Zannichellia Palustris a modern and paleoecological indicator of human disturbance in Chesapeake Bay

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

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TASK 23

ZANNICHELLIA PALUSTRIS: A MODERN AND PALEOECOLOGICAL

INDICATOR OF HUMAN DISTURBANCE IN CHESAPEAKE BAY

William B. Hilgartner

Johns Hopkins University
Baltimore, MD 21218

November 1991

This research was partially funded through a grant provided by the
Coastal Zone Management Act of the of f ice of Ocean and Coastal
Resource Management, NOAA.

QK
122.8
H55
1991

Abstract

Title: Zannichellia valustris: A Modern and

Paleoecological Indicator of Human Disturbance in

Chesapeake Bay

An examination of Zannichellig palustris from disturbed

and undisturbed habitats in Chesapeake Bay reveals

differences in size , and seed production of two

.populations, referred to as tall and short forms. The

tall form of Zannichellia produces a mean of 13 fruits

per plant and the short form 4 fruits per plant, while

surface sediments yield 150 Zannichellia seeds per 100 cc

where the tall form grows, and 13 seeds per 100 cc where

the short form occurs. Seeds of the two forms are

indistinguishable morphologically.

Robin Cove, a relatively undisturbed embayment, is the

habitat of five species of submerged aquatic vegetation

(SAV) including the tall and short forms of Zannichellia.

The tall form grows in predominantly clay sediment beyond

the intertidal zone, and is the dominant SAV where it

occurs during May and June. The short form Inhabits the

sandier substrates in shallower water and intertidal

zones. It is subdominant to Elodea nuttallii. In addition

to Zannichellia, surface sediments in Robin Cove

contained seeds of 16 species of plants, 71% occurring

within a 30 meter radius of the transect.

The short form of Zannichellia is the only SAV growing in

Brewer Creek, a tributary heavily used for recreation,

where it is found in a range of water depths and occurs

predominantly in sand. Surface sediments of Brewer Creek

contained 6 species of shoreline flora adjacent to the

transects.

.Distributions of Zannichellia populations indicate that
the 'different forms may be used to monitor human

disturbance around an estuary. Diversity of seeds in the

sediment along with abundance of Zannichellia seeds can

also be used as historical indicators of water quality

and disturbance in the past.

VS DePartment Of Commerce
YC'A coaotal services center Litrary
2234' South Hobson Avenue
Charleston, SC 29405-2413

LISTS OF FIGURES

Figure Name Page

1 Mao of Chesapeake Bay 8

2a Location of Robin Cove 10

3a Location of Brewer Creek 12

2b Transects in Robin Cove 14

3b Transects in Brewer Creek 15

4a Cross-sections of Transects 1 & 2

in Robin Cove 17

4b Cross-sections of Transects 3 & 4

in Robin Cove is

5 Cross-sections of Transects in

Brewer Creek 19

6 Morpholoqy of Fruits of Zannichellia 20

7 Core SR3a from Brewer Creek 39

8 Distribution of Z.p. from Cores 44

9 Core CHR6c from Langford Creek 46

10 Core SRla from Severn River 47

LIST OF TABLES

Table Name Page

la Percent Cover, SAV in Robin Cove 23

lb Percent Cover, SAV in Brewer Creek 24

2 Mean Water Depth 26

3 Number of Seeds in Sediment 27

4 Percent Cover Percent Seed Number 29

5 Fruit Length Rostrum/Fruit Ratio 32

6 Stem Length Number of Fruits 33

7 Height of Zannichellia Plants 34

8 Sediment Descriptions 35

TABLE OF CONTENTS

Page

ABSTRACT ii

ACKNOWLEDGEMENTS iv

LIST OF FIGURES v

LIST OF TABLES vi

TABLE OF CONTENTS vii

INTRODUCTION..................................................1

GROWTH FORMS & ECOLOGY OF ZANNICHELLIA........................4

AREA OF STUDY.................................................6

Robin Cove...............................................9

Brewer Creek.............................................8

METHODS.....................................................13

Field Sampling..........................................13

Laboratory Analysis.....................................16

Statistical Analysis....................................22

RESULTS......................................................22

DISCUSSION...................................................36

CONCLUSION...................................................48

LITERATURE CITED.............................................50

VITA.........................................................57

INTRODUCTION

Declines in populations of submerged aquatic vegetation

(SAV) in Chesapeake Bay, particularly in the 1960's and

1970's, have been attrib uted to changes in water quality,

primarily increased eutrophication and turbidity from

anthropogenic activity (Orth and Moore 1983; 1984;

Stevenson and Confer 1978). The decline in areal coverage

and number of species affected has been unprecedented.

However, studies of SAV in lakes have shown that

eutrophication and turbidity are not always followed by

a decline in SAV abundance. Rather this kind of

disturbance tends to promote the growth of

environmentally tolerant, eutrophic species with a

widespread distribution, at the expense of local, endemic

species, while a concomitant decline in species diversity

occurs (Ehrenfeld 1983; Jupp and Spence 1977; Morgan and

Phillip 1986; Stuckey 1971; 1978).

One such SAV which is widespread geographically and

appears to have a tolerance to eutrophication and human

disturbance is Zan_nichellia palpstris. It is nearly

cosmopolitan in distribution, occurring over a wide

temperature and- salinity gradient. Populations have

increased or remained constant in eutrophic waters of

western Europe (Van Vierssen 1982b), Scotland (Jupp and

Spence 1977), and in lakes in the central United States

(Lind and Cottam 1969; Stuckey 1971). In Chesapeake Bay,

Zannichellia is not present in some eutrophic tributaries

and since the 1960's has been increasing in other.

nutrient-rich areas, leading some investigators to

believe that Z., palustris is an ephemeral, pioneer

species (Stevenson and Confer 1978).

