United States
Environmental Protection
Agency
Environmental Research
Laboratory
Gulf Breeze FL 32561
Research and Development
EPA-600/S4-81-022 Aug. 1981
Project Summary
Water Quality and Mangrove
Ecosystem Dynamics
Samuel C. Snedaker and Melvin S. Brown
Many of the ecological and physio-
logical mechanisms that permit the
mangrove ecosystem to thrive in
intertidal coastal environments that
are influenced by freshwater inflow
from upland areas are poorly under-
stood. This report describes key mech-
anisms that permit development of
the mangrove ecosystem in certain
coastal areas and considers flow vari-
ables of pollutants entrained in water
circulating within the forest.
Data from Florida and Puerto Rico
were combined with data from studies
done by the author in other countries.
It was concluded that poorest forest
development occurs in arid climates
with relatively little ground water or
freshwater inflow, and in sedimentary
carbonate environments. Highly de-
veloped forests occur where tidal
amplitude and fresh water inflow
assure frequent and extensive inunda-
tion and flushing. Synthetic organic
pesticides were never found in water,
sediment, or plant tissues, but low to
moderate concentrations of heavy
metals were ubiquitous. Heavy metals
were concentrated in sediments and
plant tissues. Highest concentrations
of metals were associated with fossil-
fuel burning power plants, agriculture,
and highway runoff.
It was shown that nitrate and sulfate
are key components of water and
sediments required for development
of mangrove. Nitrate may be most
important as an oxidant in anaerobic
decomposition of reduced organic
matter accompanied by release of
nutrients to the rhizosphere and the
formation of ammonia. Sulfate appears
to act as an oxidant that penetrates
deeply into anaerobic sediments during
flushing. It can also combine with
metals, making them unavailable for
uptake by mangroves.
A model for the pathway, storage,
uptake, and turnover rates of copper is
given as an example of the dynamics
of a pollutant as it relates to the
dynamics of a mangrove ecosystem.
This Project Summary was devel-
oped by EPA's Environmental Research
Laboratory, Gulf Breeze. FL, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Mangrove forests are extensive and
important ecosystems in the intertidal
zone of the Gulf of Mexico and the
Caribbean Sea. They are very productive
systems composed of plants and animals
adapted to life along the shore, and they
export large amounts of detritus that
help support other alongshore and
offshore ecosystems. Productivity and
other ecological and physiological proc-
esses of the mangrove ecosystem are
closely related to physiognomy, topo-
graphy, frequency of inundation, circu-
lation patterns and water quality of an
area. Consideration of these features
has led to identification of seven types of
forest. In decreasing order of productivity,
these types are: riverine, fringe, over-
wash, hammock, basin (flushed), dwarf,
and basin (impounded).
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From surveys of mangrove stands in
the United States, Puerto Rico, Mexico
Costa Rica, Australia, Pakistan, Ban-
gladesh, and Thailand, it was found that
mangrove ecosystem dynamics could
not be related to current theory. Vigorous
growth was found in what was thought
to be nutritionally poor environments,
and mangroves showing severe growth
restrictions were found in ostensibly
optimal environments. Although char-
acter of the substratum was important,
it appeared that water quality, as it
related to the reduction of allochthonous/
autochthonous organic matter, precipi-
tation of carbonates, and mass transport
of clastic materials, also caused local
variations in mangrove structure and
function.
Water quality in its largest context
has never previously been evaluated
with respect to the dynanics of mangrove
function. This project attempted to fill
the apparent gap in knowledge of water
quality and mangroves, and to incorpo-
rate considerations of the role of water-
borne pollutants in the mangrove eco-
system.
The purposes of this project were to
define the empirical relationship that
exists between water quality and man-
grove ecosystem dynamics, and to
evaluate that relationship as a two-way
interaction. The many ramifications of
this overall purpose are detailed below
as specific working objectives.
Objective 1: Use the existing literature
and information on man-
groves to develop the hy-
pothetical relationship
between water quality
and mangrove dynamics
as an overall guiding
hypothesis for the specific
research tasks.
Objective 2: Define the quality of
those waters associated
with the highest quality
mangrove ecosystems.
and, conversely, the
poorest quality mangroves.
