1. The implications of Hurricane Katrina and altered nutrient regimes on long-term trends of
marsh elevation at Big Branch National Wildlife Refuge, Louisiana
Reed, A. D. 1, M. Commagere 1, Hester 2
University of New Orleans 1. University of Louisiana Lafayette2.
INTRODUCTION Table 1. Rate of elevation change (mm/yr) plus the standard error pre-Katrina, Katrina, post-Katrina,
The tidal marshes in Louisiana are threatened by sea level rise, increased frequency and and the full time series for each treatment. An asterisk indicates the slopes failed to be statically
intensity of tropical storms, as well as erosion and subsidence. These issues are potentially significant at the 5% confidence interval. The Katrina time series was not statistically analyzed.
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 Rate of elevation change (mm/yr)
example of emergent wetlands threatened by such risks. In St. Tammany Parish alone, Pre-Katrina Katrina Post-Katrina Full Time Series
Treatment
residential/urban land use increased over 159 % between 1982 and 2000 resulting in the (7/15/04 - 8/22/05) (8/22/05 – 10/21/05) (10/21/05 – 4/10/08) (7/15/04 – 4/10/08)
conversion of 4.4 km2 of marsh to urban development (Beall et al. 2001). These marshes are Control 22 ± 8 1174 ± 249 -49 ± 10 50 ± 5
essential for the valuable functions they provide, including water quality improvement, storm P 5 ± 8* 724 ± 146 -31 ± 5 29 ± 3
protection, and wildlife and fisheries habitat. Consequently, understanding the processes N 11 ± 8* 231 ± 237 -36 ± 10 1 ± 4*
required to best manage them is critical. The goal of this project was to identify the effects of N+P 57 ± 15 767 ± 189 -43 ± 9 39 ± 4
hurricane impacts and nutrient input on an oligohaline marsh at Big Branch National Wildlife Lethal 44 ± 10 602 ± 134 -39 ± 6 31 ± 3
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 5 mm/yr
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
9 mm/yr
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. Lake
Pontchartrain
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). Figure 1. Study area. Yellow circle indicates the location of SETs. Red line represents
Post-Katrina exhibited significant negative rates of elevation change for all treatments. However, the path of Hurricane Katrina
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, 300 Figure 3. Isopleth map of relative sea level rise rates adapted from
from July 2004 to August 2005 there was a net change in elevation of 26mm. From August 2005 Gagliano 1999.
250
Change in Elevation (mm)
to October 2005, there was a net change of 215mm. Post-Katrina, a large decrease in elevation N
was evident, however, the long term net change (July 2004 to April 2008) was positive for all 200 N+P
treatments, except for the N plots, which showed a slight decrease of 4 mm. 150 P
100 Control
50 ACKNOWLEDGEMENTS
Lethal Thanks to Carol Wilson, Jen Roberts, Brendan Yuill, Laura Dancer, and all those
0 who aided in the collection of data over the years. This work was supported by
9/22/04 8/22/05 1/8/07 6/6/07 NOAA grant #NA06NOS4630026.
CONCLUSIONS -50 3/8/05 10/21/05 4/10/08
• Complex patterns of elevation change indicate some effect from nutrient loadings prior to -100
5/23/06 REFERENCES
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