INTRODUCTION Mangrove wetlands are the salt marshes of tropical and subtropical regions of the world. They are most commonly found along favorable coastlines between 25o N and 25o S latitude (Kuenzler, 1974). Since they lie at the interface of land and sea, mangrove wetlands, like salt marshes, display ranges in salinity from fresh to brackish to marine (and even to hypersaline in highly evaporative areas). Mangrove species dominate these ecotones because they have evolved several mechanisms that allow them to be successful under these highly variable salinity regimes (see Chapman, 1975; Clough, 1984). Even though they appear to do quite well in freshwater habitats where there is little competition, mangrove species are better adapted to a variable (freshwater-saltwater) or constant salt lifestyle. Salinity has even been suggested as a possible agent of vegetation zonation within mangrove wetlands (Bunt et al., 1982). As a result of this, in the recent past, salinity and salinity gradients have been considered the most important factor in determining the development of mangrove systems. However, a few have maintained a position that salinity may not be so important in mangrove zonation (Kjerfve, 1990) or development (Sternberg, in press).

Although many investigators have focused on salinity in coastal wetland zonation and development, fewer have looked at the two factors (geomorphology and hydrology) that underlie salinity and a multitude of other abiotic factors in coastal systems. Geomorphology is a physical/geological characterization of a system, and hydrology is a description of the dynamics of the water in the system. A system's geomorphology and hydrology are determined by a number of abiotic elements including: local geology, sea-level change, tide, freshwater inputs, shoreline structure, watershed morphometry, groundwater influence, natural disturbance regimes, and climate. Therefore, these two features not only govern the development of a system (Orson and Howes, 1992; Woodroffe, 1993; Kjerfve, 1990) they should also control rates of primary production (carbon fixation) and, as a result of this, the cycling of organic carbon. However, since these are aquatic systems, we would expect hydrology to be the most important.

Mangrove wetlands, in general, are considered to be highly productive systems. Nonetheless, there is much variability in these data among the various "types" of mangrove systems. For example, Lugo and Snedaker (1974) reported that different mangrove forest types in south Florida and Puerto Rico had gross primary productivity values anywhere from 1.4 to 13.9 g C m-2 day-1. More recently, Brown and Lugo (1982) reported GPP values (in various south Florida mangrove systems) of 2.8 to 24.0 g org. matter m-2 day-1. The discrepancies in these data are not a result of variation in time they are merely a function of the different mangrove forest types, not only found in Florida and Puerto Rico, but around the world. These differences in forest types are believed to be a result of the topographic gradients and hydrologic variation within a given mangrove system. In areas where topographic gradients are steep, geomorphology appears to have more of an impact on mangrove physiognomy (Davis, 1940). And, where these gradients are not as visible, local hydrology seems to determine physiognomy (Lugo and Snedaker, 1974).

This idea led to a classification scheme in south Florida (a relatively flat topography) that is commonly used throughout the neotropics (Lugo and Snedaker, 1974; Pool et al., 1977). This classification scheme separates mangrove systems on the basis of physical appearance (forest structure), hydrology, and productivity. Riverine forests, the most productive, are those that lie along river and creek channels. These forests are the largest in stature and experience a constant flow of water both in the dry season (through daily tidal activity) and wet season (from terrestrial runoff). Fringe forests are moderately productive intertidal mangrove wetlands that occupy protected shorelines and the mouths of channels. Fringing mangroves are known for their capacity to trap sediments from both marine and terrestrial sources. Overwash forests are subtidal to intertidal marine-dominated systems that have productivity values resembling fringe forests. This forest type is commonly found in the form of a small island that is constantly washed by tides. Basin forests are also moderately productive forests that are found in more inland areas. These mangroves are rarely inundated by tidal action nor terrestrial runoff. Dwarf (scrub) forests are the least productive. This forest type is dominated by a stunted form of mangrove (usually Rhizophora mangle). These wetlands, although long in hydroperiod, are low in both nutrients and hydrologic energy. Hammock mangrove wetlands appear as tree islands along fringing coastlines. They grow in depressions and have characteristics similar to basin and dwarf mangroves (Lugo and Snedaker, 1974). Recently, a few investigators have modified this into a three category scheme: riverine, fringing (including overwash), and basin (including dwarf and hammock) forests (Cintron et al., 1985). Although this scheme has pan-tropical applications and is open to modifications, its use has been confined to the New World. Old world mangrove systems (i.e. Australia, Africa, Malaysia, Thailand, etc.) tend to be classified on the basis of, geomorphic setting, mangrove zonation, and mangrove associations.

