lessons learned from a decade of assessment and restoration studies of benthic invertebrates and...

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Lessons Learned from a Decade of Assessment and Restoration Studies of BenthicInvertebrates and Submersed Aquatic Vegetation in Lake PontchartrainAuthor(s): Michael A. Poirrier, Elizabeth A. Spalding, and Carol D. FranzeSource: Journal of Coastal Research, Number 10054:88-100. 2009.Published By: Coastal Education and Research FoundationDOI: http://dx.doi.org/10.2112/SI54-005.1URL: http://www.bioone.org/doi/full/10.2112/SI54-005.1

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Lessons Learned from a Decade of Assessment andRestoration Studies of Benthic Invertebrates andSubmersed Aquatic Vegetation in Lake Pontchartrain

Michael A. Poirrier{, Elizabeth A. Spalding{, and Carol D. Franze{

{Estuarine Research LaboratoryDepartment of Biological

Sciences and thePontchartrain Institute forEnvironmental Sciences

University of New OrleansLakefront Campus

2000 Lakeshore DriveNew Orleans, LA 70148, [email protected]

{Louisiana Sea Grant Marine Extension/LSU Ag Center Research & Extension

21549 Old Covington HighwayHammond, LA 70403, U.S.A.

ABSTRACT

POIRRIER, M.A.; SPALDING, E.A., and FRANZE, C.D., 2009. Lessons learned from a decade of assessment andrestoration studies of benthic invertebrates and submersed aquatic vegetation in Lake Pontchartrain. Journal of CoastalResearch, SI(54), 88–100. West Palm Beach (Florida), ISSN 0749-0208.

Studies of Lake Pontchartrain benthic invertebrates and submersed aquatic vegetation (SAV) from 1996 through 2005provided the opportunity to determine community responses to severe disturbances including: recovery from shelldredging; effects of anoxia and hypoxia from saltwater intrusion; the 1997 Bonnet Carre Spillway opening; a prolongeddrought resulting from an El Nino Southern Oscillation; and effects of Hurricane Katrina. We included reviews andupdates of our recently published work, an integrated analysis of findings, and restoration recommendations. Duringthis decadal study there were no prolonged periods of ‘‘normal’’ conditions. Instead, benthos and SAV were experiencingor recovering from significant temporal disturbances. This made it difficult to use the abundance of a particular set oforganisms at a point in time to evaluate habitat quality or restoration success without considering past disturbanceeffects. Benthos species diversity and the relative abundance of stress tolerant groups such as annelids were goodindicators of short-term adverse conditions, but rapid changes occurred in response to salinity and dissolved oxygen.Rangia cuneata was a good indicator of the extent of hypoxia and long-term damage from hurricanes. The distributionand abundance of SAV decreased with turbidity and nutrient increases, and Vallisneria americana and other freshwaterSAV decreased with prolonged salinity increases. Resistance to and resilience after disturbances and natural changesduring long term cycles have to be considered in evaluating habitat condition and restoration success.

ADDITIONAL INDEX WORDS: Estuary, shell dredging, spillway, ENSO, drought, hurricane, salinity, turbidity,water clarity, saltwater intrusion, hypoxia, Rangia cuneata, Ischadium recurvum, species diversity, SAV, Ruppiamaritima, Vallisneria Americana.

INTRODUCTION

Lake Pontchartrain is a large, shallow, oligohaline estuary

located in a deteriorating delta of the Mississippi River. The

estuary is disturbed by both natural and human factors. Lake

Pontchartrain (Figure 1) has a mean salinity of 3.9 ppt

(Francis et al., 1994), a mean depth of 3.7 m, and a surface

area of 1630 km2 (Sikora and Kjerfve, 1985). Salinity

increases from west to east (Swenson, 1980). Freshwater

enters via streams to the west and north, outfall canals on the

south shore, and periodic openings and leakage of the Bonnet

Carre Spillway. Higher salinity water enters from eastern

tidal passes and the Inner Harbor Navigation Canal (IHNC),

which is connected to the Mississippi River Gulf Outlet

(MRGO). From 1933 to 1990, accumulated Rangia cuneata

shells were dredged from the lake bottom and used for the

construction of roads, parking lots, levees, and the production

of cement (Sikora and Sikora, 1981, 1982; USACE, 1987).

Current adverse effects include urban and agricultural

runoff, shoreline modification, Mississippi River flood diver-

sions (Penland et al., 2001), and saltwater intrusion that

causes salinity stratification and bottom water anoxia and

hypoxia (Abadie and Poirrier, 2000).

Benthic invertebrates are generally accepted as meaning-

ful indicators of the environmental health of estuaries

(Boesch and Rosenberg, 1981; Diaz and Rosenberg, 1995;

Reish, 1986; Wilson, 1994). Because they cannot move away

from adverse conditions, they are continuously exposed to

environmental stress and therefore must respond to episodic

environmental disturbances (Bilyard, 1987; Moore, 1992).DOI:10.2112/SI54-005.1.

Journal of Coastal Research SI 54 88–100 West Palm Beach, Florida Fall 2009

Also because of their sessile nature, differences in their

temporal and spatial community composition are easily

sampled and quantified. Oligohaline assemblages have low

species richness (Remane and Schleiper, 1971) and are

controlled more by abiotic factors than persistent biotic

interactions (Boesch and Rosenberg, 1981; Gunter, 1960). They

are composed of biologically diverse taxa that provide impor-

tant links in food chains and water quality processes. Rangia

cuneata (Bivalvia:Mactridae) or ‘‘common rangia’’ is the

dominant member of the benthos in oligohaline Gulf Coast

estuaries based on number of individuals (Hopkins and

Andrews, 1970), total biomass (Cain, 1975; Odum, 1967), life

span, and ecosystem interactions (Darnell, 1958; LaSalle and

de la Cruz, 1985; Peterson and Heck, 1999; Vaughn and

Hakenkamp, 2001). It was a principal indicator of the condition

of bottom habitat in our studies.

The first report on Lake Pontchartrain benthic inverte-

brates was by Suttkus, Darnell, and Darnell (1954). They

found an average of 95 large R. cuneata per square meter, and

their study included areas near the mouth of IHNC. In later

studies conducted from 1969 to 1972, Tarver and Dugas

(1973) found that the average density of large R. cuneata in

the lake was 39 clams/m2, and they did not find clams larger

than 16 mm in areas that were continually dredged. Poirrier

(1978) documented salinity stratification and low dissolved

oxygen associated with saltwater intrusion from the IHNC

and mentioned possible adverse effects on benthic organisms.

In an ecological characterization of the benthic community of

Lake Pontchartrain, Sikora and Sikora (1982) found that

total number of species in Lake Pontchartrain was lower than

expected for brackish systems and that all measures of

benthic community structure indicate a stressed community.

