Figure 13.5 Fluxes of organic matter and nutrients in a mangrove ecosystem, including exchange with the estuary (tN = inorganic nutrients). A diagram of a mangrove forest with soil nutrient resources is also presented to describe the spatial linkages in these ecological processes

1987). If this increase of nitrogen is proportionately greater than the loss of leaf mass during decomposition, then there will be a net input of nitrogen to mangrove soil. The source of this nitrogen may be absorption and adsorption processes by bacterial and fungal communities (Fell and Master 1973; Rice and Tenore 1981; Rice 1982), and nitrogen fixation (Gotto and Taylor 1976; Zuberer and Silver 1978; Potts 1979; Gotto et al. 1981; van der Valk and Attiwil! 1984). Twilley el al. (1986) found that this process of nitrogen immobilization was more significant in decomposing Rhizophora mangle leaf litter than in leaf litter of Avicennia germinans in a basin mangrove forest, The enrichment of decomposing litter with new nitrogen is apparently a function of demand for this substrate by microorganisms colonizing the detritus. Leaf litter with high C:N ratios (>30) usually has a higher potential for nitrogen immobilization since nitrogen concentrations are low relative to carbon, requiring high demand by microorganisms. C:N ratios of Rhizophora litter are usually double the levels in Avicennia litter, and accordingly there is a lower decomposition rate and higher nitrogen immobilization in forests dominated by Rhizophora trees (Twilley et al. 1986).

The C:N ratios of leaf litter are linked to recycling processes in the canopy whereby nutrients are reabsorbed or retranslocated prior to leaf fall (Figure 13.5) (Ryan and Bormann 1982; Vitousek 1982). Steyer (1988) found that retranslocation of Rhizophora was higher than that of Avicennia. contributing to the higher C':N ratio of leaf litter in the former genus. This indicates that more nitrogen may be recycled in the canopy of a mangrove forest dominated by Rhizophora compared with Avicennia. Higher recycling of nutrients in the canopy could improve nutrient use efficiency and thus result in less demand for nitrogen uptake by the tree. In the Rhizophora forest, nitrogen immobilization in leaf litter during decomposition as result of the higher C:N ratio will result in less remineralization on the forest floor, compared with Avicennia forest where nitrogen regeneration occurs during litter decomposition. Higher nitrogen remineralization in litter of Avicennia would supply the higher demand for nitrogen in the canopy of this genus. Experimental tests are needed to demonstrate if there exist cause and effect linkages in nutrient recycling due to the relative dominance by Rhizophora and Avicennia in a mangrove forest. However, these ideas suggest that shifts in the species composition of mangrove forests could contribute to different patterns of nitrogen dynamics between the canopy and soils of mangrove ecosystems.

13.3.2 Ecological type and litter dynamics

Productivity of mangroves, both primary and secondary, is usually associated with the concepts of outwelling in cstuarine ecosystems (Twilley

1988). One reason for this may be related to the greater tidal amplitude, as in Ecuador or Australia, and by higher runoff in some tropical deltaic systems, as in Mexico, Brazil or Venezuela, used to study mangrove export compared with temperate intertidal wetlands. The productivity of mangroves may be strongly related to the ccological type of mangrove (sec discussion in Section 13.2), since processes may be specific to riverine, fringe or basin mangroves according to their respective hydrologic characteristics (Twilley 1988, 1995). This conclusion is based mainly on organic matter exchange in mangroves, although there are indications that nutrient recycling may also vary along a continuum in hydrology.

In mangroves, the residence time of litter on the forest floor is largely controlled by tidal flooding frequency. Trends for litter productivity and export suggest that as geophysical energy increases, the exchange of organic matter between mangroves and adjacent estuarine waters also increases. The average rate of carbon export from mangroves is about 210 gC m~2 year"1, with a range from 1.86 to 401 gC m~2 year 1 (based on 10 estimates in the literature; Twilley et a!. 1992). Total organic carbon (TOC) export from infrequently flooded basin mangroves in southwest Florida is 64 gC m~2 year and nearly 75% of this material is dissolved organic carbon (DOC) (Twilley 1985). Particulate detritus export from fringe mangroves in south Florida was estimated at 186 gC m 2 year"' (Heald 1971), compared with 420 gC m"2 year"1 for a riverine mangrove forest in Australia (Boto and Bunt 1981). Estimates of average tidal amplitude in these three forests types are 0.08 m, 0.5 m and 3 m, respectively. Rates of organic carbon export from basin mangroves are dependent on the volume of tidal water inundating the forest each month, and accordingly export rates arc seasonal in response to the seasonal fluctuation in sea level. Rainfall events may also increase organic carbon export from mangroves (Twilley 1985), especially DOC. Accordingly, as tidal amplitude increases, the magnitude of organic material exchanged at the boundary of the forests increases (Twilley 1985).

Geophysical processes alone do not control the fate of leaf litter in mangrove ecosystems. Litter productivity in a riverine forest in Ecuador is similar to that in a riverine forest in south Florida at about 10 Mg ba~' year"1. Leaf litter turnover rates in the two sites were different by factor of 10, which fits the model discussed above, since the tidal amplitude in Ecuador is 3-4 m compared to < 1 m in Florida. Yet observations in the mangroves in Ecuador suggest that most of the leaf litter on the forest floor is harvested by the mangrove crab, U tides oecidentalis, and transported to sediment burrows rather than exported out to the estuary (Twilley et al. 1990). The influence of mangrove crabs on litter dynamics has been described in other mangrove ecosystems with high geophysical energies and rates of litter turnover >5 year"1 (Malley 1978; Leh and Sasekumar 1985; Robertson and Daniel 1989; see Section 13.4.1). Thus, the patterns of leaf litter export from mangroves are not restricted to just geophysical forcings such as tides; in some locations there arc important biological factors that influence litter dynamics. In these examples, high rates of litter turnover do not reflect the coupling of mangroves to coastal waters, but the conservation of organic matter within the forest. This demonstrates the complex nature of the relative importance of geophysical processes and biodiversity on the ecological functions of mangrove ecosystems.

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