&EPA
United States Environmental
Protection Agency
Off ice of Water
Washington, DC 20460
EPA-822-R-02-024
March 2002
METHODS FOR EVALUATING WETLAND CONDITION
#16 Vegetation-Based Indicators of
Wetland Nutrient Enrichment
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United States Environmental Office of Water EPA-822-R-02-024
Protection Agency Washington, DC 20460 March 2002
METHODS FOR EVALUATING WETLAND CONDITION
#16 Vegetation-Based Indicators of
Wetland Nutrient Enrichment
Principal Contributor
Indiana University, School of Public and Environmental Affairs
Christopher Craft
Prepared jointly by:
The U.S. Environmental Protection Agency
Health and Ecological Criteria Division (Office of Science and Technology)
and
Wetlands Division (Office of Wetlands, Oceans, and Watersheds)
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NOTICE
The material in this document has been subjected to U.S. Environmental Protection Agency (EPA)
technical review and has been approved for publication as an EPA document. The information
contained herein is offered to the reader as a review of the "state of the science" concerning wetland
bioassessment and nutrient enrichment and is not intended to be prescriptive guidance or firm advice.
Mention of trade names, products or services does not convey, and should not be interpreted as
conveying official EPAapproval, endorsement, or recommendation.
APPROPRIATE CITATION
U.S. EPA. 2002. Methods for Evaluating Wetland Condition: Vegetation-Based Indicators of
Wetland Nutrient Enrichment. Office of Water, U.S. Environmental Protection Agency, Washing-
ton, DC. EPA-822-R-02-024.
This entire document can be downloaded from the following U.S. EPA websites:
http://www.epa.gov/ost/standards
http://www.epa.gov/owow/wetlands/bawwg
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CONTENTS
FOREWORD v
LIST OF MODULES vi
SUMMARY 1
PURPOSE 1
INTRODUCTION 1
VEGETATION-BASED INDICATORS OF NUTRIENT ENRICHMENT 2
METHODS FOR ASSESSING NUTRIENT ENRICHMENT 6
CASE STUDIES ll
REFERENCES 19
GLOSSARY 22
LIST OF TABLES
TABLE l: LEVEL I AND LEVEL II INDICATORS OF NUTRIENT
ENRICHMENT IN WETLANDS 12
TABLE 2: COMPARISON OF VEGETATION-BASED INDICATORS OF
PHOSPHORUS (P) ENRICHMENT IN UNENRICHED AND
EUTROPHIC SAWGRASS, CLADIUM JAMAICENSE,
COMMUNITIES OF THE FLORIDA EVERGLADES 14
TABLE 3: LEVEL I AND LEVEL II INDICATORS OF PHOSPHORUS (P)
ENRICHMENT OF SAWGRASS, CLADIUM JAMAICENSE,
COMMUNITIES OF THE FLORIDA EVERGLADES 15
TABLE 4: LEVEL I AND LEVEL II INDICATORS OF NITROGEN (N)
ENRICHMENT OF SPARTINA ALTERNIFLORA DOMINATED
SALT MARSHES ALONG THE ATLANTIC (NC, GA) COAST 17
in
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TABLE 5: LEVEL I AND LEVEL II INDICATORS OF NITROGEN (N)
AND PHOSPHORUS (P) ENRICHMENT OF A WET SEDGE
TUNDRA (ERIOPHORUM ANGUSTIFOLIUM) IN ALASKA AFTER
5 YEARS OF FERTILIZATION
18
FIGURE l
FIGURE 2:
FIGURES:
LIST OF FIGURES
DISTRIBUTION OF PLANT SPECIES AND HYDROPERIOD
ACROSS A SOUTHEASTERN BOTTOMLAND FORESTED
WETLAND
DISTRIBUTION OF PLANT SPECIES AND HYDROPERIOD
ACROSS A FRESHWATER MARSH WETLAND
PROTOCOL FOR SAMPLING VEGETATION FOR WETLAND
EUTROPHICATION ASSESSMENTS
1O
IV
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FOREWORD
In 1999, the U. S. Environmental Protection Agency (EPA) began work on this series of reports entitled
Methods for Evaluating Wetland Condition. The purpose of these reports is to help States and
Tribes develop methods to evaluate (1) the overall ecological condition of wetlands using biological
assessments and (2) nutrient enrichment of wetlands, which is one of the primary stressors damaging
wetlands in many parts of the country. This information is intended to serve as a starting point for States
and Tribes to eventually establish biological and nutrient water quality criteria specifically refined for
wetland waterbodies.
This purpose was to be accomplished by providing a series of "state of the science" modules concerning
wetland bioassessment as well as the nutrient enrichment of wetlands. The individual module format
was used instead of one large publication to facilitate the addition of other reports as wetland science
progresses and wetlands are further incorporated into water quality programs. Also, this modular
approach allows EPA to revise reports without having to reprint them all. A list of the inaugural set of
20 modules can be found at the end of this section.
This series of reports is the product of a collaborative effort between EPAs Health and Ecological
Criteria Division of the Office of Science and Technology (OST) and the Wetlands Division of the
Office of Wetlands, Oceans and Watersheds (OWOW). The reports were initiated with the support
and oversight of Thomas J. Danielson (OWOW), Amanda K. Parker and Susan K. Jackson (OST),
and seen to completion by Douglas G. Hoskins (OWOW) and Ifeyinwa F. Davis (OST). EPArelied
heavily on the input, recommendations, and energy of several panels of experts, which unfortunately
have too many members to list individually:
• Biological Assessment of Wetlands Workgroup
• Wetlands Nutrient Criteria Workgroup
More information about biological and nutrient criteria is available at the following EPA website:
http ://www. epa. gov/ost/standards
More information about wetland biological assessments is available at the following EPA website:
htto ://www.epa. gov/owow/wetlands/bawwg
V
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LIST OF "METHODS FOR EVALUATING WETLAND
CONDITION" MODULES
MODULE # MODULE TITLE
1 INTRODUCTION TO WETLAND BIOLOGICAL ASSESSMENT
2 INTRODUCTION TO WETLAND NUTRIENT ASSESSMENT
3 THE STATE OF WETLAND SCIENCE
4 STUDY DESIGN FOR MONITORING WETLANDS
5 ADMINISTRATIVE FRAMEWORK FOR THE IMPLEMENTATION OF A
WETLAND BIOASSESSMENT PROGRAM
6 DEVELOPING METRICS AND INDEXES OF BIOLOGICAL INTEGRITY
7 WETLANDS CLASSIFICATION
8 VOLUNTEERS AND WETLAND BIOMONITORING
9 DEVELOPING AN INVERTEBRATE INDEX OF BIOLOGICAL
INTEGRITY FOR WETLANDS
10 USING VEGETATION TO ASSESS ENVIRONMENTAL CONDITIONS
IN WETLANDS
11 USING ALGAE TO ASSESS ENVIRONMENTAL CONDITIONS IN
WETLANDS
12 USING AMPHIBIANS IN BlOASSESSMENTS OF WETLANDS
13 BIOLOGICAL ASSESSMENT METHODS FOR BIRDS
14 WETLAND BIOASSESSMENT CASE STUDIES
15 BIOASSESSMENT METHODS FOR FISH
16 VEGETATION-BASED INDICATORS OF WETLAND NUTRIENT
ENRICHMENT
17 LAND-USE CHARACTERIZATION FOR NUTRIENT AND SEDIMENT
RISK ASSESSMENT
18 BlOGEOCHEMICAL INDICATORS
19 NUTRIENT LOAD ESTIMATION
2O SUSTAINABLE NUTRIENT LOADING
VI
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SUMMARY
INTRODUCTION
TV' egetation-based attributes of wetland func-
V tion (e.g., energy flow, nutrient cycling) and
structure (species composition) that respond to
nutrient enrichment and eutrophication are pre-
sented below. Attributes consist of Level I and
Level II indicators that respond quickly to nutri-
ent enrichment and are relatively easy to use.
