Microbial Processes And Ecosystem Function

The biodiversity of microbial communities in mangrove ecosystems is poorly documented, but the biomass and metabolism of this guild is well established for tropical intertidal environments (Fell and Master 1980; Newell et al. 1987; Alongi 1988, 1989; Alongi and Sasekumar 1992). The microbial ecology of mangrove sediments and its influence on ecological processes, particularly decomposition, has been related to the chemical diversity of mangrove leaf litter (Benner and Hodson 1985; Benner et al. 1986; Benner et al. 1990a,b). As discussed above for meio-fauna, chemical compounds such as tannins may influence the population dynamics of microbial communities on specific leaf litter. This, in turn, influences the fate of organic matter and nutrients on the forest floor of mangrove ecosystems. Most of the research relating the chemical ecology of mangroves with specific ecological processes has been done with Rhizophora, while there may be other interesting comparisons among the different mangrove tree species. A potentially important area of study is the effect of species richness of trees on the chemical ecology of mangrove litter and soils, and how this can influence the ecosystem functions of mangrove forests.

Nitrogenasc activity has been observed on decomposing leaves and root surfaces (prop roots and pneumatophores) and in sediment. This enzyme makes an important contribution to the nitrogen budget in mangrove systems (Kimball and Teas 1975; Gotto and Taylor 1976; Zuberer and Silver 1978; Potts 1979; Gotto et al. 1981). Results from studies of mangrove sediments in south Florida indicate the nitrogen fixation rates range from 0.4 to 3.2 g N m~2 year 1 (Kimball and Teas 1975; Zuberer and Silver 1978). These studies have shown that decomposing mangrove leaves are sites of particularly high rates of fixation, and thus account for some of the nitrogen immobilization in leaf litter (Gotto et al. 1981; van der Valk and Attiwill 1984). For example, Gotto et al. (Í981) found that nitrogen fixation in Avkennia leaves was nearly twice that in Rhizophora leaves. Thus, the contribution of this ecological process to the fertility of mangrove ecosystems may depend on the nutrient status of litter among different types of mangrove ecosystems, as discussed above. Whereas mangrove forests also fix and store carbon in wood and organic-rich sediments, the total carbon sequestration in tropical coastal ecosystems is unknown, but may represent a potentially important sink of'carbon in tropical coastai ecosystems (Twilley et al. 1992).

The colonization of microbial communities on leaf litter can influence the exchange of nutrients at the boundary of mangrove ecosystems. Rivera-Monroy et al. (1995) observed that there was a net uptake of ammonium and nitrate during tidal exchange in a flume constructed in a fringe forest in Terminos Lagoon, Mcxico. Organic nitrogen was exported from this flume to the estuary at rates equal to the uptake of inorganic nitrogen. Thus this mangrove zone functioned as a transformer of inorganic nitrogen to organic detritus, which is similar to processes observed in salt marsh ecosystems. Based on these flume studies of nutrient exchange, nitrate uptake was assumed to be denitrified to gaseous form, representing a sink of nitrogen from the system. However, studies with nitrogen-¡5 labeled nitrate showed that very little of the amended nutrient was transformed to nitrogen gas (Rivera-Monroy et al. 1995; Rivera-Monroy and Twilley 1996). In addition, nearly all of the enriched ammonium N-15 could be accounted for in sediments; thus no coupled denitrification occurs in these wetland soils. Loss of nitrogen via denitrification was low, apparently due to the high nitrogen demand in decomposing leaf litter on the forest floor (Rivera-Monroy et al. 1995; Rivera-Monroy and Twilley 1996). These studies further indicate the significance of the quality of leaf litter to nitrogen cycles in mangrove forests (Figure 13.5), as has been observed in other forest ecosystems (see Section 13.3).

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