Wetlands mitigation evaluation vegetation studies Final report to the city of Norfolk from the Virginia Institute of Marine Science, School of Marine Science, The College of William and Mary

[From the U.S. Government Printing Office, www.gpo.gov]

WETLANDS MITIGATION EVALUATION

VEGETATION STUDIES

Final Report to

The City Of Norfolk

from

The Virginia Institute of Marine Science
School of Marine Science
College of William and Mary
Gloucester Point, Virginia 23062

Principal Investigator
Walter I. Priest, III

July 1989

WETLANDS MITIGATION EVALUATION

VEGETATION STUDIES

INTRODUCTION

Coastal wetlands in Virginia represent a finite resource which is being

subjected to ever increasing development pressures. As a means of reducing

these losses while accomodating necessary economic development, the policy

of wetlands mitigation through compensation is increasingly being utilized

by both regulatory agencies and developers. This practice generally

involves the grading of an upland area to the appropriate elevation and

planting it with wetlands vegetation to replace a marsh being lost in

another area.

The technology to plant and grow marsh vegetation for this and other

purposes has been well demonstrated. In as few as two growing seasons the

appearance and primary productivity can be very similar to natural marshes,
but the lengih of time necessary for them to become fully functional in an

ecolgical sense is unknown (Woodhouse et al, 1974). This question remains

unanswered and the need still exists to conduct both shorL and long term

studies of planted marshes to evaluate their success at replacing th-e

wetlands resouces being lost to development. Tbes& 6tudies need to include

not only the plant community ?,lso the physical environment and the

110

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utilization of these areas by invertebrates, fishes, birds, and mammals

(Zedler, 1984).

In an effort to address some of these questions this portion of the

study was designed to 1) compare the vegetative characteristics of a man-

made marsh with those of similar natural marshes and 2) investigate the role

of elevation and tidal inundation in the development of the marsh.

STUDY SITES

The primary site chosen for this study is a marsh constructed by the

U.S. Navy in an old spoil disposal area called Monkey Bottom adjacent to

Willoughby Bay in Norfolk, Virginia (Fig. 1). As a condition of the permit

to reuse the disposal area, the Navy was required to replace 7.6 acres of

tidal wetlands which had developed in the center of the disposal area

(Priest et al, 1982).

The new tidal wetland was designed for a parcel of the disposal area

adjacent to a four foot diameter culvert under the 1-64 causeway that

connected the area to Willoughby Bay. Because of the extensive stands of

common reed, Phragmites australis, present in the disposal area, the new

marsh was designed to support saltmarsh cordgrass, Spartina alterinflora, at

and below the elevation of mean high water. It was hoped that tb--*,,, lo-it,

design elevation would prevent the colonization of the compensation area by

common reed.

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Figure 1. Location Of Monkey Bottom Disp0sal Area

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Construction of the marsh began in the summer of 1984 concurrent with

the construction of the berm for the new disposal area. The compensation

area was graded to elevations at and below mean high water. Drainage was

accomplished by sloping the area to four lateral ditches which emptied into

the main ditch that connected to the culvert under 1-64 (Fig. 2). During

the grading and planting, the area was isolated from tidal inundation.

The marsh was planted with saltmarsh cordgrass, Spartina alterniflora,

during September and October 1984, using a tree planter. The two sections

closest to the City of Norfolk Visitor's Center were planted on two foot

centers with transplants from the site. The two sections furthest from the

Visitor's Center were planted with six month old seedlings also on two foot

centers. The entire area was broadcast with an especially prepared 19-5-12

slow release fertilizer at the rate of one ounce per planting. The

fertilizer was mechanically raked into the soil prior to planting. The

entire area was planted, even areas above and below the expected successful

elevation, so that good coverage was ensured.

The natural marshes used for comparison were a small pocket marsh on
Willoughby Bay (Willoughby Bay) (Fig. 3) adjacent to the disposal area and a

a small embayed marsh on the Lafayette River (Larchmont Pond) (Fig. 4). The

natural comparison marshes were selected on the basis of proximity to the

planted marsh, similar physiography and hydrology and plant communities

which appeared similar in composition.

