United States
 Environmental Protection
 Environmental Research
 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

  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).

  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
  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-
  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

  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

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
  '  (2)
Figure 1.     General process model for heavy metals.

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
  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.

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
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-
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
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.

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
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