Level I indicators consist of remotely sensed data
to assess change in wetland plant communities
over time as well as field measurements of stem
height and leaf C:N:P ratios. Level I indicators
are recommended for coarse assessment of indi-
vidual wetlands or large-scale surveys of many
wetlands. Level II indicators include
aboveground biomass/litterfall, standing dead
C:N:P, nutrient resorption efficiency and profi-
ciency, nutrient use efficiency, and nutrient-tol-
erant and -intolerant species. These indicators
are used for more detailed assessment of wet-
land eutrophication. Their sound application re-
quires that similar sample collection protocols
be used for both "targeted" (potentially eutrophic)
and reference (unenriched) wetlands.
Observational and experimental studies confirm
the reliability of vegetation-based indicators for
identifying eutrophication in nutrient-enriched
and unenriched areas of the Florida Everglades,
salt marshes, and wet sedge tundra. Until the
indicators are tested elsewhere, however, they
should be applied cautiously to assessments of
eutrophication in other types of wetland and in
different geographic regions.
PURPOSE
r I The purpose of this module is to identify veg
J. etation-based indicators that can be used by
wetland regulatory and natural resource manag-
ers to determine the nutrient status (eutrophic or
unenriched) of freshwater and estuarine wetlands.
TT'T' etlands improve surface water quality by
V V intercepting sediment, nutrients, and other
pollutants transported from terrestrial areas and
upstream aquatic ecosystems (Johnston 1991,
Mitsch and Gosselink 1993). Wetlands become
sinks for nitrogen (N) by sequestering it in accu-
mulating soil organic matter (Craft 1997) and by
microbially converting nitrate (NO3~) to atmo-
spheric N2 (denitrification) (Groffman 1994).
Wetlands serve as sinks for phosphorus (P) by
trapping sediments; by sorption to iron (Fe), alu-
minum (Al), and calcium (Ca) minerals; and by
plant uptake (Craft 1997). The ability of wet-
lands to remove nutrients has led to its wide-
spread use, in both natural and artificial forms,
to remove N and P from secondarily treated
wastewater, septic effluent, and nutrient-enriched
agricultural drainage (Johnston et al. 1991, Craft
and Richardson 1993, Kadlec and Knight 1996).
When natural wetlands receive excessive nutri-
ent loadings, ecosystem processes, such as plant
productivity and nutrient cycling, are altered in
measurable ways. The structure of the plant com-
munity also may change as slower growing na-
tive species are replaced by faster growing spe-
cies that take advantage of high nutrient levels to
increase growth (Davis 1991).
The threshold where significant alteration in
wetland function and structure occurs is referred
to as the "assimilative capacity" of the system.
When the assimilative capacity of a wetland is
exceeded, the ecosystem responds by increasing
nutrient uptake that translates into increased
growth. Sometimes the result is a shift in plant
species composition, as natives are displaced
by aggressive interlopers like cattail (Typhd).
This phenomenon is known as cultural eutrophi-
cation and is caused by excessive nutrient load-
ings from anthropogenic sources. Because N and
P are the primary nutrients limiting productivity
in wetlands (Schlesinger 1991, Vitousek and
1
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Howarth 1991, Bridgham et al. 1996), these nutri-
ents usually are responsible for changes in ecosys-
tem function and structure that occur when wetland
assimilative capacity is exceeded (Carpenter et al.
1998).
Wetland vegetation responds to nutrient addi-
tions by increased storage of N and P in plant
tissue and by increased net primary production
(NPP) (Craft et al. 1995, Bridgham et al. 1996).
Increased NPP and nutrient storage in turn affect
ecosystem processes including decomposition
(Valiela et al. 1982, Davis 1991, Rybczyk et al.
1996), accumulation of soil organic matter, and
organic carbon export (Craft and Richardson
1993, 1998, Morris and Bradley 1999). Over
time, plant species composition may shift as na-
tive species decline and are replaced by species
that take advantage of high nutrient levels to in-
crease growth (Craft et al. 1995). Nutrient en-
richment often results in replacement of uncom-
mon or rare species by species tolerant of high
nutrient loadings (e.g., Typha, Phragmites)
(Davis 1991, Chambers etal. 1999, Galatowitsch
et al. 1999). Such changes in community
composition and ecosystem processes
compromise wetland ecological integrity by
altering energy flow, nutrient cycling, and
niche/habitat characteristics that in turn affect
wetland fauna assemblages.
This chapter describes vegetation-based indi-
cators that can be used to determine whether a
wetland's ecological integrity has been impaired
by nutrient enrichment and eutrophication. In-
dicators are described for structural and func-
tional responses to both low and high nutrient
loadings. Functional indicators include leaf N
and P content and metrics of NPP (biomass pro-
duction, stem height). Structural indicators con-
sist of the presence/absence of "sentinel" spe-
cies that reflect ambient (low nutrients) and im-
paired (high nutrients) nutrient loading regimes.
Methods for sampling vegetation and analytical
techniques to assess the degree of nutrient enrich-
ment also are described.
VEGETATION-BASED
INDICATORS OF
NUTRIENT ENRICHMENT
~\ Tutrient enrichment affects both structural and
-L V functional attributes of wetlands. Structural
attributes include characteristics of the commu-
nity, or of individual species, whereas functional
attributes relate to energy flow and nutrient cy-
cling. Changes in wetland function that occur in
response to nutrient enrichment include increased
N and P uptake, NPP, and decomposition.
Changes in structure occur through shifts in plant
species composition, including replacement of
nutrient-intolerant species with those adapted to
high nutrient conditions. In this module, func-
tional indicators are emphasized because energy
flow and nutrient cycling processes respond
quickly and dramatically when nutrient loadings
are increased.
