Nekton (free-swimming organisms) food webs represent faunal guilds that utilize mangrove habitats for food and protection at different stages of their life cycle. Most of these organisms are migratory (while there may be some residents), and the ephemeral nature of the periods when these organisms utilize mangrove habitats contributes to the poor understanding of their ecology. Robertson and Blaber (1992) reviewed the results of four mangrove fish community studies in northern Australia where species richness varied from 38 to 197 species. In the neotropics, extensive surveys of the composition and ecology of mangrove nekton have found 26-114 species of fish (from Table 9 in Robertson and Blaber, 1992). Tropical estuarine fishes in mangrove ecosystems, as secondary consumers, can be important in energy and nutrient flow in several ways. They can be stores of nutrients and energy, control rates and magnitude of energy flow through grazing of food sources, and move energy and nutrients across ecosystem boundaries (Yanez-Arancibia 1985; Robertson and Duke 1990).
The structural and ecological functions of mangroves sustain nearshore marine habitats and provide food and refugia to a variety of organisms at different trophic levels (Odum and Heald 1972; Thayer et al. 1987; Yanez-Arancibia et al. 1988, 1993; Robertson and Duke 1990; Rojas et al. 1992; Sasekumar et al. 1992). This is clearly reflected in the description of nekton food webs in mangroves (Figure 13.7). The complexity of food sources found in fish stomachs documents changes in food diversity and fish preferences as fish grow. Often the diet of a single species comprise more than 20 different (or diverse) food types in mangrove areas. The whole trophic structure does not comprise specific trophic levels, as fish eat food from a diversity of sources in the mangrove ecosystem (Figure 13.7). In summary, the general characteristics of feeding relationships among "mangrove-related fishes" are: (1) flexibility of feeding in time and space; (2) sharing of the common pool of most abundant food resources among a diverse pool of species; (3) the taking of food from different levels of the food web by each species; (4) the changing of the diet with growth, food diversity, and locality within the estuary; (5) the use of both the pelagic and benthic pathways by a given species.
One of the key questions about mangrove ecology is the significance of these habitats to the dependence of marine organisms on estuaries during the juvenile or adult stages of their life cycle (Robertson et al. 1992; Yanez-Arancibia et al. 1994). The seasonal pulse of primary production along with temporal variation in physical constraints (e.g. temperature and salinity) provide a unique set of conditions for estuarine-dependent life cycles (Rojas et al. 1992). Three primary types of migration have been documented: diel. seasonal and ontogenetic. Rooker and Dennis (1991) report both die! and seasonal migrations at a mangrove key near Puerto Rico, suggesting that diel movements are primarily the result of feeding habits, while seasonal migrations "may be related to changes in environmental parameters (i.e. salinity, temperature, turbidity) that alter habitat characteristics, or to aspects of life-history (e.g. reproduction, recruitment) (Williams and Sale 1981; Williams 1983)". More often reported are migrations due to ontogenetic causes (Odum et al 1982; Ogden and Gladfelter 1983; Gilmore and Snedaker 1993). Ontogenetic migrants generally spend their juvenile stage in the relative protection of the mangrove habitat, and subsequently move offshore as they grow too large to effectively utilize the mangrove structure as shelter (Odum et al. 1982; Ogden and Gladfelter 1983). For the most part, however, very little detail is known about the life-history characteristics of many of these multi-habitat fishes, especially the larval phase (Voss et al. 1969).
The sequential pulsing of primary production by planktonic and macro-phyte communities, coupled with seasonal export of mangrove detritus, suggest that the delivery of organic matter sustains high estuarine secondary production and species diversity of estuarine-dependent consumers (Figure 13.8) (Rojas et al. 1992; Yànez-Arancibia et al. 1994). The seasonal coupling of primary and secondary production in mangrove ecosystems, along with variation in environmental conditions, shows how the functional assemblages of fish use tropical lagoon habitats in time and space to reduce the effect of competition and prédation. Assemblages of macroconsumers within functional groups play an important ecological role by coupling life-history strategies with the environmental gradients within the estuary (Figure 13.8). In Figure 13.8 we can see the two main mangrove habitats, fringe (SMS) and riverine (FLS), where the same fish population assemblages utilize the habitats in a sequential manner from one season to the next. This diversity in behavior suggests that the lag in fluctuations of total biomass through the year, as is common in high-latitude estuaries, is a consequence of the
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