Junot, Poirrier, and Soniat (1983) documented an adverse

impact on benthos near the mouth of the IHNC due to low

dissolved oxygen from salinity stratification. Poirrier et al.

(1984) collaborated with Schurtz and St. Pe (1984) in a study

of bottom dissolved oxygen concentrations resulting from

salinity stratification from the IHNC and effects on benthic

invertebrates. They attributed the ‘‘dead zone’’ reported by

Sikora and Sikora (1982) to low dissolved oxygen and other

conditions associated with salinity stratification. The USACE

(1987) prepared a report on the effects of shell dredging that

summarized the literature and possible impact on benthic

organisms. In a study of the Duncan outfall canal, Powers,

Poirrier, and Yund (1992) found that nematodes, oligo-

chaetes, and capitellid polychaetes are abundant in areas

affected by urban runoff, but the densities of the molluscan

species R. cuneata, Texadina sphinctostoma, and Probythi-

nella protera were low and increased with distance from the

source of pollutants. Spalding Walker, and Poirrier (2007)

quantified the benefits of restoring R. cuneata through

complete closure of the MRGO using clam clearance rates of

four phytoplankton species, silts and clays, and Escherichia

coli. They estimated the time it would take R. cuneata to filter

a volume of water equal to the lake before and after

restoration using clam density, relationships between length

and dry weight biomass, and filtration rates.

The reestablishment of historic submersed aquatic vege-

tation (SAV) is a realistic and measurable Lake Pontchar-

train restoration goal. The former dominant species, Vallis-

neria americana, declined by over 50% since first studied in

1953 (Burns and Poirrier, 1996). With SAV abundance

rapidly decreasing, further disturbance, whether it is

natural or anthropogenic, could possibly decimate remaining

populations. Factors that control SAV distribution and

abundance in Lake Pontchartrain are complex, but the

principal direct driver is light availability (Cho and Poirrier,

2005a; Dennison et al., 1993), which is adversely affected by

increased turbidity and algal epiphyte growth. Eutrophica-

tion increases turbidity and epiphyte growth. Thus, SAV is a

good indicator of water clarity, eutrophication trends, and

results of restoration efforts (Cho and Poirrier, 2005a,

2005b).

Submersed aquatic vegetation distribution in Lake

Pontchartrain was initially described in a report by Suttkus,

Darnell, and Darnell (1954). Vallisneria americana and R.

maritima occurred on the north shore at a site just east of

the Tchefuncte River and from Lewisburg eastward to the

Rigolets. On the southshore, SAV was observed from Indian

Beach to just east of South Point; however, the eastern lobe

of the lake was not surveyed. Montz (1978) surveyed the

lake in 1973 to determine the effects of the Bonnet Carre

Spillway opening on SAV and reported no effects of the

opening on SAV in the lake. The 1973 diversion occurred

coincident with shell dredging in the lake, which maintained

high turbidity and probably prevented cyanobacteria blooms

observed with nutrient introductions after shell dredging

ceased (Brammer et al., 2007). Montz (1978) found two

additional species, Najas guadalupensis and Potamogeton

perfoliatus. Poirrier (personal observation) observed an

exotic species, Myriophyllum spicatum, after the 1973

Spillway opening.

Turner, Darnell, and Bond (1980) compared the surveys by

Suttkus, Darnell, and Darnell (1954), and Montz (1978)

documented a 25% to 35% decrease in SAV. In 1985, Mayer

(1986) conducted a lakewide survey similar to Montz (1978)

Figure 1. Benthic invertebrate and submersed aquatic vegetation survey

sites in Lake Pontchartrain, Louisiana.

Benthic Invertebrates and SAV in Lake Pontchartrain 89

Journal of Coastal Research, Special Issue No. 54, 2009

and found a 50% decrease in SAV between 1973 and 1985.

The maximum depth distribution for SAV in 1985 was 1.2 m

(Mayer, 1986), whereas maximum depth distribution report-

ed from earlier studies was 1.8 m (Montz, 1978; Suttkus,

Darnell, and Darnell, 1954). Burns, Poirrier, and Preston

(1993) conducted SAV surveys in 1991 and 1992. A compar-

ison of their findings to those of Mayer (1986), showed a 17%

decrease from 1985 to 1992. During the 1991–1992 surveys,

SAV foliar cover was greatest in the 0.3- to 0.6-m water depth

contour (Burns, Poirrier, and Preston, 1993). Total areal

cover for SAV in 1991–1992 was 88 ha (24 ha when corrected

for patchiness). Submersed aquatic vegetation foliar cover

along the north shore totaled 76 ha and along the urbanized

south shore it totaled 12 ha (Burns, Poirrier, and Preston,

1993). A comparison of the 1992 and 1993 surveys showed a

74% reduction of SAV areal coverage after passage of

Hurricane Andrew in August 1992 and a 95% reduction on

the south shore after the passage of a severe winter storm in

March 1993 (Burns and Poirrier, 1996). Potamogeton perfo-

liatus, a rare Louisiana plant whose westernmost distribution

is Lake Pontchartrain, was observed at five sites during the

1991 to 1993 surveys (Burns, Poirrier, and Preston, 1995).

Benthos and SAV provide essential habitat for sport and

commercial finfish, shellfish, and wildlife including waterfowl

and the manatee (Cottam, 1939; Darnell, 1958; Ebersole and

Kennedy, 1995; LaSalle and de la Cruz, 1985; Orth, Heck,

and van Montfrans, 1984; Perry and Uhler, 1981, 1988;

Richman and Lovvorn, 2004). In addition to long-term

monitoring and assessment, our studies focused on ecological

factors and environmental stressors that determine the

distribution and abundance of these critical habitats. An

important aspect of our studies was the determination of the

cause of habitat loss and provision of strategies needed for

sustained restoration. Restoration of these essential habitats

will result in expanded fish and wildlife resources, and

aquatic recreational opportunities.

Major research objectives were to distinguish anthropo-

genic stressors from natural abiotic and biotic ecological

drivers. Often anthropogenic perturbations work in concert

with natural environmental drivers, making organisms more

sensitive to natural stress and producing fragile communi-

ties that are unable to rapidly recover from natural

disturbances. Some of these disturbances cause chronic

stress, producing gradual long-term changes; others might

be acute, but episodic, producing periodic severe disturbanc-

es. Our studies focused on the effects of severe natural and

anthropogenic disruptions that included recovery from shell

dredging; effects of anoxia and hypoxia from MRGO

saltwater intrusion; the 1997 opening of the Bonnet Carre

Spillway; a prolonged drought resulting from an El Nino

Southern Oscillation (ENSO); and a severe disturbance by

Hurricane Katrina. Each of these disturbances challenged

the resistance and resilience of the benthic invertebrate and

SAV communities, and provided natural experiments on the

response of species populations and community structure to

disturbance. We present original research incorporated with

reviews and updates of our recently published work to

provide an integrated analysis of findings from 1996 through

2005.