FUNCTIONAL INDICATORS
When nutrients are limiting, wetland vegeta-
tion responds to nutrient additions by incorpo-
rating N and P into growing or "green" tissue
and increasing NPP (Shaver and Melillo 1984,
Craft et al. 1995, Koerselman and Meuleman
1996, Shaver et al. 1998). Changes in nutrient
uptake and NPP affect wetland energy and nutri-
ent cycles by altering rates of uptake, storage,
and release of C, N, and P.
Net primary productivity
Net primary productivity is the amount of car-
bon fixed during photosynthesis that is incorpo-
rated into new leaves, stems, and roots. NPP is
often expressed as amount of biomass produced
per m2 of wetland surface per year (g/m2/yr). Most
2
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techniques to measure NPP focus on production of
aboveground biomass and discount root produc-
tion that sometimes accounts for half or more of the
NPP. The simplest way to measure aboveground
biomass is by harvesting all of the standing material
at the end of the growing season (Broome et al.
1986). This method is useful for measuring NPP of
herbaceous emergent vegetation, especially intem-
perate climates where there is a distinct growing
season. However, measurements of aboveground
or "standing crop" biomass typically underestimate
NPP because they do not include biomass losses
to herbivory and stem mortality during the growing
season. Nondestructive methods such as tagged
stems or the use of external markers like wire are
used to measure NPP of Sphagnum (Clymo and
Hay ward 1982) and coastal Louisiana marsh plants
(Hopkinsonetal. 1980). These methods account
for stem mortality and herbivory, and provide a truer
estimate of NPP than the harvest method. How-
ever, they are much more labor-intensive, and time-
consuming, and are not recommended for wetland
eutrophication assessment. Enhanced biomass pro-
duction is reflected by increased height and, some-
times, stem density of herbaceous emergent veg-
etation (Broome et al 1983). Increased stem den-
sity, however, may reflect other factors like vigor-
ous clonal growth, so it is not recommended as an
indicator of nutrient enrichment.
Woody vegetation is not as good an indicator
of enrichment as is herbaceous vegetation.
Woody plants grow slowly and have a longer
life cycle than herbaceous plants, resulting in a
slower response to nutrient loading. In wetland
dominated by trees with little herbaceous veg-
etation, leaf litterfall is a common means to esti-
mate NPP (Chapman 1986). Measurements of
litterfall involve the periodic (usually monthly)
collection of leaf litter that collects in littertraps
placed on or above the forest floor. Like the
harvest method, litterfall is an index of NPP be-
cause it estimates the portion of NPP that goes into
producing photosynthetic tissue. One drawback,
however, is that the litterfall method is labor-inten-
sive and time-consuming. But, for wetlands where
herbaceous vegetation is unimportant, litterfall is the
best method to measure NPP.
Biogeochemical cycling
Indicators of biogeochemical cycling describe
the uptake, storage, and release of N and P in
plant tissue. Nutrient enrichment of wetlands
leads to increased uptake and storage of N and/
or P, depending on the causative nutrient
(Verhoeven and Schmitz 1991, Shaver et al.
1998). In wetlands where P is limiting, leaf tis-
sue P is the first indicator to respond to nutrient
enrichment (Craft et al. 1995). Increased P up-
take by plants is known as "luxury uptake" be-
cause P is stored in vacuoles and used later
(Davis 1991). Like P, leaf tissue N increases in
response to N enrichment (Brinson et al. 1984,
Shaver et al. 1998). However, most N is used
directly to support photosynthesis and growth of
new tissue, so luxury uptake of N is not always
observed (Verhoeven and Schmitz 1991).
The ratio of carbon to nitrogen (C:N) in
aboveground biomass or leaves can be used to
determine whether a wetland is N-limited or
whether there is excess N in the system. Under
conditions of N enrichment, plants assimilate
more N, increasing leaf N and decreasing C:N
(Shaver and Melillo 1984, Shaver et al. 1998).
Likewise, P enrichment results in increased leaf
P and decreased C:P (Craft et al. 1995). Appli-
cation of C:N and C:P ratios requires that
baseline measurements are made using vegeta-
tion collected from unenriched areas or from the
same area prior to eutrophi cation.
Leaf N:P also has been used to determine
whether a wetland is N-limited or P-limited
(Koerselman and Meulemen 1996, Verhoeven et
al. 1996). It has been hypothesized thatN:P< 15
(weightweight basis) indicates N limitation whereas
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N:P>15 indicates P limitation (Verhoeven et al.
1996). Assuming that this hypothesis is valid, this
information is useful for determining whether a wet-
land is at risk for either N or P enrichment. For
example, wetlands with vegetation N:P< 15 may be
susceptible to N enrichment whereas wetlands with
vegetation N:P>15 may be susceptible to P limita-
tion. Sometimes N:P ratios are presented on a
mole:mole basis. In this case, N:P <33 indicates N
limitation whereas N:P>33 suggests P limitation.
Development of techniques to identify N versus P
limitation is important because, for a given wetland,
the nutrient that limits ecosystem productivity usu-
ally is the cause of eutrophication.
Three useful measures of nutrient availability
and eutrophication are nutrient resorption effi-
ciency, nutrient resorption proficiency, and nu-
trient use efficiency (Shaver and Melillo 1984,
Killingbeck 1996). Nutrient resorption efficiency
is a measure of nutrient conservation and limita-
tion. Under low nutrient conditions, plants re-
sorb and translocate nutrients from senescing tis-
sue and store them in belowground tissue to be
used later (Aerts et al. 1999). Vegetation grow-
ing in nutrient-poor wetlands translocate large
quantities of nutrients to belowground tissue and,
thus, possess high nutrient resorption efficiency.
Plants growing in nutrient-enriched wetlands of-
ten possess low nutrient resorption efficiency
because of high nutrient availability in soil and
water. Nutrient resorption efficiency (RE) is
defined as:
RE =
N or P (g/m2) in green biomass
minus N or P (g/m2) in standing
dead biomass
N or P (g/m2) in green biomass
where biomass is aboveground clipped (har-
vested) material.
Resorption efficiency also may be calculated us-
ing the concentration of N or P (mg) per individual
leaf (Aerts et al. 1999) or per cm2 leaf material
(Feller etal. 1999).
Nitrogen and P in senesced or standing dead
leaves also are used as measures of nutrient resorp-
tion proficiency. Nutrient resorption proficiency is
defined as the absolute levels to which nutrients are
reduced in senesced leaves (Killingbeck 1996).
Resorption is highly proficient in plants that reduce
N and P concentrations below 0.7% and 0.05%,
respectively. It is thought that high resorption pro-
ficiency is an evolutionary adaptation that enables
plants to conserve nutrients in infertile environments
(Killingbeck 1996).
