1. Denitrification in an Urban Tidal Freshwater Wetland of the Delaware
T. Quirk, A. Smyth, M. Piehler, B. Horton, J. Mead, and D. J. Velinsky
February 2010

2. Tidal Freshwater Wetlands
Along coastal rivers
Salinity less than 0.5 ppt
Salt intolerant species
Globally rare habitat
Receive and process nutrient loads

3. Organic N NO3- NH4+ flux N2 flux N2 (g) NO3+ flux NH4+OO
SEDIMENT
WATER
Organic N O
NO3-
Net Nitrification
NH4+ flux
N2 flux O
N2 (g)
Denitrification
NO3+ flux
Permanent removal O
NH4+
Net Ammonification
adapted from Kemp et al. 1990

4. Denitrification varies temporally and spatially
Hypothesis
Denitrification varies temporally and spatially
Elevation
Season
Vegetation
Soil and water N

5. The Darby-Cobbs watershed
Cobbs Creek
Darby Creek
Stony Creek
Muckinipattis Creek
Hermesprota Creek

6. Watershed
Marsh
Phragmites australis

7. Low tide
High tide

8. Spatial variation
- upstream, main marsh, downstream
- low, mid-, high marsh plant communities / elevations

9. Water Column Characteristicsb

10. Seasonal aboveground biomass

11. Denitrification Seasonal effectB
Seasonal effect
Little spatial variation but a few hotspots of DNR
No significant relationship between DNR and elevation or biomass

12. APRIL Net flux of N from sediment to water columnOO
SEDIMENT
Organic N
WATER O
NO3-
Net Nitrification
NH4+ flux
N2 flux O
N2 (g)
Denitrification
NO3+ flux
+ 85 ± 27
µmol m-2 hr-1
+ 474 ± 26
µmol m-2 hr-1
+ 55 ± 29
µmol m-2 hr-1 O
NH4+
Net Ammonification
Net flux of N from sediment to water column
Low rates of denitrification

13. JULY Net flux of nitrate from water to sedimentOO
SEDIMENT
Organic N
WATER O
NO3-
Net Nitrification
NH4+ flux
N2 flux O
N2 (g)
Denitrification
NO3+ flux
+ 80 ± 21
µmol m-2 hr-1
- 293 ± 53
µmol m-2 hr-1
+ 396 ± 43
µmol m-2 hr-1 O
NH4+
Net Ammonification
Net flux of nitrate from water to sediment
High rates of denitrification

14. OCTOBER Ammonium flux low and variableOO
SEDIMENT
Organic N
WATER O
NO3-
Net Nitrification
NH4+ flux
N2 flux O
N2 (g)
Denitrification
NO3+ flux
+ 4 ± 9
µmol m-2 hr-1
- 327 ± 92
µmol m-2 hr-1
+ 100 ± 9
µmol m-2 hr-1 O
NH4+
Net Ammonification
Ammonium flux low and variable
Moderate rates of denitrification

15. Positive relationship between NH4+ flux and N2 flux in July

16. Sediment O2 Demand
Increase in SOD - Reduces O2 availability - Limits nitrification
Implications
Both nitrification and water column NO3+ fueling DNR in July
Water column NO3+ fueling DNR in Oct

17. Conclusions Little spatial variation in DNR
Seasonal effects on plant community composition and % of biomass AND DNR
April- many locations with no DNR
July- greatest DNR with coupled nitrification-DNR + water column NO3+ fueling DNR
October- lower DNR but water column NO3+ fueling DNR
Calculation of % N load undergoing DNR limited by lack of adequate streamflow data