MATERIALS AND METHODS

Benthos

Infaunal invertebrates were sampled November 1996–

November 2005 along an east-to-west transect at five fixed

sites and March 1997–November 2005 along a north-to-south

transect at six fixed sites as part of a long-term monitoring

program (Figure 1). Additional benthic invertebrate samples

were collected at 33 sites along 8 transect lines spaced evenly

from west to east across the lake in 1997, November and April

2004, and 2005 (Figure 1). Salinity and dissolved oxygen were

measured during each sampling episode with a YSI 85 S-C-T,

D.O meter. These sites have been used in our previous studies

of benthic invertebrates (for specific methods see Abadie and

Poirrier, 2000; Brammer et al., 2007; Poirrier, Rodriguez del

Rey, and Spalding, 2008).

Three replicate infaunal samples were obtained with a

15 cm 3 15 cm petite Ponar dredge, rinsed in the field

through a 0.6-mm sieve, and the retained material fixed in a

10% buffered formalin solution and stained with Rose bengal.

In the laboratory, samples were rinsed on a 0.5-mm sieve, and

macrofauna were identified to the lowest feasible taxon and

enumerated using a dissecting microscope. Taxonomic desig-

nations followed the Integrated Taxonomic Information

System. Clams were identified to species, sized into 5-mm

classes, and counted. Because of our inability to visually

distinguish the difference between R. cuneata and Mulinia

lateralis, clams smaller than 6 mm were excluded.

We also conducted studies to quantify possible benefits of

restoring R. cuneata through closure of the MRGO (Spalding,

Walker, and Poirrier, 2007). Static, clam clearance rate

experiments using four different phytoplankton species, silts

and clays, and Escherichia coli were used to quantify effects

of clam filtration.

Submersed Aquatic Vegetation

Surveys of SAV abundance to determine areal coverage

along a littoral depth gradient were conducted during late fall

from 1996 through 2005 at Lacombe, Pointe aux Herbes, and

Fontainebleau State Park. A location at Goose Point was

added in 1999, and Lincoln Beach was added in 2000

(Figure 1). These locations have been used in our previous

studies of SAV depth distribution and areal coverage (for

specific methods see Burns and Poirrier, 1996; Burns,

Poirrier, and Preston, 1993; Cho, 2003; Cho and Poirrier,

2005a, 2005b, 2005c). At each location, five random transects

were surveyed. Transects were perpendicular to the shoreline

and extended from the shoreline to a water depth of

maximum colonization. The line intercept method was used

(Gertz, 1984). While snorkeling, species of SAV intercepted by

the transects were identified and distance covered by each

species was recorded in centimeters by direct observation.

Statistical Analyses

Plymouth Routines in Multivariate Ecology (Clark and

Gorley, 2001) were used to analyze macroinvertebrate

community structure. Univariate analyses of variance (AN-

90 Poirrier, Spalding and Franze


OVA) were performed using general linear model procedures

in SPSS to test for differences in bottom salinity, Shannon-

Wiener (loge) species diversity, and R. cuneata density from

1996 to 2005. Species diversity data were limited to the period

from May 1998 through August 2002 to determine the effect

of the ENSO-induced drought. Data were tested for meeting

parametric analysis criteria and a level of 0.05 was used to

determine significance. Although species diversity and R.

cuneata density exhibited mild departures from normality,

the ANOVA technique is considered robust, and data

exhibiting mild departures from normality may be considered

acceptable for analysis without transformation (Neter, Was-

serman, and Kutner, 1990). Post hoc pairwise comparisons

were made to determine significant differences in means

using the Bonferroni method.

ArcMap GIS software kriging function was used to spatially

analyze the distribution of R. cuneata biomass and the extent

of dissolved oxygen concentration at the mouth of the IHNC

in August 2006. Measured data values were weighted to

derive a prediction for unmeasured locations based on the

distance between the measured sites; the prediction locations;

and the overall spatial arrangement among the measured

sites. Semivariograms were created and then fitted to the

most appropriate model to analyze the spatial correlation

between sites. Microstructure and measurement error were

estimated using replicate samples at each transect site.

Standard error maps were created and evaluated, and cross

validations were performed.

RESULTS

We integrated published and unpublished data on changes

in benthic invertebrates and SAV communities to examine

trends over a 10-year period. A popular overarching assump-

tion is if environmental conditions in Lake Pontchartrain are

improving, population trends in these communities should be

shifting to the historic distribution and abundance of key

species and over time indices of community structure should

indicate less stress. However, during this study it became

clear that communities were regularly affected by diverse

disturbances that challenged evaluation of long-term trends.

This synthesis examines responses to disruptions and

extracts trends, ecologic drivers, and restoration strategies

from the long-term data set.

Recovery from Shell Dredging

Benthos

Abadie and Poirrier (2000) compared densities of large

($21 mm) R. cuneata in studies before and after 1990 when

shell dredging ended to determine if a density increase had

occurred. In 1954 R. cuneata were abundant (95 clams/m2).

They were less abundant in 1973 (39 clams/m2), 1982 (2

clams/m2), and 1984 (41 clams/m2) as the intensity of shell

dredging increased. Samples obtained in 1996 showed an

increase (126 clams/m2) that was comparable to densities in

1954. Clam densities in 1954 and 1996 were statistically

different from 1973, 1982, and 1984, but not different from

each other. These recovery studies indicated that shell

dredging had a significant impact on the bottom community.


Submersed aquatic vegetation distribution and abundance

were much lower when this study began in 1996 (Figure 2)

than historic levels (Suttkus, Darnell, and Darnell, 1954).

Hurricane Andrew damaged north shore beds in 1992 and a

winter storm damaged south shore beds in 1993. Recovery

from these weather events occurred, but beds did not increase

to 1991 levels (Burns and Poirrier, 1996). Surveys in 1996 did

not indicate an increase in SAV due to cessation of shell

dredging (Figure 2), and an overall increase in SAV did not

occur until 1998 (Cho and Poirrier, 2001).

