The Effects of Siltation on Tropical Coastal Ecosystems Miguel Fortes CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Responses of Seagrass to Siltation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Effects of Siltation on Seagrass Depth Distribution and Abundance. . . . . . . . 94 Effects of Siltation on Diversity, Biomass, and Survival . . . . . . . . . . . . . . . . . 95 Effects of Siltation on Seagrass Growth and Primary Productivity . . . . . . . . . 96 Effects of Siltation on Seagrass Morphology. . . . . . . . . . . . . . . . . . . . . . . . . . 98 Responses of Corals to Siltation/Sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Effects of Sedimentation on Coral Abundance, Diversity, and Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Effects of Sedimentation on Coral Growth and Productivity. . . . . . . . . . . . . 101 Modeling Reef Status and Sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Responses of Mangroves to Siltation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Effects of Siltation on Seagrass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Effects of Sedimentation on Coral Reefs. . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Effects of Siltation on Mangroves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 INTRODUCTION Coral reefs, seagrass beds, and mangroves are the major ecosystems in coastal Southeast Asia. They are experiencing widespread deterioration, largely as a result of siltation (Fortes, 1988). During the past 25 years rates of siltation in the region have increased substantially and are among the highest in the world (Milliman & Meade, 1983; Milliman & Syvitski, 1992). These have been caused largely by human distur- bances such as land reclamation or changes in land use (Fortes, 1988 and 1995; Short & Wyllie-Echeverria, 1996). The rapid progression of coastal development, near and offshore mining, agricultural land use, and deforestation have led to increasing silt load and eutrophication. These brought about dramatic changes in the development 7 93 © 2001 by CRC Press LLC of coastal plant and animal communities in both tropical and temperate waters (Orth & Moore, 1983; Cambridge & McComb, 1984; Onuf, 1994; Terrados et al., 1998). The aim of this chapter is to describe the changes in depth distribution, abun- dance, growth and photosynthetic performance, and morphological changes in sea- grasses and corals along siltation gradients. In addition, the effect of siltation on the demography of mangrove seedlings is briefly discussed. It is hypothesized that the reduction in light availability brought about by siltation or sedimentation is the most operationally significant factor forcing changes in species composition and commu- nity distribution along gradients of siltation. Hence, at less perturbed sites, a change in species composition along a gradient should parallel a similar change with increas- ing depth. RESPONSES OF SEAGRASS TO SILTATION Seagrasses are submerged angiosperms that can fulfil their entire life cycle under water, forming extensive meadows on sandy to muddy sediments in shallow coastal waters (den Hartog, 1970; Valiela, 1984). Among the most productive components of coastal ecosystems (Hillman et. al., 1989), these meadows are an important link between land and ocean (Holligan & de Boois, 1993; Hemminga et al., 1994) and support a high primary production (Valiela, 1984; Hillman et al., 1989; Duarte, 1989). Seagrass leaves and stems add considerable three-dimensional structure to the seabed, providing habitat, feeding, and breeding grounds as well as nurseries for a diverse array of fauna (e.g., sirenians, birds, fish, and invertebrates: Jacobs et al., 1981; Bell & Pollard, 1989; Howard et al., 1989; Klumpp et al., 1993). Seagrass meadows also act as sediment traps (Bulthuis et al., 1984; Ward et al., 1984; Fonseca & Fisher, 1986; Fonseca, 1989) and as breakwaters offering natural shoreline protec- tion (Fonseca et al., 1982; Hemminga & Nieuwenhuize, 1990). The effects of siltation on seagrasses are manifested in their depth distribution, abundance, species composition, growth, primary productivity, and changes in mor- phology. These changes are briefly discussed below. EFFECTS OF SILTATION ON SEAGRASS DEPTH DISTRIBUTION AND ABUNDANCE Seagrass beds are subject to both direct and indirect influences of man’s interference in the coastal