https://www.vims.edu/newsandevents/topstories/2019/oil_spill_grass.php
[links and images in online article]
Study shows continuing impacts of Deepwater Horizon oil spill
by David Malmquist | April 18, 2019
Findings reveal that restoration of marsh vegetation is key to overall
recovery
Nine years ago tomorrow—April 20, 2010—crude oil began leaking from the
Deepwater Horizon drilling rig into the Gulf of Mexico in what turned
out to be the largest marine oil spill in history. A long-term study
suggests the oil is still affecting the salt marshes of the Gulf Coast,
and reveals the key role that marsh grasses play in the overall recovery
of these important coastal wetlands.
Conducting the study was a multi-institutional research team funded in
part by the Gulf of Mexico Research Initiative, a 10-year independent
program established through a $500 million financial commitment from BP.
The team began sampling soon after the spill was finally contained, and
continue their work today. Their most-recent article—in Estuaries and
Coasts—reports on the first six and a half years of sampling post-spill.
Lead author on the study is John Fleeger, an emeritus professor at LSU.
Co-authors are Rita Riggio, Irving Mendelssohn, Qianxin Lin, and Aixin
Hou of LSU; David Johnson of William & Mary’s Virginia Institute of
Marine Science; Donald Deis of Atkins North America; Kevin Carman of the
University of Nevada-Reno; Sean Graham of Nicholls State University; and
Scott Zengel of Research Planning, Inc.
Johnson, an assistant professor at VIMS and expert in salt-marsh
invertebrates, says “Our study highlights the crucial role that plants
play in the recovery of important links in the Gulf of Mexico’s coastal
food web.” Those links ultimately connect to the fish and shellfish that
support the region’s economy and culture.
Two plants dominate healthy Gulf Coast salt marshes—the smooth cordgrass
Spartina alterniflora and the black needlerush Juncus roemerianus. Also
abundant on the marsh surface are single-celled, plant-like organisms
that scientists collectively refer to as benthic microalgae, while a
suite of small invertebrates—amphipods, copepods, nematodes, snails,
worms, and others—swim, hop, and crawl among the grass blades or burrow
in the underlying root zone.
The team studied these organisms by measuring their abundance and
biomass in heavily oiled, moderately oiled, and oil-free areas of
Louisiana’s Barataria Bay, using both surface plots and shallow cores.
Sampling took place at roughly 6-month intervals between 2011 and 2016.
The researchers’ early sampling showed that nearly all the plants in
heavily oiled areas died, while benthic microalgae and burrowing
invertebrates suffered significant reductions. Their later sampling
showed that marsh recovery was led by benthic microalgae and
Spartina—which began to show significant above-ground growth within two
to three years.
Importantly, it was only after Spartina started its comeback that
recovery of the invertebrate community began in earnest. “Plants are the
foundation of salt marshes,” explains Johnson. “Marsh grasses facilitate
colonization by burrowing invertebrates, fuel the food web, provide
animal habitat, bind the soil, and slow water flow. Without plants there
is no marsh, and there is no marsh recovery following a spill without
plants leading the way.”
If you plant it, they will come
The team’s findings have important implications for responding to any
future spills. Fleeger says “our findings indicate that mitigation
strategies for any future spills should include the planting of
foundation species such as Spartina.”
Mendelssohn, a VIMS alumnus (M.A. '73), says that foundation species
“enhance recovery by providing habitat and reducing sediment erosion.”
Over the longer term, he says, “plant growth enhances recovery by
improving soil quality. Plants generate organic matter that accumulates
below ground, while their roots and rhizomes release oxygen, bind
sediments, and increase sediment volume. Breakdown of plant tissues also
provides nutrients that further stimulate plant growth and beneficial
microbial processes in the marsh.”
A slow road to full recovery
Tempering the promise of marsh recovery via planting of grasses such as
Spartina is the team’s discovery that heavily oiled marsh sites remained
less healthy than moderately oiled and oil-free sites more than 6 years
after the Deepwater Horizon spill. Heavily oiled sites still had
elevated concentrations of oil and its breakdown products, and showed
slower growth of black needlerush, lower production of plant detritus
and below-ground organic matter, and altered soil density. Populations
of worms, juvenile snails, and other small invertebrates had also failed
to fully recover.
Particularly troubling was the continued rarity of the polychaete worm
Manayunkia aestuarina. One of the most abundant single species in the
invertebrate community, this tube dweller is important to the health of
marsh sediments, and plays a key role in the marsh food web as a major
prey item for crabs, shrimp, and fish. “The near absence of this species
could indicate significant alteration of ecological function at heavily
oiled sites,” says Johnson.
Also troubling is that projecting the observed pace of mash recovery
into the future suggests that complete recovery at moderately and
heavily oiled sites will likely take much longer than a decade. This is
slower than reported in many previous studies of oil spills and their
impacts on the marsh community.
“Previous work shows that oil spills in salt marshes can impact
bottom-dwelling invertebrates for more than four decades,” says Fleeger.
“Long-term exposure to oil and its breakdown products may also decrease
the sensitivity and resilience of these organisms to future spills,” he
adds.
On a brighter note, a previous study by Johnson and colleagues suggests
that fertilization of Spartina plantings can enhance growth of both its
stems and roots, thus aiding marsh recovery in the long term. “We’re
starting to see the salt marsh in the Gulf of Mexico rebound,” says
Johnson, “but it will likely be a decade or more before we see complete
recovery.”
Fleeger, J.W., Riggio, M.R., Mendelssohn, I.A. et al. Estuaries and
Coasts (2019) 42: 204. https://doi.org/10.1007/s12237-018-0443-2
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