Effects of Anoxia and Hypoxia from MRGOSaltwater Intrusion

Benthos

Abadie and Poirrier (2000) demonstrated increased abun-

dance of large R. cuneata after shell dredging ended and also

found that large R. cuneata ($21 mm) were absent from a large

area north of the IHNC. Later studies by Abadie and Poirrier

(2001a, 2001b) determined that the affected area was approx-

imately 250 km2 (100 mi2) and was caused by episodes of low

dissolved oxygen due to salinity stratification from more saline

water entering from the MRGO through connections with the

IHNC. Over the course of this study, episodic hypoxia has been

documented as far north as site NS2, 24 km (15 mi) from the

mouth of the IHNC, and hypoxia has occurred regularly at site

NS3B, 17 km (10.5 mi) north of the IHNC (Figure 3). Periodic

samples of benthic invertebrates from north–south transects

through the ‘‘dead zone’’ from 1997 through 2006 demonstrated

the regular occurrence of Shannon-Wiener species diversity

values often in the range of 0.0 to 1.0 (Figure 4). Anoxia and

hypoxia typically is found at the mouth of the IHNC as in

August 2006 (Figure 5). Abadie and Poirrier (2001a) compared

the abundance of large R. cuneata with other indicator taxa and

concluded that these could be used to monitor the area affected

Figure 2. Average annual abundance of submersed aquatic vegetation

(m) by dominant species from fall 1996 to spring 2005 (1996 to 1998, n 5

15; 1999, n 5 20; 2000 to 2006, n 5 25).



by anoxia and hypoxia. Using R. cuneata biomass estimates

from April 2004, we can easily see the extent of the area

affected by episodic hypoxia (Figure 6). Low species diversity

values and the absence of large R. cuneata documented in

numerous previous studies demonstrated a persistent negative

impact on the infauna.

Studies were conducted to quantify possible benefits of

restoring R. cuneata through closure of the MRGO (Spalding,

Walker, and Poirrier, 2007). Static clam clearance rate

experiments using four different phytoplankton species, silts

and clays, and E. coli were used to quantify effects of clam

filtration. We found a relatively high average clearance rate

for Ankistrodesmus sp. and a low value for Anabaena sp. We

found that clams were also able to clear the water column of

silts or clays and E. coli. We used clam density data from

lakewide surveys, relationships between length and dry

weight biomass, and filtration rates based on dry weight

biomass to determine the time that it takes clams to filter a

volume of water equivalent to Lake Pontchartrain before and

Figure 3. Surface and bottom dissolved oxygen values from March 1997

to November 2005 along the north–south transect.

Figure 4. Shannon-Wiener species diversity index (loge) from March 1997

to November 2005 along the north–south transect. The NS transect

intersects the ‘‘dead zone.’’

Figure 5. Spatial interpolation of hypoxic conditions at the mouth of the

IHNC on August 2, 2006. Inset illustrates area of the lake sampled.

Figure 6. A map showing spatial interpolation of the lakewide distribu-

tion of Rangia cuneata biomass (g/m2) in April 2004.



after restoration. Total lake volume clearance time decreased

for all suspended particles, indicating that restoring clams in

the 250 km2 dead zone would reduce phytoplankton blooms

and fecal coliforms, and increase water clarity.


Vallisneria americana has persistently decreased in abun-

dance since SAV was first surveyed in 1953 (Burns, Poirrier,

and Preston, 1993). Stress from changes in the salinity regime

due to saltwater intrusion from the MRGO may be contribut-

ing to stress on freshwater species such as V. americana, N.

guadalupensis, and P. perfoliatus. Freshwater species have

become rare along the southeastern shoreline near the IHNC,

a source of saltwater intrusions (Cho and Poirrier, 2005b).

1997 Bonnet Carre Spillway Opening

Benthos

Brammer et al. (2007) found that the opening of the Bonnet

Carre Spillway from March 17 through April 18, 1997, caused

an abrupt decrease in salinity (Figure 7), species diversity

(Figure 8), and R. cuneata abundance (Figure 9). Infaunal

macroinvertebrate, surface and bottom salinity, and dissolved

oxygen data obtained from November 1996 through Novem-

ber 1998 from five sites on the east–west transect were

analyzed in this study. A community composed of oligohaline

taxa persisted during the freshwater period, but changes in

dominance, and to a lesser extent, composition, occurred over

time and among sites. Prior to the Spillway opening,

hydrobiid snails were most abundant and after the opening,

oligochaetes became most abundant. There was a pronounced

spatial effect related to the distance of the sites from the

Spillway and from tidal passes (Brammer et al., 2007).

Mean diversity values significantly decreased during the

Spillway opening and remained lower than preopening values

for over 1 month after the Spillway was closed. A significant

change from these low values did not occur until salinity

returned to preopening levels in November 1998. Diversity

values ranged from 0.83 at Site 4 on October 29, 1997, to 2.24

at Site 5 on November 3, 1998. Macrofaunal abundance did

not significantly decrease during the Spillway opening.

However, it significantly declined by May 1997, over 1 month

after the Spillway was closed. Abundance did not significantly

increase from these low May 1997 values until July 1998.

Because of its numerical dominance and size, R. cuneata

comprises most of the benthic macroinvertebrate biomass.

Rangia cuneata biomass significantly declined between April

1997 and January 1998 (Brammer et al., 2007).


Poirrier et al. (1999) studied the effects of the March 1997

Bonnet Carre Spillway opening. A M. spicatum bed near the

mouth of Bayou St. John north of the flood control structure

was monitored in addition to the established survey

locations. They found a significant decrease in photosynthet-

ically active radiation (PAR) from severe cyanobacterial

blooms due to the spillway opening. There was no change in

V. americana coverage, but significant decreases in R.

maritima occurred by the 1997 post–spillway opening survey

(Figure 2). A 2000 m2 M. spicatum bed in Bayou St. John

died due to overgrowth by the green filamentous alga,

Cladophora spp. Decomposition of accumulated cyanobacte-

ria, algae, and SAV resulted in anoxic conditions (Poirrier et

al., 1999) and an associated fish kill. They also found a

decrease in combined foliar coverage for V. americana and R.

maritima (total SAV) after the spillway opening. Abundant

growth of the alga Cladophora spp. occurred on R. maritima

and M. spicatum, but not V. americana. Cladophora spp.

growth on R. maritima made it susceptible to shading from

phytoplankton and uprooting by wave energy (Poirrier et al.,

1999). This resulted in R. maritima being selectively lost

Figure 7. Salinity values from November 1996 to November 2005 across

the east–west transect starting with EW1 in the westernmost portion of

the lake to EW5 in the eastern portion of the lake. In addition, hurricanes

and other disturbances are noted.