zone. Urbanization, large-scale reclamation and shore protection works, increased sediment delivery by rivers draining watersheds with changing land-use practices, eutrophication, and increased fishing pressure have severely affected the depth distribution, density, and areal extent of seagrass meadows (Cambridge et al., 1986; Fortes, 1988; Shepherd et al., 1989; Giesen et al., 1990; Holligan & de Boois, 1993; Lundin & Linden, 1993). Distribution and abundance of seagrasses are controlled by a range of environ- mental conditions including light availability (Dennison & Alberte, 1985; Dennison, 1987), nutrient availability (Short, 1987), water motion (Fonseca & Kenworthy, 1987), and grazing (Lanyon et al., 1989). Of these, light availability is considered one of the 94 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC more important environmental parameters, controlling the depth to which seagrasses can grow and excluding seagrasses from areas with low light conditions (Dennison et al., 1993; Abal & Dennison, 1996; Bach, 1997; Bach et al., 1998; Duarte et al., 1997). Siltation is a major factor that limits light availability for benthic organisms The relationships between light conditions and depth distribution of temperate seagrasses clearly document that light availability is the prime regulating factor for plant performance (e.g., Bulthuis, 1983; Dennison, 1987; Olesen, 1996). Silt from rivers and land reduces underwater light penetration by increasing both light absorp- tion and scattering (Kirk, 1983; Onuf, 1994). Increase in nutrient load, similarly asso- ciated with an increase in silt load (Malmer & Grip, 1994), favors the growth of microalgae and epiphytes (Sand-Jensen & Borum, 1991; Duarte, 1995), thereby reducing light availability to seagrass. In turn, reduced seagrass abundance decreases the ability of the plants to protect surface sediments (Fonseca et al., 1982), enhanc- ing sediment resuspension (Bulthuis et al., 1984). Deterioration of the underwater light climate for the remaining seagrass stands results. At Cape Bolinao, northwestern Philippines, the depth penetration of the mixed seagrass beds declined systematically with increasing siltation (Bach, 1997 and 1998; Terrados et al., 1998). At the control site, leaf growth of Thalassia hemprichii, Cymodocea rotunda, and Cymodocea serrulata responded clearly to artificial reduc- tion of light. However, in natural stands of T. hemprichii, C. serrulata, and Enhalus acoroides growing along the siltation gradient, there was no differential leaf growth to variations in light regime. They responded only moderately to reduced light with increasing depth. EFFECTS OF SILTATION ON DIVERSITY, BIOMASS, AND SURVIVAL While siltation smothers and buries benthic organisms (Duarte et al., 1997), at the same time it increases the nutrient load in both water and the sediments (Malmer & Grip, 1994; Mitchel et al., 1997). These changes in the water and sediment conditions are particularly detrimental for seagrasses (Giesen et al., 1990; Duarte, 1991; Sand-Jensen & Borum, 1991; Duarte, 1995). At Cape Bolinao, the diversity of the mixed seagrass beds was reduced with increasing silt load (Bach et al., 1998). From the most to the least tolerant, the seagrass species could be ranked after their tolerance to siltation as: Enhalus acoroides Ͼ Cymodocea serrulata Ͼ Halodule uninervis Ͼ Thalassia hemprichii Ͼ Halophila ovalis Ͼ Cymodocea rotundata Ͼ Syringodium isoetifolium. This sequential loss of species agrees well with that found in a related study among sea- grass beds along siltation gradients in the Philippines and Thailand (Terrados et al., 1998), suggesting that the sequence may represent a general pattern of tolerance to sil- tation among Southeast Asian seagrass species. At the initial phase under conditions of severe light reduction some seagrasses exhibit a rapid loss of biomass. Leaf densities of Heterozostera tasmanica (Bulthuis, 1983) and Posidonia sinuosa (Gordon et al., 1994) decreased by 70% during the first month of exposure to 2 and 1% of ambient light, respectively. H. pinifolia, on the other hand, can survive long periods of light deprivation, a feature of great impor- tance for the species especially in the Southeast Gulf of Carpentaria (Australia) The Effects of Siltation on Tropical Coastal Ecosystems 95 © 2001 by CRC Press LLC which periodically receives monsoonal