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). ------- 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 ------- 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. ------- 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 ------- 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 ------- ------- United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Postage and Fees Paid Environmental Protection Agency EPA 335 Official Business Penalty for Private Use $300 PS 0000329 ------- |