Figure 8. Shannon-Wiener species diversity index (loge) from November

1996 to November 2005 along the east–west transect starting with EW1 in

the westernmost portion of the lake to EW5 in the eastern portion of the

lake. Bottom salinity and disturbances are noted.



from grass beds. In September 1998, Hurricane Georges

struck east of Lake Pontchartrain, and salinity in the lake

increased from the tidal surge (Figure 7). Comparisons of

pre- and poststorm surveys showed no statistical differences

in SAV abundance (Cho and Poirrier, 2001). High salinity

persisted after the passage of Hurricane Georges due to lack

of rainfall from a drought (Cho and Poirrier, 2001; Zheng et

al., 2003).

1999–2001 Drought resulting from El Nino

Benthos (Cho, 2003)

A strong ENSO event occurred between 1997 and 2001

(Zheng et al., 2003) producing a drought from 1999–2001 in

southern Louisiana. Bottom salinity varied significantly over

time (Figure 7; F(15,63) 5 33.20; p , 0.001). The drought

resulted in a significant lakewide salinity increase across the

east–west transects through fall 2000. Post hoc pairwise

comparisons indicated salinity levels in October 1999, July

2000, and October 2000 were significantly different from all

other values with the exception of salinity levels after

Hurricane Katrina. During July 2000 and through April

2001, salinity values were nearly the same across the east–

west transect (Figure 7). The hooked mussel Ischadium

recurvum and other high salinity species increased as salinity

increased during the drought. Shannon-Wiener species

diversity from May 1998 to August 2002 (Figure 8) differed

significantly over time (F(9,98) 5 4.07; p , 0.001) but not site

(F(4,98) 5 1.170; p 5 0.33). Species diversity values in the

western portion of the lake increased and became similar to

those of the eastern portion. Density of R. cuneata signifi-

cantly differed over time (F(14,149) 5 21.83; p , 0.001). Post

hoc pairwise comparisons indicated clam density values in

April 2001, August 2002, and November 2003 were signifi-

cantly different from all other values with the exception April

2004 and after Hurricane Katrina.

With a decrease in salinity, a decrease in species diversity

is expected, but the low values that occurred in April 2001 in

all but the easternmost portion of the lake are indicative of

other sources of stress. This is supported by the concomitant

shift in species composition to an annelid-dominated benthic

community and a severe loss of R. cuneata during April 2001

and August 2002 indicating severe lakewide stress.

Prior to 1999, small numbers of I. recurvum were confined

to the easternmost area of the lake (density ranged from 0–20

mussels/m2). As the density of R. cuneata decreased, I.

recurvum increased lakewide, including the western portion

of the lake. This resulted in the rapid increase of not only

small mussels (,5 mm) but also the establishment and

growth of larger mussels (.35 mm) on R. cuneata shells that

formed small spherical reefs about 30 cm in diameter. We

found I. recurvum at a mean lakewide density of 2284

mussels/m2 in April 2001. Ischadium recurvum gradually

decreased as salinity in the lake decreased.


Cho and Poirrier (2005b) determined the effects of an ENSO

event on SAV. Relatively low levels of R. maritima occurred

Figure 9. Average Rangia cuneata density (clams/m2) 6 SE of clams greater than or equal to 21 mm across the east–west transect from November 1996 to

November 2005 (n 5 15).



from 1996 through 1998, but a significant increase occurred in

1999 that persisted through 2002 (Figure 2). They discovered

causal links between the El Nino to La Nina climate shift and

SAV change. Reduced rainfall increased salinity (Figure 7) and

water clarity, which produced a rapid increase in the

euryhaline species R. maritima in deeper water and historic

locations where SAV had not been found since 1953. Increased

salinity caused a decrease in the freshwater species V.

americana and loss of N. guadalupensis and P. perfoliatus.

There was a shift in species dominance from V. americana to R.

maritima from 1997 through 2002. Although V. americana and

N. guadalupensis increased significantly between 2001 and

2002, R. maritima continued to dominate in 2002 because of its

dramatic increase at Goose Point (eightfold increase) and

Lacombe (fivefold increase). The increase and abundance of

SAV during these years rivaled the historic distribution of SAV

(Suttkus, Darnell, and Darnell, 1954).

Data obtained in the study were used to develop a model to

predict potential SAV habitat with changes in water clarity

(Cho and Poirrier, 2005a). Three drivers of potential SAV

habitat were (1) water clarity controls SAV colonization depth;

(2) fluctuation in annual mean water level and wave mixing

determines SAV minimum colonization depth; and (3) differ-

ences in shoreface slope determine SAV areal coverage under

comparable water quality conditions. They compared empirical

data to values predicted by the model to validate the model.

Hurricane Katrina

Benthos

A major disturbance of benthic community structure was

caused by Hurricane Katrina, which made landfall as a

category 3 hurricane near eastern Lake Pontchartrain on

August 29, 2005, and produced a storm surge of 3.7 to 4.9 m

along the northeastern shore, which decreased to 1.5 to 3 m

along the western shore (Poirrier, Rodriguez del Rey, and

Spalding, 2008). It caused an increase in salinity (Figure 7),

decrease in Shannon-Wiener species diversity at most sites

(Figures 4 and 8), and a decrease in density of R. cuneata

(Figure 8). Salinity stratification and low dissolved oxygen

were present at three north shore sites on September 30, 2005

(Poirrier, personal observation). Large (.21 mm) R. cuneata

that recently died were a major component of newly formed

shoals and ridges along the shoreline.

Benthic invertebrate samples obtained in November 2004

prior to Hurricane Katrina were compared with posthurri-

cane November 2005 samples to assess damage (Poirrier,

Rodriguez del Rey, and Spalding, 2008). There was a major

shift in the rank order of dominant species. After Hurricane

Katrina, the usual dominant species including the hydrobiid

snails Probythinella protera and Texadina sphinctoma, the

isopod Cerapus benthophilis, and the polychaete Amphicteis

floridus were reduced to less than 1% of the total number of

individuals present. Oligocheates and the polychaete Stre-

blospio benedicti became dominants. Rangia cuneata de-

creased from the most abundant species prior to Hurricane

Katrina to the fifth most abundant species after the

hurricane. A comparison of the number of species, the

number of individuals, species diversity, and R. cuneata

biomass between 2004 and 2005 samples using ANOVA found

significant site and time interactions. This indicates that

Hurricane Katrina caused changes in all major measures of

community structure. The widespread defaunation caused by

Hurricane Katrina is best demonstrated by the loss of R.

cuneata and other community dominants from 50% (800 km2)

of the lake bottom at depths greater than approximately the

3.7-m depth contour. Bottom disturbance from past shell

dredging and enhanced saltwater intrusion from the MRGO

may have contributed to the severity of this impact.