Nutrient use efficiency (NUE) is a measure of
the effectiveness by which plants use nutrients
to produce biomass. High NUE corresponds to
high rates of nutrient resorption because large
amounts of biomass are produced with little loss
of nutrients in litterfall (Vitousek 1982). Nutri-
ent-poor wetlands often possess lower litter N
and P concentrations, resulting in a higher NUE
than in nutrient-rich wetlands (see Table 2).
Nutrient use efficiency is defined as:
Aboveground biomass production
(e.g., litterfall, standing dead biomass)
NUE =
Nutrient (N or P) in litter/all,
standing dead biomass
where litterfall/biomass and nutrients are
expressed in g/m2 (Vitousek 1982).
A simpler means to measure NUE is by calcu-
lating the inverse of the concentration of N or P
in standing dead biomass/litterfall (Aerts et al.
1999). The maj or advantage of this method over
the standard NUE method is that litterfall need
not be measured, only tissue N and/or P content.
Nutrient use efficiency varies widely among differ-
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ent growth forms such as grasses, conifers, and
deciduous trees and shrubs. For example, conifers
typically have much higher N and P use efficiency
than deciduous and graminoid wetland plants (Aerts
etal. 1999). For this reason, it is important to com-
pare NUE, RE, and the resorption proficiency of
similar growth forms and species collected from
nutrient-enriched and unenriched wetlands to as-
sess the vegetation response to eutrophication.
STRUCTURAL INDICATORS
Structural indicators of nutrient enrichment con-
sist of community-level attributes like the pres-
ence or absence of specific species. Cattail
(Typha), for example, encroaches on and colo-
nizes areas that have undergone soil disturbance,
nutrient enrichment, and hydrology alteration
(Apfelbaum 1985, Urban etal. 1993). Other spe-
cies are common in nutrient-poor wetlands, but
decline or disappear during eutrophi cation as they
are overcome displaced (Davis 1989, Craft et
al. 1995, Jensen et al. 1995). Attributes that de-
scribe vegetation structure, like biomass and
stem height, also are useful indicators of
enrichment.
Structural indicators of nutrient enrichment in-
clude biomass, stem height (discussed earlier),
dramatic/widespread change in plant community
composition over time, and the presence or ab-
sence of species adapted to either nutrient-en-
riched (nutrient-tolerant) or unenriched (nutri-
ent-intolerant) conditions. Biomass and stem
height also provide an index of NPP, which in-
creases in response to enrichment. Other pos-
sible structural indicators include species domi-
nance, richness, and the presence of rare and in-
vasive (nonnative) species.
Structural indicators may be less reliable than func-
tional indicators for wetland eutrophi cation assess-
ments because plants respond to environmental fac-
tors other than nutrients. Light, moisture/waterlog-
ging, acidity, and other stressors (e.g., salinity, sul-
fides, fire) affect plant community composition
(Smith 1996). In addition, different plant species
possess variable life history traits, such as seed pro-
duction, dispersal, viability, and germination, that
determine their distribution across the landscape (van
derValk and Davis 1978). Environmental factors
and life history traits interact to regulate the geo-
graphic distribution of plant species. Structural in-
dicators, described above, also can be used to as-
sess the overall biological integrity of wetlands.
Module 10: Using Vegetation to Assess Environ-
mental Conditions in Wetlands provides a detailed
overview of using vegetation to assess biological
integrity (Fenessy in press).
Anthropogenic disturbances to wetlands often
are manifested in a dramatic and widespread
change in plant species composition over time.
Aerial and satellite photography can be used to
detect coarse changes in wetland plant commu-
nities in response to eutrophi cation (Jensen et
al. 1987, 1995). Remote sensing techniques can
detect changes in the aerial extent of wetlands,
the percent cover of vegetation, as well as the
replacement of one plant community by another.
Remote sensing requires field verification, how-
ever, to calibrate plant community types with
patterns discerned from aerial and satellite im-
ages. (See Module 17: Land-Use Characteriza-
tion for Nutrient and Sediment Risk Assessment,
for further information on this topic.) Field-based
measurements of NPP and biogeochemical indica-
tors also are needed to verify whether eutrophica-
tion and not some other type of disturbance (e.g.,
hydrologic alteration) is the causative agent.
During the eutrophication process, large-scale
shifts in plant species composition occur in response
to the addition of the limiting nutrient. For example,
cattail encroaches on and eventually can become
the dominant species in areas of the Everglades that
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receive large loadings of P, the primary limiting nu-
trient (Davis 1991). In wetlands where surface
water is present, duckweed (Lemna sp.) often in-
creases in density and coverage in response to in-
creased nutrients (Portielje and Roijackers 1995,
Janse 1998, Vaithiyanathan and Richardson 1999).
Another species that may invade in response to in-
creased nutrients is Phragmites (Chambers et al.
1999, Galatowitschetal. 1999). It is important to
be aware that some species invade in response to
other anthropogenic alterations like changes in
hydroperiod (e.g., Typha sp., Phragmites aus-
tralis, Phalaris arundinacea) and salinity (e.g.,
Phragmites, Typha angustifolia) and soil dis-
turbance (Typha sp., Lythrum salicaria,
Phragmites, Phalaris) (Apfelbaum 1985, Urban
et al. 1993, Chambers et al. 1999, Galatowitsch
et al. 1999). Thus, when using species-specific
indicators, it is important to identify anthropo-
genic disturbances in and around the wetland to
ensure that changes in plant species composition
are the result of nutrient enrichment and not some
other type of disturbance.
In contrast to nutrient-tolerant species, some
species are adapted to low nutrient or olig-
otrophic conditions. In the Everglades, emer-
gent (sawgrass, Cladiumjamaicense) and float-
ing aquatic (bladderwort, Utricularia sp.) veg-
etation dominate unenriched areas but are re-
placed by cattail and other species in eutrophic
areas (Davis 1989, Urban et al. 1993). The dis-
appearance of submerged aquatic vegetation
(SAV) also may be a useful indicator of nutrient
enrichment. Over the past few decades, SAV
declined dramatically in many estuaries of the
eastern United States. (Orth and Moore 1983).
The decline of SAV has been linked to light
attenuation caused by eutrophication and sedimen-
tation (Dennisonetal. 1993). It should be noted
that species identified as nutrient intolerant may be
limited to specific wetland types and geographic
regions.
A national database of wetland plant sensitivities
to nutrient enrichment and hydrologic alteration is
being produced for USEPA by Paul Adamus of
Oregon State University (Adamus and Gonyaw
2000). The database, which is to include both ex-
perimental and observational studies, assesses the
responses of various species to eutrophication and
alteration of wetland hydrology. The database may
serve as a guide to identify wetland species that are
indicators of nutrient enrichment.