The fossil record of Zannichellia palustris seeds

however, shows that the species has been a persistent

occupant of Chesapeake Bay for over 2000 years, with some

fluctuations in populations related to climatic events

and human activities (Brush, Hilgartner and Thor ton in

prep). Changes in fossil seed numbers indicate a general

expansion during European colonization in the mid-1700 I a,

when increased siltation and nutrients from runoff of

agricultural land occurred throughout much of Chesapeake

Bay (Brush 1984a; 1984b; 1986; 1989; Brush and Davis

1984). Following this period and until about 1970, seed

influxes decreased with intensification of agriculture

and expansion of'urban development. Since the 197019, the

range of Zannichel-lia Valustris has expanded in the

lower tributaries of the western shore and in Chester

River (Fig. 1), while declining in the upper stretches of

many tributaries.

While an appreciable amount of information concerning the

autecology of Zannichellia is available (see Stevenson

and Confer 1978; Van Vierssen 1982a; 1982b; 1982c),,

knowledge of its precise ecological requirements in North

America is lacking. Both growth forms have been described

from Europe, but the ecology in Europe differs from what

has. been observed in Chesapeake Bay (Hilgartner pers.
obs.). For example, Z. palustris in Europe is generally

confined to freshwater, a stratification period of 2
months at 40C is required for the seeds to germinate in

early spring, and the tall form is considered a perennial

while the short form is considered an annual (Van

Vierssen 1982a). In Chesapeake Bay, 1. @alustris is most

common in brackish water with a salinity tolerance of up

to 20 ppt.,- germination appears to occur in late summer

when no stratification period is required, and the tall

form is evidently annual, dying back by early June

(Stevenson and Confer 1978; Hilgartner, pers. obs.) . The

relationship of disturbance,, sediment, wave action, water

depth, and water quality as well as seed production and

fossilization, with growth patterns of Zannichellia,

Palustris needs to be more clearly defined.

The purpose of this study was to explore the possibility

of using population characteristics of Z. palustris as

an indicator of water quality in Chesapeake Bay.

Zannichellia vegetation and associated SAV were compared

in two sites that differed in the degree of anthrop ogenic,

disturbance. The relationship of plant cover, seed

production and seed abundance in surface sediments of

Zannichellia was compared to plant cover and seed

abundance of associated species. The broad spatial

distribution of Zannichellia, its abundant seed record in

sediments, a nd response . to changes in water quality

suggest that a few simple measurements of plant growth

and/or seeds in surface sediments might serve as a water

quality monitoring tool. Additionally, seed numbers along

with associated flora, could be used to refine analyses

of environmental changes in the paleoecological record.

GROWTH FORMS AND ECOLOGY OF ZANN ICHELLIA PALUSTRIS

zannichellia palustris, is cosmopolitan in distribution,

occurring throughout Eurasia and the Sahara, North

America from Alaska to Guatemala in the northern

hemisphere, and in South Africa, India, Madagascar, and
New Zealand in the southern hemisphere, being absent only

from Australia and the tropics (Haynes and Holm-Nielson

1987; Ridley 1930). This wide distribu
tion of.

zannichellia has presented problems concerning its

taxonomy and ecology (Van Vierssen 1982a) . In western

Europe, where there are four taxa. of Zannichellia,

palustris occurs in two growth forms (Van Vierssen

1982a,1982b). The short, creeping form, Z-12. repens, is

an annual that occurs in northern latitudes. in a wide

range of water depths from near water edge to depths

greater than 1 meter. These plants are tolerant of

disturbances by current and wave action. The tall form,

7.p. palustris, grows more than one meter in length in

water depths of 1.85 m, and behaves as a perennial. Both

forms are found primarily in fresh water, being

intolerant of salinities > 3.5 ppt. Seeds must undergo a
stratification period of two months at 4 0 C before

germination the following spring.

By contrast, in North America, Zannichellia consists of

a single annual species YL. valustris, which reproduces

solely by seeds (Haynes 1988; Haynes and Holm-Nielsen

1987). Like its counterpart in Europe,, there are two

growth forms in Chesapeake Bay, a prostrate form and an

upright form (Hurley 1990). Stevenson and Confer (1978)

noted that the species is never found where wave action

is significant.

Zannichellia generally grows in shallow, quiet water 0.5

to 1.5 m deep and occurs in a wide salinity range from

freshwater lakes high in sulfate ions in Minnesota to

mesohaline waters of estuaries in salinities up to 20 ppt

(Lind and Cottam 1969; Moyle 1945; Stevenson and Confer

1978; Hurley 1990). Seed germination can occur during the

year of deposition. Two fruiting and growth periods, one

in late spring and one in late summer have been reported

(Stevenson and Confer 1978), but Z. palustris has been

observed in fruit each month from May to October in

Chesapeake Bay (Hilgartner, personal observation).

It is possible that the short and tall forms in

Chesapeake Bay are homologous to the two subspecies

Z-2.repens and Z-2-palustris found in Europe. However, it

has been suggested that these two forms could also

represent two species (Haynes, pers comm) . Also possible

is that these forms represent ecotypes or climatic races

(Clausen et al 1941) or environmentally induced growth

forms called ecophenes by Mooring et al (1971), similar

to those which occur in the two growth forms of gRartina

alterniflora.

AREA OF STUDY

To assess the effects of human activity on Zannichellia

palustris, populations were examined in a disturbed and.

undisturbed tributary. In this study, disturbance

includes heavy recreational use and proximity to

-residential development. Recreational factors that

directly affect submerged vegetation include motorboats,

which tear and uproot plants and produce exhaust gases

and engine oil which have detrimental effects on plants.

Dredging, retaining walls, boat docks and foot traffic

all affect shallow-water habitats (Stuckey 1971; 1978).

The residential community also stresses organisms by

affecting water quality through septic systems and

runoff. The Chester River on the eastern shore and Severn

River on the western shore of Chesapeake Bay were chosen

for the study (Fig. 1) . Human activity along the Chester

River is primarily rural. Large tracts of privately owned

land are used for agriculture, game management' and

private use. There are numerous isolated coves and

tributaries. On the other hand, heavily residential

development including the city of Annapolis characterize

the environs of the Severn River.