Hurricane Katrina produced a storm tidal surge in eastern

Lake Pontchartrain that increased salinity and damaged

littoral habitats. Surveys conducted in October 2005 found

major decreases in SAV aboveground biomass at all study

locations (Figure 2). SAV beds were reduced to small patches

consisting mainly of V. americana. SAV was eliminated at all

locations but Lacombe. Spring 2006 surveys indicated recovery

of R. maritima, but not V. americana. After Hurricane Katrina,

relatively high salinity due to decreased rainfall adversely

affected V. americana, a freshwater species, but did not affect

R. maritima, a euryhaline species that grows rapidly from

seeds (Cho and Poirrier, 2005c). Submersed aquatics in streams

north of Lake Pontchartrain and marshes in the path of the

storm were also severely affected by Hurricane Katrina (M.A.

Poirrier, personal observation).

DISCUSSION

The analyses of benthic invertebrate distribution and

abundance from 1996 through 2005 indicated large fluctua-

tions in benthic invertebrates (Figures 4 and 8) and SAV

(Figure 2) populations. During the 10-year study there was

no prolonged period that could be regarded as ‘‘normal’’

conditions. Instead, these communities were always experi-

encing or recovering from a significant temporal disturbance.

In addition, persistent spatial perturbations and other

chronic disturbances with greater time, but lesser impact

scales were present. These persistent long-term perturba-

tions included natural deterioration of the Bernard Delta;

disruption of wetland hydrology including Mississippi River

inflow, eutrophication, background toxic pollution from

agriculture and urban runoff; and changes in the general

habitat quality of the watershed, lower basin, and the Gulf of

Mexico (Penland et al., 2001). These multiple stressors might

have produced more fragile communities with reduced

diversity, resistance, and resiliency.

Benthic invertebrate species diversity values from 1996

through 2005 fluctuated as a result of changes in salinity,

hypoxia, and damage from a severe hurricane (Figures 3 and

4). In coastal marine systems, diversity increases with

salinity, and low salinity estuaries, such as Lake Pontchar-

train, are characterized by relatively low diversity (Moore,

1992; Remane and Schleiper, 1971). Because salinity gener-

ally increases from west to east in Lake Pontchartrain, there

is a corresponding increase in species diversity. Exceptions to



this pattern occurred during the 1997 Bonnet Carre Spillway

opening and during July 2000 when salinities throughout the

lake were similar (Figures 7 and 8). In 1997, salinity across

the lake was the same due to the massive influx of Mississippi

River water and in 2000 due to the lack of stream flow from a

severe drought. Species diversity also generally increased

along our north–south transect with distance from the mouth

of the INHC even though salinity decreased (Figure 4). This

was due to stress from anoxia and hypoxia that had a greater

effect on the community structure than salinity. This pattern

rapidly changed when stratification was disrupted and

recruitment of higher salinity opportunistic species occurred.

The presence, absence, and relative abundance of indicator

taxa showed strong correlations with shifts in salinity and

dissolved oxygen. Most species are small and opportunistic,

forming different assemblages that rapidly recover from

disturbances. Large R. cuneata require several years to

recover from episodes of harmful dissolved oxygen. Therefore,

clam distribution can be used in annual surveys to assess

areas affected by anoxia and hypoxia.

Recovery from Shell Dredging

Benthos

Our recovery studies conducted after shell dredging ended

in 1990 indicate that shell dredging decreased large clam

density and water clarity (Abadie and Poirrier, 2000, 2001b).

After shell dredging ended, a significant increase in the

density of large R. cuneata indicated recovery to levels

equaling the 1954 baseline data. A study of water clarity

based on Secchi disc visibility observations from three sites on

the Lake Pontchartrain Causeway found that significant

visibility increases occurred at the north and midlake sites,

but no change occurred at the south shore site (Francis and

Poirrier, 1999). This increase in visibility appears to be

related to the increase in R. cuneata biomass after shell

dredging ceased. This is supported by the low density of clams

near the south shore due to saltwater intrusion, where water

clarity did not increase; by the increase in water clarity that

peaked 3 years after shell dredging was stopped, the length of

time that it takes for clams to mature; and by a recent study

(Spalding, Walker, and Poirrier, 2007) indicating that high

clam biomass has a significant effect on water clarity.

Although recovery of large R. cuneata from shell dredging

occurred, episodes of anoxia and hypoxia associated with

saltwater intrusion from the MRGO also causes loss of large

clams (Figure 6). Because high salinity bottom water flows

along depth contours, shallow channels cut by shell dredges

may have facilitated more widespread movement of water low

in dissolved oxygen in the lake.


There was no immediate increase in SAV abundance on the

north shore associated with the water clarity increase in 1993

after shell dredging was stopped (Francis and Poirrier, 1999).

A significant SAV increase did occur in 1998 (Cho and

Poirrier, 2005b). This increase may have been delayed due to

damage from Hurricane Andrew in 1992 (Burns and Poirrier,

1996). Water clarity dynamics in the littoral zone probably

differed from the pelagic zone where water clarity was

studied. Littoral habitats may have been degraded from

watershed increases in nutrients and turbidity. Even if

turbidity from suspended silts and clays decreased, algal

epiphyte growth would increase and shade SAV without

nutrient reduction. These effects persisted after shell dredg-

ing ended, but decreased during the drought. Low runoff

during the drought resulted in relatively clear, low nutrient

conditions and a rapid expansion of SAV as evidenced by the

increase of R. maritima to near historic levels during the

spring of 1999 (Cho and Poirrier, 2005b).

Effects of Anoxia and Hypoxia from MRGOSaltwater Intrusion

Benthos

A major and continuing impact to Lake Pontchartrain is

saltwater intrusion via the IHNC from the MRGO. Unnatu-

rally high salinity bottom water has entered Lake Pontchar-

train through the MRGO since it was completed in 1963. This

saline water produces stratification and periodic bottom

anoxia and hypoxia (Junot, Poirrier, and Soniat, 1983;

Poirrier, 1978). Following the recovery of large R. cuneata

after shell dredging ceased, clams greater than 21 mm

remained absent from a 250 km2 area as a result of anoxia

and hypoxia facilitated by salinity stratification (Abadie and

Poirrier, 2001a). This area has become known as the ‘‘dead

zone’’ (Figure 6). Episodes of high salinity and low dissolved

oxygen may extend beyond this zone and affect benthic

invertebrate populations lakewide. Earlier studies (Sikora

and Sikora, 1982) attributed the stressed benthic community

to toxic substances from urban runoff and chemical spills.

Later studies demonstrated that outfall canals have a

localized effect (Poirrier et al., 1984; Powers, Poirrier, and

Yund, 1992), and the principal cause of stress is low bottom

dissolved oxygen due to saltwater intrusion.

