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Nutrient Processing and Floodplain Connectivity Following Restoration in Urban Streams

Sara McMillan1, Gregory Noe2, Alea Tuttle1,3, Gregory Jennings4

1University of North Carolina at Charlotte, 2US Geological Survey, 3Wildlands Engineering,

4North Carolina State University

Pristine Impaired

Restored?

1. Can we achieve any measurable improvements in water quality with current design and construction practices?

2. Identify those restoration features (both in the stream and adjacent floodplains) that enhance nutrient transformations.

Stream Restoration & Field of Dreams Hypothesis

Current restoration goals AND practices focus on improved channel stability and reduced sediment transport

Natural Channel Design includes morphology adjustments and engineered structures to achieve stability, grade control and bank stabilization.

How do these physical changes influence ecosystem processes?

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Stream Restoration in Urban Watersheds

Challenges:

• Flashy hydrology, channel incision

• Increased nutrient, sediment and contaminant transport

Solutions:

• Low impact development, stormwater control measures, stream restoration

Constraints:

• Existing infrastructure, competing project goals, funding

Little Sugar Creek, Charlotte, NC: Greenway and stream restoration (CMSWS, 2010)

1North Carolina Ecosystem Enhancement Program (NCEEP)

764 stream restoration projects in NC

Total cost of $488,209,4601

Retention Hydrologic Retention

Biological Retention

Physical/ Chemical Retention

Mass Residence

time Flux (mass/time)

• Assimilation

• Mineralization

• Production/ respiration

• Sorption

• Dissolution

• Floodplain reconnection

• Meanders

• 2-stage channel

• Bed complexity

Restoration

Stream restoration & nutrient retention

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Key stream-floodplain features

Restoration Age

Geomorphic Complexity

Hydrologic Connectivity

Methods: nutrient biogeochemistry

• Reach-scale uptake – Nutrient spiraling approach (Stream Solute

Workshop 1990) – Short-term solute (NO3

-, PO43-) release with Cl-

tracer until steady state

• Denitrification – Ambient rates: Denitrification (acetylene +

chloroamphenicol) and N2O flux – Potential rates: Denitrification enzyme activity

(DEA) assay using acetylene block (Groffman, 1999)

• Net N & P mineralization rates – In situ cores deployed for 30 days – Net flux (NH4

+, NO3-, SRP) in soil

anion/cation resin surrounding the soil core (Noe 2011)

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Methods: hydrology and loading

• Sediment & nutrient loading – Triplicate plots in floodplain – Monthly sedimentation rates via tile

deposition (Noe and Hupp, 2009) – Monthly DIN/DIP loading via modified

anion/cation resin bags (Binkley and Hart, 1989)

• Hydrologic connectivity – Stage gages installed at each site – Detailed survey to determine flooding

frequency

PDF OF PLAN SURVEY

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

0 5 10 15

Sed

imen

t C

arb

on

Restoration Age (years)

0%

10%

20%

30%

40%

50%

60%

70%

80%

0 2 4 6 8 10 12

Can

op

y C

over

Restoration Age (years)

R2 = 0.508

p = 0.004

Newly restored sites with high autochthonous production -> increased sediment carbon and water temperature

AGE

R2 = 0.479

p = 0.006

Effect of restoration age on instream retention

McMillan, S. K., A. K. Tuttle, G. D. Jennings, and A. Gardner (2013). “Influence of restoration age and riparian vegetation on reach-scale nutrient retention in restored urban streams”. Journal of the American Water Resources Association, In Press.

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Effect of age + geomorphic complexity

Reach-scale uptake velocity, Vf = efficiency of nutrient removal from water column

NITRATE PHOSPHATE

Predictor variable R2 = 0.82; p<0.001 R2 = 0.73; p=0.006

Channel complexity +

Canopy cover + -

Temperature +

Sediment carbon +

Nutrient Concentration +

Velocity x depth +

MLR variables tested: nutrient concentration, canopy, temperature, sediment carbon, channel complexity and velocity x depth

McMillan, S. K., A. K. Tuttle, G. D. Jennings, and A. Gardner (2013). “Influence of restoration age and riparian vegetation on reach-scale nutrient retention in restored urban streams”. Journal of the American Water Resources Association, In Press.

