COLLEGE STATION, TX – In a sampling trip in September,
Louisiana’s marshes showed dissolved organic carbon (DOC) levels twice
as high compared to when they were sampled in July – just after the
spill. This is likely attributed to the low DOC levels that were present
when the Davis Pond Freshwater Diversion Structure was pumping
Mississippi River water into the marshes at full capacity to prevent
even more oil entering from the Gulf, according to results of a study by
a Texas A&M University professor who recently returned from the
region.
Thomas Bianchi, professor of chemical oceanography who
has conducted research on Louisiana marshes for years, says that areas
he examined around Barataria Bay – east of the city of Houma – show that
these river diversions can be good in keeping oil out. Still, there is a
change in the overall ecosystem as measured by a significant increase
in DOC, observes Bianchi, who adds he expected the DOC to be higher this
time when Mississippi Water from the Davis Pond diversion structure was
not being pumped in.
Bianchi’s work is funded by a National
Science Foundation, Division of Chemistry (Environmental Sciences)
grant. He is joined in the project by Robert Cook of Louisiana State
University, Michael Perdue of Georgia Tech and Alex Kolker of the
Louisiana Universities Marine Consortium.
“The marshes are going
into their winter season, and they have started to senesce (turn brown)
for the winter season,” Bianchi explains.
“Many of these areas
still have small traces of oil. While only a small percentage of the
marsh areas has been hit pretty hard with heavy oil contamination, many
seem to be in good shape.
“We found a lot of DOC and in many
cases, the amount was about double of what we had seen back in July. We
think that is related to marsh decay, natural rise in wetland DOC levels
compared to river water that was being diverted at the time, and
possibly to some breakdown products from petroleum.”
He notes
there has been a contentious debate “over the effects of river
diversions in this region, adding “our results now add more details on
the chemistry of the water changes after a diversion.”
Bianchi
says the marshes and wetlands have numerous places for oil to collect on
the vegetation, which could result in higher DOC levels. Higher amounts
of DOC can also cause lower oxygen levels in the water, producing fish
kills.
“The marshes have a lot of places for the oil to get
‘snagged’ or collected onto the plants, which is very different from the
open gulf,” he points out. “So it can definitely be said that oil is
still present in many of these wetland areas.”
“It (oil) tends to
gets trapped by particles and settles in around the base of the marshes
where it is not easily seen. This is very different from oil on the
surface of the open ocean; it’s there for everyone to see, but down in
the reeds and root systems, there’s a lot more structure for it to stick
to. But there’s no doubt it’s still there, and the longer it remains in
the system the greater the chance for this material to be incorporated
into the wetland food web.”
Bianchi adds that after several
months of winter, it will be necessary to survey the area again to see
how much of the marshes survived the cold fronts that bring more erosion
of marsh material, and to check the overall “health” of the wetlands
area.
“One thing to point out is that some of these marshes have
not rebounded, and they have, in part been, also impacted by the
‘cleaning’ approach used by some oil companies,” he notes.
“They
use high pressure water systems, which on rocks is fine, but not on
plant material. So, this has likely cut them back so short that changing
water levels with tides and storms will make them more vulnerable in
the upcoming months. The high DOC we are observing also was likely
impacted by this ‘washing’ technique.
“When we examined the areas
earlier this summer, you could see a lot of ‘chocolate mousse-like’
matter in the marshes where the oil was really thick,” the Texas A&M
oceanographer adds. “It’s not that bad now. Much of this has been
washed away by the tides or has been decomposed by bacteria, but the
lack of regret in some of the marshes suggests that the toxicity levels
in a small number of marshes are still high enough to prevent plant
growth.
“Will these marshes and wetlands bounce back? That’s a
big question we need to find out, but we can say that the effects of the
spill on marsh erosion will still likely pale in comparison to the
channelization, river levees and oil and gas exploration that have
plagued this region and enhanced wetland erosion for over 50 years,” he
adds.
For more information, go to Bianchi’s research site at http://ocean.tamu.edu/profile/TBianchi.
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