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