Protecting bays from ocean acidification
Research shows that submerged vegetation helps to offset Chesapeake Bay acidification

Date:
June 12, 2020
Source:
University of Delaware
Summary:
As oceans absorb more human-made carbon dioxide from the air,
a process of ocean acidification occurs that can have a negative
impact on marine life. But coastal waterways, such as Chesapeake
Bay, can also suffer from low oxygen and acidification. New research
identifies one way to protect these waterways -- the presence of
submerged aquatic vegetation (SAV).



FULL STORY ==========================================================================
For many years, the world's oceans have suffered from absorbing human-made carbon dioxide from the atmosphere, which has led to the decreasing pH
of saltwater, known as ocean acidification, and threatened the health
of marine organisms and ecosystems. While this process has been well documented, the acidification process is complicated and poorly understood
in coastal waters.


==========================================================================
For example, the main stem of Chesapeake Bay, the largest estuary in the
east coast, has suffered from low oxygen and acidification for years in
its bottom waters. Unlike ocean waters, acidification in estuaries like Chesapeake Bay is driven by both fossil fuel-derived carbon dioxide as
well as carbon dioxide released from the intense decomposition of algae
spurred by nutrient inputs from surrounding land. Although scientists
are improving their understanding of the causes of acidification, the
ways in which coastal waters like Chesapeake Bay fight back and resist acidification are less known.

One possible way the Chesapeake Bay is combating ocean acidification
comes in the form of an already present ally: submerged aquatic vegetation (SAV). While there was a bay-wide decline of SAV from the 1960s through
the 1980s, restoring these once-abundant SAV beds has been a primary
outcome of efforts to reduce loads of nutrients and sediments to the
estuary and SAV cover has increased by 300 percent from 1984 to 2015.

One of the largest recovered SAV beds lies in an area of the bay known
as the Susquehanna Flats -- a broad, tidal freshwater region located
near the mouth of the Susquehanna River at the head of the bay.

The University of Delaware's Wei-Jun Cai was part of a research group that recently conducted a study of the bay, including in the Susquehanna Flats,
in order to understand how the Chesapeake Bay uses a defense mechanism
against acidification -- known as buffering -- to help reduce carbon
dioxide and acidification in its waters during the summer time.

The research team included researchers from Xiamen University in China,
St.

Mary's College, Oregon State University and the University of Maryland
Center for Environmental Science's Chesapeake Biological and Horn Point Laboratories.



==========================================================================
They found that strong photosynthesis by the plants in SAV beds at
the head of the bay and in other shallow, nearshore waters can remove
nutrient pollution in the bay, can generate very high pH, and elevate
the carbonate mineral saturation state, which facilitates the formation
of calcium carbonate minerals. When these calcium carbonate particles and
other biologically produced carbonate shells are transported downstream,
they enter acidic subsurface waters where they dissolve.

This dissolution of the carbonate minerals helps to "buffer" the water
against pH decreases or even support pH increases. "Just like people take
Tums to neutralize the acids that cause heartburn, the idea is that SAV
beds send carbonate minerals to the lower Bay to neutralize acids there,"
said Jeremy Testa of the University of Maryland Center for Environmental Sciences and a co- author of the study.

The research was recently published in Nature Geoscience. The first
author, Jianzhong Su, was a UD-Xiamen University Dual Degree doctoral
student and had Cai as an adviser.

Calcium carbonate dissolution In previous work, Cai, the Mary
A.S. Lighthipe Professor in the School of Marine Science and Policy in
UD's College of Earth, Ocean and Environment, showed there was a lot of
calcium carbonate dissolution in the subsurface water of the lower bay
but they didn't know where that carbonate was coming from.



========================================================================== "This paper shows unique evidence that the carbonate comes from these
submerged aquatic vegetation beds," said Cai. "Shallow waters in the
upstream heads and nearshore areas can have a vast amount of submerged
aquatic vegetation." In these areas during summer time, sunlight
combines with nutrients to allow dense SAV beds to initiate high rates
of photosynthesis that causes the pH in the water to increase, meaning
the water is less acidic.

Because the pH is so high, the researchers were able to collect and
measure the carbonate particles on the surface of the leaves, which
they could scrape and analyze. Co-authors Chaoying Ni, professor in
UD's Department of Materials Science and Engineering and Director of
the W.M. Keck Center for Advanced Microscopy and Microanalysis, and
Yichen Yao, who was a master's level student in materials engineering,
did the mineral analysis.

"The lab did an image for us and showed the carbonate in these sediments
and the sediment on the leaves, the particles, their concentration was
a lot higher than the bottom sediment," said Cai.

Theoretical carbon formations When the researchers went to a shallow
area upstream of the Susquehanna Flats, they also found the carbonate,
which led them to their theory that the carbonate forms in one location, particularly, in the SAV bed of the Susquehanna Flats, and then it's transported to the lower bay.

"We know there is a lot of carbonate dissolution in the lower bay,
and we know the upper bay is where the carbonate is formed. So in the
paper, we hypothesize that it's that formation in the SAV bed that gets transported downstream and dissolves and we reproduce this downstream
transport with a numerical model," said Cai. "This carbonate that is transported from upstream actually acted as a way to resist, to buffer
the pH of the system." There are important ecological ramifications of
this finding in that coastal nutrient management and reduction not only
help to fight against low oxygen stress but also acidification stress
to the environments and organisms that live there via the resurgence of submerged vegetation.

Cai said that while their preliminary results are encouraging, the next
steps are to determine if the carbonate particles are really transported
by the currents and tides to the lower bay and if so, how fast and under
what conditions this happens. He wants to go back to the Bay to nail down
the missing link between where the carbonate forms and where it dissolves.

"This is a very interesting thing," Cai said. "People talk about ocean acidification and very rarely talk about what resists it, what can buffer
the system against ocean acidification. So that's what we want to find."

========================================================================== Story Source: Materials provided by University_of_Delaware. Original
written by Adam Thomas.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Jianzhong Su, Wei-Jun Cai, Jean Brodeur, Baoshan Chen, Najid
Hussain,
Yichen Yao, Chaoying Ni, Jeremy M. Testa, Ming Li, Xiaohui Xie,
Wenfei Ni, K. Michael Scaboo, Yuan-yuan Xu, Jeffrey Cornwell,
Cassie Gurbisz, Michael S. Owens, George G. Waldbusser, Minhan Dai,
W. Michael Kemp.

Chesapeake Bay acidification buffered by spatially decoupled
carbonate mineral cycling. Nature Geoscience, 2020; 13 (6): 441 DOI:
10.1038/ s41561-020-0584-3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200612172206.htm

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