Subaqueous Soil Science in the Indian River Lagoon

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Subaqueous Soil Science in the Indian River Lagoon

• A focus on Halophila johnsonii habitats

L. Rex Ellis UF Mary. E. Collins UF Garett
Lipps - FDOT
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Halophila johnsonii

• Halophila johnsonii (Hj) is Johnsons Seagrass
• Habitat range is in the IRL, south of Sebastian
  inlet
• Not known to reproduce sexually
• Spreads by vegetative growth or by fragments
• Dispersal is limited, thus it is listed as
  endangered

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Halophila johnsonii

• Can grow in any water depth that other seagrasses
  grow, but is usually confined to the shallow and
  deep extremes
• Also found in disturbed areas

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Halophila johnsonii

• Question What are the characteristics of shallow
  habitats supporting Hj?
• This is an important question if we desire to
  create Hj habitat.
• Additionally, we can use this information to
  forecast where Hj will occur in newly constructed
  habitats.

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A constructed segrass habitat SL 15

• Seagrass and mangrove damages resulting from the
  Jensen Beach Causeway expansion resulted in a
  need for mitigation.
• Remove a spoil island to create new mangrove and
  seagrass habitat.

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Jensen Beach Causeway
Jensen Beach Causway
IRL
Stuart
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Jensen Beach Causeway
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Mangrove Impacts and Mitigation

• Impacts 1.36 acres of mangroves directly
  impacted by construction
• Mitigation Paid into the C-18 Canal Mitigation
  Fund
• Fund for future restoration of mangroves along
  the C-18 Canal in northern Palm Beach County

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Seagrass Impacts
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Seagrass Mitigation

• Offsite Mitigation SL-15 Spoil Island
• Ratio of 21 for in-kind mitigation
• Ratio of 31 for out-of-kind mitigation
• Creation of Habitat
• 3.28 ac seagrass
• 4.74 ac mangrove saltmarsh cordgrass
• Restoration of 2.38 ac of upland habitat

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SFWMD Permit Requirements

• Annual seagrass surveys
• Post-construction survey
• Mitigation for mangrove and seagrass impacts

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SL 15 Concept

• Spoil islands were created in the 1950s by
  dredging the IRL channel and placing dredge-spoil
  material onto existing seagrass flats.
• These islands have been in place for 50 yrs.
• Remove spoil islands to restore/create seagrass
  and mangrove habitat.

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SL 15
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SL-15 Pre-Mitigation Habitats
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SL15 Restoration Plan
4 ft NGVD
1 ft NGVD
-1.5 ft NGVD
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Barge Transport of Fill

• Project Permit was then modified to allow spoil
  to be piled and dried at the mitigation site.
• Spoil was then transported by conveyor belt into
  barges, which then carried the fill to a mainland
  site adjacent to the original drying lagoon.

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SL 15 Progression
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Semi-permanent monitoring quads
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Seagrass Area

• What are the spatial patterns of seagrass natural
  recruitment?
• Do other benthic vegetation show similar
  patterns?
• Which grasses colonize first?

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Vegetative Succession
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Vegetative Succession
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Vegetative Succession
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Drift Algae
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Drift Algae
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Drift Algae
75 Cover
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Drift Algae Density (2006)
Percent Cover
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Drift Algae Density (2006)
Percent Cover
100
0
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Drift Algae Density (2006)
Percent Cover
Percent Cover
100
Green areas are patches of seagrass delineated in
the field
0
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Seagrass Density (2006)
Shoots / m2
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Seagrass Density (2006)
Shoots / m2
0.8
0
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Seagrass Density (2006)
Shoots / m2
0.8
Green areas are patches of seagrass delineated in
the field
0
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Seagrass Density (2006)
cm NGVD
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Soil Elevation
cm NGVD
-100
-180
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Soil Elevation
cm NGVD
-100
-180
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Incoming Tide
Velocity (cm/s)
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0
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Outgoing Tide
Velocity (cm/s)
30
0
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Flow in
Flow out
Drift algae could be carrying in Hj. Shoot
patterns of other grasses reflect similar drift
patterns. What is the grass source prevailing
easterlies or proximity to western disturbance?
Shoots
Macroalgae
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SL 15 Conclusions Seagrass

• It appears as if currents bring algae and grass
  into the seagrass area.
• These are trapped on the western shore.
• Need to understand whether this happens because
  of predominant winds and currents or because of
  the large seagrass bed to the east.

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Mangrove Area
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Mangrove Area

• Transect monitoring has show low survivorship in
  some areas.
• What are the spatial patterns to survivorship?
• What can we infer from these patterns?

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Mangrove Density (2007)
Mangroves / m2
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Mangrove Density (2007)
Mangroves / m2
1.75
0.1
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Mangrove Survivorship (2007)
Surviving
25 - 50
50 - 75
75 - 100
100 - 125
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Mangrove Survivorship (2007)
Surviving
Above 80
Below 80
Below
Above
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SL 15 Conclusions Mangroves

• Survivorship is highest along the berm edge.
• Could be trapping new recruits.
• Could be less flooded.
• Need to compare to recruitment in spartina areas.
• Need to test by creating berms in middle of
  mangrove area.

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External Factors

• Has the grass outside SL 15 declined?
• What have been the water quality trends since
  2005?
• Casual observations suggest more algae and
  epiphytes this year.

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Site Locations
Ft. Pierce Inlet
Locate sites where clear High and Low edges of Hj
habitat can be observed. Avoid disturbed
sites. Sites at distance from inlets do no seem
to exist.
St. Lucie Inlet
Jupiter Inlet
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A Typical Transect
High Tide
Thalassia testudinum
Halophila johnsonii
Halodule wrigtii
No seagrass
Halophila johnsonii
Low Tide
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A Typical Transect
Hj Low
Hj High
High Tide
Hj High Elevation
Hj Low Elevation
Low Tide
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Elevations of Hj Habitat
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Elevations of Hj Habitat
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Elevations of Hj Habitat
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Soil Characterization
These are extremely sandy soils with very little
organic matter. The sand is in the fine and very
fine fractions and is dominantly siliceous.
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Soil Temperature
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Soil Temperature
Cool flat
Hot flat
Edge of Hj
Hot flat
Hot flat
Hot flat
Hj patch in mangrove shade
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Soil Temperature

• Mangroves are typically thought to shade out
  seagrasses.
• Hear it appears that Hj prefers the shade.
• Could the flat be too hot for Hj?
• Could the mangroves provide temperature relief?
• At what depth should soil temperature be measured?

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Soil Temperature
Water Surface
Soil Surface
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Soil Temperature
Temperatures may converge around 15cm depth
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Soil Temperature

• For comparative studies, perhaps measure T at
  shallow (e.g. 1-3cm) and at deep (e.g. 15-20cm)
  points in the soil.
• Also need to analyze the effect on tide and air
  temperature.
• Do this throughout the day and throughout the
  year.

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Conclusions

• Hj and other seagrasses will colonize a
  mitigation site within one season.
• Colonization probably affected by ability of
  seeds and fragments to enter and settle.
• Vegetative growth does not appear viable in the
  short term.
• Hj target elevations a function of distance from
  inlet.

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Conclusions

• Cannot directly compare to existing Hj elevations
  unless many proximate observations are made and
  analyzed.
• If far from an inlet, do not expect Hj to
  persist.
• Soil types should be fine sands w/ low OM.
• Must determine T controls on Hj, so designs can
  avoid high temp flats.

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Thank You
Kelly Fischler UF Graduate Student Todd Osborne
UF Research Scientist FDOT Research Funding
and Logistical Support ESciences Seagrass patch
mapping w/ FDOT