METHODS FOR
ASSESSING NUTRIENT
ENRICHMENT
A ssessing wetland to detect nutrient enrich-
-Zl ment is different from assessing it for biologi-
cal integrity. The goals of an Index of Biological
Integrity (IBI) are to assess the health of the bi-
otic or living components of the ecosystem, us-
ing metrics such as species richness, and to re-
late biotic health to anthropogenic stressors, such
as land clearing, drainage, and runoff, that affect
the community undergoing study. The purpose
of this module, in contrast, is to identify vegeta-
tion-based indicators of a specific stressor—
eutrophication—whose effects are manifested as
altered ecological structure and function, espe-
cially energy flow and nutrient cycles. Despite
their differences, however, both the IBI and the
assessment for nutrient enrichment share a simi-
lar framework for achieving their respective
goals. For both types of assessments, it is im-
portant to follow the procedures outlined in Mod-
ule 6: Developing Metrics and IBIs. Like IBIs, as-
sessments for nutrient enrichment should (1) define
clear objectives, (2) classify wetlands into regional
classes, (3) carefully select reference sites and
sample sites, and (4) collect information on wet-
land characteristics (e.g., hydrology, wetland veg-
6
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etation, soils) and surrounding land use. In particu-
lar, landscape level and local disturbances that con-
tribute to nutrient enrichment should be identified.
Wetland assessment for eutrophication requires
collection of remotely sensed and field data. Aerial
and satellite photography are used to document
changes in aerial coverage and percent cover of
wetland vegetation as well as coarse changes in plant
community composition over time. Remotely
sensed data are obtained from a variety of govern-
ment agencies, including NASA; USDA-NRCS;
State highway departments; and local tax, planning,
and zoning offices. Information for obtaining these
data are found in Module 17: Land-Use Charac-
terization for Nutrient and Sediment Risk Assess-
ment.
Field data consist of Level I and Level II indica-
tors of nutrient enrichment that are based on at-
tributes of wetland ecosystem function and struc-
ture.
• Level I indicators (stem height, plant tissue N
and P) are relatively easy to measure.
• Level II indicators (aboveground biomass, nu-
trient use efficiency, presence/absence of nu-
trient-tolerant and intolerant species) require
greater effort, but better characterize the re-
sponse of specific structural and functional at-
tributes to nutrient enrichment.
FIELD SAMPLING
In addition to remotely sensed data, field and
laboratory measurements are needed to assess
changes in wetland structure and function that oc-
cur in response to nutrient enrichment. The best
way to document change in structure and function
is by monitoring the site over time, before and dur-
ing the eutrophication process. This opportunity
rarely occurs because, in most cases, wetland
eutrophication began decades ago following wide-
spread application of inorganic fertilizers.
A widely used approach to assess anthropogenic
impacts is to compare the ecological integrity of
potentially enriched wetlands with unenriched "ref-
erence" wetlands. For example, unenriched areas
of the southern Everglades have been used as a
reference in documenting the effects of enrichment
in areas of the northern Everglades that receive ag-
ricultural drainage (Davis 1989, 1991, Craft and
Richardson 1993,1998, Reddy etal. 1993, Quails
and Richardson 1995). The ideal reference wet-
land is one that is relatively undisturbed and pos-
sesses the same abiotic template, except for nutri-
ents, as the enriched wetland. When using refer-
ence wetlands, a key assumption is that the refer-
ence site contains the same assemblage of biota that
enriched wetland contained prior to eutrophication.
Usually, reference wetlands are selected from the
same watershed or from a watershed nearby so
that both enriched and reference wetlands possess
the same abiotic template of climate, geomorphol-
ogy, geology, hydrology, and soil type.
It is nearly impossible to find wetlands that have
been unaffected by at least some human activity.
Wetlands that are minimally disturbed usually
represent the best approximation. Sampling more
than one reference wetland is highly recom-
mended to fully characterize the natural variabil-
ity among a particular wetland type and to mini-
mize the effects of human disturbance inherent in
one or more reference sites.
Protocols for field sampling should be designed
to capture the spatial and temporal variability in-
herent in both candidate and reference wetlands. It
is critical that the same experimental design, fre-
quency of sampling, field measurements, and labo-
ratory methods be used for both nutrient-enriched
and reference wetlands. Because hydrology exerts
7
-------
a controlling influence on wetland structure and func-
tion, a stratified sampling approach is needed to
encompass the spatial variation in inundation pat-
terns. Sampling is stratified into deep-, mid-, and
shallow-water zones or from more to less frequently
flooded areas (Figures 1 and 2) (see also Module
6: Developing Metrics and IBIs). Using hydrology
to stratify sampling captures the distinct patterns of
zonation of plant and animal communities that often
are observed in wetlands. Deep-water zones, which
are in contact with water (and nutrients) longer than
shallow-water zones, may respond to enrichment
relatively quickly compared with higher elevations
that are flooded less frequently.
In some wetlands, the presence of inundation
or soil saturation occurs only for short periods
during the growing season. When sampling these
wetlands, it may be necessary to design a sam-
pling plan that encompasses the temporal vari-
ability in inundation that affects seasonal changes
in plant and animal communities.
Selection of sampling locations depends on the
size and habitat complexity of the wetland. For
small-sized wetlands or surveys of many wet-
lands, sampling is stratified according to habitat
complexity or water-level depth. In depressional
or shoreline wetlands, samples and measurements
are taken near the center (deepwater), middle,
and edge (shallow) of the wetland (see Figure
2). In floodplain wetlands, sampling is strati-
fied based on habitat complexity with samples
collected from the levee, oxbow, and terrace
habitats (see Figure 1). Large wetlands or com-
prehensive studies require greater sampling ef-
fort. Comprehensive assessments require repli-
cate sampling points within a given location or
habitat and repeated measurements at a given
location to accurately assess changes in nutrient
indicators over time.
Collection of vegetation for nutrient and carbon
analysis requires careful selection of leaf samples.
When comparing nutrient-enriched and reference
wetlands, it is important that the same species be
sampled in each wetland because different species
possess different amounts of N and P in their leaves
(Craft et al. 1995, Shaver et al. 1998, Aerts et al.
1999). Herbaceous species preferably are sampled
(unless it is a forested wetland) because they grow
faster and respond to enrichment faster than woody
species. The two or three dominant species based
on percent cover or biomass are sampled. It also
is important to collect similar-aged green leaves from
each site, as young leaves usually have higher N
and P content than older leaves (Schlesinger 1991).
In the case of herbaceous vegetation, similar-aged
leaves are selected by clipping leaves from nodes
of a similar distance below the terminal bud. From
woody vegetation, green leaves are selected simi-
larly by sampling a fixed number of nodes (or
branches) below the terminal bud. Replicate leaf
samples are collected from several individual plants
to encompass the variability in leaf N and P within
populations.
A "flow" diagram describing steps for field
sampling of wetland vegetation is provided in
Figure 3.
ANALYTICAL METHODS
Different methods are required to measure NPP
in herbaceous as opposed to woody vegetation.
