Phosphorus cycling in a sedimentation of a constructed

Santiago Clerici Michael Krom, Robert Mortimer and Sally MacKenzie WWT Slimbridge

River Severn •Effluents high in N, P, Organic Matter and suspended solids

• downstream The treatment wetland

•Sedimentation pond to remove

Ditch suspended matter

Inflow •PO and excess suspended Cascade 4 Reserve matter taken up by reed beds 100 mts Failing of the treatment wetland

Excess P is being exported from the system

Palmer Felgate et.al 2011:

Sedimentation pond becomes source of P in summer

 Collapse of algal blooms in early Summer:

. P released by redox reactions when oxygen levels go down

. P released by decomposition of algal matter

Objectives

 To find the dominant processes controlling P cycling in the sedimentation pond in Spring and Summer

 To know whether the sediment is a source or a sink of Phosphorus?

 To find the major processes controlling P cycling in the sediment and whether they change seasonally?

Model of P cycling in the water column

INLET

Part P Inflow PO4 Inflow

Algal uptake PO Outflow Part P Outflow POND POND 4

Part P Respiration PO4

Resuspension Sediment fluxes Settling

Moles P / day Chlorophyll and Oxygen levels

DO - March 2011

125 Bloom 100

75

50

25 DO (% saturation)DO

0 12:00 18:00 00:00 06:00 12:00 18:00 time

Chl.a

450 Bloom DO - June 2011

125 300 100

75

Chl.a (μg/l) 150 Crash 50 Crash 25

DO (%DO saturation) 0 0 12:00 18:00 00:00 06:00 12:00 18:00 Mar-11 Apr-11 May-11 Jun-11 date time

Night hours Sampling P at inlet and pond

INLET

Part P Inflow PO4 Inflow

Algal uptake Part P Outflow POND POND PO4 Outflow Part P Respiration PO4

Settling Sediment fluxes Resuspension Calculating water column fluxes

INLET Obscured incubation

Part P Inflow PO4 Inflow 25 Only respiration Algal uptake Part P Outflow POND POND PO4 Outflow

Part P Respiration PO4

(μM P) 4

Settling Sediment fluxes PO Resuspension 20 12:00 18:00 00:00 06:00 12:00 18:00 time

Open incubation

25 Photosynthesis + respiration

(μM P)

4 PO

20 12:00 18:00 00:00 06:00 12:00 18:00 time Calculating sediment fluxes

INLET

PO4 in Benthic Chamber #1 March Part P Inflow PO4 Inflow 1.5

Algal uptake Part P Outflow POND POND PO4 Outflow

Part P Respiration PO4 (μM P)

Settling Sediment fluxes 4

Resuspension PO 0 12:00 18:00 00:00 06:00 12:00

time

PO4 in Benthic Chamber #1 June

35

25

(μM P)

4 PO

15 12:00 18:00 00:00 06:00 12:00 18:00 time P fluxes March

INLET

Part P Inflow PO4 Inflow 14 2

Algal uptake Part P Outflow PO4 Outflow 1 Pond Part P Pond PO4 1 12 14 moles 2 moles

Settling Sediment fluxes 4 0.1/0.2

Fluxes in moles P / day P fluxes June

INLET

Part P Inflow PO4 Inflow 16 47

Algal uptake Part P Outflow PO4 Outflow 2 13 Pond Part P Pond PO4 55

9 moles Respiration 35 moles Settling 1 Sediment fluxes

6 10

Fluxes in moles P / day Sediment phosphorus cycle (after Slomp et al 1996)

PO4 To outlets Water Sediment Pore water Organic P Fe~P Ca~P PO4

Apatite P

Water: it transports PO4 Solid: it retains P but it can be released it back into water Solid: it retains P indefinitely Description of the solid phase

5 0.75 1 0

-5 sediment content 0

0 50 100 20x increase

depth(cm) depth(cm) -10

-5 -15

% O2 saturation porosity Organic P ↔ Pore water PO4

PO4 Increased respiration in June

uM NH4 Pore water 5 Organic P PO4

200 moles P released between March and June 0 400 Organic P (μ mole P / litre) 0

-5 0 16000 depth (cm)

-8 depth (cm)

March june -15 June -16 march Iron~P ↔ Pore water PO4

PO4 Increased dissolved iron in June

uM Fe Pore water 5 Fe~P PO4

25 moles P released between March and June 0 300 Iron~P (μ mole P / litre)

-5 0

0 16000 depth (cm)

-8 depth (cm) june March -16 march -15 June Apatite P ↔ Pore water PO4

PO4

Consumption of calcium in June Pore water Ca2+ (μ mole Ca / litre) PO4 0

0 8000 Apatite P

100 moles P consumed between march and June

Apatite P (μ mole P / litre) 0

-6 0 16000 depth (cm)

-8 depth (cm)

june -12 -16 march Pore water PO4 ↔ Water column PO4

PO4 (μM) - March PO4 (μM) - June

5 5

0 80 0 600

-5 -5

depth (cm) depth (cm)

-15 -15

Calculated: 0.3 mole P / day Calculated: 7 moles P / day Summary: control of P in the water column

 Pond retains 15% of Particulate P, both in March and June

 Algal bloom in March  Oxygen saturation  Algae and sediments take up SRP in March

 Collapse of algal bloom  Oxygen levels very low in June  Large plume of SRP flows in, in June  Important release of P from sediments in June Summary: release of P from sediments

 Release of PO4 mainly by consumption of accumulated organic matter

 Redox related dissolution of Fe also contributes to the

release of PO4

 Precipitation of apatite counteracts, in part, the release of

PO4