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Source: Data from Newell and Fell (1992).

Source: Data from Newell and Fell (1992).

dominated by Lulworthia grandispora, may comprise 50% of the fungal community.

Export of plant detritus from mangrove ecosystems to the oceans is an important contribution to the nutrient loading of oceans (Lee, 1995). Lee demonstrated the importance of the decomposition of plant litter in coastal communities and the consequent nutrient mineralization that supplied nutrient to the ocean. Outwellings of water from mangrove swamps to the ocean can result in a transfer of between 60 and 260ty"1 of carbon, which is exported mainly as dissolved organic carbon (DOC). This may be an important component of nutrient additions to near-shore waters and a process in which fungi play a major role. Hyde and Lee (1995) point out, however, that there are still many gaps in our knowledge of the role of fungi in nutrient cycling in mangrove ecosystems. They suggest that the rates of chemical transformations are dependent upon the age of the mangrove stand, the diversity of mangrove and terrestrial tree flora, and the proportion of the various microhabitats within an area. They also suggest that the end product of fungal decomposition is likely to be dissolved organic matter rather than particulate organic matter, of which there is scant understanding of its origins and movement in distribution in marine estuarine ecosystems. In addition, fungi are exported from mangrove ecosystems. The mangrove tree (Rhizophora mangle ) produces viviparous seedlings. These seedlings develop as the fruit germinates on the tree, falls off into the water, and is carried by water currents. These drifting seedlings are vehicles for the dispersal of marine fungi. Kohlmeyer and Kohlmeyer (1979) report occasions where Keissleriella blepharospora and Lulworthia spp. have been transported in this manner from the tropics to the coast of North Carolina by the Gulf Stream.

2.2.2 Freshwater Ecosystems

According to Wong et al. (1998) there are more than 600 species of aquatic fungi, many of which have specific morphological and physiological adaptation to allow them to live in aquatic ecosystems. Fungi have been isolated from spores

suspended in the water column of streams, ponds, and lakes, growing on decaying vegetation and utilizing suspended organic matter in deep aquifers (Kuehn and Koehen, 1988). Aquatic hyphomycetes occur on almost all substrates in freshwater systems (Barlocher, 1992). Fungi biomass is usually greater than bacterial biomass on decomposing leaf litter in aquatic ecosystems. Plant litter inputs into headwater streams in forested catchments can be on the order of 500 g dry massm"2 and reach peaks of over 100 gm"2 (Weigelhofer and Waringer, 1994). The success of fungal species that are adapted to live in aquatic habitats is that terrestrial fungi entering the system along with the plant material are unable to macerate the resources when submerged, and colonization of plant litter by tetraradiate and sigmoid spores of aquatic fungal species is more efficient than by rounded spores of terrestrial fungi, which are adapted for wind dispersal (Wong et al., 1998). Measures of rates of decomposition of plant litters by aquatic fungi suggest that a range of carbohydrate resources can be utilized (Bergbauer et al., 1992), but also that the rate of decomposition is reduced in mixed-species fungal assemblages compared to single species. Bergbauer et al. (1992) attribute this reduction in decomposition to the production of antimicrobial compounds that result in nonnutritional competition between fungal species. Wood is an important resource, representing up to 20% of the total plant litter input (Table 2.20). Shearer (1992) estimates that some debris enters stream systems of old-growth temperature coniferous forests and 40 to 130 tons ha21 in mixed hardwood forests. This woody material is important in that its residence time is much greater than leaf litter, and therefore forms a more stable environment for fungal community development. Despite the lack of white and brown rot fungi, which occur in terrestrial ecosystems, lignolytic aquatic hyphomycetes of the genera, Tricladium, Anguillospora, and Dendrospora are dominant colonizers of woody material in streams. The main addition of woody material to a stream ecosystem occurs in the winter as a result of windthrow and weather damage to branches and twigs. It is thus available for fungal colonization in the spring and throughout the year, whereas deciduous leaf litter enters as a pulse in the fall and is usually degraded before the next leaf fall or has already been exported downstream. Approximately one-third (86 species) of aquatic hyphomycetes have been isolated from wood. Shearer (1992) also shows that many species express a wide range of enzymes related to the decomposition of woody material. Of 20 species, seven were shown to produce enzymes to degrade carboxymethyl cellulose, cellobiose, amylose, xylan, xylose, lignin, and pectin, while another four species could utilize five or more resources. The community of fungi colonizing woody resources alters along the length of a water course (habitat selection) and over time at the same location (resource succession). Gessner et al. (1997) provide a review of the role of fungi in plant litter decomposition in aquatic ecosystems. They provide a conceptual model of the interactions among the internal controls (litter quality), external controls u

Table 2.20 Annual Total Coarse Particulate Organic Matter Entering a Variety of Streams in a Beech-Dominated Austrian Forest

Site

Input location

Leaves (%)

Wood (%)

Miscellaneous (%)

Total (g dw)

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