rains that result in highly turbid floodwaters covering over the seagrass beds (Shepherd et al., 1989). In contrast to the high tolerance of H. pinifolia, H. ovalis has a low tolerance to darkness, death occurring after only 38 days in the dark. A similar intolerance to light deprivation has also been demonstrated for monospecific H. ovalis plants growing in sub-tropical waters (Longstaff et al., 1999). This long-term survival strategy of Halophila species to perturbations has also been suggested to occur elsewhere (Kenworthy, 1992). The explanation is that seagrasses growing under reduced light conditions allocate a lower fraction of photosynthetic products to underground tis- sues (Madsen & Sand-Jensen, 1994) and formation of new shoots. This results in low shoot density. For the remaining shoots, however, light becomes more available because of a concomitant reduction in self-shading among them. Prolonged condi- tions of improved (Williams, 1987) or reduced light availability (Zieman et al., 1989) induce changes in shoot density and biomass. Species loss may also result indirectly from the effect of siltation on sediment grain size, resuspension, and resistance to oxygen diffusion (Duarte et al., 1997). Fine-grained sediments are more readily resuspended, and therefore seagrass beds in silted areas more often experience partial burial. The large Enhalus acoroides and species which grow profusely via their vertical rhizomes (e.g., Cymodocea serrulata) can comparatively tolerate both silt and burial (Vermaat et al., 1997), while smaller species (e.g., Halodule uninervis and Syringodium isoetifolium) cannot survive bur- ial (Duarte et al., 1997). Seagrasses also respond differently to changes in redox potential of the sediment, depending on their morphology and ability to maintain root oxygen supply (Smith et al., 1988). Hence, the integrated response of mixed seagrass beds to siltation is likely to be caused by changes in both water and sediment condi- tions it brings about. EFFECTS OF SILTATION ON SEAGRASS GROWTH AND PRIMARY PRODUCTIVITY The relationships between siltation, the light conditions it brings about, and growth and photosynthesis of seagrasses clearly demonstrate that light is the prime factor regulating plant performance (e.g., Bulthuis, 1983; Dennison, 1987; Olesen, 1996). Seagrasses generally require a higher quantity of light in comparison to other marine and terrestrial flora (Dennison et al., 1993; Duarte, 1991; Abal et al., 1994). However, as to the actual amount of light required for long-term survival, more studies have yet to be done. Estimates of light requirements of seagrasses differ between species (e.g., 4.4 to 29% of surface light) and within a species (e.g., 5 to 20% of surface light) (Dennison et al., 1993), while an average requirement of seagrasses as a group of plants has been calculated to be 11% of surface light (Duarte, 1991). In tropical seas, productivity of shallow seagrass stands seems to be limited largely by the availability of nutrients (e.g., Agawin et al., 1996). However, nutrient availability is low in fine-grained carbonate sediments (Short et al., 1985; Short, 1987) but increases in coarse-grained carbonate and terrigenous sediments (Erftemeijer, 1994; Erftemeijer & Middelburg, 1995). These findings suggest that not all tropical seagrass meadows might be nutrient limited (Erftemeijer et al., 1994). The 96 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC nutrient status of seagrasses, however, may be reduced by a decrease in the availabil- ity of light (Abal et al., 1994), thereby reducing the nutrient requirements of some tropical seagrasses. Three photosynthetic parameters have been found to respond strongly to both the gradient in natural light and light deprivation, conditions which are associated with siltation: chlorophyll a-to-b ratio, leaf amino acid concentration, and leaf 13 C value. Decreasing chlorophyll a:b with depth has been observed in a number of seagrass species including Zostera marina (Dennison & Alberte, 1985), H. ovalis (Longstaff et al., 1999), Halophila spp., Halodule spp., Syringodium filiforme, and Thalassia testudinum (Wigington & McMillan, 1979; Lee & Dunton, 1997). A decrease in the chlorophyll a:b has been considered an adaptive response that increases the light absorption efficiency of seagrass (Abal, 1996; Lee & Dunton, 1997). Changes in amino acid