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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