Post on 26-Jun-2020
The Grass is Always Greener: Seagrass Ecology in South Florida
R.P
. van D
am
James W. Fourqurean Biological Sciences and Southeast Environmental Research Center, Florida International University
Ocean Life Lecture Series, Florida Keys Cultural Center, Ley Largo February 17, 2012
Outline
• What are seagrasses?
• What is the global status of seagrass ecosystems?
– Seagrass services
– Seagrass history
– Seagrass pressures
– Seagrass response
– Human response
• What is the local status of the seagrasses in south Florida? – signs of trouble in paradise
Seagrasses are flowering plants that live submerged in the sea
The seagrasses are an ecological group, NOT a taxonomical unit
A plant is called a seagrass if it: 1. Grows when fully submerged 2. Is securely anchored by a root system 3. Is adapted to live in salt water 4. Has flowers that are pollinated under water 5. Can compete with other organisms in the sea
Seagrass or seaweed?
300 BC: Seagrasses are a type of “seaweed”
- Theophrastus (300 BC), Enquiry into Plants
No. Species Seagrasses •50-60 spp. Flowering plants •235,000 spp. Marine plants •60,000 spp. Phytoplankton • 50,000 spp. Macroalgae - seaweeds • 10,000 spp. Seagrasses (No. Genera) •7 tropical (Halodule, Cymodocea, Syringodium, Thalassodendron, Enhalus, Thalassia, Halophila), •5 temperate (Zostera, Phyllospadix, Heterozostera, Posidonia, Amphibolis) •1 cosmopolitan (Ruppia)
Evolution
Seed
Male flower
Female flower Juvenile
Juvenile
Fruit
LIFE CYCLE of
Thalassia testudinum
Male clone
Female clone
Seagrasses are abundant in tropical and temperate regions
Halophila:
Bocas del Toro, Panama
Thalassia:
Kuna Yala, Panama
Ruppia:
Morro Bay, USA
Zostera:
Ria Formosa, Portugal
Seagrass provides critical food source in tropical regions
R.P
. van D
am
J. K
enw
ort
hy
Manatee (Trichechus)
In Thalassia meadow,
Puerto Rico
Green Sea Turtle (Chelonia)
In Thalassia and
Syringodium meadow,
Yucatan, Mexico
Seagrass provides critical habitat in temperate regions
G. K
endrick
G. P
erg
ent
Seahorse (Hippocampus)
In Cymodocea meadow,
Mediterranian Sea
Zebra fish (Girella)
In Posidonia meadow,
Perth, Western Australia
Seagrass provides valuable fisheries resources
Nutrient processors: Seagrass beds absorb and transform nutrients in the marine environment
Sediment stabilizers: seagrasses efficiently hold
sediments in place, preventing resuspension and
movement of sediment deposits-
They reduce shoreline erosion and keep the water
clear!
Seagrasses need:
• Clear water!
• Sandy or muddy bottom
• Stable salinity
• Moderate waves and currents
Seagrasses are valuable and threatened compared to other major marine habitats
Seagrasses evolved in a very different marine environment from today
Hydorcharitaceae
Cymodoceacea
Posidoniaceae
Zosteraceae
Seagrasses evolved in a very different marine environment from today
Hydorcharitaceae
Cymodoceacea
Posidoniaceae
Zosteraceae
“An inconvenient truth” timeline
Seagrasses evolved in a very different marine environment from today
Hydorcharitaceae
Cymodoceacea
Posidoniaceae
Zosteraceae
Pressures to seagrass: human population
Egypt ca. 10 000 years ago
Pacific northwest, seagrass use
Washington DC, 2006
Pressures to seagrass: human population
Egypt ca. 10 000 years ago
Pacific northwest, seagrass use
Washington DC, 2006
Pressures to seagrass: human population
Egypt ca. 10 000 years ago
Pacific northwest, seagrass use
Washington DC, 2006
Pressures to seagrass: increased invasive species
Panama Canal, 1907 Ballast pumping
Caulerpa
taxifolia
Pressures to seagrass: increased invasive species
Panama Canal, 1907 Ballast pumping
Caulerpa
taxifolia
Pressures to seagrass: increased nutrient input (Fertilizer and sewage)
Fertilizer applied in agriculture Fertilizer applied to lawns Wastewater Treatment Plant
Fishing practices may also be changing seagrass meadows around the world
Reports of seagrass losses and the rates of decline are increasing dramatically
Waycott et al. 2009 PNAS
Drivers of seagrass loss
1. Water quality degradation from poor land use practices
3. Invasive species
2. Dredging and filling
Research effort on seagrasses increasing, but lagging behind other coastal habitats.