These zonation patterns and "associations" are thought to be an artifact of several unique ecosystem properties that mangroves possess. As individuals, mangrove species have a number of unique features (both morphologically and physiologically) that separate them from other flora. The summation of these features over a large spatial scale result in something that not only benefits the individual mangrove but also a number of other species that utilize these habitats. Mangroves offer suitable habitat to a wide variety of organisms including many species of vertebrates and invertebrates (see Odum and Heald, 1972). They have also been historically viewed as "land builders". Davis (1940) offered anecdotal as well as empirical evidence to support this claim. Mangrove communities usually have a complex network of prop roots and pneumatophores that slow water and trap sediments of marine, terrestrial-upland, and mangrove origination resulting in a net accretion of the marsh. However, recent studies have shown that this may not always be the case. Sediment type, seasonal variations in hydrology and natural disturbance regimes may be more important in terms of sedimentation (Wanless, 1974). Also, a more recent study by Wolanski et al. (1980) showed that the density of prop roots and pneumatophores in a tidally driven mangrove wetland was most important in determining whether the system was eroding or accreting. Finally, mangroves are also known for their sediment stabilization capacity. Sediments are not only trapped by these complex networks of aboveground root systems, they are also maintained and used to build upon.

In the past few decades, mangroves have been investigated as a source of energy to mangrove fauna and to adjacent coastal communities. After the Georgia salt marsh work of Odum and de la Cruz (1967), estuaries were viewed as a source of carbon to adjacent coastal systems. It was believed that carbon (in the form of detritus) was "outwelled" to coastal systems by way of tidal action. Prior to the pioneering work of Odum and Heald (1975A) and Odum et al. (1972), the importance of mangrove-derived detritus in both mangrove and adjacent coastal systems, however, was not well-understood. After investigating the feeding habits and gut contents of a number of different species in and around coastal mangrove wetlands, these investigators found that mangrove-derived detritus was an important part of the diets of numerous primary and even secondary consumers (Odum and Heald, 1975A & B). These organisms were then the source of food for higher consumers (many of which are endangered or commonly hunted by man).

During this same time, another hypothesis was offered concerning the fate of estuarine detritus (whether it was outwelled or not). The concept of detritus food chains (Odum, 1980) explained the tight coupling between bacterial/meiofaunal communities and plant detritus. This close association was believed to result in a more efficient cycling of nutrients and was used as an explanation for high estuarine (salt marsh) production. Mangroves are no different. However, it appears that the primary production and the export or import of organic carbon into mangrove systems are governed by the movement of water and the geophysical characteristics of the system.

This paper will attempt to show the importance of mangrove wetland geomorphology and hydrology in determining mangrove production and the cycling of mangrove-derived organic carbon. Few investigators have taken this approach in the past; therefore, most of the data will come from past outwelling and production studies but the analysis will be done ad hoc. Odum et al. (1979) introduced this idea in terms of import/export of organic carbon, but I will try to expand on this by utilizing a variety of examples from around the tropics. However, before I get into the actual data I would like to first discuss the range of geomorphological settings and hydrodynamics that are currently in the mangrove literature.

GEOMORPHOLOGICAL SETTINGS AND PHYSICAL PROCESSES There are four commonly encountered geomorphic settings in which coastal mangroves exist. Deltas are the most common. These low-lying, alluvial systems are favorable to mangrove colonization for a few reasons. First, riverine inputs contribute both sediment and nutrients to these systems. They are also environments of low wave energy that serve as a protective barrier against high winds, tide, and storm surges. Estuaries are another geomorphic setting of mangroves. These drowned river mouth systems are similar to deltas in that they have both a riverine and marine input but they are different in that they offer no impediment to riverine sediments. Lagoon mangrove systems are those that form in and around a semi-enclosed body of water. These, most often fringing communities, are typically driven by rates of oceanic exchange, seasonal changes in freshwater delivery, and wind. Finally, coastal fringing mangroves are neither associated with a semi-enclosed basin nor a river. These systems seem to be driven purely by tidal forcing (Thom, 1967; Odum et al., 1979; Kjerfve, 1990).

