July14, 2023
The water off South Florida
is over 90 degrees Fahrenheit (32 Celsius) in mid-July, and scientists are
already seeing signs of coral bleaching off Central and South America.
Particularly concerning is how early in the summer we are seeing these high
ocean temperatures. If the extreme heat persists, it could have dire
consequences for coral reefs.
Just like humans, corals
can handle some degree of stress, but the longer it lasts, the more harm it can
do. Corals can’t move to cooler areas when water temperatures rise to dangerous
levels. They are stuck in it. For those that are particularly sensitive to
temperature stress, that can be devastating.
Two photos shows a coral on
two different dates, one healthy and reddish in color, the other white.
I lead the Coral Program at
the National Oceanic and Atmospheric Administration’s Atlantic Oceanographic
and Meteorological Lab in Miami, Florida. Healthy coral reef ecosystems are
important for humans in numerous ways. Unfortunately, marine heat waves are
becoming more common and more extreme, with potentially devastating
consequences for reefs around the world that are already in a fragile state.
Why coral reefs matter to
everyone
Coral reefs are hot spots
of biodiversity. They are often referred to as the rain forests of the sea
because they are home to the highest concentrations of species in the ocean.
Healthy reefs are vibrant
ecosystems that support fish and fisheries, which in turn support economies and
food for millions of people. Additionally, they provide billions of dollars in
economic activity every year through tourism, particularly in places like the
Florida Keys, where people go to scuba dive, snorkel, fish and experience the
natural beauty of coral reefs.
If that isn’t enough, reefs
also protect shorelines, beaches and billions of dollars in coastal
infrastructure by buffering wave energy, particularly during storms and
hurricanes.
But corals are quite
sensitive to warming water. They host a microscopic symbiotic algae called
zooxanthella that photosynthesizes just like plants, providing food to the
coral. When the surrounding waters get too warm for too long, the zooxanthellae
leave the coral, and the coral can turn pale or white – a process known as
bleaching.
If corals stay bleached,
they can become energetically compromised and ultimately die.
When corals die or their
growth slows, these beautiful, complex reef habitats start disappearing and can
eventually erode to sand. A recent paper by John Morris, a scientist in my lab
in Florida, shows that around 70% of reefs are now net erosional in the Florida
Keys, meaning they are losing more habitat than they build.
Unfortunately, these
critical coral reef habitats are in decline around the world because of extreme
bleaching events, disease and numerous other human-caused stressors. In the
Florida Keys, coral cover has decline by about 90% over the past several decades.
Coral bleaching in 2023
In the Port of Miami, where
we have found particularly resilient coral communities, a doctoral candidate in
my lab, Allyson DeMerlis, documented the first coral bleaching of her
experimentally outplanted corals on July 11, 2023.
Other scientists we work
with have reported coral bleaching off of Colombia, El Salvador, Costa Rica and
Mexico in the eastern Pacific, as well as along the Caribbean coasts of Panama,
Mexico and Belize.
We have yet to see
widespread coral death associated with this particular marine heat wave, so it
is possible the corals could recover if sea surface temperatures cool down
soon. However, global sea surface temperatures are at record highs, and large
parts of the Atlantic and eastern Pacific are under bleaching alerts. At this
point, the evidence points to the potential for a very negative outcome.
El Niño is contributing to
the problem this year, but the longer-term trends of rising ocean heat are
driven by global warming fueled by human activities.
To put that into context, a
paper by NOAA scientist Derek Manzello showed that in the Florida Keys, the
number of days per year in which water temperatures were higher than 90 F (32
C) had increased by more than 2,500% in the two decades following the mid-1990s
relative to the prior 20 years. That is a remarkable increase in the number of
days that corals are experiencing particularly stressful warm water.
What can we do to protect
corals?
First, we cannot give up on
corals.
Alice Webb, a coral reef
scientist working with our group, recently published a study based on years of
our research in the Florida Keys. She modeled reef habitat persistence under
climate, restoration and adaptation scenarios and found that protecting reefs
is going to take everything – active restoration of reefs, helping corals
acclimate or adapt to changing temperatures, and, importantly, human curbing of
greenhouse gas emissions.
Major restoration efforts
are underway in the Florida Keys as part of the NOAA-led Mission Iconic Reefs.
We are also assessing how different coral individuals perform under stress,
hoping to identify those that are particularly stress-tolerant by combing
through the massive amounts of data from restoration projects and coral
nurseries.
We are also evaluating
stress-hardening techniques. For example, in tide pools, corals are exposed to
large swings in temperature over short periods, making them more resilient to
subsequent thermal stress events. We are exploring whether it’s possible to
replicate that natural process in the lab, before corals are planted onto
reefs, to better prepare them for stressful summers in the wild.
Coral bleaching on a large
scale has really been documented only since the early 1980s. When I talk to
people who have been fishing and diving in the Florida Keys since before I was
born, they have amazing stories of how vibrant the reefs used to be. They know
firsthand how bad things have become because they have lived it.
There isn’t currently a
single silver-bullet solution, but ignoring the harm being done is not an
option. There is simply too much at stake.
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