Chesapeake BayGeology, Chemistry, Physics and Biology

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Chesapeake BayGeology, Chemistry, Physics and
Biology
Lecture 22
OEAS-306
April 23, 2009

• Outline
• Geologic History of Chesapeake Bay
• Physics of Estuarine Circulation
• Hypoxia in the Bay
• Water Quality and SAV
• Oysters
• Crabs

Good PBS show about Chesapeake Bay FrontlinePois
oned Waters
http//www.pbs.org/wgbh/pages/frontline/poisonedwa
ters/
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Chesapeake Bay Watershed

• Chesapeake Bay is the largest estuary in the US
• Watershed covers 64,300 square miles including
  the District of Columbia and Part of 6 States
  (VA, WV, MD, DE, PA, and NY)
• Over 16 million people live in the Bay watershed.
• More than 150 rivers and streams drain into the
  Bay.
• Main stem of the Bay is about 200 miles long.
• There are over 11,500 miles of shoreline in the
  Bay and tributaries.
• Average depth of the Bay is less than 7 meters
  (21 feet).
• Bay is home and nursery to numerous commercially
  valuable species including blue crabs, oysters,
  stripped bass, menhaden, white perch.

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A large meteorite hit the region that is now
lower Chesapeake Bay about 35 million years ago.
Companies were exploring beneath Chesapeake Bay
for structures that might contain oil and gas. As
part of that search, they collected a network of
seismic reflection profiles in the bay. These
profiles showed clearly that a huge impact crater
is buried beneath the bay centered near the town
of Cape Charles. The crater is 90 km in diameter
and 1.3 km deep. It covers an area twice the
size of Rhode Island and is nearly as deep as the
Grand Canyon
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• Sediment cores revealed large hand-size to
  person-size chunks (clasts) of clay, limestone,
  and sand.
• There were also Tektites--millimeter-to-centimeter
  -size glass beads derived from sediment melted by
  the impact

breccia is a rock composed of angular fragments
or minerals or rocks in a matrix

• Center of the crater is filled with an impact
  breccia and all around the crater is an ejecta
  blanket.
• Dating the materials in the cores did not make
  sense in terms of normal geological processes.

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Geologic History of the Chesapeake Bay
Calvert Cliffs, Maryland
At one time, a warm, shallow sea covered all of
Southern Maryland. When the sea receded 20-25
million years ago, marking the end of the Miocene
period, cliffs were exposed and began eroding.
Today these cliffs reveal the remains of
prehistoric species, including sharks, whales,
rays and seabirds that were the size of small
airplanes.
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Fossils Found at Calvert Cliffs, MD
Megalodon was a giant shark that lived in
prehistoric times. The oldest remains of this
species are about 18 million years old. Probably
became extinct about 1.5 million years ago.
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Sea-level Variations over the past 200,000 years
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Response of Chesapeake Bay to Sea-Level Variations
Lowstand pre-500,000 yrs ago
Lowstand 430,000yrs ago
Highstand 200,000 yrs ago
Lowstand 150,000 yrs ago
Highstand 400,000 yrs ago
Lowstand 340,000 yrs ago
Highstand 120,000 yrs ago
Lowstand 18,000 yrs ago
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Today, Chesapeake Bay is largely filled in with
Sediment.
High turbidity areas.
Estuaries are very effective at trapping
sediment.
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Physical Processes in Chesapeake Bay.
Chesapeake Bay is an estuary
rivers
Estuaries are semi-enclosed bodies of water with
a free connection to the open sea (tides), where
sea water is measurably diluted with fresh water
derived from land drainage
rivers
ocean
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Tides flow in (flood) and out (ebb) of the
Chesapeake Bay every 6 hours.
But the mean circulation (days) is in at the
bottom and out at the surface.
Baroclinic
Barotropic
Tides
Estuarine or Gravitational Circulation
Mean circulation maintains salt in estuary. Also
effectively traps sediment
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Hypoxia in Chesapeake Bay
FRESH
SALTY
1) The mean residual circulation brings salty
(dense) water up along the bed and fresh
(lighter) water out at the surface. 2) This leads
to strong density stratification. 3) Stratificatio
n prevent the mixing of oxygen from the surface
to the area below the pycnocline.
Phytoplankton bloom (photosynthesis gives off O2)
Phytoplankton die and sink to bottom
Decomposition of dead phytoplankton uses up O2
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Map of Mean Dissolved Oxygen -- Summer 2005
DOlt1mg/L
Many organisms are negatively impacted when
oxygen concentrations are below 2 mg/L.
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Sub-merged Aquatic Vegetation (SAV).

• More than 16 species of underwater bay grasses
  also called submerged aquatic vegetation or SAV
  are found in the Bay.
• SAV are an excellent measure of the Bays overall
  health because they are not harvested and their
  health is closely linked to water quality.
• SAV provide shelter and habitat for many fish,
  shellfish and waterfowl.
• SAV serve as protective nurseries for crabs and
  juvenile fish
• SAV add oxygen to the water through
  photosynthesis, trap and anchor sediment that
  would otherwise cloud the water and bury benthic
  organisms.
• SAV help absorb excess nutrients

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• Increased nutrient and sediment inputs, reduces
  water clarity.
• This reduces that amount of SAV in the Bay.
• With less SAV, sediments arent stabilized and
  fewer nutrients are absorbed.
• This results in greater phytoplankton blooms,
  which further reduces water quality.

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Both overfishing, disease and poor water quality
have nearly wipe out oysters in Chesapeake Bay
At peak abundance (prior to 1870) it was
estimated that oysters could filter the water of
the bay every 3.3 days. Now it is estimated that
it would take over 300 days. In addition to
filtering algae and particulates from the water
column, the Eastern oyster forms
three-dimensional reefs that provide habitat for
other species in the bay
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Should we introduce a non-native Asian oyster
into Chesapeake Bay?
Crassostrea ariakensis
Crassostrea virginica

• Almost 90 of native oysters are wiped out by
  disease (MSX and dermo) before reaching maturity.
• The Asian oyster is thought to be more resistant
  to MSX and dermo.
• However, there are many concerns
• 1) Would Asian oyster introduce new diseases?
• 2) Would it completely wipe out native oysters?
• 3) Would it perform the same ecological function
  as the native oyster?
• 4) What unforeseen impacts could they have on
  the environment?

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With few oysters, there has been increased
fishing pressure on blue crabs in the Bay

• Estimates of the crab population were down
  significantly in 2008.
• This prompted tighter restrictions on crabbing,
  including banning winter dredging in Virginia.
• This years survey indicated that the population
  increased by about 40.
• Should the ban on winter dredging be lifted?

Millions of pounds landed
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Calculating the Number of Crabs in the Bay
Winter Dredge Survey

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Human development has increased nutrient and
sediment inputs
Without human interference, the Bay operates as
a massive self-filtering ecosystem regulating its
own water quality.
Fewer SAVS, oysters and other filter feeder
results in higher nutrient and sediment
concentrations
Increased algae blooms
Decrease in SAV beds
Decreased oyster, crab, fish populations.
More sediment and higher nutrient
concentration/less habitat for fish/crabs/etc..
Larger hypoxic dead zones