Objective 3: Evaluate the fates of
selected organic and
metal-based toxic mate-
rial within the mangrove
ecosystem, and report the
concentrations of such
pollutants in mangrove
ecosystems in relation to
water quality.
Objective 4: Select and evaluate a key
parameter of mangrove
ecosystem dynamics that
can be related in an em-
pirical manner and used
as an index relative to
water quality and the
potential productivity of
the environment.
Objective 5: Identify and evaluate the
most critical factor or
factors associated with,
and contributing to, water
quality that have the
greatest influence on the
dynamics of the mangrove
ecosystem.
Conclusions
Objective 1. Sufficient information
exists within the literature to assemble
a variety of conceptual models portraying
the general structure and functioning of
the mangrove ecosystem or specific
processes therein. Eighteen such models
have been developed and reported, from
which a general model was constructed
to guide the research on this project. In
contrast to the rather complete qualita-
tive knowledge of the structure and
functioning of the mangrove ecosystem,
reliable, quantitative data are almost
non-existent. Such data are also poorly
documented and are expressed in units
which make their incorporation into a
model very difficult. In this limited hard
data pool, more quantitative information
exists on state variables (structural
features) than on flow variables (time-
dependant functions expressed as rates).
The temperate salt-marsh literature
contains significantly more hard data of
the type useful to the understanding of
the ecosystem; but it too, is deficient in
flow variables. Specifically, in the sub-
tropical portion of the United States,
there exist little data from which con-
clusions can be drawn to develop a
quantitative understanding of the man-
grove ecosystem and the consequences
of water quality changes, or pollution,
therein. An example of the data defi-
ciency is apparent in the parameteriza-
tion of the element copper.
Objective 2. Overall, the structure of
all mangrove forests studied, and per-
ceptions of their functioning, are re-
markably uniform despite large differ-
ences, particularly in water quality. The
poorest developed structures (low stand
density, short stature of mature trees,
relatively open canopy and absence of
surface leaf litter), and therefore inferred,
poorest dynamics (low rate of community
metabolism and specifically, a low rate
of net primary productivity), are consist-
ently found only in arid climates (low
rainfall), environments with insufficient
ground water or fresh surface water,
and in sedimentary carbonate environ-
ments. The best (in the sense of high
density of individuals, tall stature,
closed canopy, and conspicuous leaf
litter suggestive of a high rate of net
primary productivity) mangrove forests
tend to be found where there are mod-
erate soil salinities due to the availability
of fresh water and to tidal amplitude that
ensures frequent and extensive inunda-
tion and flushing. Marginal environ-
ments are those with either uniformly
high or low annual salinity regimes,
exposure to excessive silt loading,
and/or in areas in which the tidal
amplitude is normally small or has been
attenuated by natural or man-induced
forces. In the marginal and poor quality
environments, vigorous stands of man-
groves, nevertheless, can be observed
in association with anaerobic organic
soils, or underlying peat bodies. In
general, mangroves appear to be re-
markably tolerant of a wide range of
water quality conditions, as if water
quality were not a controlling factor.
Certainly, a review of the literature now
demonstrates that mangroves are ba-
sically freshwater plant forms that
possess a unique ability to tolerate salt
better than other plant species. In this
regard, normal salinity regimes are the
factors which prevent invasion by, and
competition from, freshwater species,
thus allowing mangroves to maintain
competitive dominance in the intertidal
zone. One key aspect of water quality
management in this environment is the
maintenance of salinity and tidal flush-
ing patterns to perpetuate the domina-
tion and high productivity of mangroves.
Objective 3. Samples of water, sedi-
ment, and mangrove tissues were anal-
yzed for ten synthetic organic com-
pounds (aldrin, dieldrin, DDT, DDE,
ODD, lindane, heptachlor, mirex, para-
thion, and PCB's). The compounds were
not detected in the 180 samples collected
from 18 stations in southern Florida and
9 stations in Puerto Rico. Unknown
compounds in certain groups of the
samples were subsequently identified
as the active ingredients in a commercial
insect repellent used by the field crew.