Along with geomorphic settings, there are a number of other ecosystem attributes that work to shape mangrove systems. Geographical features such as drainage basin size and proximity to anthropogenic influences are going to determine the types of materials (contaminants, nutrients, etc.) that are going to be imported to or exported from the mangroves. This will obviously have more of an impact in riverine systems. Next, topography and bathymetry will determine both the extent of mangrove development and the rate of material exchange between the mangrove wetland and its adjacent habitats. Finally, local geology will determine not only the kinds of materials that are being transported to the mangroves but also the water and sediment chemistry.

HYDROLOGICAL FACTORS Water is of obvious importance in an aquatic system. However, there are a number of processes and factors (many of which have already been mentioned) that affect the rates of materials transport, mixing and circulation in mangrove wetlands. First, the presence or absence of freshwater inputs can have profound affects on a mangrove system. As previously mentioned, mangroves are facultative halophytes, this means that they can be quite productive in freshwater habitats, but they are limited to coastal brackish environments probably as a result of outcompetition in the more diverse freshwater upland habitats. This environment forces mangroves to ultrafilter salts at the root-water interface in times of high salinity. Some mangroves, however, experience seasonal and even daily shifts in salinity as a result of freshwater input. With recent empirical evidence of surface water utilization by a neotropical species of mangrove (Lin and Sternberg, 1994), it would be wise to assume that some mangroves might be more "active" in their uptake of water and nutrients during periods of freshwater input. This has been recently shown to be the case in the Taylor River mangrove system (Childers and Davis, unpublished data).

In 1984, W.E. Odum proposed a dual-gradient hypothesis that could be used to explain the transport and transformation of detritus in mangrove wetlands. He believed that salinity gradients were important in terms of the flocculation of riverine dissolved organic matter in estuaries. This flocculation is the result of ionic changes caused by the mixing of fresh and salt water and results in the precipitation of organic carbon (see Sholkovitz, 1976). Also, marsh stream order, he hypothesized, was important in determining the source of the organic material. In high order mangrove creeks, DOC concentrations in the water tend to be lower than in low order creeks. High DOC in the low order streams results from the high inputs of the surrounding mangrove vegetation (with little input from marine sources). Low values in the high order streams are due to dilution at the marine/mangrove interface. These hypotheses can therefore be used to explain the patterns of organic carbon cycling within a estuarine or deltaic mangrove system. However, not all mangrove systems are found in these two geomorphic settings so these hypotheses will only be noted at this point.

Freshwater also mixes with saltwater in the lower reaches of many riverine mangrove systems resulting in a pressure gradient. This gradient leads to mixing and circulation patterns that can be a flushing mechanism as well as a vital source of nutrients and sediments to the surrounding mangroves (Kjerfve, 1990). Francis (1992) investigated the impact of mixing in the Rufji Delta, Tanzania and found that river discharge was trapped as a result of this pressure gradient. He hypothesized that this process resulted in an increase in the retention time of river-borne and mangrove-derived detritus. This trapped material thus increased the productivity and even enhanced the regeneration of the mangrove (Francis, 1992). A similar study by Ridd et al. (1990) in Australia found that a similar process caused an increase in the retention time of water above the mouth of a tidal creek-dominated system, but also found that water and material at the mouth of the creek was dispersed quite efficiently to the near-shore zone. Finally, in systems with little or no freshwater input, tidal activity seems to dominate for the most part. In a semi-enclosed, tidal creek system (non-riverine) in Japan, Mazda et al. (1990) found that circulation was blocked by the seasonal presence of a sill. With the sill present, circulation was reduced and the sediments became anaerobic resulting in the release of nutrients to the overlying water. This caused an increase in algal production in the water column and a subsequent decrease on the sediment surface. These events continued until storm surges or excessively high tides breached the sill resetting the system. In another tidally driven system in Kenya, Kitheka (1996) observed trapping of turbid brackish water around the mangroves. This, he found, was the result of the combination of onshore wind and alongshore tidal currents.