Our study of R. cuneata restoration through closure of the

MRGO found that clam restoration would have widespread

positive contributions to the holistic rehabilitation of Lake

Pontchartrain (Spalding, Walker, and Poirrier, 2007). In

addition to direct benefits of reduction of phytoplankton

blooms, turbidity, and fecal coliforms by increased clam

filtration, increased water clarity would increase SAV. Clam

biomass restoration would add shells that would stabilize the

mud bottom and reduce shoreline erosion. Clam restoration

will also provide more food for fish, crabs, and waterfowl

(Darnell, 1958; Ebersole and Kennedy, 1995; Richman and

Lovvorn, 2004) and increase nutrient cycling through clam

excretion, biodeposition of feces and pseudofeces, and biotur-

bation of sediment (Dame, 1996; Newell, 2004; Vaughn and

Hakenkamp, 2001). Overall, R. cuneata restoration would

lessen eutrophication, improve recreational water quality,

and greatly expand essential fish habitat. Work is in progress

to close the MRGO. Natural recruitment of R. cuneata will

occur in areas that are currently adversely affected. The

annual increase in clam abundance and size classes will

provide restoration milestones and the presence of mature,



large clams will provide a measureable endpoint in the

restoration process.


Elevated salinities from saltwater intrusion have affected

SAV community structure (Figure 2) by favoring the growth

of the opportunistic, euryhaline species, R. maritima, and

contributing to stress on freshwater species, including the

rare species P. perfoliatus (Cho and Poirrier, 2005b). A major

benefit of closing the MGRO will be SAV restoration through

improved water clarity on the southeastern shore due to clam

restoration. Monitoring should be continued to document the

recovery of SAV and clams after the MRGO closure.

1997 Bonnet Carre Spillway Opening

Benthos and SAV

The 1997 spillway opening appeared to affect both benthos

and SAV. Based on decreases in species diversity, abundance,

and the number of taxa, we concluded that the 1997 spillway

opening had a deleterious impact on the benthos (Brammer et

al., 2007). The cause of these changes cannot be attributed

with certainty to any one factor, but we speculate that they

were the result of reduced salinity, cyanobacterial blooms,

and hypoxia or anoxia. Although recovery of benthos

occurred, based on overall changes in macroinvertebrate

abundance and R. cuneata biomass, no increase in benthic

invertebrate productivity was detected. The decline of overall

SAV abundance, especially R. maritima, and the loss of M.

spicatum near the mouth of Bayou St. John after the 1997

spillway opening indicated that the opening also had a

negative effect on SAV (Poirrier et al., 1999). These results

should be considered in the operation of the Bonnet Carre

Spillway and the design of high volume diversions that

discharge directly into Lake Pontchartrain.

1999–2001 Drought Resulting from an El NinoSouthern Oscillation

Benthos

The increase in species diversity, the decrease in R. cuneata,

and the increase in I. recurvum associated with an increase in

salinity during the 1999–2001 drought demonstrated that

climate shifts can affect benthic community structure and

dominant bivalve abundance. However, it is unlikely that

increased salinity alone was responsible for the R. cuneata

decline. The decline occurred during a period of decreasing, not

increasing salinity, and at this time, species diversity was low

and samples were dominated by oligochaetes. The salinity was

well within the known salinity tolerance of R. cuneata, which

can reproduce and grow at salinities up to 15 ppt (Cain, 1973,

1974) and survive salinities as high as 25 ppt (Hopkins,

Anderson, and Horvath, 1973; LaSalle and de la Cruz, 1985;

Swingle and Bland, 1974). The underlying factor causing the

decline was the interaction of stress from episodic hypoxia and

abrupt salinity changes. Organic material and plant nutrients

may have accumulated in the watershed during the drought

and were flushed into the lake with rains, which rapidly

decreased salinity after the drought. Henry, Mangum, and

Webb (1980) reported 50% mortality of R. cuneata after 5–7 d

under deoxygenated conditions in both 2 and 20 ppt salinities.

Those that survived initially did not recover after being

returned to normoxic conditions. In addition, they found that

survival decreased to 3 d when given a hypo- or hypersaline

shock (18 ppt change).

Ischadium recurvum are commonly found in more saline

conditions than R. cuneata (Holland, Mountford, and Mi-

hursky, 1977; Read, 1964). The average low salinity levels in

Lake Pontchartrain are not conducive to recruitment,

establishment, and growth of I. recurvum. Establishment of

I. recurvum was also aided by differences in life history

strategies, which gave this species a competitive edge over R.

cuneata. Ischadium recurvum are epifaunal, attaching to

hard substratum (Holland, Mountford, and Mihursky, 1977),

while R. cuneata are infaunal, burrowing in the sediment.

During periods of hypoxia, it is likely that I. recurvum were

not as severely affected as R. cuneata by low oxygen because

of their higher position in the water column. The die-off of R.

cuneata ended competition from R. cuneata for food and other

resources. This resulted in the rapid increase of not only

small mussels (,5 mm) but also the establishment and

growth of larger mussels (.35 mm). The number of I.

recurvum rapidly decreased with decreasing salinity, and R.

cuneata recovery was slow, indicating that factors other than

salinity affected recovery.


Based on SAV dynamics observed during the drought, it

appears that climate shifts may cause cyclic changes in SAV

with salinity controlling species composition and water

clarity controlling SAV distribution and abundance (Cho

and Poirrier, 2005b). SAV restoration could be accomplished

by improving water clarity. We also learned that the response

of R. maritima to improved water clarity is rapid. The

increase in SAV observed during 1 year of this study was

nearly equal to SAV loss in 40 years of decline. Comparable

natural shifts in SAV likely occurred in the past. However,

this increase in SAV probably would not have occurred if shell

dredging had continued because widespread high-turbid

waters would have continued to shade the littoral zone

(Francis and Poirrier, 1999). The model we developed can be

used to quantitatively predict potential SAV habitat changes

caused by changes in water clarity (Cho and Poirrier, 2005a).

Hurricane Katrina

Benthos

Sampling water quality immediately after Hurricane

Katrina was difficult. However, Pardue et al. (2005) obtained

samples of New Orleans storm water in early September

2005. They found that Hurricane Katrina floodwaters were

typical of storm water with the exception of elevated levels of

lead. Our studies of the effects of Hurricane Katrina

demonstrated that severe hurricanes have direct and indirect

effects on benthos (Poirrier, Rodriguez del Rey, and Spalding,



2008). Direct effects included the storm surge and wave

energy disrupting the bottom and displacing and burying

benthic organisms. Indirect effects included the defaunation

of over 800 km2 of lake bottom extending from the deepest

water near the center of the lake to about the 3.7-m contour.

