine if gas exchange or rainfall can be related to factors other than sex determination, like embryonic survivability.
What we can do about it
If you live in a tropical country or state that is known to have migrating sea turtle nests on their beaches, then you can help raise awareness locally. Many academic and governmental institutions still don’t understand the effect that accumulated Sargassum could have on the local turtle populations. Speaking out about the importance of algal bloom beach cleanups and the implications this biomass could have on sea turtles due to their peculiar sex determination could help prevent future turtle declines. It is not too late to minimize the effects that ocean warming and algal blooms are having on sea turtle populations and their nesting habits.