With this in mind, it is obvious that various hydrological factors including: current velocity, tide, pulsed events (storm surge, hurricanes, etc.), groundwater, and even sea-level rise are going to have an impact on mangrove production. Current velocity is going to simply determine rates of sedimentation and resuspension and the direction of flow will determine the direction of material transport (import or export). The importance of tide is well-documented and has been discussed at length already. Pulsed events and groundwater, however, have not received much attention in the literature. These are two very important components of any coastal system, yet due to logistical and methodological problems they cannot be assessed directly. Finally, changes in sea-level are going to be most important in determining mangrove distribution especially over a large temporal scale. In order to persist mangroves must accumulate enough sediment to keep pace with the rapid increase in sea-level (approx. 28 cm 100 yr-1). Ellison (1993) showed that many low-lying coastal mangroves in Bermuda are accumulating sediment at a rate of 8.5 to 10.6 cm 100 yr-1. This will, no doubt, result in the loss of mangroves at the seaward margin and erosion of sediment. She hypothesized that this will result in the landward migration of mangroves over the next several centuries (Ellison, 1993).

ORGANIC CARBON DYNAMICS It has been previously mentioned that mangrove systems can be quite productive. That production goes towards maintenance and the building of new biomass. However, losses must also be accounted for. Mangrove losses occur through respiration, herbivory, litterfall, and woodfall. These losses contribute to pools of both organic and inorganic carbon. This paper will discuss only the organic pool.

Organic carbon generally falls under two categories. It is either in a particulate (> 0.45 µM) or in a dissolved state (< 0.45 µM). The size of the organic carbon depends on a number of factors which will be discussed later, however, it should be mentioned that each form has merit in terms of importance to mangrove systems and the quantities of each usually depend on the stage of decomposition. Since the majority of mangrove production goes into the formation of new leaves and since mangroves lose their leaves on a regular or event-driven (high winds, etc.) basis, leaf litter is considered to be one of the biggest losses in mangroves. Once these leaves fall, they begin to be degraded by a number of bacterial, fungal, and meiofaunal organisms.

The first stage of decomposition in mangroves involves the leaching of soluble materials (mostly carbohydrate). These compounds are, as a result of their dissolved state, readily available for uptake by bacteria. Benner and Hodson (1985) found that bacterial assimilation of the leachable components of red mangrove leaves in an aerobic environment was fairly high (approx. 30%). In an anaerobic situation, this efficiency decreased 10 to 30-fold (Benner and Hodson, 1985). In the Bahamas, Benner et al. (1986) even found that the leachable components of red mangrove leaves were utilized by their associated microflora at a higher rate (> 18X) than planktonic microflora. This meant that red mangrove leaf leachate was taken up immediately following dissolution in water. Therefore, it is obvious that this first step is quite rapid and seems to support a number of detritus associated organisms.

Physical and biological decomposition are much slower processes. Physical decomposition involves the weathering of mangrove-derived material over time. This process occurs until all of the components have been broken down or deposited and locked into peat. Biological factors are also at work to break down and utilize all available components of the remaining material. There are organisms that feed directly on mangrove detritus, and there are those that feed on the organisms that are feeding on the mangrove detritus. This can result in a complex trophic pathway that has been previously described (detritus food webs). Detritus pathways are not just limited to leaf material. They also include: stem and twig wood, prop roots, belowground roots, flower parts, propagules, and stipules. The rates of the decomposition of these mangrove parts generally depend on the nutrient content of the material. Following the review work of Enriquez et al. (1993), a clear case was made for this hypothesis. Upon looking at a number of past studies, these investigators found that the C:N:P content of the plant material determined the rate at which it would be decomposed. In the case of mangroves, they found that the phosphorus content of the material accounted for more than 53% of the variability associated with decomposition rates (Enriquez et al., 1993). Since woody tissues are mostly structural in nature (high lignocellulosic content), they decompose much more slowly than do non-woody components. In fact, Benner and Hodson (1985) found that rates of lignocellulosic mineralization were approximately 10 times lower than the mineralization rates of the leachable fraction. In New Zealand, Albright (1976) found that the degradation of mangrove parts (leaves, roots, and pneumatophores) was not related to phosphorus content. In fact, additions of nitrogen seemed to accelerate the decomposition of these parts.

Herbivory is another loss of fixed carbon to mangroves. Herbivory is simply defined as the direct grazing of living biomass by a consumer. As a result of their high phenolic content (a possible defense mechanism), mangroves are not commonly subjected to high rates of herbivory (Onuf et al., 1977). Onuf et al. (1977) found that, in south Florida, herbivory of red mangrove leaf material was a function of bird use. Trees that were utilized by birds had higher rates of herbivory, possibly as a result of the increased nutrient input from guano. Similarly, in a Belize dwarf mangrove forest, Feller (1995) tested the hypothesis that herbivory in mangroves was a function of nutrient availability. She found that herbivory was higher in areas where phosphorus (limiting nutrient) was more available and lower in areas where tissues were higher in phenolic compounds (Feller, 1995). Although it can be quite important in some areas, herbivory of mangrove tissues simply hasn't received much attention in the literature.