We postulate that this indirect effect was caused by

widespread, higher-salinity bottom water, which formed a

nonmixing stratified layer that became deoxygenated due to

the input of organic material. This further damaged the

benthic community in the entire lake at depths greater than

3.7 m. Some recovery by opportunistic species has occurred,

but R. cuneata recovery has been slow due to high salinity

conditions produced by the storm surge and low rainfall after

Hurricane Katrina.


We found that severe hurricanes such as Hurricane Katrina

directly destroy SAV and recovery rate depends on conditions

after the storm. Ruppia maritima and N. guadalupensis are

more susceptible to damage from wave energy because they

have a bushy growth form with branches and numerous

separate leaves, while V. americana has tapelike leaves.

Ruppia maritima, however, recovered more rapidly than V.

americana because of abundant seeds in the seed bank and

better tolerance of higher post-Hurricane Katrina salinities

than V. americana. Burns and Poirrier (1996) reported rapid

recovery of north shore SAV species after damage from

Hurricane Andrew in August 1992. Hurricane Georges, which

made landfall in October 1998, did not directly damage SAV,

and R. maritima increased in abundance after Georges (Cho

and Poirrier, 2001). Chabreck and Palmisano (1973) also

reported an increase in R. maritima in the Mississippi River

delta marshes after the passage of Hurricane Camille. Poirrer

and Handley (2007) attributed major temporal changes in

seagrass populations in Chandeleur Island Sound mainly to

the direct effects of tropical storms and hurricanes.

Both benthic invertebrates and SAV can serve as endpoints

for the evaluation of habitat quality and restoration status.

However, the distribution and abundance of these oligohaline

organisms does not always follow paradigms established in

higher salinity systems or convey a specific set of ‘‘good or bad’’

values. While the open ecosystem dynamic nature of estuaries

causes frequents disruptions, it also provides mechanisms for

rapid recovery. Unraveling the effects of multiple stressors in

these complex systems is difficult and will require decades of

further study. However, the more we learn about the biology of

indicator species and communities, the better they will serve as

indicators of environmental change.

CONCLUSION

The overall lesson learned in this study is that an increase or

decrease in a particular set of benthic organisms were not

simply determined by persistent, long-term anthropogenic

changes in habitat quality, but were often driven by short-

term disturbances and natural cycles. Measures of community

structure and indicator species such as R. cuneata and V.

americana were useful, but periodic disturbances made using

them to evaluate environmental health during a particular

period difficult. The effects of past disturbances and the degree

and nature of recovery have to be considered. The oligohaline

estuarine zone is composed of opportunistic species that

respond rapidly to dynamic abiotic factors such as salinity

and dissolved oxygen, and populations can rapidly recover by

recruitment from adjacent coastal waters. Resistance and

resilience to periodic habitat disturbances, the presence of

natural cycles in distribution and abundance, and the occur-

rence of alternate community states have to be considered in

evaluating habitat quality and restoration success.

Specific lessons learned pertaining to benthic inverte-

brates:

N Shell dredging had a significant, disruptive effect on

benthic organisms and water clarity, and it should

continue to be banned.

N Saltwater intrusion from the MRGO through the IHNC

and associated episodic anoxia and hypoxia are the

principal causes of stress in the benthic community.

N The widespread stress observed in the benthic community

in earlier studies is due to saltwater intrusion and

associated anoxia and hypoxia, and not toxic substances

from urban runoff. Discernable effects of runoff on the

benthic community are limited to sites near discharges.

N Complete closure of the MRGO would reduce salinity

stratification and associated anoxia and hypoxia and result

in the return of R. cuneata to a 250 km2 (100 mi2 area).

Clam restoration would reduce eutrophication effects,

improve recreational water quality, and greatly expand

essential fish habitat.

N Large R. cuneata take several years to recover from

episodes of harmful dissolved oxygen. Therefore their

distribution and abundance can be used in annual surveys

to assess areas affected by anoxia and hypoxia.

N The 1997 Bonnet Carre Spillway opening disrupted the

benthic invertebrate community by abruptly lowering

salinity, facilitating cyanobacterial blooms, and decreasing

dissolved oxygen. Contrary to common management

paradigms, there was no increase in benthic invertebrate

productivity associated with the 1997 diversion.

N The 1999–2001 ENSO drought produced high bottom

salinity throughout the lake and episodic hypoxia was also

widespread. These hypoxic events coupled with increased

salinity resulted in lakewide changes in species diversity, a

decrease in R. cuneata clams, and an increase in I. recurvum.

N Hurricane Katrina directly killed benthic organisms and

pushed high salinity bottom water that was low in

dissolved oxygen up to the 3.7-m contour, severely

disrupting the structure of the benthic community.

Specific lessons learned pertaining to SAV:

N Light availability is the primary factor that controls SAV

distribution and abundance in the lake. Turbidity reduc-

tion is thus required for SAV restoration. Natural increases

in abundance would not have occurred if shell dredging

had not been banned and associated turbidity decreased.

N Potential SAV area gained from increased water clarity can

be quantitatively predicted using our SAV habitat model.



N Increased R. cuneata density as a result of the MRGO

closure would improve water clarity. Clams remove

suspended solids from the water column, thus increasing

SAV distribution and abundance.

N Closure of the MRGO will reduce episodes of high salinity,

increase the abundance of V. americana and other

freshwater SAV in the southeastern lake, and may help

restore P. perfoliatus.

N Nutrients increase phytoplankton and epiphyte growth,

which limit light availability resulting in a decrease in

SAV. This should be considered in the design of freshwater

diversions.

N Salinity controls SAV species composition by limiting the

distribution of freshwater species. Potamogeton perfoliatus

was extirpated during the drought from Lake Pontchar-

train, the only known habitat in Louisiana.

N Maintaining and increasing SAV ‘‘friendly shorelines’’ and

source populations through V. americana culture and

transplanting will enhance SAV restoration.

N Seeds are important in the life cycle of the pioneer species

R. maritima. Seeds should be incorporated in restoration

strategies.

N Hurricanes can directly damage SAV, but recovery de-

pends upon species adaptations and posthurricane effects.

Ruppia maritima is known to increase after hurricanes.

ACKNOWLEDGMENTS

The preparation of this paper and associated analyses were

funded by grants from the National Oceanic and Atmospheric

Administration through the Pontchartrain Restoration Pro-

gram. Freeport-McMoran, Inc., funded benthic invertebrate

sample collections and preliminary data analyses prior to

2001. The Louisiana Department of Wildlife and Fisheries

and the Lake Pontchartrain Basin Foundation provided funds

for SAV monitoring and restoration studies prior to 2001. We

would like to thank the following individuals who assisted in

these studies: Chip Crews, Jesse Hobson, Wendy Hobson,

Ryan Poirrier, Zoe Rodriguez del Rey, and Ashley Walker.

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