The implications of Hurricane Katrina and altered nutrient regimes on long-term trends of marsh...
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The implications of Hurricane Katrina and altered nutrient regimes on long-term trends of marsh elevation at Big Branch National Wildlife Refuge, Louisiana Reed, D. 1 , A. Commagere 1 , M. Hester 2 University of New Orleans 1 . University of Louisiana Lafayette 2 . Table 1. Rate of elevation change (mm/yr) plus the standard error pre-Katrina, Katrina, post-Katrina, and the full time series for each treatment. An asterisk indicates the slopes failed to be statically significant at the 5% confidence interval. The Katrina time series was not statistically analyzed. Rate of elevation change (mm/yr) Treatment Pre-Katrina (7/15/04 - 8/22/05) Katrina (8/22/05 – 10/21/05) Post-Katrina (10/21/05 – 4/10/08) Full Time Series (7/15/04 – 4/10/08) Control 22 ± 8 1174 ± 249 -49 ± 10 50 ± 5 P 5 ± 8* 724 ± 146 -31 ± 5 29 ± 3 N 11 ± 8* 231 ± 237 -36 ± 10 1 ± 4* N+P 57 ± 15 767 ± 189 -43 ± 9 39 ± 4 Lethal 44 ± 10 602 ± 134 -39 ± 6 31 ± 3 INTRODUCTION The tidal marshes in Louisiana are threatened by sea level rise, increased frequency and intensity of tropical storms, as well as erosion and subsidence. These issues are potentially exacerbated by increased urbanization and development, which may lead to increased nutrient loading (Nicholls et al. 2007). The north shore of Lake Pontchartrain provides an excellent example of emergent wetlands threatened by such risks. In St. Tammany Parish alone, residential/urban land use increased over 159 % between 1982 and 2000 resulting in the conversion of 4.4 km 2 of marsh to urban development (Beall et al. 2001). These marshes are essential for the valuable functions they provide, including water quality improvement, storm protection, and wildlife and fisheries habitat. Consequently, understanding the processes required to best manage them is critical. The goal of this project was to identify the effects of hurricane impacts and nutrient input on an oligohaline marsh at Big Branch National Wildlife Refuge (Figure 1). Research plots had been established and monitored two years prior to hurricane Katrina, which allowed us to investigate the emergent wetlands’ response and resilience to hurricane impacts. METHODS • Surface elevation tables (SET) were used to measure changes in marsh elevation. Fifteen SETs were evenly distributed across three separate areas and within each area were randomly assigned a nutrient treatment. Nutrient treatments applied to SET plots included 40 g nitrogen m -2 yr -1 , 30 g phosphorous m -2 yr -1 , a combination of these levels of N and P, a lethal disturbance using Rodeo herbicide, and controls with no treatments. • Baseline measurement was taken July 17, 2004 and subsequent measurements continued semi-annually until April 10, 2008. • For analysis, the dataset was divided into pre-Katrina and post-Katrina subsets. The entire time series was also analyzed for overall trends in rates of elevation change. Net change in elevation refers to the change in elevation relative to the initial measurement. RESULTS The pre-Katrina dataset exhibited significant positive rates of elevation change (i.e., the slope was significantly different from zero) for the control, combined N and P, and lethal plots (Table 1). Post-Katrina exhibited significant negative rates of elevation change for all treatments. However, the entire data set exhibited a significant positive rate of elevation change for all treatments except the N plots. A large increase in elevation occurred directly after hurricane Katrina (October 2005) in all treatment plots and was 2 to 8 times higher than the net change in elevation that occurred immediately prior to hurricane Katrina (Figure 1). For example, in the control plot, from July 2004 to August 2005 there was a net change in elevation of 26mm. From August 2005 to October 2005, there was a net change of 215mm. Post-Katrina, a large decrease in elevation was evident, however, the long term net change (July 2004 to April 2008) was positive for all treatments, except for the N plots, which showed a slight decrease of 4 mm. CONCLUSIONS • Complex patterns of elevation change indicate some effect from nutrient