Within widely accessed media, reports of seagrass are lacking
Bottom line: less seagrass research done AND it isn’t broadly publicized
Seagrasses on a global scale
• Extensive seagrass losses have occurred in
temperate and tropical regions
• Nutrients and sediment inputs are the primary
pressures
• Research and understanding of seagrass
communities is increasing…
• BUT – despite a lot of effort, the importance
and issues of seagrass habitats are still
lagging behind other coastal communities in
effective communication to the public and
ultimately policy makers
A broad variety of seagrass habitats in south Florida
Thalassia testudinum Turtle grass
Many (most) seagrasses are dioecious
Syringodium filiforme Manatee grass
Halodule wrightii Shoal grass
Halophila decipiens
A monoecious seagrass
Halophila engelmanni
Halophila johnsonni
Ruppia maritima Widgeon grass
Synoptic Benthic Habitat Surveys
Synoptic Surveys: Species distributions
Thalassia testudinum
Halodule wrightii Syringodium filiforme
Halophila decipiens
As nutrient inputs to seagrass beds increase, species shifts occur and seagrasses get displaced by seaweeds, then by phytoplankton
Long-term changes in seagrass bed composition suggest nutrients are becoming more plentiful
1996 1998 2000 2002 2004 2006 2008 2010
Bra
un
Bla
nq
uet
Den
sit
y
0.0
1.0
2.0
3.0
4.0
5.0Site 273 Thalassia
SyringodiumHaloduleCalcareous Green*
1996 1998 2000 2002 2004 2006 2008 2010
Bra
un
Bla
nq
uet
Den
sit
y
0
1
2
3
4
5 Site 260
Changes in relative abundance of primary producers At 19 of 30 sites, species composition has shifted in a manner
consistent with increased nutrient availability
Green: Thalassia getting denser
Red: Thalassia decreasing in relative
importance
Progressive eutrophication or light reduction
Sea
gra
ss L
eaf
N:P
20
30
40
50
60
0.05 0.10 0.15 0.20 0.25
0.5
1.0
1.5
2.0
2.5
3.0
Nutr
ient-
reple
te
Oligotrophic
P-limited
Oligotrophic
N-limited
Eutrophication
Eutrophication
N:P
= 3
0:1
Leaf tissue P content (% of dry weight)
Lea
f ti
ssu
e N
con
ten
t (%
of
dry
wei
gh
t)
As nutrient inputs increase, the amounts of important plant nutrients in the seagrass leaves
changes – providing a sentinel for nutrient addition
N:P of leaves
Thalassia testudinum
10 20 30 40 50 60 70 80 90 100
P-limitedN-limited
The relative importance of the two important plant nutrients nitrogen and phosphorus change across the
landscape in south Florida
Some seagrass meadows in south Florida are changing in ways that indicate nutrient pollution
1996 1998 2000 2002 2004 2006 2008 2010
N:P
of T
ha
lassia
20
25
30
35
40
45
50
Site 267 Y=1032.6-0.5x
r2=0.171, p=0.002
Long-term changes in N:P consistent with increases in nutrients
Changes in water management in south Florida will cause a change in the distribution of seagrass species – and the animals that live in the seagrass beds
0 1 2 3 4 5
Kilometers
Organic C content (% of dry wt)
0 1 2 3 4 5 6
Dep
th (
cm
)
0
50
100
150
200
250
Organic C content (% of dry wt)
0 1 2 3 4 5 6
Dep
th (
cm
)
0
50
100
150
200
250
Trout Cove
Russel Bank
Organic C content (% of dry wt)
0 1 2 3 4 5 6D
ep
th (
cm
)
0
50
100
150
200
250
Organic C content (% of dry wt)
0 1 2 3 4 5 6
Dep
th (
cm
)0
50
100
150
200
250
Nine Mile Bank
Bob Allen Keys
Corg generally
decreases
downcore in
Florida Bay
seagrass soils.
Buried peats
have high Corg
Deep, C-rich soils underlay seagrass meadows
Ec
os
ys
tem
C s
tora
ge
(Mg
Co
rg h
a-1
)
0
200
400
600
800
1000
1200
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Boreal
Temepra
te
Tropic
al
Upland
Oceanic
Mangro
ve
Seagrass
Living Biomass
Soil Corg
Seagrasses store about as much C as forested ecosystems
There are about 18,000 km2 of seagrass beds in south Florida
A very rough estimate of carbon stored in the top meter of seagrass soils in south Florida: 18,000 km2 of seagrasses 594 tons CO2e ha-1
1 x 109 tons CO2e stored in the soils!
Anthropogenic CO2e flux is about 29 x 109 tons y-1
Summary points – Seagrasses in south Florida •The Keys are home to some of the most expansive and important seagrass beds on earth
•Rapid population increases adjacent to oligotrophic marine ecosystems in south Florida may have deleterious effects on those ecosystems, as in so many other places in the world •Changes are occurring in south Florida seagrass beds that are consistent with increased nutrient availability in the system. •These changes are relatively subtle, we have not witnessed large-scale permanent loss of seagrass beds in the Keys. There is time to act!
Are we describing locally-
induced changes, responses to
larger-scale processes, or
natural cycles?
• Sister taxa in Atlantic and Indopacific