Biophysical Control of Nitrogen Cycling in the Upper Mississippi River

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Biophysical Control of Nitrogen Cycling in the
Upper Mississippi River
USGS Nutrient Workshop
William Richardson, E. Strauss, D. Soballe, L.
Bartsch, J. Cavanaugh, J. Rogala, D. Bruesewitz,
and H. Imker.
Upper Midwest Environmental Sciences Center,
La Crosse,
Wisconsin
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Why is Nitrogen a High Priority?

• Nitrogen enrichment is a highly visible,
  multi-scale, management issue of national
  importance.
• Nitrogen has negative effects on valued aquatic
  species.
• Processing of nitrogen within the Upper
  Mississippi River is poorly understood and is a
  key aspect of system function and sustainability.

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Major Questions

• What are the processes controlling temporal
  patterns of nitrogen movement?
• How do processes vary spatially? (By habitat)
• What are the effects on river biota (mussels,
  noxious algae, macrophytes)?
• What are the management priorities?
• What can be done?

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Trempealeau NWF Refuge
Two Rivers NWF Refuge
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River flood plains are ideal sites for
bacterially mediated nitrogen removal
Sediments tend to be highly organic, moist, and
combination of aerobic and anaerobic conditions.
Connections with main channel water can replenish
nitrate in backwater wetlands.
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Approach

• Combine
• Process studies (mensurative field and lab
  experiments).
• Long term monitoring data (Long Term Resource
  Monitoring Program).
• GIS Geospatial Modeling.
• Link to management priorities.

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Variation in river discharge
1. Redistribution of nitrogen and sediment
throughout the flood plain.
2. Controls sediment moisture, oxygen, and redox
dynamics (through sediment drying/wetting).
3. Plays a role in temperature dynamics.
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Hydrologic Control Of Distribution Nitrate
Low flows (Fall 1999) nitrate highest in main
channel (regions of low sediment moisture)
High flows (SP 2001) higher nitrate
concentrations farther from main channels (areas
of higher sediment moisture backwater lakes,
marshes).
Floods homogenize nitrate concentrations across
the pool Typical low summer flows result in
little transport of nitrate to backwaters.
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Denitrification and Denitrification Enzyme
Activity in Pool 8, Mississippi River
1. Denitrification rates across reach are very
low.
2. Backwater lakes with extremely low
denitrification rates.
3. Impounded and channel areas with elevated
sediment carbon show higher rates.
4. Enyzme activity (DEA) extremely high in
backwaters.
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Linked spatial and temporal patterns of nitrate
distributions and denitrification
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Experimental Evaluation of Carbon and Nitrate
Limitation
Little effect of carbon (glucose) addition (10
mg/l) except in low carbon sediments.
Nitrate additions (2 mg/l) resulted in
significant increase in denitrification in all
river sediments.
Combined treatments resulted in slight decrease
in denitrification possibly due to nitrate
limitation?
Results support our hypothesis that
denitrification in backwater areas (high carbon
depositional sediments) is nitrate limited and
channel areas tend to be both C N limited.
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Nitrification in pool 8

• Results
• nitrification was low in or near main channel
• nitrification was variable away from main channel
• no strong seasonal pattern

• Method
• top 5 cm of sediment
• nitrapyrin technique
• 3day incubation
• ambient temperature

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Nitrification/Denitrification Coupled?
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Vertical distribution of oxygen in sediments
Depth (mm)
Percent oxygen saturation
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Schematic of hydrologic control of nitrate
distribution and denitrification rates in the
Upper Mississippi River.
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Conclusions

• Denitrification is well below potential maximum.
• Nitrification limited by oxygen tends to result
  in build-up of sediment ammonia.
• Coupled nitrification/denitrification important
  in some habitats but not critical for supply of
  nitrate to denitrifiers.
• Nitrogen management may be possible.
• manipulation of hydrologic exchange
• management of water levels to stimulate growth of
  plants and enhance processes at sediment-water
  interface.

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Future Directions - Whats Next?

• Improve estimates of nitrogen removal and removal
  potential in the River.
• Evaluate role of aquatic plants and water level
  manipulations on N cycling.
• Quantify local/regional effects of N enrichment.
• Document casual mechanisms.
• Evaluate effects of sediment ammonia on biota.
• Move research efforts up into watersheds and
  small streams where management is most effective.

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Old River
New and improved River
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Vertical distribution of nitrification in
sediments
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Link Spatial and Temporal Patterns
Nitrate concentrations tend to be highest near
main channel lowest in backwaters equitable
across habitats during floods.
Nitrate
Denitrification (actual)
Actual denitrification rates tend to be extremely
low but highest near main strongly nitrate and
temperature dependent
Denitrification Potential
Potential denitrification tends to be highest in
backwaters strongly nitrate and carbon dependent.
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Control of Denitrification Rates
1. Nitrate concentrations
2. Sediment carbon
3. Coupled nitrification-denitrification
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Nitrification as a source of nitrate for
denitrification
N2
NO3-
Up-river / Surface Water
Denitrification
NO3-
NH3
Diffusion
NO3-
Nitrification
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Sed. carbon
WaterVelocity
Depth