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www.frontiersinecology.org © The Ecological Society of America
Sargassum accumulation may spell
trouble for nesting sea turtles
Thigh-high mounds of sargassum seaweed line the
northern section of a beach on Long Island, Antigua –
an island in the West Indies – that hosts nesting hawksbill
sea turtles (Eretmochelys imbricata) every summer. A meter
offshore – although it is difficult to tell where the shore
begins and the sea ends – the seaweed is crisp, even
crunchy underfoot. We plod through it to skirt a mangrove
tree during nightly patrols for nesting hawksbills, but the
thick seaweed soup makes wading slow and difficult. The
mass of seaweed varies with the prevailing winds and cur-
rents but often stretches over 10 m into the water. Closer
to shore, it is a warm, fly-infested mush in various states of
stinking decay. It collects on the beach to form a low but
substantial wall, at times approaching 1 m in height and
well over 2 m in width (Figure 1). And the sargassum keeps
coming. Unfortunately, this image of sargassum is becom-
ing widespread across much of the Caribbean region.
Sargassum, a genus of macroalgae that has garnered much
research attention in recent years, has been dubbed the
“floating rainforest” of the sea (Laffoley et al. 2011). The pri-
mary reason for this attention and the nickname lies in the
middle of the North Atlantic Subtropical Gyre: the Sargasso
Sea, where predominant ocean currents create a vortex that
amasses huge quantities of the seaweed. This unique habitat
is essential for a diverse array of fauna (Laffoley et al. 2011);
for instance, open-ocean sargassum mats provide important
nurseries for sea turtles such as loggerheads (Caretta caretta;
Mansfield et al. 2014). But, over the past half-decade, sargas-
sum has been proliferating in unprecedented quantities and
is affecting areas far from the Sargasso Sea. Massive amounts
have appeared on coasts from Florida to South America,
even reaching western Africa. As a result, sargassum is gen-
erating substantial interest among the travel and tourism
sectors, commercial enterprises seeking to harvest the sea-
weed, and conservation organizations (Smetacek and
Zingone 2013). There is some speculation as to the source of
all of this biomass, but evidence from a peak year in 2011
points to a possible origin off the coast of Brazil, north of the
Amazon River mouth (Gower et al. 2013).
This coastal influx begs the question: what does all this
seaweed mean for the region’s populations of nesting sea
turtles? Although sargassum in the open sea is a critical
nursery ground, how will coastal accumulation affect
Caribbean nesting beaches? The wider Caribbean region
hosts numerous nesting sites for six of the seven extant
marine turtle species (Dow et al. 2007); globally, these six
species are categorized from Vulnerable to Critically
Endangered by the International Union for Conservation
of Nature (IUCN 2015).
The benefits of sargassum are largely recognized among
members of the conservation community. As mentioned,
oceanic sargassum is a key habitat for neonatal and juve-
nile sea turtles (Mansfield et al. 2014), and increased preva-
lence may improve prospects of survival for these age
classes. When washed onto nesting beaches, seaweed may
serve to stabilize against erosion. It also represents a mech-
anism for the transport of marine nutrients to terrestrial
ecosystems (Polis and Hurd 1996), which further enhances
stabilization by promoting the growth of sand-holding
plants. These are important functions, given that healthy
nesting beaches are critical components of population
recovery efforts, and especially considering that only an
estimated 1 in 1000 sea turtle eggs hatch and survive the
decade or more required to reach maturity (Frazer 1986).
Based on recent experiences at our
study site on Long Island, Antigua,
however, we suspect that sargassum
may pose an emerging threat on
some nesting beaches, where it can
act as a barrier. Crescent-shaped
Pasture Bay, the primary nesting
beach on Long Island, is windward
facing (in contrast to many other
hawksbill beaches) and oriented
north–northeast. This positioning
makes it an ideal area for accrual of
sand and a choice landing spot for
female hawksbills seeking a nesting
site. Unfortunately, the geography
and the typical prevailing currents
also mean it is a perfect sargassum
trap.
Nearly 90 individual hawksbills laid
NATURAL HISTORY NOTES NATURAL HISTORY NOTES
Figure 1. Immense quantities of sargassum have been washing ashore on Long Island,
around Antigua, and across the Caribbean in waves over the past 5 years.
