Transplanted Oyster (Crassostrea virginica) Beds as Self-Sustaining Mechanisms for Water Quality Improvements in Small Tidal Creeks: A Pilot Study

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Transplanted Oyster (Crassostrea virginica) Beds
as Self-Sustaining Mechanisms for Water Quality
Improvements in Small Tidal Creeks A Pilot Study
Kimberly A. Nelson
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OYSTER FILTRATION

• Oysters filter up to 120 L of H2O daily.
• In flume studies, oysters may remove up to 50
  of the seston from the overlying water column.
• Oysters may decrease the risk of eutrophic
  conditions, thereby improving light penetration
  and water clarity.

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HYDRODYNAMICS
Low Velocity Shadow Zone
Flow
REEF

• Increased turbulence at reef crest
• Increased flow velocities at reef crest
• Low velocity shadow zones
• downcurrent and on reef edges

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OYSTER RESTORATION

• Current Methods
• Large subtidal reefs of high-relief, to improve
  reef health and survival.

Piankatank River, Virginia

• PROBLEMS
• Subtidal reefs may not allow the majority of the
  water column to reach the benthos for filtration.
• High velocities may decrease filtration
  capabilities.

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OBJECTIVES

• Establish self-sustaining oyster reefs in two
  small tidal creeks.
• Examine hydrodynamic changes associated with the
  placement and subsequent growth of the oyster
  reefs.
• Investigate downstream changes in suspended
  particulate matter, chlorophyll a and ammonium as
  function of reef placement and growth.

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STUDY AREA
UPLAND
LOWER CREEK
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METHODS

• REEF PLACEMENT
• September 2000
• 2 m X 3 m
• PVC frame with
• hardware cloth
• Mature oysters (3)
• 125 live oysters per m2

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WATER QUALITY ANALYSIS

• Monthly Samples
• Total Suspended Solids (TSS)
• Chlorophyll a
• Ammonium

Ebbing Tide
1 m
3 m
5 cm
REEF
4-6 cm
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SAMPLING SCHEME Upland

• Flow Measurements
• Reef Channel Profile
• Sediment Analysis
• Reef Characteristics
• Density
• Growth
• Mortality

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OYSTER PARAMETERS
70
length
60
width
50
40
Mean size (mm)
30
20
10
0
Aug - 00
Dec - 00
Jun - 01
JUN - 01
DEC - 00
Total mortality survivorship
20.3 43.8
79.7 56.1
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Mean TSS - Lower Creek
Reef Control
Reef Control Channels
TSS mg L-1
Above Below
Above Below Reef
TSS mg L-1

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Mean TSS Upland
Reef Control
Reef Control Channels
TSS mg L-1
Above Below
Above Below Reef
TSS mg L-1


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CHLOROPHYLL a

• Phytoplankton is the preferred food source of
  Crassostrea virginica.
• Oysters may deplete 20 to gt75 of chlorophyll a
  concentrations from the particulate supply over
  oyster beds in flume studies.
• By active filtration of phytoplankton and
  nutrients (phosphate, nitrogen, silica), oysters
  may decrease the potential for eutrophication in
  estuaries.

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Mean Chlorophyll a Lower Creek
Reef Control
Reef Control Channels
Chl a µg L-1
Above Below
Above Below Reef
Chl a µg L-1
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Mean Chlorophyll a - Upland
Reef Control
Reef Control Channels
Chl a µg L-1
Above Below
Above Below Reef
Chl a µg L-1
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FLOW DYNAMICS
Discharge
Velocity
Max impact
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Hour 3 Chlorophyll a - Lower Creek Reef to
Control Channels
Reef Control
Chl a µg L-1
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Hour 3 Chlorophyll a - Lower Creek Above to Below
Reef
Above Below
Chl a µg L-1
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Hour 3 Chlorophyll a - Upland Reef to Control
Channels
Reef Control
Chl a µg L-1

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Hour 3 Chlorophyll a - Upland Above to Below Reef
Above Below
Chl a µg L-1



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AMMONIUM (NH4)

• After consumption and processing, seston is
  deposited as feces or pseudofeces.
• Oysters may contribute 15 to 40 of the
  dissolved organic nitrogen within in situ domes.

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Ammonium - Lower Creek Reef to Control Channels
Reef Control
Ammonium µg L-1
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Ammonium - Lower Creek Above to Below Reef
Above Below
Ammonium µg L-1
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Ammonium - Upland Reef to Control Channels
Reef Control
Ammonium µg L-1
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Ammonium - Upland Above to Below Reef
Above Below
Ammonium µg L-1
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Hour 3 Ammonium - Lower Creek Reef to Control
Channels
Reef Control
Ammonium µg L-1
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Hour 3 Ammonium - Upland Above to Below Reef
Above Below
Ammonium µg L-1
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Hour 3 Ammonium - Upland Reef to Control Channels
Reef Control
Ammonium µg L-1
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GRAIN SIZE ANALYSIS
Mode(µm) Mean(µm)
Reef Matrix 16.40 103.8 Downstream 19.76 1
30.0 Upstream 140.1 120.2 Control 140.1 152.
4
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LAST THOUGHTS

• Small, viable oyster reefs can be established
  and maintained over periods of at least ten
  months duration in small upland tidal creeks.
• An optimal ratio of reef size to flow discharge
  exists whereby filtration is maximum.
• Additional studies are needed to determine the
  volume of reef ratio and the reef geometry that
  would achieve desired results utilizing maximum
  resources.