For herbaceous vegetation, stem height and bio-
mass are used to measure NPP. Biomass is de-
termined by end-of-season harvest of aboveground
plant material in small (0.25 m2) plots (Broome et
al. 1986). The stem height of individuals of domi-
nant species is measured in each plot. The height
of the 5 to 10 tallest stems in each plot has been
shown to be a reliable indicator of NPP (Broome
8
-------
River Channel
Oxbow
Second Terrace
Upland
river birch (Betula nigra) American elm (Ulmus americana) bald cypress willow oak (Quercusphellos) loblloly pine
cottonwood (Populus deltoides) sweetgum (Liquidambar styraciflua) axo lum ironwood (Carpinus caroliniana)
bald cypress (Taxodium distichum) sycamore (Platanus occidentalis) $weetgam(Liquidambai
white oak
(Nyssa aquatica)
(Quercus alba)
FIGURE l: DISTRIBUTION OF PLANT SPECIES AND HYDROPERIOD ACROSS
A SOUTHEASTERN BOTTOMLAND FORESTED WETLAND.
Rarely Flooded
grasses
Occasionally Flooded
Emergents
sedges (Carex)
arrowhead (Saggitaria
spp.)
Frequently Flooded
Emergents
Continuously Flooded
Floating Aquatics
cattail (Typha spp.) water lily (Nymphaea spp.)
bladderwort (Utricularia
spp.)
FIGURE 2: DISTRIBUTION OF PLANT SPECIES AND HYDROPERIOD
ACROSS A FRESHWATER MARSH WETLAND.
9
-------
Identify "Impacted" and
Reference Wetlands
V
Within Each Wetland, Stratify
Sampling by Plant Community
or Vegetation Zone. Try to
Identify Vegetation Zones
Common to Each Wetland
V
Within Each Zone, Sample Species/Life
Forms That are Common to Both
"Targeted" and Reference Wetlands
V
For Nutrient Analysis, Clip Similar Age
Leaves From Several Species in Each
Zone. Collect Leaves from 5-10
Individuals of Each Species.
FIGURE 3: PROTOCOL FOR SAMPLING VEGETATION FOR WETLAND
EUTROPHICATION ASSESSMENTS.
10
-------
et al. 1986) and one that saves the time it takes to
measure the height of all stems in a plot.
Aboveground biomass is clipped at the end of the
growing season, in late summer or fall. If vegeta-
tion is dense, 0.25 m2 plots are sufficient for clip-
ping. Clipped material is separated into live (biom-
ass) versus dead material then dried at 70°C to a
constant weight. For stem height and biomass sam-
pling, 5 to 10 plots per vegetation zone are col-
lected.
For woody vegetation, litterfall is the best tech-
nique for measuring NPP Litterfall is measured by
collecting leaf litter that falls into 0.25 m2 screen or
mesh traps placed on the wetland surface (Chapman
1986). Collections are made periodically (every
1 -2 mos.) throughout the year, although collection
during peak litterfall season (Sept-Dec.) may be
adequate for some assessments.
Leaves are collected and analyzed for N, P,
and organic C. Leaf analyses are performed on
samples that are dried at 70°C. Nitrogen and
organic C are measured by dry combustion us-
ing a CHN analyzer. Phosphorus is measured by
spectrophotometry in acid (H2SO4-H2O2) digests
(Allen et al. 1986). Many land-grant universi-
ties, state agricultural testing laboratories, and
environmental consulting laboratories perform
these analyses. Contact your local USDA office
or land-grant agricultural extension office for in-
formation on laboratories that perform plant tis-
sue nutrient analyses.
Nutrient resorption efficiency, resorption pro-
ficiency, nutrient use efficiency, and C:N:P are
calculated from the C, N, and P concentrations
measured previously. Resorption proficiency
and RE require that N and P are analyzed for
both senesced and green tissue (Killingbeck
1996, Aerts etal. 1999). Nutrient use efficiency
requires measurements of productivity (litterfall,
aboveground biomass) and leaf N and P (Vitousek
1982).
For emergent vegetation, community-level indi-
cators (nutrient-tolerant and intolerant species) are
measured in larger plots (2-10 m2), or by estimat-
ing percent cover of each species using plot or plot-
less sampling techniques (Mueller-Dombois and
Ellenberg 1974). The use of community-level indi-
cators with woody vegetation requires larger plots
(0.1-1 ha) or longer transects (Mueller-Dombois
and Ellenberg 1974).
MINIMUM MONITORING REQUIREMENTS
Minimum monitoring requirements to assess
nutrient enrichment of wetlands consist of (1) aerial/
satellite photography of the wetland and (2) field-
based measurements of Level I indicators of en-
richment. Level I indicators describe attributes of
wetland structure (stem height) and function (stem
height, leaf C, N, P and C:N:P ratios) that are rela-
tively easy to measure (Table 1).
Remotely sensed data are used to assess coarse
changes in wetland communities over time. Field
measurements such as stem height and leaf N and
P are useful because they respond to nutrient en-
richment relatively quickly (Craft et al. 1995,
Chiang et al. 2000). Herbaceous vegetation is a
better indicator of nutrient enrichment than woody
vegetation because herbaceous plants complete
their life cycle in less time and, thus, respond
more quickly to enrichment.
CASE STUDIES
Case studies using the Florida Everglades,
Atlantic coast estuarine salt marshes, and
Alaskan wet sedge tundra are presented below to
describe the response of vegetation-based indica-
tors to nutrient enrichment. The response of Ever-
glades plant communities to N and P additions is an
example of phosphorus limitation and eutrophica-
tion. The response of the estuarine salt marshes,
1 1
-------
TABLE l: LEVEL I AND LEVEL II INDICATORS OF NUTRIENT
ENRICHMENT IN WETLANDS
MEASUREMENT
EASE OF USE
METHOD
Level I
Functional Indicators
Stem height (E) '• 2
Leaf C and N, C:N
LeafP,C:PN:P
Easy
Moderate
Moderate
Clip plots
CHN analyzer
Acid digestion & spectrophotometer
Structural Indicators
Coarse change in wetland plant communities
over time
Easy
Aerial & satellite photography
Level II
Functional Indicators
Aboveground biomass (E)
Standing dead biomass (E)
Litterfall (W)1
SenescedleafQKQN3
SenescedleafP,C:PN:P3
N & P resorption efficiency
Nutrient use efficiency
Moderate
Moderate
Difficult
Moderate
Moderate
—
—
Clip plots
Clip plots
Litter traps
CHN analyzer
Acid digestion & spectrophotometer
Calculated
Calculated
Structural Indicators
# of nutrient tolerant species 4
% of nutrient tolerant species 4
# of nutrient intolerant species 4
% of nutrient intolerant species 4
Moderate
Moderate
Moderate
Moderate
Plot sampling
Plot sampling
Plot sampling
Plot sampling
1 E = emergent vegetation, W = woody vegetation.
2 Structural indicator also.
3 Resorption proficiency.
4 Reliable nutrient-tolerant and -intolerant species for most wetlands and geographic regions have not been identified yet.