concentrations in seagrasses are brought about by a num- ber of environmental variables. Two of these which are associated with siltation are water depth and nutrient addition. Depth has been shown to affect amino acid con- centrations in Posidonia oceanica (Pirc, 1984), although this was not in the case of Thalassodendron ciliatum (Parnik et al., 1992). Ambient sediment nutrient concen- tration and sediment nutrient addition can also have a significant effect on amino acid concentrations (Udy & Dennison, 1997a and b). The increase in concentration at depth is linked to a response to reduced light availability and could be related to the balance of nutrient against light limitation of seagrass growth, the light condition bringing about the elevated amino acid content in the plants. In response to shading and increased water depth, the carbon isotope ratio ( 13 C) of H. pinifolia leaves became more negative (Abal, 1996; Grice et al., 1996; Longstaff et al., 1999). This may be due to a more rapid uptake of 12 C in relation to 13 C, the preferential rate occurring because 12 C uptake requires less energy in com- parison to 13 C (Abal & Dennison, in press; Grice et al., 1996; Longstaff et al., 1999). Whether reductions in light availability have significant effects on seagrass growth and survival depends primarily on the efficiency with which light energy is used in the autotrophic accumulation of plant biomass. These are often described using photosynthesis-irradiance (or P-1) curves (Drew, 1979). Species that are able to physiologically acclimate to reduced light by adjusting their P-1 curves will have a better chance to survive severe siltation events. Measured P-1 curves of different species revealed that in the Philippine Enhalus acoroides and Thalassia hemprichii, variation in the compensation depth (i.e., the depth at which daily respiratory demand and photosynthetic oxygen supply are just in balance) with water depth and turbidity correlates well with predicted maximum colonization depth. An important observation may then be derived from the colo- nization depth-turbidity curve which would suggest that small reduction in water clarity may dramatically affect seagrass performance in relatively clear waters of the Mediterranean and the Philippines (K Ͻ0.5 m Ϫ1 ). Furthermore, it would suggest that moderately eutrophicated waters subjected to increases in turbidity may not allow seagrasses to colonize deeper parts. Recent shading studies have shown that the survival period of a seagrass below the minimum light required may be altered by adaptations in photosynthetic parame- ters (e.g., increased chlorophyll content, changes in the chlorophyll a:b ratio, The Effects of Siltation on Tropical Coastal Ecosystems 97 © 2001 by CRC Press LLC increased canopy height and shoot thinning: Abal et al., 1994; Lee & Dunton, 1997). This was the case with H. pinifolia which demonstrated an increased chlorophyll con- tent, decreased chlorophyll a:b ratio, and an increased canopy height under condi- tions of shading (Longstaff et al., 1999). EFFECTS OF SILTATION ON SEAGRASS MORPHOLOGY Traditionally changes in the morphology of seagrasses have been used as an indica- tor of an adverse environmental effect on a seagrass community (e.g., Posidonia sin- uosa, shoot density and leaf length) (Gordon et al., 1994). In the present study, the morphological responses to siltation under consideration include decreases in bio- mass, shoot density, and canopy height. It should be noted, however, that physiolog- ical responses can detect declining seagrass health and impending seagrass die-off before substantial morphological changes occur. Sediment dynamics over a seagrass bed may range from a gradual, continuous deposition to a sudden storm-related event (Marba et al., 1994a), and from a homo- geneous rate over large areas to small-scale variability associated with sand ripples or dunes (Marba et al., 1994b). Seagrasses may respond to the latter via horizontal rhizome growth. On the other hand, the species respond to homogeneous sedimenta- tion rates only via vertical stem elongation or re-establishment from seeds. Vertical stem growth, even in Philippine seagrasses, has been shown to be sea- sonal (Duarte et al., 1994; Vermaat et al., 1995): during the growing season, longer internodes are formed and this often also occurs at a higher rate than at other, less