Brewer Creek,, a small tributary of the Severn River,

supporting heavy boat traffic and dense residential use,

is the study site for the disturbed tributary. Robin

Cove, near the confluence of the Corsica and Chester

Rivers, surrounded by marshland and loblolly pine is the

study site for the relatively undisturbed environment. Z.

nalustris grows in both tributaries.

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Figure 1. Map of Chesapeake Bay, showing location of Chester and Severn Rivers

Robin Cov2 (39*04"N and 76*09'W) (Fig.2a)

This protected tidal tributary runs through Corsica Neck,

a peninsula at the confluence of the Corsica and Chester

Rivers. It is underlain predominantly by Quaternary

lowland deposits of gravel, sand, silt and clay (Cleaves

et al 1968), and surrounded by tidal marsh on the north,

east and south sides, and forest dominated by Pinus taeda

(loblolly pine) and Licruidambar styraciflua (sweetgum) on

the southeast side. The cove has been privately owned at

least since 1928 (Peter Sheaffer,, personal

communication). Maps from 1860, 1915., and 1951 show

virtually no change in the configuration of Robin Cove or

in the surrounding land use during.the past 130 years. On

the Besley Map of 1915, the pine forest is clearly shown

where it exists today. On the north and east sides of the

cove is a private game management farm abutting the

surrounding marsh. The closest residence is about 0.5 km

distant. A wooden structure built in the 1950's,

prevents boats from entering the mouth of the cove, but

does not disrupt the natural ebb and flow of tides.

The sediment where SAV grows is predominantly soft clay

mixed with some sand, into which a person can sink to 25

cza in some places. Dominant emergent vegetation growing

in, or just beyond the intertidal zone includes Spartina

alterniflora, patens, fi. cyansuroides, Lva frutescens.

LANGFORD CREEK

.76
-7,

CHESTER RIVER CORSIC

J-1

ROBIN COVE

Corsica Nee

Figure 2a. Map showing location of Robin Cove

Acnida cannibina and Aster tenuifolius. Five species of
submerged angiosperms were observed growing along the

northeast shore: Zannichellia palustris (both short.and

tall forms), Elodea cf. nuttallii, Potamogeton

perfoliatus, Myriophyllum spicatum and Ruppia inaritima.

The above suite of species is typically found in an

average annual salinity range of 9-14 ppt (Phillip and

Brown 1965).

Elodea nuttall-A was distinquished from canadensis on

the basis of leaf size and salinity Aolerance.

nutal lii is found in salinities of 14 ppt; B. canadensis

cannot tolerate salinities > 3.6 ppt (Cook and Urmi-Konig

1985; Phillip and Brown 1965).

Brewer Creek (39002'N and 76*32'W) (Fig.3a)

Brewer Creek, a tidal tributary of the Severn River, is

surrounded by steep slopes of upper Cretaceous fine to

coarse grained sand (Cleaves et al 1968; Vokes and

Edwards 1974). Sherwood Forest, a community of summer

homes established in 1920's is surrounded by mature

Quercus spp. (oaks) and Liriodendron tulipifera, (tulip

tree). Boat traffic is heavy and is accomodated by boat

marinas. There is frequent foot traffic on the firm, fine

ZRMnM CR=

SL"n= RIVER

Figure 3a. Map showing location of Brower Creek

intertidal sand. Scirpum americanus and an unidentified

grass are the dominant emergent plants growing in the

intertidal zone. The short form of Z. nalust ris was the

only SAV observed at the site during the summers of 1987

and 1990.

METHODS

Field sampling

Vegetation: Percent cover for all SAV was determined

from 1 m quadrats spaced at 5 m intervals along transects

ranging in length from 10 to 40 m. In Robin

Cove, 16 quadrats were sampled along 4 -transects. The

transects were made by walking one meter away from the

transect line in order to minimize disturbance of plants

within the quadrat. Two transects, were run through beds

of tall Zannichellia. palustris adjacent to a sand bar,

and 2 transects, were run through beds of short 7,L.

valustris in shallower water (Fig. 2b). In Brewer Creek,

nine quadrats were sampled along three transects

positioned perpendicular to the shoreline and 1 transect

parallel to shore (transect 4) (Fig.3b). Data collected

from quadrats in transect 4 included only plant cover and

buried seeds. Each 1 m2 quadrat was divided into one

4
5

high marsh

Tr eect 3

wooden Zplf ZpT
bulkhead

Tr &an 0 e"o t Zp?

Trangect 2 ROBIN COVE
high marah" -L

loblolly pine
woodlan

ZpS, Zamichellia plLustris (short form)
ZpTi Zannichellia p1ju_strie (Tall form)
4 Quadrat

Fl&re 2b. Location of transects in Robin Cove

Tulip-oak forest

zPS 1 2 3

zPS boat pier

Transoot 3 Transoot I
Traftsect 2

b oat piers

ZpSs Zamichellia yalustrLe Cabe" form)

*-I Quadrat

Figure 3b. Locations of transects in Brewer Creek

hundred 10 cm X 10 cm units. The number of units in which

a species occurred in each quadrat provided the measure

of percent cover for that particular species. Total

percentage in each quadrat could exceed 100%. Height of

plants within quadrats and along transects was also

measured: Depth of water at high tide was measured.

Samples of both forms, of Zannichellia were collected.

other species growing at the site but not included in the

transects were recorded. Transect cross-sections are

shown in Figs. 4 and 5.

Sediment: Surface sediment samples were collected from

randomly chosen points within each quadrat for seed

analysis. A short tube with a piston was inserted into

the sediment,, which when extruded, provided a sample 2 cm

long. The sediment sample was placed in a ziploc bag,
labeled, and stored at 4*C in the laboratory.