The ability to detect traces of the con-
taminant, but not the synthetic organic
compounds of interest, suggests that
they are not present in any detectable
quantity in the 27 mangrove areas
sampled. As a result, this phase of the
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investigation was concluded, the em-
phasis shifted to heavy metals, and a
general process model for heavy metals
was developed (Fig. 1}.
Low to moderate concentrations of
metals appear to be ubiquitous compo-
nents of the mangrove study areas in
southern Florida and Puerto Rico. Com-
pared to the concentration of metals in
local waters, metals appear to be con-
centrated several orders of magnitude
in mangrove sediments and up to six to
seven orders of magnitude more con-
centrated in mangrove tissues. With
respect to the general environmental
concentrations of metals in the man-
grove environment, the observed varia-
tions reflect the geochemistry of the
regional watershed. For example, metals
in general are higher in concentration in
Puerto Rico than in southern Florida,
where there are no geologic sources for
metals in drainage and leachate water
entering the coastal zone. Highest
concentrations of metals in Florida
mangroves can be associated with
fossil-fuel burning power plants, agri-
cultural usage, and highway runoff.
Despite the magnitude of biological
concentration, no evidence was found
to suggest that the absolute levels
constituted a toxic hazard to the health
of mangroves. However, the appearance
of biologically concentrated metals in
the leaf litter destined to become part of
detrital foodwebs raises a question
concerning dose rates and body burdens
in nearshore ma/me animals. Although
the greatest concentrations of metals in
the physical environment were found in
the sediments, no evidence was obtained
concerning whether mangroves take up
metals from the sediment versus the
ambient water. It is likely that the metals
are sequestered as sulfides in the
anaerobic environment in which case
they are unavailable for uptake so long
as salinity, pH and redox potential
remain constant. Water quality manage-
ment again emphasizes the maintenance
of site-specific salinity regimes (through
normal mixing of fresh and marine
waters) and temporal and spatial patterns
of tidal inundation.
Objective 4. Width: length ratios, and
leaf litter production were evaluated as
key indices of overall mangrove dyna-
mics associated with environmental
and water quality conditions. The com-
plexity index proved highly useful in
comparative studies of mangrove areas
of the western hemisphere, but it was
judged unsuitable for the purposes of
this project. Mangrove leaf measure-
ments were made at 34 sites in Mexico,
Florida, Haiti, and Puerto Rico, using an
average of 139 sun leaves otRhizophora
mangle from each site. Although there
was a great variation in size, e.g., 14.7
cm to 6.4 cm in length and 8.9 to 3.0 cm
in width, the length:width ratio always
approximated 2.1.1. The cause of the
variation in absolute size is not under-
stood, but it is believed to reflect both
population isolation (mangroves of the
Weathering
' (2)
(P)
Figure 1. General process model for heavy metals.
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Pacific coast of Mexico had consistently
larger leaves than those of the Caribbean
area) and the variation in the local
climatic character of regional environ-
ments. Because the reason for leaf-size
differences could not be established
without a prohibitively expensive re-
sampling program, this index was deleted
from further consideration.
The index that proved to be most
reliable in reflecting general considera-
tions of the mangrove ecosystem is the
biweekly rate of leaf litter production
because it: (1) can serve as a calibrated
index of mangrove net productivity, (2)
reflects the broad characteristics of the
physical environment and integrates
both physical and biological measures,
and (3) appears to be a precise and
accurate measure of mangrove dynam-
ics. A leaf litter production record for 9
stations in southern Florida maintained
over a period ranging from 18 months to
6 years was evaluated in this project.
Based on this parameter, productivity
indices of seven forest types can be
ranked:
Forest Type
Litter Production
g/m2. year
Riverine 1120
Fringe 1032
Overwash 1024
Hammock 750
Basin (flushed) 741
Dwarf (scrub) 220
Basin (impounded) 0
The most pertinent interpretation of
the leaf litter production record arose
from the comparison of rates for fringe
forests in two contrasting environments:
one in southwestern Florida in a nutrient-
rich moderate salinity environment and
the second in southeastern Florida in a
relatively nutrient-poor high salinity
environment. The original hypothesis
stated that the former would show a
consistently higher rate of leaf litter
production than the latter. In fact, the
record showed the reverse was true and
led to the explanation given above of the
importance of sulphate reduction in
anaerobic subsurface peats.