U.S. NAVY
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SECTION 11. WILLOUGHBY BAY b

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Figure 3. Location of Willoughby Bay study site

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LAMBERTS POINT TERMINAL

SECTION IV-C. LAFAYETTE RIVER
LAMBERTS POINT

Figure 4. Location of Larchmont Pond study site

METHODS

At Monkey Bottom the transects were established along the centerline of

each lateral ditch. The sampling plots were selected by a random distance

along the centerline and a random distance to one side or the other. At the

Willoughby Bay and Larchmont Pond sites the centerline line transect was

established down the center of the marsh and the plots randomly selected in

the same manner as at Monkey Bottom.

Each plot was sampled with a .25 m2 square circular quadrat for cover,

density and peak standing crop. Percent cover for each species within the

quadrat was visually estimated. All of the stems within each quadrat were

counted and clipped at ground level to determine stem density. The clipped

stems were put in plastic trash bags and returned to the laboratory. The

samples were washed lightly to remove extraneous inorganic material and oven

dried to a constant weight to determine the estimate of peak standing crop.

The center of each quadrat was marked with a stake which was used as a

reference point in determining the elevation of the quadrat. This was done

by a surveying crew from the City of Norfolk. The surveying crew also

provided the elevations of the upper transition zone between the cordgrass

and the common reed, Phragmites australis, the lower limit of -he cordgrass,

the ditch bottom and the ridge between the maa:sh sections at Monkey Bottom.

They also obtained the quadrat elevations, upper transition zone and lower

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cordgrass limits at Larchmont Pond. Only the quadrat elevations were

obtained at the Willoughby Bay site. At Monkey Bottom and Willoughby Bay,

the elevations were related to the tidal survey performed by ODU for this

study. At Larchmont Pond the elevations were related to available NOS data.

RESULTS

A summary of the vegetation characteristics measured at the three study

sites is presented in Table 1 and graphically depicted in Figure 5.

The highest average percent cover was 64% at Willoughby Bay, Larchmont

Pond followed with 58% and Monkey Bottom was third with 46%. At Monkey

Bottom and Willoughby Bay the only species present in the quadrats was

Spartina alterniflora. The Larchmont Pond quadrats included the perennial

saltmarsh aster, Aster tenuifolius, within the cover estimates where it

represented 9% of the reported 58% total cover.

2
Stem density was highest at the Larchmont Pond site with 465/m

2
Monkey Bottom was next with a density of 340/m . Willoughby Bay had the

2
lowest stem density with 308/m . As with the cover estimates, the densities

at Willoughby Bay and Monkey Bottom were strictly Spartina alterniflora. In
the quadrats at Larchmont Pond the Aster tenuifolius density averaged 92/m 2

2
out of the average total density of 465/m

Table 1. Summary of the vegetation data for the three study
2 2
sites (Cover- %, Density- stems/m , Standing Crop- g/m
Elevation- MLW).

MONKEY BOTTOM

Mean Std Dev Min Max N

Cover 45.73 28.55 0 85 26

Density 340.00 220.42 0 744 26

Standing Crop 591.43 392.45 0 1119.68 26

Elevation 1.54 .40 .96 2.43 24

WILLOUGHBY BAY

Cover 64.17 13.57 40 80 6

Density 308.00 .112.74 180 456 6

Standing Crop 559.18 .213.32 176.0 769.52 6

Elevation 2.01 .18 1.75 2.23 5

LARCHMONT POND

Cover 57.86 13.18 40 80 7

Density 464.57 87.95 392 648 7

Standing Crop 448.10 144.38 263.9 712.64 7

Elevation 2.60 .12 2.37 2.74 7

Comparison of Vegetation Parameters
Natural vs. Man Made Marshes
Cover - Density Standing Crop
Ob
800- 500
Cb

--450 wtanding crop

500-- cover
--400

dexudty

--350
400--

Soo

Soo- 250

200

200--

--150

100
100-- 3 cc
LO

--50
Figure 5. Summary of
0 0 Vegetation Characteristics
Uonkey Bottom WS"oughby Day Larclxmont Pond
Sampling sit" - 19613

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The highest average peak standing crop of organic matter, 591.43 g/m2
was found at Monkey Bottom. The next highest was 559.18 g/m 2 at Willoughby

2
Bay. The lowest value, 448.10 g/m , was found at Larchmont Pond. The Aster

tenuifolius found in the quadrats at Larchmont Pond was not analyzed

separately and is included with the Spartina alterniflora standing crop.