Laboratory Analysis

Seeds (fruits) on plants: The fruiting structure of

Zannichellia has been referred to as an endocarp, achene,,

and nutlet (Fernald 1970; Gleason 1952; Haynes and Holm-

Nielsen 1987; Montgomery 1977; Pierce and Tiffney 1986;,

Ridley 1930). The general term 'fruit' is used here (Fig.

6). Morphometrics conforming with Van Vierssen (1982a)

include number of fruits, fruit length, fruit rostrum

mm man

Zannichellia 'Dalustris (Ull form)

Transect I Quadrat I Quadrat 2 Quadrat 3
10 A 15 a

+
S 41 *a--

2
F-med sand j I pasid I sort =CAY

Transect 2 Quadrat I Quadrat 2 Quadrat 3
10 a 15A

2 a

mod *an?i I I sand I soft CIOX

Zannichgllift RAIUSIcks 44 Xyrimllyll- siAc

mode nutta"" UAW& G&CLIL"

Figure 4 a. -Crose-s*ctions ok Transects I and 2 in Robin Cove

Zasmichellia Ralustri (short @orm)

Transect 3
Qu@dr I Qua rat W.I. at high tide
74a

si6nd im clay

Transect 4 4U&4rat I Quadrat 2 Quadrat 3
5a 10 a 15 a

peat son clay# silt and sand

Quadrat Quadrat 6 Quadrat 7
25's 30 a 35a

Soft GI!q, Gilt WA SAA4 r P*&

Figure 4 b. Cross-sections of Trans*c-ts 3 and 4 In Robin Cove

Quadrat I Quadrat 2
Tmneect 1 5 a 10
bank lal

L 2 a

Quadrat I QuWat 2
Transect 2 10

Quadrat 2
Transect Quadrat 1 10 a

silt and sar

pi (ah
klustris ort form)

SeIrpus americanue

Figure 5. Cross-section's of transects In Brewer Creek
bmk

Figure 6. Morphology of fruits of Zannichellia Palustris. Fruits on
left show location of rostrum and fruit length measurements.
Fossil seed on right shows the typical appearance of seeds
found in sedimentl with teeth along the convex margin.

length, and the ratio of rostrum to fruit length. Number

of fruits per cm of internode is multiplied by the mean

height of plants in the quadrats to arrive at an estimate

of the mean number of fruits per plant. Morphological

features, such as teeth along the convex margin of the

fruit, were also noted, in an attempt to distinquish the

seeds of the two growth forms.

Seeds in Sediment: The term 'seed' includes any

angiosperm fruiting body retrieved from surface

sediments, often referred to in paleoecological

literature as macrofossils. A subsample of each sediment

sample was placed in a measured volume of 10% HN03. The

volume of the subsample was measured by the amount of

displaced liquid. The sediment was allowed to remain in

the acid for 2 hours, whereupon it was wet-sieved through

two nested sieves. Seeds were isolated and examined under

1OX and 40X magnification. Identification was

accomplished using the seed reference collection at the

Johns Hopkins University and seed identification keys

(Gleason 1952, Martin and Barkley 1973, Montgomery 1977).

Sediment: Sediment samples from selected transects were

examined under 15X magnification to estima te the percent

sand in the substrate. Randomly chosen sand grains from

each sample were measured with a dial caliper to arrive

at a mean grain size for sand.

Statistical Analysis

A Pearson-r test was used to examine the correlation

between seed numbers in the sediment and percent cover of

Zannichellia in each quadrat. A t-test was used to test

the difference in means between rostrum-fruit ratios of

the short and tall forms of Zannichellia, the null

hypothesis being that the rostrum: fruit -ratios are the

same for both forms. A t-test was also used to test the

difference between mean number of seeds of Zannichellia

in all sediment samples in Robin Cove and Brewer Creek,

the null hypothesis being that there is no difference

between seed numbers in either area.

RESULTS

SAV ComRosition And Cover (Tables la and lb):

Five species of SAV were identified in Robin Cove where

greatest diversity occurred along transect 4. Percent

relative cover was highest for Zannichellia palustris in

transects 1 and 2, and Elodea.nuttallii in transects 3

and 4. Potamogeton Derfoliatus. Mvriophyllum spicatuM and

Ruppia maritima were each represented by < 10 relative

cover. Zannichellia palustris was the only SAV observed

in Brewer Creek.

Table IA.. Percent cover and percent relative cover of SAY in Robin Cove and Brewer Creek.
Zps Zanh@cheljja galustris (ZpS short form* ZPT - tall form)l Rm a RUDDia marlilmat
Me a Myrio llum spicatuml En Elode nuttalliii PP - Potamoxeton T)erfolTatus.

Robin Cove (Percent cover)
Tall form beds Short form beds
Transects Tr I Tr 2 Tr 3 Tr 4
Quadrats* 1 2 3 1 2 3 4 1 2 1 2 3 4 5 6 7

ZP 24 68 20 60 34 20 43 0 52 25 0 0 20 10 40 0
RM 0 0 0 0 0 0 0 0 0 0 0 0 17 15 0 0
Ms 1 8 13 2 0 0 2 3 2 3 1 12 8 2 6 0
En 100 1 0 58 16 12 0 12 0 19 26 4o 8 14 50 100
PP 0 0 0 0 15 0 0 0 20 0 24 0 0 20 0 0
Robin Cove (Percent relative cover of SAV)
Zp ig 88 61 50 52 62 96 0 70 53 0 0 38 16 42 0
Rm 0 0 0 0 0. 0 0 0 0 0 0 0 32 24 0 0
Ms 1 10 39 2 o o 4 20 3 6 2 23 15 3 6 o
En 80 1 0 48 23 37 0 80 0 40 63 77 13 23 52 100
PY, 0 0 0 0 23 0 0 0 27 0 58 0 0 33 0 0
Robin Cove (Mean percent cover for each transect) (@Iean percent relative cover - tranGects)
Transects 1 2 3 4-- Mean 1 2 3 4. Mean
ZpT 37 Z9 0 0 38 36 63 0 0 60
zPS 0 0 26 V+ 20 0 .0 35 21 28
RM 0 0 0 5 1 0 0 0 8 2
Me 7 1 2 5 4 17 15 11 8 9
En 34 21 6 37 24 27 28 40 39 33
Pp 0 4 io 6 5 o 6 13 13 8

