SPARTINA SPECIES ZONATION ALONG AN ESTUARINE GRADIENT IN GEORGIA

214 Pages·2004·4.68 MB·English

SPARTINA SPECIES ZONATION ALONG AN ESTUARINE GRADIENT IN
GEORGIA: EXPLORING MECHANISMS CONTROLLING DISTRIBUTION
by
SUSAN N. WHITE
(Under the Direction of Merryl Alber)
ABSTRACT
The general paradigm for the observed plant zonation in salt marshes is that a
combination of abiotic stress and competition drive vegetation patterns within a single
marsh, but there have been few studies investigating the bankside vegetation changes that
occur along the longitudinal salinity gradient of estuaries. The main objectives of the
research presented in this dissertation were to examine whether the same controls that
explain the distribution of Spartina alterniflora in the salt marsh can be applied without
modification to a longitudinal salinity gradient and to evaluate changes in Spartina
distributions under drought conditions. Reciprocal transplant studies, greenhouse
experiments, species removals in mixed Spartina stands, and vegetation surveys were
conducted in the estuary of the Altamaha River, GA, where S. cynosusroides occurs
upstream of S. alterniflora. In reciprocal transplant experiments, each plant survived and
performed best in its natural habitat. The presence of conspecific neighbors slightly
reduced S. alterniflora plant performance in the salt marsh whereas S. cynosuroides
showed little response in either environment. The results of these and other experiments
suggest the lower estuarine distribution of Spartina cynosuroides is controlled by abiotic
conditions (salinity or sulfide concentrations). The upper estuarine distribution of S.
alterniflora is not well understood, but seems to also be primarily controlled by abiotic
factors (possibly a sulfate requirement). These outcomes challenge results from previous
investigations of zonation controls in salt marshes and suggest that modifications to the
salt marsh paradigm are necessary when describing vegetation distribution along an
estuarine gradient. During an extended drought (2000-2002), Spartina alterniflora
density increased to a greater extent than S. cynosuroides in mixed stands and the
location where Spartina cover was 50 % S. cynosuroides and 50 % S. alterniflora shifted
approximately 3 km upriver, suggesting that Spartina communities can respond rapidly to
increasing estuarine salinity. These studies improve our understanding of the ecological
linkages in estuaries and can aid coastal policymakers in making better management
decisions and predictions concerning how changes in freshwater inflow might impact the
distribution of estuarine organisms.

INDEX WORDS: Spartina alterniflora, Spartina cynosuroides, Plant zonation,
Freshwater inflow, Salt marsh, Brackish marsh, Altamaha River
estuary, Georgia coast
SPARTINA SPECIES ZONATION ALONG AN ESTUARINE GRADIENT IN
GEORGIA: EXPLORING MECHANISMS CONTROLLING DISTRIBUTION

by

SUSAN N. WHITE
B.S., Duke University, 1997

A Dissertation Submitted to the Graduate Faculty of The University of Georgia in Partial
Fulfillment of the Requirements for the Degree

DOCTOR OF PHILOSOPHY

ATHENS, GEORGIA
2004
© 2004
Susan N. White
All Rights Reserved
SPARTINA SPECIES ZONATION ALONG AN ESTUARINE GRADIENT IN
GEORGIA: EXPLORING MECHANISMS CONTROLLING DISTRIBUTION

by

SUSAN N. WHITE

Major Professor: Merryl Alber

Committee: Alice Chalmers
Lisa Donovan
Samantha Joye
Steve Pennings
Catherine Pringle

Electronic Version Approved:

Maureen Grasso
Dean of the Graduate School
The University of Georgia
August 2004
DEDICATION
For their unwavering encouragement, realistic perspectives, and endless love, I
dedicate this dissertation to my parents, Richard and Norma White, my grandmother,
Katharine White, and my sister and brother, Karen and Richard White.
iv
TABLE OF CONTENTS
Page
CHAPTER
1 INTRODUCTION...................................................................................................1
OVERVIEW......................................................................................................1
SALT MARSH STUDIES.................................................................................2
BRACKISH MARSH STUDIES.......................................................................6
OBJECTIVES....................................................................................................8
LITERATURE CITED......................................................................................9
2 EXTENDING THE SALT MARSH PARADIGM: CONTROLS OF SPARTINA
DISTRIBUTIONS ALONG A GEORGIA ESTUARINE GRADIENT...............16
INTRODUCTION...........................................................................................16
METHODS......................................................................................................23
RESULTS........................................................................................................28
DISCUSSION..................................................................................................34
LITERATURE CITED....................................................................................43
3 SPARTINA ALTERNIFLORA AND S. CYNOSUROIDES GROWTH
RESPONSES UNDER MANIPULATED ABIOTIC AND BIOTIC
CONDITIONS.......................................................................................................75
MATERIALS AND METHODS.....................................................................80
RESULTS........................................................................................................85
v
DISCUSSION..................................................................................................94
CONCLUSION..............................................................................................109
LITERATURE CITED..................................................................................110
4 THE RESPONSE OF SPARTINA SPECIES TO PROLONGED DROUGHT IN
THE ALTAMAHA RIVER ESTUARY.............................................................149
METHODS....................................................................................................153
RESULTS......................................................................................................158
DISCUSSION................................................................................................164
LITERATURE CITED..................................................................................176
5 CONCLUSIONS..................................................................................................197
vi
CHAPTER 1
INTRODUCTION
OVERVIEW
The intertidal marshes that border riverine estuaries grade from freshwater
marshes (where salinities are <0.5 practical salinity units, or psu), to brackish marshes
(with salinities between 0.5-18 psu), to salt marshes (with salinities >18 psu), each of
which has a characteristic vegetation pattern. The factors that control plant distribution
patterns in salt marshes have received the most attention, and the generally accepted
paradigm is that a combination of abiotic stress (specifically salinity) and competition
drive vegetation patterns within a single marsh. However, there have been few studies
investigating the vegetation changes that occur along the longitudinal salinity gradient of
estuaries and it is unclear if the same mechanisms that control the distribution of plants in
salt marshes can be applied.
In the southeastern United States, Spartina cynosuroides dominates riverbank
habitats in brackish areas, whereas S. alterniflora is dominant in salt marshes. The
central goal of this dissertation was to investigate the mechanisms that control the
distribution of Spartina species along the longitudinal axis of an estuary. Specific
research objectives were to document the distribution of S. alterniflora and S.
cynosuroides in response to changing inflow conditions and to determine the effects of
both abiotic (salinity and sulfate concentrations) and biotic (neighbor presence) factors on
plant performance. This type of study is important for improving our understanding of
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the ecological linkages in estuaries and will also help us to predict how changes in
estuarine salinity regimes, mediated by changes in freshwater inflow, might affect the
distributions of estuarine organisms.
SALT MARSH STUDIES
Spartina alterniflora is the dominant plant species in southeastern United States
salt marshes. Spartina alterniflora is tallest on the creekbank and plant height decreases
with distance from the creekbank to upland areas where mixed marsh communities of S.
alterniflora and Juncus romereianus are located. Early work focusing on plant
community structure in salt marshes identified various abiotic mechanisms that structure
these habitats, including plant response to salinity, inundation (resulting in anoxic
sediments), and nutrient availability (Valiela and Teal 1974, Mendelssohn 1979, King et
al. 1982, Howes et al. 1986, Pennings and Bertness 2001). More recent research has
focused on combinations of the above abiotic mechanisms with biotic interactions (such
as competition or facilitation) in order to more fully explain observed plant distributions
in the field (Bertness and Ellison 1987, Bertness and Yeh 1994, Hacker and Gaines 1997,
Gough and Grace 1998, Pennings and Bertness 2001). In the salt marsh, both inter- and
intra-specific competition for light, nutrients and water influence plant community
structure. However, competitive abilities are influenced by an organism’s physiological
ability to tolerate environmental stressors. Several studies have found that competitively
dominant organisms monopolize physically benign habitats while the less competitive
(but often more physiologically hardy) organisms dominate physically stressful habitats
(Connell and Slayter 1977, Adam 1990, Bertness and Pennings 2000).
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