Ecosystem Processes

Succession

Succession in mangroves has often been equated with zona-tion, wherein 'pioneer species' would be found in the fringe zones, and zones of vegetation more landward would 'recapitulate' the successional sequence toward terrestrial communities. Zonation in mangrove communities has variously been accounted for by a number of biological factors, including salinity tolerance of individual species, seedling dispersal patterns resulting from different sizes of mangrove propagules, differential consumption by grapsid crabs and other consumers, and interspecific competition. Snedaker proposed the establishment of stable monospecific zones wherein each species is best adapted to flourish due to the interaction of physiological tolerances of species with environmental conditions. Geological surveys of the intertidal zone of Tabasco, Mexico, demonstrated that the zonation and structure ofmangrove wetlands are responsive to eustatic changes in sea level, and that mangrove zones can be viewed as steady-state zones migrating toward or away from the sea, depending on its level. Thus, both monospecific and mixed vegetation zones of mangrove wetlands represent steady-state adjustments rather than successional stages. Many models of mangrove succession are based on how gap dynamics influence spatial patches of community dynamics across the landscape.

Productivity and Litter Dynamics

Tree height and aboveground biomass of mangrove wetlands throughout the tropics decrease at higher latitudes, indicating the constraint of climate on forest development in the subtropical climates. In addition, mangrove biomass can vary dramatically within any given latitude, an indication that local effects of regulators, resources, or hydroperiod may significantly limit the potential for forest development at all latitudes. The primary productivity of mangroves is most often evaluated by measuring the rate of litter fall, as recorded for other forested wetlands. Regional rates in litter production in mangroves are a function of water turnover within the forest, and rank among the ecological types is as follows: riverine > fringe > basin > scrub.

The dynamics of mangrove litter, including productivity, decomposition, and export, can determine the coupling of mangroves to the secondary productivity and biogeochemistry of coastal ecosystems. Patterns of leaf-litter turnover have been proposed to vary among ecological types of mangroves with greater litter export in sites with increasing tidal inundation (riverine > fringe > basin). However, several studies in the Old World tropics in higher-energy coastal environments of Australia and Malaysia have emphasized the influence of crabs on the fate of mangrove leaf litter, rather than geophysical processes. In these coastal environments, crabs consume 28-79% of the annual leaf fall. A similar biological factor was observed in the neotropics where the crab Ucides occidentaliss in the Guayas River estuary (Ecuador) processed leaf litter at similar rates observed in Old World tropics. Differences in litter turnover rates among mangrove wetlands are a combination of species-specific degradation rates, hydrology (tidal frequency), soil fertility, and biological factors such as crabs.

Nutrient Biogeochemistry

The nutrient biogeochemistry of mangrove wetlands as either a nutrient source or sink depends on the process of material exchange at the interface between mangrove wetlands and the estuary, which is largely controlled by tides (tidal exchange, TE, in Figure 3). Nutrient exchanges may occur either with coastal waters (TE) or with the atmosphere (atmosphere exchange, AE), depending on whether the nutrient has a gas phase or not (Figure 3). Substantial amounts of carbon and nitrogen can exchange with the atmosphere, resulting in very complex mechanisms both at the interface with coastal waters and with the atmosphere that influence the mass balance of these nutrients. In addition, there are internal processes, including root uptake (UT), retranslocation (RT) in the canopy, litter fall (LF), regeneration (RG), immobilization (IM), and sedimentation (SD) (Figure 3). The balance of these nutrient flows will determine the exchanges across the wetland boundary.

There are very few comprehensive budgets of carbon, nitrogen, or phosphorus for mangrove ecosystems. Mangrove sediments have a high potential in the removal of N from surface waters, yet estimates of denitrification have a large range from a low of 0.53 p,molm~ h~ , to 9.7-261 mmolNm^h-1 in mangrove forests receiving effluents from sewage treatment plants. Small amendments of 15NO3 followed by direct measures of 15N2 production have shown that denitrification accounts for

Figure 3 Upper panel: Schematic of the various fluxes of organic matter and nutrients in a mangrove ecosystem, including exchange with the estuary (IN = inorganic nutrients). Lower panel: A diagram of a mangrove wetland with soil nutrient resources describing the various processes associated with intrasystem cycling and exchange. From Twilley RR (1997) Mangrove wetlands. In: Messina M and Connor W b0665 (eds.) Southern Forested Wetlands: Ecology and Management, pp. 445-473. Boca Raton, FL: CRC Press.

<10% of the applied isotope suggesting that NO3 is accumulated in the litter via immobilization on the forest floor rather than a sink to the atmosphere. The other nutrient sink in mangrove wetlands is the burial of nitrogen and phosphorus associated with sedimentation. A survey of sedimentation and nutrient accumulation among five sites in south Florida and Mexico indicates patterns associated with the ecological types of mangroves, with rates of about 5.5gm~2yr~\ This rate is higher than nitrogen loss via denitrification, indicating the significance of burial as nitrogen sink in mangrove ecosystems. Intrasystem nutrient recycling mechanisms in the canopy may be a site of nitrogen conservation in mangroves and, together with leaf longevity, could influence the nitrogen demand of these ecosystems. The significance of this ecological process to the nutrient budget of different mangrove wetlands has not been determined.

Surveys of nitrogen exchange demonstrate some of the principles of determining the function of mangrove wetlands as a nutrient sink. The largest nitrogen flux of nitrogen from sites in Mexico and Australia is export of particulate nitrogen, consistent with organic carbon representing the largest flux from most mangroves (Figure 3). Compared to other flux studies of mangroves, there seems to be a pattern of net inorganic fluxes into the wetlands and corresponding flux of organic nutrients out. The best summary may be that mangrove wetlands transform the tidal import of inorganic nutrients into organic nutrients that are then exported to coastal waters. Carbon export from mangrove ecosystems ranges from 1.86 to 401 g Cm-2 yr-1, with an average rate of about 210 gCm~ yr~ . Carbon export from mangrove wetlands is nearly double the rate of average carbon export from salt marshes, which may be associated with the more buoyant mangrove leaf litter, higher precipitation in tropical wetlands, and greater tidal amplitude in mangrove systems studied.

Mangrove Food Webs

The function of mangrove wetlands as a source of habitat and food to estuarine-dependent fisheries is one of the most celebrated values of forested wetlands. There are several excellent reviews that describe the secondary productivity of tropical mangrove ecosystems. The original 'outwelling hypothesis' of mangroves has been revised from the original paradigms based on comparisons among different mangrove estuaries using natural isotope abundance to trace mangrove organic matter through estuarine food chains. There are seasonal and spatial differences in the amount of mangrove detritus that can be measured in shrimp and fish that inhabit mangrove estuaries. If the distance from the source of mangrove detritus increases, the proportion of carbon in the tissue of shrimp from mangrove detritus decreases as the signal of carbon phytoplankton increases. The seasonal timing of mangrove export of detritus relative to the migration of estuarine-dependent fisheries may also dilute the contribution of mangrove detritus from the food webs among diverse sites. The migratory nature of many of the nekton communities and the seasonal pulsing of both organic detritus input and in situ productivity result in very complex linkages of mangroves with estuarine-dependent fisheries. In addition, mangrove detritus low in nitrogen relative to carbon may be modified by the microbial community and then utilized by higher trophic levels, masking the direct utilization of this organic matter as an energy source.

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