The short form of YL. 2alustrin (ZpS) occurred in

transects 3 and 4 in Robin Cove, and in all transects in

Brewer Creek. The tall form of yL. 2alustris (ZpT)

occurred in transects 1 and 2 in Robin Cove and was not

observed in Brewer Creek. At Robin Cove, Zp T was dominant

in transects 1 and 2 until ' 15 June; &. nuttallii was

dominant and ZpS was second dominant in transects 3 and

4. After - 15 June when ZpT died back, Elodea became

dominant in transects 1 and 2. Although ZpS populations

changed somewhat in transects 3 and 4 throughout the

sum,mer, Elodea remained dominant.

Water Depth and the Occurrence gf Zannichellia (Table 2):

In Robin Cove, ZpT grows in depths ranging f rom. 0.70 and

1.20 meters, but grows most abundantly about 1 meter

(between 0. 96 and 1. 10 m) . ZpS grows in shallower water

ranging from 0. 35 to 0. 60 m, occurring most abundantly at

0.4 m. In Brewer Creek, where only ZpS was observed, it

was found in water ranging from 0. 3 to 1. 4 ri and was

most abundant between 0.65 to 0.70 m.

Seed Num bers in Surface Sediments (Table 3):

six species of SAV and eleven species of emergent plants

are represented by seeds buried in surface sediments at

Table 2. Mean depth at high tide for all transects perpendicular to shore, and depth at which
Zannichellia has the highest percent cover along each transect. Tidal range is 0.50 m.

Robin Cove Brewer Crook

2 3 2 3

LenjIth of transect (a) is 20 38 to 10 10'
Mean depth along Transect. (a) 0.94 0.94 o.48 o.46 0.78 0.78 0.72
Standard deviation (t) 0.23 0.24 0.07 0.12 0.39 0.45 0.39
Range of depth where Zp occurs (m) 0.71-1-19 0.94-1.14 0.35-0-46 0.42-0.59 0.33-1-40 0.32-0.94 0.29-0-87
Depth of highest % cover - ZPT (m) 1.10 o.96
'Depth of high'est % cover - ZPS (a) o.43 0.44 0.71 o.68 o.65
Highest cover 68 60 52 40 79 82 25

Table 3. Number of weds 2W m' of emorpent and submerged veptafion in "diments aloog WSMOCEL. Oubdrats am 5 mcccr inter4ah, cxccpt in trw@ 3.
the 2 quadrats am 7 vactm apaxt kadvAuk ") Rum- XMIUMA M4116). EQMM= 201WWW (E. Md,). Pot-o-tan 22%iMW M 921Z).
Eotamoret2a 6P. M OP,). HUM OAdd=MVi (U@6 SO= &M M&). SO= M?l (& SP6). In frutesce- (Lf.). rbagmitu Sp. Irphraff- Sp.). G
(Gram.), P-ly-on- 02ga= M21m 2MI.). Cyporacew (CYPcrw-). CAM SP- fkQdWkdA 2M32k CP-&)- RRbM RIM V- LldQdMW= DJiWfM G;&)- Q=
lan" (Q.). ZWMMM OP- (EMU SP-N IMM OP- MM SOAQ= AWQ9W

Robin Crock

Tr I Tr 2 Tr 3 Tr 4
quads 2 3 1 2 3 4 1 2 1 2 3 4 5 6 7

M2. 173 11S 2V 144 71 223 13 64 67 105 4W 197 &S 83 69
a.m. 50 20 12 83 35 64 9 59 48 24 91 25 6 12 20
E-22d- 9 10 25 6 12 5 4 14 24 9 17 13 4
E-22Z. .13 12
5 4 5
5 5 19 27 29 73 14 19 74 4 9 4 4
12. 5 35 17 18 29 06
5-Sp. 4 9 24
1-f- 5 4 5 9
Eunip. 9 5 5 5
Gram. 5 9 9 9 4
f2bi.m. 5 5 4 4
Cypcrac. 9 9 4
Cum 'p- 5
Liz. 9 13 4 4
Rub 9
EUM Sp. 4
Unknown 12 6 5 9 5 26 4

Bnmor Crock

Tr I Tr 2 Tr 3 Tr 4
quadrats 1 2 1 2 1 2 3 1 2 3

L2- 24 17 25 11 10 6 6 11 21 5
8 11 5 17
4
5
Gram. 5
EM-sp- 6
10

Robin Cove. Greatest diversity occured along transect 4,

where 16 species of seeds were recovered. Seeds of

Zannichellia represented the highest percentage of buried

seeds in all .4 transects. Seeds of Ruppia were the next

most common. Seeds of emergent species represented 13% of.

all seeds recovered, but represented 23% of the seeds in

transect 4. The most abundant emergent species

represented by seeds was Scirpus olneyi.

The only SAV species represented by seeds in Brewer Creek

is Zannichellia. This is-consistent with the plant cover

for Brewer Creek. In addition,, six other species

represented by a few seeds included sedges, grasses, and

tulip tree from the neighboring shoreline vegetation.

RelationshiR Of Pla6t, Cover to Seed Number (Table 4):

A greater number of species were represented by seeds

than by plants at both locations. In Robin Cove, 5

species of plants were recorded in the vegetation and 17

species by seeds in the sediment. The five species

represented in the vegetation quadrats were SAV, while 6

of the 17 species from seeds belonged to SAV. The

remaining eleven species were represented by seeds of

nearby marsh emergents not represented in the quadrats.

