mean level of channel water al low tide
Figure 13.6 Vertical zonation and abundance of epibenthos on Rhizophora apiculata trees, Phuket Island (Taken from Alongi and Sasekumar 1992 as modified from Frith et al. 1976). Species codes: 1, Sea anemone sp.A; 2, Sea anemone sp.B; 3, Nemertine sp.A; 4, Lepidonotus kumari; 5, Betroiisthes sp.; 6, Cibanarius padavensix; 7, Diogenes avarus; 8. Leipocten sordidulum; 9, Tylodiplax tetra-tylophora; 10, Batanas amphitrite; 11, Chthamalus withersii; 12, Ligia sp.; 13, Sphaeroma walker i ; 14, Nerita birmanica; 15, Littorina carinifera; 16, Littorina scabra; 17, Assiminea brevicu/a; 18, Cerithidea eingulata; 19, Cerithidea obtusa; 20. C. breve; 21, C. patulum; 22, Capulus sp.; 23, Murex capueinus; 24, Nassarius jaeksonianus; 25, Onehidium sp.; 26, Brachidontes rostratas; 27, Isognomon ephip-pium; 28, Enigmonia aenigmatiea; 29, Saccostrea eucidlata; 30, Diplodonta globosa; 31, Teredo sp.; 32, Xylophaga sp.; 33, Trapezium sublaevigatum the unique architecture of mangrove trees, such as prop roots and pneumatophores, may facilitate biodiversity in the tropical intertidal zone. However, chemical leachates from mangrove trees, especially tannins from the genus Rhizophora, may reduce the density and diversity of benthic communities. Alongi (1987a) demonstrated that the density of meio-fauna was negatively correlated with concentrations of soluble tannins in mangrove sediments. In addition, laboratory feeding experiments have shown that population dynamics of meio-fauna may differ when fed detritus of Avicennia compared with Rhizophora, apparently owing to higher concentrations of tannin in the latter genus (Alongi 1987b). Infaunal communities can also be regulated by tannin concentration (Giddins el al. 1986; Neilson el al. 1986). However, other studies by Tietjen and Alongi (1990) present evidence that the negative effects of tannins may be reduced with higher nitrogen concentrations of mangrove detritus. It is evident however that there may be important linkages in the chemical ecology of specific mangrove tree species to the biodiversity and ecological function of benthic communities.
There is an increasing knowledge of nutrient fluxes (Figure 13.5) within mangrove forests and surrounding areas of tropical coastal ecosystems (see reviews by Alongi 1989, 1990). However, the functional ecology of benthos relative to the biogeochemistry of these ecosystems is not clear. Specifically, the role of benthic faunal guilds in the exchange of nutrients across the sediment-water interface is poorly understood, while this ecological function has been described in temperate estuaries. There may be some interesting linkages in the species-specific chemistry of mangrove sediments, benthos density and diversity, and nutrient cycling properties of mangrove ecosystems.
Sponges, tunicatcs and a variety of other forms of epibionts on prop roots of mangroves are highly diverse (Sutherland 1980; Ruetzler and Feller 1988; Ellison and Farnsworth 1992), especially along mangrove shorelines with little terrigenous input. The diversity and biomass of these communities and associated ecological functions may be limited to specific geomorphological types that are protected from turbid waters. The ecology of these communities has been dominated by studies of species distribution and population dynamics, although there are a few studies on ecosystem function. For example, the growth rates of prop roots can be reduced by root-boring isopods (Perry 1988; Ellison and Farnsworth 1990, 1992); however, sponges and ascidians that colonize prop roots prevent the invasion of these isopods and enhance root productivity (Ellison and Farnsworth 1990). There is evidence that epibionts on prop roots may be a source of nutrition for higher-level predators as well as influencing various processes in mangrove fringe forests. These processes of nutrient regeneration associated with sponge communities that colonize aerial root systems of mangroves have received comparatively little attention, but they may influence the productivity of fringe mangrove forests, as well as enhance the exchange of nutrients with coastal waters (Ellison et al. 1996; R. Twilley and T. Miller-Way, unpublished data for 1993.-1995 in Belize). The specific contribution of these productive and diverse epibiont communities in predominately carbonate environments may demonstrate an important linkage between biodiversity and ecosystem function.
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