Note: Level I indicators represent the minimum requirements for assessing enrichment.
12
-------
which are N-limited, is an example of wetland re-
sponse to N enrichment. The response of wet sedge
tundra to nutrient enrichment is an example of co-
limitation by N and P. In the tundra wetland, veg-
etation-based indicators of nutrient enrichment show
a response to both N and P additions.
water and nutrients, both N and P. From among
these data, it was difficult to separate the effects of
nutrients from the hydroperiod, and to determine
which responses were due to N versus P. Thus the
causative agent of the observed differences was not
pinpointed.
EVERGLADES
During the past 10 years, the Florida Everglades
has been the "poster child" for wetland eutrophica-
tion. Numerous studies documenting the effects of
N- and P-enriched agricultural drainage on Ever-
glades community structure and ecosystem pro-
cesses have been published (Davis 1989, 1991,
Craft and Richardson 1993a, 1998, Craft et al.
1995,Reddyetal. 1993, Urban etal. 1993). Eco-
logical changes attributed to nutrient enrichment in-
clude increased NPP, tissue P uptake, decomposi-
tion, peat accretion, and nutrient accumulation as
well as cattail encroachment into sawgrass and
slough communities (Craft and Richardson 1998,
Quails and Richardson 2000). Functional indica-
tors of enrichment including abovegroundbiomass,
stem height, leaf N (standing dead only), and leaf P
were higher in eutrophic areas compared with
unenriched areas (Table 2). Stem density did not
differ between eutrophic (40/m2) and unenriched
sites (38/m2) (Miao and Sklar 1999).
Community-level indicators also responded to in-
creased nutrient loadings. Nutrient- tolerant spe-
cies including cattail, duckweed (Lemna), and other
species were abundant in eutrophic areas but infre-
quent in unenriched areas (Table 2), (Craft and
Richardson 1997, Vaithiyanathan and Richardson
1999). Nutrient-intolerant species (e.g., Utricu-
laria spp. and other species) were abundant in
unenriched areas but absent from eutrophic areas
(Table 2) (Vaithiyanathan and Richardson 1999).
The findings presented above were based on
observational data collected from areas of the north-
ern Everglades that receive enormous amounts of
In 1990, a field experiment was initiated to in-
vestigate the effects of N versus P on native Ev-
erglades plant communities (Craft et al. 1995,
Chiang et al. 2000). The experiment applied
controlled amounts of N, P, and N+P to plots in
an area of the Everglades unaffected by agricul-
tural water and nutrient loadings. During the first
year of nutrient additions, it became apparent that
P, not N, was the limiting nutrient and, thus, was
responsible for the changes in wetland structure
and function observed in eutrophic areas. After
2 years of P additions, many functional indica-
tors responded to increased P, including in-
creased aboveground biomass, standing dead
material, stem height, and leaf P (Table 3). Leaf
C:P, N:P, resorption efficiency, and NUE (P)
decreased in response to P additions (Table 3).
Sentinel species that reflected low nutrient re-
gimes also responded to P. Utricularia, a float-
ing aquatic plant, declined in response to P and
eventually was replaced by the macroalgae, Chora.
During the 4-year period, no change in species rich-
ness or cattail/duckweed encroachment was ob-
served, although other emergents like leather fern
(Acrostichum danaeifolium) increased in response
to P (Table 3). More leaf P and less Utricularia
indicated incipient P enrichment during the first year
of P additions. The response of aboveground bio-
mass to P was not statistically detectable until the
end of the second growing season. Under
unenriched conditions, sawgrass was highly profi-
cient at resorbing P based on standing dead P (70
|ig/g) concentrations that were less than 500 |ig/g P
as suggested by Killingbeck (1996). There was no
response of vegetation to N additions (data not
shown), indicating that P, notN, limits productivity
and leads to eutrophication in this wetland system.
13
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TABLE 2: COMPARISON OF VEGETATION-BASED INDICATORS OF PHOSPHORUS (P)
ENRICHMENT IN UNENRICHED AND EUTROPHIC SAWGRASS, CLADIUM JAMAICENSE,
COMMUNITIES OF THE FLORIDA EVERGLADES
INDICATOR
UNENRICHED
EUTROPHIC
Functional Indicators
Stem height (cm)1
Aboveground biomass (g/m2)1
Stem density (number/m2)1
160
976
38
205
1958
40
Leaf N (o/o)1
LeafPtug/g)1
LeafNiPtwtwt)1
0.70
250
28
0.70
650
11
Standing dead N (%)2
Standing dead P (ug/g)2
Standing dead N:P (wt:wt)2
0.40
80
38
0.56
375
15
Structural Indicators
Course change in wetland plant
communities over time3
No change
Decreased sawgrass, increased cattail
Nutrient-intolerant species
Utricularia sff.4
Abundant
Absent
Nutrient-tolerant species
Typha>'s
LemnO4
Acrostichum danaeifolium4
Infrequent
Absent
Infrequent
Abundant
Abundant
Common
1 From Miao and Sklar (1999)
2 From Davis (1991)
3 Jensen et al. (1995)
4 From Vaithiyanathan and Richardson (1999)
5 From Craft and Richardson (1997)
Note: Numerical values in bold reflect statistically significant differences (p>0.05) between unenriched and
eutrophic conditions.
14
-------
TABLE 3: LEVEL I AND LEVEL II INDICATORS OF PHOSPHORUS (P) ENRICHMENT
OF SAWGRASS, CLADIUM JAMAICENSE, COMMUNITIES IN THE
FLORIDA EVERGLADES
INDICATOR
RESPONSE
Unfertilized
(noP)
Fertilized
(4.8 g P/mYyr)
RESPONSE TIME
Level I
Stem height1
LeafP(ug/g)
LeafC:P(wt:wt)
Leaf N:P (wtwt)2
—
210
2100
29
—
530
840
12
1-2 years
-------
Results from the Everglades experiment indicate
that (1) reliable vegetation-based indicators of P
enrichment exist and (2) some indicators (e.g., leaf
P, Utricularid) respond more rapidly to increased
P than others. The rapid and consistent responses
of leaf P, NPP, and Utricularia suggest that these
indicators are reliable for monitoring conditions in
the Everglades. Other indicators, like cattail and
duckweed encroachment, were not observed dur-
ing the first 4 years of the experiment. The absence
of cattail encroachment into the fertilized plots prob-
ably reflects the fact that competitive displacement
is a time-dependent process (Mai etal. 1997). A
shift in emergent species composition in the fertil-
ized plots might take longer than the 4-year period
of record of the experiment. Furthermore, the re-
ported increase of cattail and duckweed in eutrophic
areas of the Everglades may be the result of in-
creased water depth in addition to P enrichment
(Urban etal. 1993).
