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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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reducing phosphorus concentration in rivers wetlands not always to the rescue

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

Phosphorus: a life-support element
limiting primary productivity
Limiting primary productivity
  • Phosphorus limitation or co-limitation of many freshwater environments
  • Phosphorus limitation of oceanic primary productivity?
limiting primary productivity1
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

Hampshire Avon

Non-limited UK rivers
  • Phosphorus enrichment

Environment Agency (2005)

enrichment costs you more
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
Contributors to phosphorus loads

Morse et al (2003)

Defra (2004)

Defra (2006)

reducing phosphorus in rivers
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
slide10
…….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
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

Kronvang et al (2005)

Wetlands at our service?
  • Nutrient attenuation function
  • Riparian zone an effective sediment and P trap
wetlands at our service1
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
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?
external nutrient loads
External nutrient loads

River Yare

Lackford Run

Environment Agency (2005)

chemical extraction of phosphorus
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

Laboratory mesocosm incubations
subsurface mrp and fe 2 release

-1

MRP (mg l

P)

Subsurface MRP and Fe2+ release

-1

2+

Fe

(mg l

)

1.0

1.5

0.0

0.5

2.0

2.5

3.0

3.5

30.0

20.0

0.0

10.0

0.0

0.0

2.5

2.5

10.0

10.0

Depth (cm)

Depth (cm)

17.5

17.5

32.5

32.5

47.5

47.5

p delivery to receiving waters
P delivery to receiving waters

Ditch

4.08

0.50

5 m

MRP

0.40

4.04

P)

0.30

-1

Water level (mAAD)

4.00

MRP (mg l

0.20

3.96

0.10

3.92

0.00

1200

0000

1200

0000

1200

0000

0000

Time (hours)

p delivery to receiving waters1
P delivery to receiving waters

0.60

4.15

1050.0

Ditch

5 m

25 m

4.10

MRP

950.0

0.45

4.05

P)

4.00

-1

850.0

Water level (m AAD)

0.30

MRP (mg l

3.95

3.90

750.0

0.15

3.85

0.00

3.80

650.0

323

325

327

329

319

321

331

Julian Day

concluding comments
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
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