Objectives. Water quality is associated
with structure and dynamics of man-
grove ecosystems although several of
the mechanisms remain poorly eluci-
dated. Best-developed structure was
associated with moderate salinity (i.e., a
source of fresh water to dilute the sea
water), water-borne nutrients, and
optimal tidal circulation and flushing. In
addition, two components of marine
water quality are suggested to be simi-
larly related and, in addition, provide a
basis for understanding the mechanisms
involved. These are nitrate and sulfate,
the latter of which is abundant in
marine water. Specifically, nitrate may
derive its greatest importance as an
oxidant involved in the anaerobic de-
composition of reduced organic matter,
accompanied by the release of nutrients
in rhizosphere and the creation of
ammonia. Likewise, sulfate may be
highly important, not only as a source of
elemental sulfur, but also as an oxidant
able to penetrate deeply into anaerobic
sediments during flushing sequences.
Like nitrate, sulfate is involved in the
anaerobic decomposition of organic
matter and in the formation of sulfides,
which can combine with metals render-
ing them unavailable for uptake by
mangroves. Irrespective of the precise
role of either compound, their positive
interaction in the mangrove environ-
ment depends on: (1) the availability of a
source, such as the sulfate in seawater,
(2) tidal action as the dominant mecha-
nism promoting mixing of fresh and salt
water, and inundation of the mangrove
environment, (3) a relatively permeable
substrate facilitating the exchange of
surface and interstitial water, and (4)
the presence of reduced organic matter
in the rhizosphere. (This biologically
mediated regeneration of nutrients
appears to be able to augment the
relatively low concentrations of primary
plant nutrients in marine waters.) In
general, it is these factors which serve
to maintain and perpetuate mangroves
over a very wide range of natural envi-
ronmental conditions and in instances
of low-level water pollution involving
either metals and/or synthetic organic
compounds. However, in this latter
regard, we continue to know little about
the role of mangroves as concentrating
and transfer agents relative to the
shunting of pollutants into estuarine
food webs.
Recommendations
1. Research on the mangrove environ-
ment will be most profitable if ori-
entation is on the functional relation-
ships of the ecosystem with full
quantification.
2. With respect to water quality in the
coastal zone relative to the natural
dynamics of the mangrove ecosystem,
there are two important aspects: a
normal pattern of mixing of fresh and
saltwater and periodic inundation of
the tidelands, and the entrained
solutes which either serve directly as
primary plant nutrients or facilitate
the in situ regeneration. In addition,
the salinity component controls the
distribution of species and preserves
the halophytic nature of the man-
grove coastal zone. Although these
aspects are generally known and
accepted, there is an absence of a
quantitative understanding which
could be used in the management of
water quality and the mangrove
community. Further research on
these mechanisms should yield prof-
itable new insights into water quality
and mangrove ecosystem dynamics
useful in management and conserva-
tion.
3. The apparent tolerance or resistance
of mangroves to water borne pollu-
tants should not be interpreted as
meaning that mangroves are immune
to their toxic effects; threshold con-
centrations need to be determined
and related to the acute and chronic
response by mangroves. More im-
portant, although mangroves may be
resistant, the associated fauna is
not. It is unknown to what extent the
biological concentration of metals by
mangroves and their transfer to
detrital foodwebs represent a poten-
tial danger to marine and estuarine
animals.
4. Further quantification with regard to
hydrology and chemistry of natural
waters in the coastal zone will greatly
affect regulation of man's activities
and the conservation of productivity
of the coastal environment.
4
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Samuel C. Snedaker and Melvin S. Brown are with the Rosenstiel School of
Marine and Atmospheric Science, University of Miami, Miami, FL 33149.
Gerald E. Walsh is the EPA Project Officer (see below).
The complete report, entitled '"Water Quality and Mangrove Ecosystem
Dynamics," (Order No. PBS 1-204 109; Cost: $9.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
Gulf Breeze, FL 32561
US.OOVEBNMENTnmmNOOFFICE. 1M1 -757-012/7240
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