NUTRIENT DYNAMICS Since the decomposition of mangrove litterfall is the dominant exchange of fixed carbon (from mangroves to environment) it is going to obviously have a big impact on the nutrient cycles in a given system. Direct uptake of dissolved organic material has already been mentioned so it will not be discussed here. I will, however, try to briefly tie the carbon, nitrogen, phosphorus, and sulfur cycles together to show the importance of organic carbon in mangrove systems.

We know that chemoheterotrophic respiration results in the oxidation of reduced (fixed) forms of carbon. We also know that there is a much higher energy return when this can be done in an aerobic environment. Many compartments of mangrove ecosystems, however, are anaerobic and utilize other, more available, terminal electron acceptors (i.e. NO3, SO4, and even CO2). Therefore, the availability of organic carbon and electron acceptors is going to determine the chemical pathways that are dominating a given environment at a given time. In slightly reducing situations, nitrate is used as a terminal electron acceptor. The process of denitrification involves the oxidation of organic matter using nitrate as the terminal electron acceptor. Many investigators have shunned the importance of this process in mangrove systems, and recent evidence from the neotropics seems to support this. Using 15N and intact sediment cores, Rivera-Monroy and Twilley (1996) and Rivera-Monroy et al. (1995) found that denitrification was not necessarily a sink of inorganic nitrogen in the sediments of Laguna de Terminos, Mexico. In fact, it accounted for less than 10% of the nitrate uptake by the sediments. Therefore, they believed that the loss of inorganic nitrogen in this fringing system was merely due to sedimentation. One non-mangrove estuarine study has shown, however, that denitrification can vary according to the availability of organic carbon (Smith and Hollibaugh, 1989). This study showed that estuarine systems can became nitrogen limiting as a result of denitrification depletion of oxidized inorganic nitrogen. Ideas such as this could be easily applied to mangrove systems with high organic carbon import. Next, as systems become more reducing, sulfate is used as a terminal electron acceptor. This process, called sulfate reduction, is believed to be the dominant pathway of anaerobic respiration in mangrove systems. In fact, Kristensen et al. (1991) found that sulfate reduction, in a mangrove wetland in Thailand, accounted for 100% of the carbon dioxide measured in a sediment. Finally, under the most reducing conditions, carbon dioxide is used as the terminal electron acceptor. This process, fermentation, is inhibited by sulfide production (a by-product of sulfate reduction) so many feel that it is of little importance to mangroves over a large spatial scale.

The presence of organic matter may also determine the mineralization rates of vital elements. Ammonification results in the production of ammonia from the breakdown of organic nitrogen. This is performed by heterotrophs ranging in size from bacteria to fish. Nearly all organic nitrogen compounds are broken down via this pathway (Alongi et al., 1992). Mineralization of phosphate also occurs in the same way. Phosphorus-containing organic compounds are ingested by heterotrophs and phosphate is excreted to the environment. In a non-mangrove estuarine system in Tomales Bay, California, Chambers et al. (1995) noted the significance of allochthonous (terrestrially-derived) phosphorus to the nutrient cycling within the estuary. Numerous other studies have also focused on the importance of particulate organic phosphorus and nitrogen transport into estuarine systems. The mineralization of these two elements in mangrove systems is especially important when considering that one or both is/are usually limiting production.

MANGROVE PRODUCTION AND EXPORT OF ORGANIC CARBON IN A VARIETY OF SETTINGS Up till this point, I have discussed the individual and ecosystem properties of mangroves, the physical settings in which they exist, the hydrodynamics that drive production and development, and finally the pathways and importance of organic carbon. Now that the stage has been set, I would like to synthesize all of this by citing several examples of past productivity and export studies in order to examine the importance of geomorphology and hydrology in mangrove systems around the world. Due to the limited number of available Old World net primary productivity studies, I am limiting this first synthesis to neotropical mangrove systems.

