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![Page 1: Reducing phosphorus concentration in rivers: wetlands not always to the rescue Ben Surridge, Catchment Science Centre Louise Heathwaite, Lancaster Environment.](https://reader030.fdocuments.net/reader030/viewer/2022032612/56649ef25503460f94c04ce8/html5/thumbnails/1.jpg)
Reducing phosphorus concentration in rivers: wetlands not always to the rescue
Ben Surridge, Catchment Science Centre
Louise Heathwaite, Lancaster Environment Centre
Andrew Baird, Queen Mary, University of London
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Phosphorus: a life-support element
• Macro-nutrient, 2-4% dry weight of most cells, mostly PO4
• Constituent of DNA and RNA
• Cell structure – phospholipids
• Cell energy – ATP and ADP
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Limiting primary productivity
• Phosphorus limitation or co-limitation of many freshwater environments
• Phosphorus limitation of oceanic primary productivity?
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Limiting primary productivity
• At what concentration does P become limiting?
• Autotrophic activity:
– Individual algal species – 0.001 to >0.30 mg l-1 P• Confounding issues e.g. luxury uptake
• Heterotrophic activity
• Habitats Directive guideline – 0.20 mg l-1 P• UK TAG EQS under the WFD – 0.12 mg l-1 P
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Non-limited UK rivers
• Phosphorus enrichment Hampshire Avon
Environment Agency (2005)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
Year
Mea
n an
nual
orth
opho
spha
te (m
g l-1
P)
North West
North East
Midlands
Anglian
Thames
Southern
South West
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Enrichment costs you more
• Increased autotrophic growth rate and biomass• Shifts in community structure: macrophyte →
epiphytic algae → benthic and filamentous algae• Damage costs ~£100 million yr-1 in England and
Wales (Pretty et al. 2003)
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Contributors to phosphorus loads
Agriculture28%
Domestic61%
Industrial5%
Background6%
Agriculture53%
Domestic30%
Other17%
Agriculture43%
Domestic43%
Industrial8%
Background6% Morse et al (2003)
Defra (2004)
Defra (2006)
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Reducing phosphorus in rivers
• Range of statutory and non-statutory instruments
– 90% of costs of these instruments borne by water industry (Pretty et al. 2003)
– UWWTD most significant – discharge limits to sensitive areas of 1-2 mg l-1 P as total phosphorus
– Capital expenditure: £50 million yr-1 between 2000-2005 on improved phosphorus removal
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Justified water industry investment?
River Kennet
Jarvie et al. (2004)
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…….but
• Macrophyte growth still affected by epiphytic and benthic algae
• Because of compounding factors – phosphorus is not the only factor affecting productivity
• Because targeting WWTPs is not sufficient – baseline and spikes in river phosphorus concentration
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The diffuse problem
• Engagement – changing nutrient management at source – Defra’s CSF
• Inducement – nutrient management and targeted mitigation – Environmental Stewardship
• Entry level – 3.5 million hectares• Higher level – 65,000 hectares
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Wetlands at our service?
• Nutrient attenuation function• Riparian zone an effective sediment and P trap
Kronvang et al (2005)
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Wetlands at our service?
• Drive to re-establish and create wetlands:• UK BAP ~18,000 ha wetland• 50-year wetland vision – 12% of Yorkshire and Humber
study area has potential for restoring wetland habitat
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A second nutrient time bomb?
• Riparian zones are productive agricultural land~30% of applied phosphorus removed in produce
~70% remains in soil or is exported
• UK floodplain sediments ~500 - >2500 mg kg-1 total phosphorus (Walling et al. 2000)
How stable is this phosphorus?Could chemical, and potentially ecological, status
be affected?
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Riparian wetlands in the Norfolk Broads
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External nutrient loads
0.0
1.0
2.0
3.0
4.0
5.0
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
Year
Orth
opho
spha
te (m
g l-1
P)
0.0
2.0
4.0
6.0
8.0
10.0
12.0
198
9
199
0
199
1
199
2
199
3
199
4
199
5
199
6
199
7
199
8
199
9
200
0
200
1
200
2
Year
Orth
opho
spha
te (m
g l-1
P)
Environment Agency (2005)
River YareLackford Run
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Phosphorus retained in sediment
0 400 800 1200 1600
1-5
5-10
10-15
15-20
20-25
25-30
30-35
35-40
40-45
45-50
Depth (cm)
Total phosphorus (mg kg-1)
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Chemical extraction of phosphorus
• Majority of TP present as organic P
• Up to 30% of TP as inorganic P:
Ca/Mg-P pH sensitive Fe-P sensitive to redox conditions
• During seasonal water table fluctuation both pH and redox change significantly
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Laboratory mesocosm incubations
• Simulate P release following reflooding
• Surface water and pore water sampling
• Analysis of sediment-P pools
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MRP release to surface and subsurface
0.0
0.2
0.4
0.6
0.8
1.0
0 200 400 600 800
Time (hours)
MRP
(mg
l-1 P)
Core A1Core A2Core A3
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.0
2.5
10.0
17.5
32.5
47.5
Depth (cm)
MRP (mg l-1 P)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.0
2.5
10.0
17.5
32.5
47.5Depth (cm
)
MRP (mg l-1 P)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.0
2.5
10.0
17.5
32.5
47.5
Depth (cm)
MRP (mg l-1 P)
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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.0
2.5
10.0
17.5
32.5
47.5
Depth (cm)
MRP (mg l -1 P)
0.0 10.0 20.0 30.0
0.0
2.5
10.0
17.5
32.5
47.5
Depth (cm)
Fe 2+ (mg l -1 )
Subsurface MRP and Fe2+ release
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Stoichiometry of MRP and Fe2+ release
MRP = 0.45 * Fe2+ + 0.0053r2 = 0.91
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Fe2+ (mmol l-1)
MRP
(mm
ol l-1
)
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Comparing field and lab P concentration
0.0 2.0 4.0 6.0
0.0
12.5
20.0
37.5
47.5
Depth (cm)
MRP (mg l-1 P)
0.0 2.0 4.0 6.0
0.0
12.5
20.0
37.5
47.5
Depth (cm)
MRP (mg l-1 P)
Laboratory Field
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P delivery to receiving waters
3.92
3.96
4.00
4.04
4.0800
00
1200
0000
1200
0000
1200
0000
Time (hours)
Wat
er le
vel (
mAA
D)
0.00
0.10
0.20
0.30
0.40
0.50
MRP
(mg
l-1 P
)
Ditch5 mMRP
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P delivery to receiving waters
3.80
3.85
3.90
3.95
4.00
4.05
4.10
4.1531
9
321
323
325
327
329
331
Julian Day
Wat
er le
vel (
m A
AD
)
650.0
750.0
850.0
950.0
1050.0
MRP
(mg
l-1 P
)
Ditch5 m25 mMRP
0.00
0.60
0.45
0.30
0.15
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Concluding comments
• Wetlands may effectively remove and store phosphorus
• Store is potentially soluble and therefore bioavailable
• Soluble phosphorus may be delivered to adjacent aquatic ecosystems – a second nutrient time bomb?
• Not all wetland functions can be restored, and restoration may have negative consequences