Highest number of both plants and seeds, were found along

.*Nam MMMMMMMM MM

Tabl e 4. Comparison of mean percent relative cover and mean PDrOGnt of relative seed number in
Ro*bin Cove. C w covers S a seeds$ (a) submerged plants (e) emergent plant.

Mean
Tr 1 Tr 2 Tr 3 Tr 4 Total
C, C S C C S C . S
56 72 63 59 35 34 21 33 .44 55
0 17 0, 20 0 30 12 2 20
MXrio h llum (a) 17 0 1 0* 11 0 0 9 0
Elodea 9 27 0 27 0 40 0 39 0 33 0
=Oamoget. Perf. (a 0 5 6 4 13 8 13 3
Potamoset. R!Lct. (S@ 0 0 0 0 0 0 0 1
FotamgAe op. (S) 0 1 0 0 0 2 0 0
0 2 0 14 0 2 0 7 0
gJaS guad (a) 0 36
*-- -- nqyl to) 0 0 0 0 0 0 0 14 0 1
-11va f 0 1 0, 0 0 2 0 1
rutes. (e) 0 0 0 0 6 0 0 0 1*
-f-hraZTt7e--q (a) a 0 7
Others (e) 0 1 0 2 0 16 0

Fruit Morphology Aad Production 2f Zannichellia:

Measurements of rostrum length and fruit length of both

forms of ZannichelliA (Fig.6) were made in an attempt to

separate the two forms by seed morphology (Table 5). The

mean ratio of rostrum length to fruit length in ZpT is

0.64 and in ZpS is 0.59. A t-test shows no significant

difference between the two means (t(81) = 0.325, p> .05).

Both forms produced fruits that were dentate or entire

along the convex margin.

Number of iruits per cm of internode was 0.2 for ZpT and

0.4 for ZpS (Table 6). When frui ts per cm of internode

are multiplied by raean height of the plants, ZpT

produces a mean of 13.2 fruits per plant, and ZpS 4.2

seeds per plant (Table 7).

Sediment Description (Table 8):

The preliminary analysis of sediment indicates that ZpT

grows primarily in mud with 5 - 15% sand. In Robin Cove,

ZpS grows in a sand-mud mixture, with sand comprising ca.

75% of the sediment. In Brewer Creek, ZpS grew in

sediment that was 100% very fine to coarse-grained sand.

Table Comparison of fruit length, rostrum length
and the ratio of rDstrum to. fruit length in
Zannichellia valustris.

Mean fruit Mean rostrum Rostrums
length (mm) length (mm) Fruit ratio
ZpT 2.99 I-.go 0.64
(n-40)
ZPS 1.69 0.59
(n=43)

ers of fruits in Zannichelli palustriS-
Table 6. Comparison of stem lenXth to numb

Total length Total Number of Mean hei4it Mean number
of all number of fruits per of plants in Of fruits
internodes fruits internode quadrats per plant
(cm) length uits/cm) (cm)
(no. fr

ZpT 25o.6 49 0.2 66.0. 13.2
ZPS 99.6 43 o.4 io.4 4.2

Table 7. Maximum beigm of *m&%%wmwU inems. P6nuinUwwd 4 w= Stesumd CVWY 5 Mdal. at SR Ww MA=Wcb- PW" Wwe acowed
at I meter intervNIL

RoWn Cove

tau fam abort form
meters trails" I umsea 2 3 amnscot 4

0 0 0 0
1 0 0 0
2 0 0 0
3 45 60 0
4 87 60 0
5 56 55 0 20
6 100 50 0
7 70 0 10
8 70 65
9 66 58
10 75 60
11 70 0
12 78 45
13 as 60
.14 LID 0
15 40
16 60
17 70
66
19 58
20 10

Bovww Crook

meters 2 auvaoct 3
0 0 0
2 0 0 0
3 0 25 0
4 13 9 8
5 9 16 20
6 8 24
7 9
8 0 A.
9 6 0 0
10 .6 0 0

Table 8. Sediment descriptions. Sediment type, estimated % sand, and sand grain size,
where Zannichellia is rooted. All measurements made at 15X magnification.

Tributary Sediment type & Estimated Sand grain
transect & general descrip. percent size
quadrat sand (mm)

ZpT Robin Cove mud; material 5 not measured
Tr 1, 15 m too fine to
discern indiv.
grains
Tr 1, 10 m mud; with some 15 range - 0.1-0.8
sand mean - 0.43

ZpS Robin Cove sand--=d 75 range - 0.25-0-70
Tr 3, 7 m mixture mean = 0.44

Tr 4. 5 m sand-mud, 75 range = 0.20-0.60@
mixture mean = 0-37

Tr 4, 30 m sand-mud 75 range = 0.10-1.10
mixture mean = 0.53

Brewer Creek fine to med 100 range O.o5-o.6
Tr 3, 5 m sand mean 0.22
Ln

DISCUSSION

Differences in seeds and plant cover of the short and

tall forms of Zannichellia nalustris suggest that this

plant may be an indicator of disturbance. In the

relatively undisturbed tributary, plant cover includes.

both ZpT and ZpS. ZpT coexists with 3 other species of

SAV and is dominant, and ZpS coexists with 4 other

species and is the second dominant. In the disturbed
tributary, ZpS is the sole SA'V. Seeds recovered from

surface sediments were significantly fewer in the

disturbed locale.