After being tested for normal distribution, homogeneity and additivity,

an analysis of variance was performed comparing cover, density and standing

crop among the three sites. The results indicated that no two groups were

significantly different at the .05 level (Table 2).

Table 2. Results of the One Way Analysis of Variance of the Vegetation

Data Among the Three Study Sites.

F ratio F Prob

Standing crop 4.853 .6195

Density 1.399 .2601

Cover 1.683 .2002

The data was also analyzed using the Pearson Correlation Coefficient to

determine any relationships between the measured variables and elevation at

each of the three sites (Table 3). The data indicate that at Monkey Bottom

there is a significant positive correlation between elevati-nn and cover,

density and standing crop. At the. twc natural control sites, however, there

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Table 3. Pearson Correlation Coefficients for elevation vs.
vegetation parameters at the three study sites.

MONKEY BOTTOM

Cover Density Standin Crop

Elevation R2 .6080 ..3985 .6292
N (24) (24) (24)
P .001 .028 .000

WILLOUGHBY BAY

Elevation R2 -.4207 .6734 .7965
N (5) (5) (5)
P .240 .106 .053

LARCHMONT POND

Elevation R2 -.2140 3805 .2691
N (7) (7) (7)
P .322 .200 .280

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was no significant correlation between any of the vegetation characteristics

measured and elevation.

The quadrats at Monkey Bottom had the lowest average elevation 1.54

feet MLW (mean low water) and the greatest range in elevations, 1.47 feet.

The highest average elevation of quadrats was 2.6 feet MLW at Larchmont Pond

as was the smallest range in elevations, .37 feet. The Willoughby Bay site

was in between with an average elevation of 2.01 feet MLW and an elevation

range of .48 feet.

At Monkey Bottom the average elevation of the center of the transition

zone between the Phragmites australis and Spartina alterniflora was 2.68

feet MLW just slightly above the mean high water (MHW) elevation of 2.47

feet. The average lower elevation of the Spartina alterniflora was 1.20

feet which was almost exactly the mean tide level (MTL) of 1.23 feet for the

2.47 foot tide range at the site.

DISCUSSION

Even thc@ugh there were no significant differences among the marshes

studied, a number of reasons for the observed differences became apparent

during the course of the analysis of the data. Cover was higher at the two

natural marshes partly because the open areas of Monkey Bottom were a more

prominent feature of the sample site and consequently inore frequently

sampled. Whereas the twe Comparison marshes had little or no naturally non-

-evt- ated areas and the creek adjacent to the Larchmont Pond site did not

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happen to get sampled during the study. If the empty quadrats are removed

from the Monkey Bottom cover estimates the percent cover increases to 54%

compared to 64% for Willoughby Bay and 58% for Larchmont Pond.

The high density values at the Larchmont Pond were attributable to a

large degree to the presence of a large number of mature plants as well as

numerous seedlings of Aster tenuifolius among the dominant.Spartina

alterniflora.

The fact that the standing crop was higher at Monkey Bottom was

probably due in large measure to the range of elevations sampled,

particularly the lower, taller, more productive portions of the marsh

(Lefors et al., 1987). A comparable range of elevations was inadvertently

either not sampled or not available at the natural sites. Also, none of the

Monkey Bottom samples was above MHW where at Larchmont Pond most of the

samples were taken at or above MHW in normally less productive portions of a

Spartina alterniflora marsh.

The fact that there were no correlations between elevation and the

vegetation parameters in the natural marshes is probably due to the fact

that they have a much narrower range of elevations (flatter) and are

generally at higher elevations. The variability that a man-made marsh can

introduce into a system can tend to provide a more diverse habitat than some

similar natural systems with less variability. However, future efforts in

marsh comparisons should require not only marshes of similar species

composition but also those with a similar elevation and range of elevations

to eliminate some unnecessary variability from the comparisons.

