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