loadings prior to Hurricane Katrina. Further analysis of soil biochemistry is needed to determine other processes that may be contributing to elevation change. • Although initially a large decrease in elevation followed the large increase in elevation post- Katrina (Figure 1), the long-term trend still shows a higher rate of elevation change than the pre- Katrina rate of elevation change for Control and P plots (Table 1). • The rates of elevation change for all but the N plots, are greater than the rates of relative sea level rise for this area estimated from Gagliano 1999 (Figure 3). ACKNOWLEDGEMENTS Thanks to Carol Wilson, Jen Roberts, Brendan Yuill, Laura Dancer, and all those who aided in the collection of data over the years. This work was supported by NOAA grant #NA06NOS4630026. REFERENCES Beall, A. D., S. Penland, and F. Cretini, Jr. 2001. Urbanization effects on habitat change in St. Tammany Parish, 1982 – 2000. Final Report submitted to the Lake Pontchartrain Basin Foundation, Metairie, Louisiana. 19 pp. Gagliano, S. M. 1999. Faulting, subsidence and land loss in coastal Louisiana, In: Coast 2050: Toward a Sustainable Coastal Louisiana, The Appendices. Louisiana Department of Natural Resources. Baton Rouge, LA. Nicholls, R.J. and others. 2007. Coastal systems and low-lying areas. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the IPCC, M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson, Eds., Cambridge University Press, Cambridge, UK, 315-356. 5 mm/yr 9 mm/yr Figure 3. Isopleth map of relative sea level rise rates adapted from Gagliano 1999. -100 -50 0 50 100 150 200 250 300 N N+P P Control Lethal Date of Measurement 9/22/04 3/8/05 8/22/05 10/21/05 5/23/06 1/8/07 6/6/07 4/10/08 Change in Elevation (mm) Figure 2. Change in elevation relative to the initial measurement over time Lake Pontchartrain Figure 1. Study area. Yellow circle indicates the location of SETs. Red line represents the path of Hurricane Katrina
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Transcript of The implications of Hurricane Katrina and altered nutrient regimes on long-term trends of marsh...
The implications of Hurricane Katrina and altered nutrient regimes on long-term trends of
marsh elevation at Big Branch National Wildlife Refuge, LouisianaReed, D.1, A. Commagere1, M. Hester2
University of New Orleans1. University of Louisiana Lafayette2.
Table 1. Rate of elevation change (mm/yr) plus the standard error pre-Katrina, Katrina, post-Katrina,
and the full time series for each treatment. An asterisk indicates the slopes failed to be statically
significant at the 5% confidence interval. The Katrina time series was not statistically analyzed.
Rate of elevation change (mm/yr)
TreatmentPre-Katrina
(7/15/04 - 8/22/05)
Katrina
(8/22/05 – 10/21/05)
Post-Katrina
(10/21/05 – 4/10/08)
Full Time Series
(7/15/04 – 4/10/08)Control 22 ± 8 1174 ± 249 -49 ± 10 50 ± 5
P 5 ± 8* 724 ± 146 -31 ± 5 29 ± 3N 11 ± 8* 231 ± 237 -36 ± 10 1 ± 4*
N+P 57 ± 15 767 ± 189 -43 ± 9 39 ± 4Lethal 44 ± 10 602 ± 134 -39 ± 6 31 ± 3
INTRODUCTIONThe tidal marshes in Louisiana are threatened by sea level rise, increased frequency and
intensity of tropical storms, as well as erosion and subsidence. These issues are potentially
exacerbated by increased urbanization and development, which may lead to increased nutrient
loading (Nicholls et al. 2007). The north shore of Lake Pontchartrain provides an excellent
example of emergent wetlands threatened by such risks. In St. Tammany Parish alone,
residential/urban land use increased over 159 % between 1982 and 2000 resulting in the
conversion of 4.4 km2 of marsh to urban development (Beall et al. 2001). These marshes are
essential for the valuable functions they provide, including water quality improvement, storm
protection, and wildlife and fisheries habitat. Consequently, understanding the processes
required to best manage them is critical. The goal of this project was to identify the effects of
hurricane impacts and nutrient input on an oligohaline marsh at Big Branch National Wildlife
Refuge (Figure 1). Research plots had been established and monitored two years prior to
hurricane Katrina, which allowed us to investigate the emergent wetlands’ response and
resilience to hurricane impacts.
