What happens to marine life when oxygen is scarce?

Date:
July 26, 2021
Source:
Woods Hole Oceanographic Institution
Summary:
In September of 2017, marine biologists were conducting
an experiment in Bocas del Toro, off the Caribbean coast of
Panama. After sitting on a quiet, warm open ocean, they snorkeled
down to find a peculiar layer of murky, foul-smelling water about
10 feet below the surface, with brittle stars and sea urchins,
which are usually in hiding, perching on the tops of coral. This
observation prompted a collaborative study analyzing what this
foggy water layer is caused by, and the impact it has on life at
the bottom of the seafloor.



FULL STORY ==========================================================================
In September of 2017, Woods Hole Oceanographic Institution postdoctoral
scholar Maggie Johnson was conducting an experiment with a colleague
in Bocas del Toro off the Caribbean coast of Panama. After sitting on a
quiet, warm open ocean, they snorkeled down to find a peculiar layer of
murky, foul-smelling water about 10 feet below the surface, with brittle
stars and sea urchins, which are usually in hiding, perching on the tops
of coral.


==========================================================================
This unique observation prompted a collaborative study explained in a
new paper published today in Nature Communications analyzing what this
foggy water layer is caused by, and the impact it has on life at the
bottom of the seafloor.

"What we're seeing are hypoxic ocean waters, meaning there is little
to no oxygen in that area. All of the macro-organisms are trying to
get away from this deoxygenated water, and those that cannot escape
essentially suffocate. I have never seen anything like that on a coral
reef," said Johnson.

"There is a combination of stagnant water from low wind activity, warm
water temperatures, and nutrient pollution from nearby plantations,
which contributes to a stratification of the water column. From this,
we see these hypoxic conditions form that start to expand and infringe
on nearby shallow habitats," explained Johnson.

Investigators suggest that loss of oxygen in the global ocean is
accelerating due to climate change and excess nutrients, but how
sudden deoxygenation events affect tropical marine ecosystems is poorly understood. Past research shows that rising temperatures can lead to
physical alterations in coral, such as bleaching, which occurs when
corals are stressed and expel algae that live within their tissues. If conditions don't improve, the bleached corals then die. However, the
real-time changes caused by decreasing oxygen levels in the tropics have
seldom been observed.

At a local scale, hypoxic events may pose a more severe threat to
coral reefs than the warming events that cause mass bleaching. These
sudden events impact all oxygen-requiring marine life and can kill reef ecosystems quickly.



========================================================================== Investigators reported coral bleaching and mass mortality due to this occurrence, causing a 50% loss of live coral, which did not show signs
of recovery until a year after the event, and a drastic shift in the
seafloor community. The shallowest measurement with hypoxic waters was
about 9 feet deep and about 30 feet from the Bocas del Toro shore.

What about the 50% of coral that survived? Johnson and her fellow
investigators found that the coral community they observed in Bocas
del Toro is dynamic, and some corals have the potential to withstand
these conditions. This discovery sets the stage for future research
to identify which coral genotypes or species have adapted to rapidly
changing environments and the characteristics that help them thrive.

Investigators also observed that the microorganisms living in the
reefs restored to a normal state within a month, as opposed to
the macro-organisms, like the brittle stars, who perished in these
conditions. By collecting sea water samples and analyzing microbial DNA,
they were able to conclude that these microbes did not necessarily adjust
to their environment, but rather were "waiting" for their time to shine
in these low-oxygen conditions.

"The take home message here is that you have a community of microbes;
it has a particular composition and plugs along, then suddenly, all of
the oxygen is removed and you get a replacement of community members. They flourish for a while, and eventually hypoxia goes away, oxygen comes back,
and that community rapidly shifts back to what it was before due to the
change in resources. This is very much in contrast to what you see with macro-organisms," said Jarrod Scott, paper co-author and postdoctoral
fellow at the Smithsonian Tropical Research Institute in the Republic
of Panama.

Scott and Johnson agree that human activity can contribute to the nutrient pollution and warming waters which then lead to hypoxic ocean conditions.

Activities such as coastal land development and farming can be better
managed and improved, which will reduce the likelihood of deoxygenation
events occurring.

The study provides insight to the fate of microbe communities on a coral
reef during an acute deoxygenation event. Reef microbes respond rapidly
to changes in physicochemical conditions, providing reliable indications
of both physical and biological processes in nature.

The shift the team detected from the hypoxic microbial community to a
normal condition community after the event subsided suggests that the
recovery route of reef microbes is independent and decoupled from the
benthic macro-organisms.

This may facilitate the restart of key microbial processes that influence
the recovery of other aspects of the reef community.

========================================================================== Story Source: Materials provided by
Woods_Hole_Oceanographic_Institution. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Maggie D. Johnson, Jarrod J. Scott, Matthieu Leray, Noelle Lucey,
Lucia
M. Rodriguez Bravo, William L. Wied, Andrew H. Altieri. Rapid
ecosystem- scale consequences of acute deoxygenation on a Caribbean
coral reef.

Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-24777-3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210726165845.htm

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