favorable times of the year (Duarte et al., 1994). It is probable that the capacity of sea- grasses to respond to burial may also be seasonal, and off-season siltation may have more dramatic effects than expected. Genera without differentiated vertical stems may respond with a redirection of the horizontal rhizome to survive excessive silta- tion and burial. Shoot size is an obvious determinant of the chance to survive a burial event: larger shoots are simply less easily buried. The largest Philippine species Enhalus acoroides, for example, has horizontal rhizome branches that curve upward to posi- tion the apical meristems at an average distance of 10 cm from the main rhizome, which is generally several centimeters above the sediment. With full-grown leaves measuring about 80 cm, the leaf canopy reaches considerably further upward (Vermaat et al., 1995). Halophila ovalis, the smallest Philippine species, also lacks vertical stems, but its oval leaf blades have petioles that may reach a length of 2 cm, a height that allows a substantial short-term sediment deposition rate over the short shoot life span of this species (1 to 2 weeks) (Duarte, 1991; Vermaat et al., 1995). For species that do have vertical stems, considerable variation exists in annual mean vertical growth rates, particularly among the Philippine species: 1.5 to 13 cm shoot Ϫ1 yr Ϫ1 (for Cymodocea rotundata and C. serrulata, respectively). Additionally, species differ in the height of their vertical stems. Stem lengths range between 1 and 8 cm. These vertical stems are partly buried in the sediment, but particularly in C. serrulata, also reach above the sediment surface. Although mean annual vertical stem growth is strictly not comparable to an instantaneous response to a short-term 98 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC sedimentation event, the former will set the order of magnitude of the short-term response of the seagrass shoot. Short-term responses, however, have been quantified in a few species only. In contrast to vertical stem growth, horizontal rhizome expansion is closely cor- related with seagrass size and longevity. Rhizome growth is slowest (2 to 5 cm yr Ϫ1 ) in larger and longer-lived species (Duarte et al., 1994; Vermaat et al., 1995; Duarte, 1991). This capacity may allow shorter-lived species to migrate away from or into newly deposited sediment forms. Horizontal expansion rates at patch edges, i.e., into newly available bare-ground, are often considerably higher than those measured in established beds. Whereas the difference between species in mean annual vertical stem growth amounts to about a factor of 10, horizontal rhizome growth differs by a factor of 30, a difference present among both Mediterranean and Philippine species. Annual horizontal growth in northern temperate Zostera species is limited, though these species have relatively short life spans and small shoots. This is mainly caused by the reduced length of the growing season (Marba et al., 1994a; Vermaat & Verhagen, 1995), since rhizome internodal lengths and growth rates during the grow- ing season are comparable to those of other small species. The slowest-growing and longest-lived Philippine species, Enhalus acoroides, as well as the oldest Mediterranean species, Posidonia oceanica, also have the largest shoots and rhizomes (Duarte, 1991; Vermaat et al., 1995). In the Mediterranean, the larger and longer-lived species showed less annual variation in photosynthetic para- meters than the shorter-lived species, supporting the suggestion of increased seasonal buffering with increased size and age (Duarte, 1991). This pattern, however, was not confirmed for the three studied Philippine species, which are all rather long-lived. Morphological adjustments may also improve light availability considerably. Longer leaves or stems raise the photosynthetic tissue closer to the water surface, an investment which will probably pay off in turbid, shallow waters where light is atten- uated exponentially. The tallest tropical seagrass Enhalus acoroides is able to lift its leaves much closer to the water surface, growing in turbid water on shallow (1 to 2 in.) mudflats close to river mouths (Nienhuis et al., 1989; Brouns & Heijs, 1991; Erftemeijer & Herman, 1994). In mixed meadows, form and size could be decisive and one would expect that the smallest species in the lower leaf canopies