NPP vs. Geomorphology and Hydrology Net primary productivity values (NPP) were take from a few studies encompassing the past 3 decades (1962-1987). These studies were conducted in a variety of neotropical mangrove habitats including south Florida, Mexico, and southern Puerto Rico. The sites for these studies vary in both geomorphology and hydrology. Table 1 shows these data in an organized manner. Although, some aspects of hydrology and geomorphology are missing from these works, we can make a best guess of each based on the structure and the physical setting of each system.

Laguna de Terminos is a large (approx. 2500 km2) lagoon on the northern coast of the Yucatan peninsula. Of the two sites in Mexico, Day et al. (1987) found that the riverine site at Boca Chica was greater than 50% more productive than the fringe site at Estero Pargo. These two sites have about the same average annual precipitation (around 1700mm yr-1) yet differ in salinity range (0-5 ppt at Boca Chica and 20-40 ppt at Estero Pargo) which would lead us to believe that the two are quite different hydrologically speaking. In fact, they are quite different. Estero Pargo is a tidally driven fringe forest that receives little or no freshwater input. Boca Chica, on the other hand, is a river-dominated system that receives inputs from both terrestrial upland and marine sources. The river at this site, Rio Palizada, has a freshwater input into Laguna de Terminos of approximately 115 m3 sec-1 (Kjerfve, 1985). This input of freshwater with its suspended load of allochthonous material, combined with the marine input from the lagoon, has most likely resulted in the differences these investigators saw between these two sites.

We also see these same patterns in Florida. Generally speaking, river-dominated forests are the most productive as a result of the continuous turnover of water and materials. In three south Florida sites, Brown and Lugo (1982) saw a significant decrease in net productivity as they moved from a riverine to basin to scrub forest. Although these three sites probably experience similar patterns in precipitation and groundwater exchange, it appears that the hydrology of these systems is driving production. Twilley (1988) noted this in his studies of south Florida by saying, "hydrologic energy influences the per area productivity and flux of materials in mangroves, and together with geomorphology, these two factors also determine the aerial extent of mangroves surrounding an estuary." In the early 1970's, Carter et al. (1972) noted these same patterns. They found that a fringing mangrove community was more productive than a nearby basin forest. Although, their productivity values were similar, the high hydrologic energy near the fringe site resulted in a higher productivity there in comparison to the basin forest.

Export of Organic Carbon vs. Geomorphology and Hydrology Several studies from around the tropics were put together to look at patterns of organic carbon export in relation to available hydrological and geomorphological data. Table 2 lists these data. Again, we find here that the export of organic matter, like primary productivity, seems to be dependent upon the hydrologic characteristics (as determined by geomorphology). For example, in comparing three south Florida studies (two done at the same site), we see that the export of carbon is a function of tidal influence. Twilley (1985) and Twilley et al. (1986) found that 0.44 and 0.17 kg C ha-1 d-1 were exported from a basin forest in Rookery Bay. He also noted that most of this export was during extremely high tidal events and that only about 20% of it was from mangrove leaf litter (Twilley et al., 1986). It is also obvious that river-dominated systems (i.e. Robertson, 1986 and Boto and Bunt, 1981), being more productive systems, export more organic carbon to adjacent coastal systems. It is important to note, however, that not all mangrove systems are net exporters of organic carbon (such as these data would suggest). Systems that have little or no hydrologic energy (scrub, hammock, etc.) may be net importers of organic carbon, it just so happens that these are the only readily available data on organic carbon flux in mangrove wetlands.

CONCLUSIONS It has been stressed throughout this paper that there are a number of abiotic factors that determine the development of a mangrove. Some of these are obviously more crucial than others. Regardless of the answer, the point of this review was to show the importance of geomorphology and, especially, hydrology in production and organic carbon flux in mangrove wetlands. However, the indirect approach of this synthesis has nonetheless weakened the conclusions the reader can draw. Therefore, I suggest that it be taken as a thought provoking tool rather than hard fact. Nevertheless, it seems that hydrology does seem to be quite important in shaping mangrove communities on a short temporal scale (days to years) in terms of fluxes and production, and geomorphology seems to be more important on a much larger temporal scale (decades to millennia) in terms of zonation and aerial extent. I feel that this approach needs to be further investigated. However, there are many aspects of mangrove ecology that are still not understood (i.e. belowground production and respiration, groundwater inputs, long term flux data, etc.).

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