The relationship of Zannichellia to water depth is

equivocal. Where both forms occur, ZpT occurs in deep

water and ZpS in shallow water and the intertidal zone,

but where ZpS occurs alone, it is found in depths ranging

from the intertidal zone to 1.4 m. ZpS is not restricted

to shallow water, and ZpT is absent from the deep water

of Brewer Creek, the disturbed area. Water depth is

important because it affects the amount of light required

for germination and growth, as well as affecting the

amount of exposure of the plants to evaporation and

increased salinity that can occur in shallow water. In

unpolluted waters 2% of the surface light, the minimum

light requirement for submerged aquatic plants, occurs at

depths of 1.3 to 1.6 'meters (Southwick and Pine 1975). In

polluted waters light becomes a limiting factor in depths

of 0. 6 to 1. 0 meters. Light does not appeat to be a

factor affecting the two growth forms of Zannichellia in

the depths studied in Robin Cove and Brewer Creek. ZpS

occurs in the well-lit shallow zone in Robin Cove as well

as depths of 1.4 meters in Brewer Creek. Shallow water

areas, including the intertidal zone, can experience

evaporation, increased salinity and increased chance of

dessication. These factors influence the growth forms of

Ruppia in New Hampshire tidal marshes (Richardson 1980).

There , plants growing in shallow pannes have a creeping,

spreading habit, while those growing in deeper water have

an ascending habit.

Sediment may be an important factor on the distribution

of Zannichellia. In both areas ZpS was found growing in

sediment that was 75% to 100% sand. ZpT was found in mud

containing < 15 -% sand. These observations, though

preliminary, are in agreement with Van Vierssen (1982a) ,

who observed an increase in stem length with increased

clay percentage for both forms of Zannichellia palustris

in Europe. The restriction of ZpS to Brewer Creek may be

due to the high percentage of sand and near absence of

clay in the substrate. A sediment core collected at the

south shore of Brewer Creek also supports this conclusion

(Brush et al in prep). Sediment deposited prior'to 1940

consisted of silt and clay, and after 1940 was

predominantly silt and sand. Seeds of Zannichellia are

common in sediment deposited between 1800 and 1930, when

there is a decline, followed by disappearance in 1940
(Fig. 7). Seeds of @annichellia reappear around 1955 and

continue in smaller numbers to the present. The core

shows a switch from mud to sand about the time the

community of Sherwood was established. Increased runoff

from construction could have resulted in high influxes of

sand, the predominant sediment type of the surrounding

hillsides, and could have led to a recent establishment

of ZpS. However, since the seeds of these two forms

cannot be distinguished morphologically, this possibility

remains conjectual.

The two growth forms of Zannichellia cannot be

distinquished on the basis of fruit morphology. Both ZpS

and ZpT produce fruits which are dentate and entire or

slightly crenate along the convex margin. Nor are there

differences in fruit and rostrum length or in the ratio

of rostrum to fruit length. Van Vierssen (1982a) reported

that the rostrum-fruit ratio is the most distinctive

morphological feature in separating several species of

Zannichellia in Europe. The mean ratio of 0.62 in this

study conforms to Zannichellia Pedunculata in Europe,

where rostrum-fruit ratios >0..5, and not to X. pal ustris,

where ratios are <0.5. The mean rostrum length of 1.69 mm

for ZpS samples in this study are identical to the' mean

Depth (CM)
year

1987 0
silt
with 1945
sand

1905 20

silt

with
1850
mud 4,0

1805
//Z 1783

60
1750
0 100 200

Seed Number/100 cc

Figure 7. Core SR3a from Brewer Creek showing changing abundance of
Zannichellia Ralustris from ca. 1750 to 1987. (from Brush.
Hilgartner and Thorton, in prep)

rostrum length of Yj. pedunculata in Europe. The taxonomic

importanc e of this requires further study, but fruit

dimensions and morphology alone indicate that the two

growth forms are variations of the same species, and may

be more closely related to Z.
Pedunculata than to Z.

Dalustris.

There are some important differences in fruit production

and preservation in sediments between.the two forms. ZpT

produces three times more fruits than'ZpS. This greater

production is reflected by a greater abundance of

Zannichellia seeds in sediments where ZpT grows. However,

it cannot be inferred that an abundance of seeds in

sediment represents ZpT. High seed numbers in sediments

where ZpS occurs in Robin Cove could have been dispersed

from nearby ZpT beds.

Duration of growth differs for the two growth forms, with

ZpT growing only during the spring, and ZpS growing well

into July and returning in fall.

The high diversity of both SAV plant cover and seeds in

the undisturbed tributary contrasted sharply with the low

diversity in the disturbed environment. Zannichellia was

the only SAV species present at the disturbed site.

Discrepancies did occur between plant cover and seeds in

sediment, with some dominant plants not being represented

by seeds such as Elodga and Myriophyllum. In other cases,,

seeds were present in the sediment where there were no

plants, e.g. Najas and Potamogeton pectinatus. Both of

these genera have been overlooked in vegetation surveys,

yet are represented by seeds in surface sediments of

lakes (Birks 1973; Birks and Birks 1980).

Despite these discrepancies between plant cover and the

seed record, the local vegetation was fairly well

represented by the seed record in terms of numbers of

species. seventy-one percent of the taxa recovered as

seeds were common components of the vegetation within a

30 meter radius of the sampling sites. Annuals were more

abundant among the seed taxa. than perennials, which

contrasts with the fact that perennials are among the

most important components of submerged and emergent

vegetation in wetlands (Van der Valk 1981). The selective

predominance of seeds of annuals buried in surface

sediments has been reported in paleoecological and seed

bank studies elsewhere (Birks 1973; Birks and Birks 1980;

Leck 1989; Parker and Leck 1985).

In this study, low diversity is indicative of

disturbance. Stuckey (1971) observed a decline in

diversity in Put-In-Bay Harbor, Lake Erie, during 70

years of human activity. Morgan and Phillip (1986) found

a similar pattern in the New Jersey Pine Barrens.

.42

Ehrenfeld (1983) found that species diversity increased

in some disturbed wetlands of New Jersey even while

eutrophic species were favored, suggesting that patterns

of plant diversity reflect a particular habitat and will

vary between sites. In this study as well' as others,

species favored by human disturbance including both

eutrophication and mechanical disturbance, are species

with broad ecological tolerances that respond positively

to increased nutrient inputs. Besides Zannichellia, one

such species is Sciryus americanus, a dominate shoreline

emergent with a broad tolerance ra nge found in a series

of sites along the Patuxent River and elsewhere (Anderson

et al 1965). This species is the dominant shoreline

emergent in Brewer Creek.