would suffer most the impact of light deprivation, e.g., Halophila ovalis, Halodule uninervis, and Syringodium isoeti- folium (Vermaat et al., 1995). However, in clear waters, Halophila species have been found to grow considerably deeper than most other seagrass species (Duarte, 1991), and for one species Drew (1979) found a comparatively low compensation point (9 ␮E m Ϫ2 s Ϫ1 for Halophila stipulacea). Hence, species from the genus Halophila may survive longer under reduced light regimes. In some areas, seagrasses have to cope with burial through sediment deposition and resuspension. Burial affects seagrasses adversely by reducing light availability to affected photosynthetic tissue, reducing diffusion of O 2 to roots and rhizomes; and mechanically counteracting the production of new leaves by deeply buried meristems (Duarte et al., 1997). Seagrass responses to increased sedimentation include adjust- ments in vertical stem elongation or horizontal rhizome expansion (Duarte et al., The Effects of Siltation on Tropical Coastal Ecosystems 99 © 2001 by CRC Press LLC 1994; Marba et al., 1994 a and b), or by recolonization from seeds (Duarte et al., 1997). Architectural differences among species result in considerable ecological advantages for survival. RESPONSES OF CORALS TO SILTATION/SEDIMENTATION Sediment deposition and suspended sediments affect coral community structure dif- ferently. The inability of coral planulae to settle in areas where soft sediments con- tinually cover the bottom support the observation that sediment deposition has generally an adverse effect on living coral (Ruitenbeek et al., 1999). Adult coral colonies of some species may survive silt cover for short periods (e.g., hours to days). However, coverage for longer periods is lethal to virtually all species (Ruitenbeek et al., 1999). On the other hand, greater coral abundance may be found in many reefs with high suspended sediment loads. Species composition in these areas may differ sub- stantially from that in areas with low suspended sediment. This is in part due to the differential ability of the polyps to eject sediment. Hence, coral reefs may exhibit wide variations in species composition in areas of differing suspended sediment loads, but coral cover may not vary significantly with suspended sediment loading (Ruitenbeek et al., 1999). Sedimentation patterns exert a significant control on reef development via their influence on both sediment deposition and suspended sediment. In St. Croix, U.S. Virgin Islands, lower transport rates of sediments permit faster reef growth (Hubbard, 1986). Annual storms (wave height ϭ 3 to 5 m), however, result in order-of-magni- tude increases in sediment transport. They periodically flush sediments and offset the usual imbalance between sediment import and export. EFFECTS OF SEDIMENTATION ON CORAL ABUNDANCE, DIVERSITY, AND DISTRIBUTION Sedimentation is among the important factors that determine coral abundance, growth, and distribution (Hodgson, 1990; Babcock & Davies, 1991). High turbidity and sedimentation decrease coral abundance, alter coral growth forms to a more branching habit, and decrease species diversity (Dodge & Vaisnys, 1977). The diver- sity of corals on all intertidal flats in the vicinity of tin dredging and smelting activi- ties around Laem Pan Qah peninsula, Phuket, was low (six genera), the dominant genera being Porites, Montipora, Acropora, and Platygyra (Brown & Holley, 1981). Dodge and Vaisnys (1977) likewise reported that analysis of coral growth patterns and populations in Bermuda reveals that living coral abundance on the reefs of Castle Harbor, a location where extensive dredging occurred during 1941 to 1943, is much reduced in comparison to external North–South reefs. In Bolinao (NW Philippines), Wesseling et al. (1997) further found that Acropora completely buried with littoral sediment (16% silt, 38% fine sand, and 38% coarse sand) experienced high mortality. This finding suggests a reduction in coral 100 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC composition in reefs subjected to intense sedimentation. Less sensitive taxa (e.g., Porites), however, were found to recover within a month of exposure. The probable causes of these events include turbidity, physical tissue damage, reduced larval recruitment and mortality, and their effects on coral survival. Turbidity reduces underwater