Post on 03-Jan-2016
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
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
Limiting primary productivity
• Phosphorus limitation or co-limitation of many freshwater environments
• Phosphorus limitation of oceanic primary productivity?
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
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
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)
Contributors to phosphorus loads
Agriculture28%
Domestic61%
Industrial5%
Background6%
Agriculture53%
Domestic30%
Other17%
Agriculture43%
Domestic43%
Industrial8%
Background6% Morse et al (2003)
Defra (2004)
Defra (2006)
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
Justified water industry investment?
River Kennet
Jarvie et al. (2004)
…….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
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
Wetlands at our service?
• Nutrient attenuation function• Riparian zone an effective sediment and P trap
Kronvang et al (2005)
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
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?
Riparian wetlands in the Norfolk Broads
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
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)
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
Laboratory mesocosm incubations
• Simulate P release following reflooding
• Surface water and pore water sampling
• Analysis of sediment-P pools
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)
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
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
)
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
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
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
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