Reducing phosphorus concentration in rivers wetlands not always

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 PO 4 • 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 Environment Agency (2005) Hampshire Avon

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 Wales (Pretty et al. 2003)

Contributors to phosphorus loads 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 20002005 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) ØHow stable is this phosphorus? ØCould chemical, and potentially ecological, status be affected?

Riparian wetlands in the Norfolk Broads

External nutrient loads River Yare Lackford Run Environment Agency (2005)

Phosphorus retained in sediment

Chemical extraction of phosphorus • Majority of TP present as organic P • Up to 30% of TP as inorganic P: Ø Ca/Mg-P p. H sensitive Ø Fe-P sensitive to redox conditions • During seasonal water table fluctuation both p. H 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

Subsurface MRP and Fe 2+ release -1 MRP (mg l P) 0. 0 0. 5 1. 0 1. 5 2. 0 2. 5 2+ 3. 0 3. 5 0. 0 2. 5 10. 0 Depth (cm) 17. 5 -1 Fe (mg l ) 17. 5 32. 5 47. 5 10. 0 20. 0 30. 0

Stoichiometry of MRP and Fe 2+ release

Comparing field and lab P concentration Laboratory Field

P delivery to receiving waters Ditch 5 m MRP 0. 50 4. 00 0. 20 3. 96 0. 10 Time (hours) 0000 1200 0000 0. 00 1200 3. 92 -1 0. 30 MRP (mg l. P) 0. 40 4. 04 0000 Water level (m. AAD) 4. 08

P delivery to receiving waters 0. 60 1050. 0 4. 15 Ditch 5 m 25 m MRP -1 4. 00 MRP (mg P) l 950. 0 0. 45 4. 05 850. 0 0. 30 3. 95 3. 90 750. 0 0. 15 3. 85 0. 00 650. 0 Julian Day 331 329 327 325 323 321 3. 80 319 Water level (m AAD) 4. 10

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