E De Neef
AS Maurer et al. Natural History Notes
395
© The Ecological Society of America www.frontiersinecology.org
Andrew S Maurer1,2, Emma De Neef1,
and Seth Stapleton1,3*
1Jumby Bay Hawksbill Project, Long Island, Antigua, West
Indies; 2Department of Applied Ecology, North Carolina
State University, Raleigh, NC; 3Department of
Fisheries, Wildlife and Conservation Biology,
University of Minnesota, St Paul, MN
*(seth@jbhp.org)
M Watkins-Gilkes
eggs at >310 nests on Long Island in 2014, with
the majority deposited along the 650-m-long
Pasture Bay. Such high nesting densities make
every section of suitable habitat valuable, and
over the past three decades, the northern por-
tion of Pasture Bay has been among the sites
most intensively used by hawksbills. During
2015, however, in those north stretches where
massive amounts of sargassum have accumulated
(~10% of Pasture Bay’s shoreline), there has
been virtually no nesting activity. We have
recorded little nesting activity in sections with
more variable presence of sargassum (an addi-
tional 10–15% of the Bay’s coastline). These
findings suggest that dense sargassum can hinder
or altogether preclude access to preferred nest-
ing locations, effectively shrinking the primary
nesting beach by as much as 25%. If the sargas-
sum persists, already high nesting densities will
climb higher in those areas unaffected by sargas-
sum, increasing the chances of nesting hawksbills digging
into previously laid nests. Such destruction of in situ
clutches has been observed here for more than a decade,
even without the sargassum further concentrating nests.
Impeded access to nesting sites is not the only potential
problem for sea turtles. We hypothesize that seaweed that
is pushed over incubating nests by waves on particularly
narrow beaches (such as northern Pasture Bay) or by
beach cleaners may create an anoxic and contaminated
incubation environment when it decomposes, and may
alter thermal conditions (our preliminary data suggest a
cooling effect, which could result in the production of
more male hatchlings since gender is temperature-depen-
dent in this taxon). Hatchlings that successfully emerge
from nests along sargassum-affected stretches of shoreline
will face further obstructions both on land and at sea
(Figure 2), and their use of wave direction to navigate
during the initial offshore migration (Salmon and
Lohmann 1989) may be compromised. These factors, in
turn, may increase mortality through hyperthermia,
exhaustion, drowning, and vulnerability to predation.
Sea turtles face a multitude of threats, including har-
vesting, degradation of key foraging and nesting sites, and
bycatch in the fisheries industries (Lutcavage et al. 1997;
Bräutigam and Eckert 2006). Although most threats are
well understood, mass seaweed stranding is a new phe-
nomenon, especially in the eastern Caribbean, and its
direct impacts on sea turtle nesting remain largely unex-
plored. While sargassum may enhance beach stability, we
do not view widespread coastal accumulation as an over-
whelming ecological positive. Indeed, the direct conse-
quences of sargassum accumulation on beaches may be a
detriment to nesting sea turtles and their offspring in
some areas. Sea turtles are ancient relics that have with-
stood the test of time, and their high reproductive output
can buffer against the occasional poor nesting season.
Nevertheless, new and sustained threats can have severe
consequences for populations that are already imperiled.
On Long Island, we have borne witness to a nearly
threefold increase in nesting numbers since the inception
of our monitoring program in 1987. This is encouraging
news for a critically endangered species (Mortimer and
Donnelly 2008). However, the altered nesting behavior
we have observed and the postulated impacts of sargas-
sum on nests and hatchlings are concerning.
Unanswered questions about the current sargassum
strandings make for an unpredictable future. Sargassum may
wash out with storms, and the rafts of seaweed may cease to
appear in nearshore waters, providing a respite to the
region’s beaches and sea turtles. Yet, any reprieve may be
short-lived; some scientists suggest that sargassum influxes
may reflect larger-scale climatic changes, so high concentra-
tions of seaweed may be a new reality that the Caribbean
region will have to contend with in the years to come.
nAcknowledgements
The Jumby Bay Hawksbill Project is generously supported
by the Jumby Bay Island Company and is a member of the
Wider Caribbean Sea Turtle Conservation Network. J
Horrocks provided useful comments on an earlier version
of this text.
nReferences
Please see WebReferences
Figure 2. Hatchlings, such as this neonatal hawksbill turtle that succumbed
in the nearshore waters of eastern Antigua, may struggle through the dense
mats of sargassum as they attempt to begin offshore migrations.