Understanding the relationship b etween plant cover and

seed production is important in interpreting the

paleoecological record -(Birks 1973; Birks and Birks 1980;

Watts 1978). Davis (1985) found Zannichellia vegetation

overrepresented by seeds in Leeds Creek, a tributary of

Chesapeake Bay, but admitted that- his values for plant

cover may have been low, since data were collected after

the peak growing period. In the undisturbed site for this

study Zannichellia represented 44% of 5 species in the.

plant cover and 55% of 17 species in the seeds in surface

,sediments. In the disturbed site, Zannichellia

represented 100% of the SAV plant cover and 72% of 7

species in the seeds recovered from sediment samples.

This indicates that Zannichellia is more closely

represented in the seed record than has been thought.

Seeds preserved in surface sediments closely approximate

plant cover in an embayment or cove, even when there is

virtually no correlation between individual quadrats and

a sediment sample taken from the quadrat. Macrofossils

within a sediment core therefore represent the vegetation

of the embayment, at least within a 30 meter radius of

the coring site, for different time intervals (different

depths) . Periods of high and low disturbance can

therefore be interpreted on the basis of the number of

fossil seeds at particular depths within the core.

Paleoecology provides a long-term perspective for modern

ecological problems (Brush 1986; Foster et al 1990). In

an extensive stratigraphic analysis of SAV in the upper

Chesapeake Bay, Brush et al (in preparation) have found

Zannichellia seeds common throughout many sediment cores,

but found that they varied in abundance through time.

Assuming that seed abundance reflects plant cover,

correlations between the abundance of fossil Zannichellia

seeds and particular periods of land u.se were used to

infer the response of Zannichellia populations to

different kinds and intensity of disturbance (Fig. 8).

The most significant increase in seed numbers occurred

Z(7nniCheffi;9 PGIUSITI@

LEGEND

SCALC

Tim* laletwols

A Pee - 1720
8
C
0 30- 70
C
No of Steds 100cm-syr-I

?5-2.00

0 2.01-4.00
0 1, 4, C*
N=
E

Figure 8. The distribution of Zannichellia Palustri from core
analy ses, showing changing abundance through time
(from Brush, Hilgartner & Thornton, in prep.).

during the period from 1650 -1780, when agriculture and

land clearance expanded. Initial land clearance by early

colonists was followed later by a clustering of small

farms into tobacco plantations during this period.

Sedimentation rates increased along with an increase in

nutrients and fertilizers. It appears that the initial

response of Zannichellia to increased nutrients was an

increase in plant cover. However, with further

intensification of land clearance and increase in the
human population from the 'mid-1800's into the 20th

century, Zannichelli steadily declined. The precipitous

decline of the 1960's and 1970's is revealed by the seed

record. Plant cover expanded again in the 1980's perhaps

in response to reduced sedimentation rates in some

tributaries in recent years. There appears to have been

a positive response of Zannichellia to moderate land use

.and disturbance, and, a negative response to intense

agriculture and urban land use. Cores from the Chester'

River and Severn River illustrate this pattern (Fig s.9

and 10) . The paleoecological pattern allows inferences to

be made about the cause and effect of disturbance. The

hypothesis that Zannichellia is a newcomer to Chesapeake

Bay (Stevenson and Confer 1978) is erroneous. Historical

perspectives therefore, provide a background in which to

analyze plant populations including SAV.

Depth (ca)

Year
1987 0-1

VIP-
1939

20-

1930

Mud
1850

1750 60-

1650

.1210 90i
0 100 200 300 400

Seed Nusb*r/ 100 a*

Figure 9. Core CHR6c from the mouth of Langford Creek near its confluence
with the Chester River,, showing the changing abundance of
Zannichellia Ralustria from ca. 1210 to 1987 AD. (from Brush.
Hilgartner and Thorton. in prep).

Depth (ca)
Year
1987 0@
black 1970
AU4

K
1880 20-%,

dark 40
foray
mud 1770

60-
1720

loom-
1200
7 80
*O&rx*

&rain

L 300 1001
0 100 200 300 400

Seed Number/ 100 cc
Figure 10. Core SRIa from the upper Severn River. showing the changing
?
opulations of Z-annichellia Palustris from ca- 300 to 1987 AD
from Brush, Hilgartner and Thorto-n@-Jn prep).;

CONCLUSION

Differences in the distributions of two growth forms of

Zannichellia valustris in a disturbed and undisturbed

estuary indicate that this species is a potential

indicator of environmental disturbance. Both the short

and.tall growth forms occur in the undisturbed estuary,

but only the short form was found in the disturbed site.

Along with this distribution, it was noted that the short

form was found growing on substrates of predominantly

sand, whereas the tall form grows on substrates that are

predominantly mud. The species was also separated by

water depth in the undisturbed area, with the tall form

growing in deeper water, but in the disturbed area, the

short form grew in a wide range of water depths.

Neither form is identifiable by seed morphology, but the

tall form produces about 3x the number of seeds as the

short form. Lack of any identifying morphological traits

prevents the identification of these forms in the fossil

record. However, a comparison of seed abundance in

sediment cores with the history of land use indicates

that the response of Zannichell ia to moderate land use is

positive, while the response to intense land use is.

negative. Also, an abundance of seeds of Zannichellia in

conjunction with a high diversity of other species in the

sediment can be used to indicate relatively undisturbed

conditions.

Both modern and paleoecological distributions of

Zannichellia Ralustris indicate strongly that the species

is.an indicator of disturbance. This conjecture is now

being tested by mapping the distributions of the two

forms over a wide range of environmental conditions,

including both substrate and disturbance history.

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