ESTUARINE SALT MARSHES
Like Everglades vegetation, salt marsh vegeta-
tion responded quickly when the primary limit-
ing nutrient, in this case N, was added. Leaf N
and aboveground biomass of Spartina
alterniflora, the dominant species in east coast
salt marshes, increased during the first year of N
additions (Table 4). Leaf C:N, standing dead N,
and NUE_n also increased relative to unfertil-
(N)
ized plots during the first year of N fertilization
(Table 4). Stem height, a Level I indicator, also
increased in response to N additions, although
the response was not significantly different from
unfertilized plots. Because nitrogen is a compo-
nent of chlorophyll, where photosynthesis takes
place, N additions quickly translate into increased
aboveground biomass. Many studies report that
leaf N and aboveground biomass increase within a
few months after N is added (Valiela and Teal 1974,
Broomeetal. 1975, Chalmers 1979). Additions
of N sometimes produce more flowering stems of
Spartina whereas P additions increase the number
of flowering stems even more (Broome et al. 1975).
WET SEDGE TUNDRA
Nutrient additions to wet sedge tundra commu-
nities in Alaska revealed that tundra vegetation
responded to additions of either N, P, or N+P
(Shaver et al. 1998) (Table 5). After 5 years of
fertilization, leaf N and aboveground biomass
increased in response to N additions, whereas
C:N and N:P decreased in N-treated plots. The
effect of P additions on functional indicators of
enrichment was even more pronounced than with
N. Leaf P and aboveground biomass increased
dramatically in response to P additions whereas
leaf C:P and N:P decreased in P-treated plots
(Table 5). The plant response to N+P additions
was greater as compared with either N or P ap-
plied singly, indicating that tundra communities
are limited primarily by P and secondarily by N
(Shaver etal. 1998). In a separate fertilization ex-
periment, Shaver and Chapin (1995) added N, P,
and K to moist tussock and wet sedge vegetation
in the Alaskan tundra. Similar to the Everglades
case study, leaf N and P increased following the
first year of nutrient additions. Increased plant
growth and biomass production was not observed
until year 2 of the study and, in year 3, flowering
increased in response to fertilizer additions.
In salt marshes and amid tundra vegetation, leaf
N concentrations declined after the first year of
N fertilization, as N was "diluted" by enhanced
biomass production (Valiela and Teal 1974,
Shaver and Chapin 1995). However, loss of sen-
sitivity of this Level I indicator was offset by
increased NPP, aboveground biomass, and stem
height. In contrast to N, in ecosystems where P
was limiting or co-limiting, leaf P concentrations
remained elevated even as NPP increased in re-
sponse to P enrichment (Shaver and Chapin 1995,
Chiang etal. 2000).
16
-------
TABLE 4: LEVEL I AND LEVEL II INDICATORS OF NITROGEN (N) ENRICHMENT
OF SPARTINA ALTERNIFLORA DOMINATED SALT MARSHES ALONG THE
ATLANTIC (NC, GA) COAST
INDICATOR
RESPONSE
Unfertilized
+N
RESPONSE TIME
Level I
Stem height (cm)1
LeafNC/o)1
LeafCNtwtrwt)1'2
77
0.82
49
103
1.05
38
<1 year
<1 year
<1 year
Level II
Aboveground biomass (g/m2)1
Standing dead N (%)3'4
NUEN
360
0.90
111
600
1.05
95
<1 year
<1 year
<1 year
1 From Broome et al (1975), N added as ammonium sulfate at a rate of 16.2 g N/m 2/yr.
2 Leaf C is assumed to be 40%.
3 From Chalmers (1979), N added as sewage sludge at a rate of 2 g N/m 2/wk.
4 Same as Resorption Proficiency.
Notes: In this study (Broome et al. 1975), N was the primary limiting nutrient and P (data not shown) was
secondarily limiting. Numerical values in bold indicate that phosphorus-fertilized and -unfertilized plots were
statistically different (p<0.05) from each other.
The results of the Everglades, salt marsh, and tun-
dra fertilization studies demonstrate that increased
leaf nutrient (N, P) content is a powerful indicator
of eutrophication because it is among the first to
respond to nutrient enrichment. Level I (stem height)
and II (aboveground biomass) metrics of NPP like-
wise respond relatively quickly to nutrient enrich-
ment. They, too, are useful detectors of incipient
eutrophication of wetlands.
17
-------
TABLE 5: LEVEL I AND LEVEL II INDICATORS OF NITROGEN (N) AND
PHOSPHORUS (P) ENRICHMENT OF A WET SEDGE TUNDRA (ERIOPHORUM
ANGUSTIFOLIUM) IN ALASKA AFTER 5 YEARS OF FERTILIZATION
INDICATOR
RESPONSE
Unfertilized
(no N or P)
+N (10 g/m2/yr)
+P (5 g/m2/yr)
N+P
RESPONSE
TIME'
Level I
Leaf N (%)
Leaf P (ug/g)
1.48 a
830 a
1.78 b
1120 b
1.46 a
2660 b
1.69 b
3870 b
<1 yr
<1 yr
Leaf C:N (wt:wt)2
Leaf C:P (wt:wt)2
Leaf N:P (wt:wt)2'3
27.0
482
17.8
22.5
357
15.9
27.4
150
5.5
23.7
103
4.4
<1 yr
<1 yr
<1 yr
Level II
Aboveground biomass (g/m2)
142 a
195 b
283 c
404 d
1-2 yr
1 Response time was based on an earlier study by Shaver and Chapin (1995).
2 No statistical analyses were performed on these parameters.
3 Leaf C was assumed to be 40%.
Notes: In this study, indicators of nutrient enrichment responded to additions of either N, P, or N+P, indicating that both N
and P limit plant productivity of this community. Numerical values within the same row separated by the same letter were not
statistically different (p<0.05) from each other.
Source: Data are from Shaver et al. 1998.
18
-------
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GLOSSARY
Ecological integrity The capacity of an ecosys-
tem to sustain essential life support services such as
energy flow, biogeochemical cycling, niche space,
and habitat.
Functional indicators Attributes that describe
ecosystem function, like energy flow (e.g., pro-
ductivity) and biogeochemical (e.g., nitrogen,
phosphorus) cycling.
Nutrient resorption efficiency (r) Amount of
nutrients (e.g., N or P) resorbed from mature
leaves divided by maximum nutrient pool in ma-
ture leaves (expressed as g/m2).
Nutrient resorption proficiency The absolute
or lowest levels to which nutrient concentrations are
reduced in senesced (dead) leaves.
Nutrient use efficiency (NUE) Aboveground
biomass production (e.g., g litterfall/m2) divided
by quantity of nutrient (e.g., g N/m2) in litterfall.
Structural indicators Attributes that describe
community-level characteristics of ecosystems
like species richness, species diversity, and
canopy architecture (e.g., stem height, vertical
stratification).
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