When comparing dato on Z:he production of marshes, the ranges of values

is trremendous. Keefe (1972) repots production values for Spartina

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2
alterniflora ranging from 250-2100 g/m . This would put the values from

2
this study, 448-591g/m , among the lower quarter of that range. However

2
when compared to values from Oviatt et al. (1977), 450-980 g/m , they fall

in the lower half of that range. Density values for the same marshes from
Oviatt et al. (1977) ranged from 230-1170 stems/m 2 as compared to the range
of 308-465 stems/m2 in this study.

When the Monkey Bottom marsh was designed, the elevations of the area

to be planted with Spartina alterniflora were kept below MHW (2.47 feet MLW)

under the premise that Phragmites australis would not grow below MHW in

mesohaline areas. If true, this might prove to be an effective means of

preventing the encroachment of this weedy undesirable species into

compensation areas and displacing the planted species (Silberhorn et al.

1974). At least in this instance, this design criteria appeared to be

appropriate because the average elevation of the transition zone between the

Spartina alterniflora and the Phragmites australis was 2.68 feet above KHW.

Furthermore, no Phragmites australis was found within any of the quadrats

sampled.

The average lower limit of Spartina alterniflora of 1.2 feet MLW or .03

feet below MTL is also an important design criteria for planting marshes.

These two elevations, MTL and MHW, in this instance, defined the effective

lower and upper limits for planting saltmarsh cordgrass.

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CONCLUSION

Whenever natural systems are studied, their natural variability makes

comparisons between sites difficult. And indeed, considerable variability

was found in the three marshes studied in this report. Monkey Bottom had

the highest standing crop but the lowest percent cover. Willoughby Bay had

the highest percent cover but the lowest density. Larchmont Pond had the

highest density but the lowest standing crop. Despite this variability in

cover, density and standing crop there were no statistically significant

differences in these parameters among the three marshes.

When compared with literature values for similar types of natural

marshes, all were near the lower ends of published ranges for density and

standing crop but still within the range of natural variability.

While not necessarily as productive as some other marshes, the Monkey

Bottom plant community appears to be a viable and productive component of

the estuarine system.

The design criteria of planting Spartina alterniflora between MTL and

MHW in mesohaline areas to successfully compete with Phragmites australis

appears to be confirmed.

ACKNOWLEDGEMENTS

I would like to extend my sincere thanks to Julie Bradshaw and Tom

Barnard for their uncomplaining help in the field.

I would also like to ',,;hank Jim Perry for his statistical analyses and

1i"-s help with the background on the planting of Monkey Bottom.

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LITERATURE CITED

Keefe, C.W. 1972. Marsh production: a summary of the literature.

Contributions in Marine Science 16:163-181.

Lefor, M.W., W.C. Kennard and D.L. Civco. 1987. Relationships of salt-

marsh plant distributions ti tidal levels in Connecticut, USA.

Environmental Management 11(l):61-68.

Oviatt, C.A., S.W. Nixon and J. Garber. 1977. Variation and evaluation of

coastal salt marshes. Environmental Management 1(3): 201-211.

Priest, W.I., C.R. Terman and W. Ihle. 1982. A natural history survey and

habitat evaluation of the Willoughby Disposal Area, U.S. Naval Base,

Norfolk, Va. Final Contract Report, Naval Facilities Engineering Command,

Norfolk, Va. 29 pp.

Silberhorn, G.M., C.M. Dawes and T.A. Barnard. 1974. Guidelines for

activities affecting Virginia Wetlands. SRAMSOE No. 46. Va. Inst. of Mar.

Sci., Gloucester Point, Va. 52 pp.

Woodhouse, W.W., E.D. Seneca and S.W. Broome. 1974. Propagation of

Spartina alterniflora for substare stabilization and salt marsh development.

Tech. Memo. No. 46. U.S. Army Corps of Engineers, Coastal Engineering

Center, Fort Belvoir, Va.

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Zedler, J.B. 1984. Salt marsh restoration: A guidebook for Southern

California. California Sea Grant Report No. T-CSGCP-009. University of

California, La Jolla, Calif.

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NOAA COASTAL, SERVICES CTR LIBRAR,,,,
3 6668 14112870 4