METHODS• Surface elevation tables (SET) were used to measure changes in marsh elevation. Fifteen
SETs were evenly distributed across three separate areas and within each area were randomly
assigned a nutrient treatment. Nutrient treatments applied to SET plots included 40 g nitrogen
m-2 yr-1, 30 g phosphorous m-2 yr-1, a combination of these levels of N and P, a lethal
disturbance using Rodeo herbicide, and controls with no treatments.
• Baseline measurement was taken July 17, 2004 and subsequent measurements continued
semi-annually until April 10, 2008.
• For analysis, the dataset was divided into pre-Katrina and post-Katrina subsets. The entire
time series was also analyzed for overall trends in rates of elevation change. Net change in
elevation refers to the change in elevation relative to the initial measurement.
RESULTSThe pre-Katrina dataset exhibited significant positive rates of elevation change (i.e., the slope
was significantly different from zero) for the control, combined N and P, and lethal plots (Table 1).
Post-Katrina exhibited significant negative rates of elevation change for all treatments. However,
the entire data set exhibited a significant positive rate of elevation change for all treatments
except the N plots. A large increase in elevation occurred directly after hurricane Katrina
(October 2005) in all treatment plots and was 2 to 8 times higher than the net change in elevation
that occurred immediately prior to hurricane Katrina (Figure 1). For example, in the control plot,
from July 2004 to August 2005 there was a net change in elevation of 26mm. From August 2005
to October 2005, there was a net change of 215mm. Post-Katrina, a large decrease in elevation
was evident, however, the long term net change (July 2004 to April 2008) was positive for all
treatments, except for the N plots, which showed a slight decrease of 4 mm.
CONCLUSIONS• Complex patterns of elevation change indicate some effect from nutrient loadings prior to
Hurricane Katrina. Further analysis of soil biochemistry is needed to determine other processes
that may be contributing to elevation change.
• Although initially a large decrease in elevation followed the large increase in elevation post-
Katrina (Figure 1), the long-term trend still shows a higher rate of elevation change than the pre-
Katrina rate of elevation change for Control and P plots (Table 1).
• The rates of elevation change for all but the N plots, are greater than the rates of relative sea
level rise for this area estimated from Gagliano 1999 (Figure 3).
ACKNOWLEDGEMENTSThanks to Carol Wilson, Jen Roberts, Brendan Yuill, Laura Dancer, and all those
who aided in the collection of data over the years. This work was supported by
NOAA grant #NA06NOS4630026.
REFERENCESBeall, A. D., S. Penland, and F. Cretini, Jr. 2001. Urbanization effects on
habitat change in St. Tammany Parish, 1982 – 2000. Final Report
submitted to the Lake Pontchartrain Basin Foundation, Metairie,
Louisiana. 19 pp.
Gagliano, S. M. 1999. Faulting, subsidence and land loss in coastal Louisiana,
In: Coast 2050: Toward a Sustainable Coastal Louisiana, The
Appendices. Louisiana Department of Natural Resources. Baton
Rouge, LA.
Nicholls, R.J. and others. 2007. Coastal systems and low-lying areas. Climate
Change 2007: Impacts, Adaptation and Vulnerability. Contribution of
Working Group II to the Fourth Assessment Report of the IPCC, M.L.
Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E.
Hanson, Eds., Cambridge University Press, Cambridge, UK, 315-356.
5 mm/yr
9 mm/yr
Figure 3. Isopleth map of relative sea level rise rates adapted from
Gagliano 1999.
-100
-50
0
50
100
150
200
250
300
N
N+P
P
Control
Lethal
Date of Measurement
9/22/04
3/8/05
8/22/05
10/21/05 5/23/06
1/8/07 6/6/074/10/08C
han
ge
in E
lev
atio
n (m
m)
Figure 2. Change in elevation relative to the initial measurement over time
Lake
Pontchartrain
Figure 1. Study area. Yellow circle indicates the location of SETs. Red line represents
the path of Hurricane Katrina