Effect of geomorphic complexity

Lautz, and Fanelli 2008, Biogeochemistry

• Pool: anoxic conditions, high retention times; high denitrification rates and nitrate removal leading to low concentrations

• Riffle: oxic conditions, low retention times; high nitrification increasing nitrate concentration

• Weir: high vertical head gradients, low retention times; transport dominated

FLOW leaves algae

Harrison et al., 2012, Ecol. Engr.

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Sedgefield Park (SP) 7/21/2010

Fabric sheet

-1.4

-1.5

-1.3

-0.5

Flow

Approach: Measure denitrification rates (acetylene block) and head gradients (piezometers in stream bed) upstream and downstream of instream features

0

100

200

300

400

500

600

700

GC SP DB EB HP SP TC

De

nit

rifi

cati

on

(u

mo

l m-2

h-1

) Upstream

Downstream

Lower head gradients than expected near weir structures with changes in water surface profile

Higher DNF in pool downstream of weir boulder structure

riffle boulder weir debris dam

*

*

p < 0.05

*

*

*

Geomorphic complexity increased denitrification

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boulder step

channelized

Invasive plants

Replanted buffer

floodplain reconnection

Steep banks

landscaping

Influence of Channel Complexity + Restoration

Tuttle, A. K., McMillan, S. K., A. Gardner and G. D. Jennings. “Geomorphologic Drivers of Denitrification Rates in Restored and Natural Urban Streams”. Ecological Engineering, In Review.

Channel complexity increased rates of denitrification (p=0.014)

Higher denitrification rates in restored streams (p=0.0006)

Locations of greater DNF reflected design and construction practices

Effect of hydrologic connectivity

Floodplain connectivity

OrgN NO3

-

NH4+

NH4+ NO3

-

NH4+ NO3

-

N2 OrgN

NO3-

NH4+

N2

0.01

0.1

1

10

100

1000

10000

LSW MC TOR WT DB

Sed

imen

tati

on

Rate

(g

m-2

d-1

) Low bench

High bench

0

1

2

3

4

5

6

7

8

-10.0 -5.0 0.0 5.0 10.0ln N

min

erali

zati

on

(m

mol

m-2

y-1

)

ln Sedimentation Rate (g m-2 d-1)

R2 = 0.416

p = 0.004

Funded by NC WRRI (2012-2013; $50,000). Collaborators: G. Noe (USGS), G. Jennings (NCSU)

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Effect of restoration age on N/P transformations

0

2

4

6

8

10

0 2 4 6 8 10

sq r

oot

DE

A (

mg m

-2 h

-1)

Restoration age (yr)

R2 = 0.203

p = 0.012

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10

ln N

min

erali

zati

on

(m

mol

m-2

y-1

)

Restoration age (yr)

R² = 0.1666

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 2 4 6 8 10

ln P

min

erali

zati

on

(m

mol

m-2

y-1

)

Restoration age (yr)

R2 = 0.435

p = 0.0002

R2 = 0.166

p = 0.031

In-situ mineralization rates of N and P increased with age

Potential denitrification rates increased with age

Lower DEA and P-min at oldest site with low connectivity

Future work to integrate multiple controlling variables (age, connectivity, nutrient loading)

Key stream-floodplain features

RESTORATION AGE: • Carbon inputs change as canopy cover matures

– Instream = greater retention in newly restored streams by highly productive autotrophs

– Floodplains = greater mineralization as sites mature and carbon pools build up

GEOMORPHIC COMPLEXITY: • Greater diversity of flowpaths increase N/P

retention (increase both retention time + microbial activity)

HYDROLOGIC CONNECTIVITY: • Floodplains that flood frequently AND retain

sediment have the greatest sediment carbon and greatest N/P processing

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Acknowledgements

• Collaborators: Sandra Clinton, Craig Allan, Anne Jefferson, Greg Jennings, Greg Noe, Philippe Vidon

• Current and Past graduate students: Alea Tuttle, Brandon Blue, Alexandra Apple, Colin Bell, Erin Looper, Jordan Gross, Molly Welsh and many undergraduate students who helped along the way

• Funding: NC WRRI, NSF, USGS