light due to scattering from sediment particles in the water col- umn. Hence, a source of energy is virtually lost. In addition, time and energy that could be used to capture food, grow, metabolize, and reproduce are likewise lost (Dodge & Vaisnys, 1977). Experimental application of sediments onto living coralline tissues has demon- strated detrimental effects including expulsion of zooxanthellae, cellular damage, and after complete burial, death (Babcock & Davies, 1991). On the other hand, they found that while higher sedimentation rates reduced the number of larvae settling on upper surfaces, total numbers of settled larvae were not significantly affected by sed- imentation regime. EFFECTS OF SEDIMENTATION ON CORAL GROWTH AND PRODUCTIVITY At 13 sites with varying siltation levels in the Philippines, studies were conducted on the responses of corals to sedimentation. At the level of the colony, the comparatively fewer number of white and dark bands observed in Porites at a more silted site indicated slower growth rate when compared to colonies with a greater number of bands observed at a less silted site (Mamaril-Villanoy et al., 1997). Barnes and Lough (1993) found that coral growth over a year is represented by adjacent dense and less dense bands which may be caused by different factors, among which are turbidity and sedimentation. At the population level, Wesseling et al. (1997) differentiated two types of lesions in corals found along siltation gradients: Type I lesions, surrounded with liv- ing tissue, and Type II lesions, at the edge of colonies. Colony size and density of lesions varied among reefs, with smaller colonies and more lesions observed in more exploited and silted areas. A relation with sedimentation rate, however, was found only for Type II lesions where it increased significantly above a sedimentation thresh- old rate of about 25 mg/cm 2 /day. Sediment affects coral metabolism by decreasing photosynthetic production, increasing relative respiration, and increasing carbon loss through greater mucus out- put (Riegl & Brance, 1995). In nine coral species investigated under simulations of natural sedimentation levels and light conditions, a severe reduction in productivity and respiration was recorded under sedimented conditions. P/R ratios of all species were above 1 in no-silt conditions. In silted conditions, on the other hand, the ratios dropped below 1. In relation to mucus secretion, it averaged 35% of daily respiration under the unsilted condition; the value rose to 65% under silt treatment (Riegl & Brance, 1995). MODELING REEF STATUS AND SEDIMENTATION Two recent procedures are used to generate a surface dose-response model of the rela- tionship among coral abundance and various inputs including sedimentation. These The Effects of Siltation on Tropical Coastal Ecosystems 101 © 2001 by CRC Press LLC are fuzzy logic procedures and watershed-based modeling. The first is linked to a non-linear economic structure incorporating technical intervention (e.g., pollution treatment) and policy interventions (e.g., taxation) (Ruitenbeek et al., 1999). The result of the optimization process gives insights into the most cost-effective means to protect reefs under different reef quality targets. In Montego Bay, Jamaica, for exam- ple, appropriate policy measures costing (US) $12 million are estimated to improve coral abundance by 10% in 25 years. At a cost of (US) $153 million, these are expected to provide up to 20% increase. Watershed-based modeling of sedimentation and inland pollution is a part of a global analysis, involving 3000 watersheds in the world. It integrates data on slope, precipitation, and land cover type to estimate “relative erosion potential” (REP) by roughly a 2-km grid cell (Burke, L., personal communication). The results are sum- marized by watershed to develop criteria for watersheds of low, medium, or high mean REP. The zone of effect for sediment discharge is estimated based upon an esti- mate of flow (discharge) for the peak rainfall month. RESPONSES OF MANGROVES TO SILTATION Siltation is of primary importance in the development of mangroves. In deltas along the coasts of Southeast Asia, mangroves cover large areas. This is largely because of high rainfall and rivers with high silt loads which combine to provide favorable con- ditions for their development (Milliman & Meade, 1983; Milliman & Syvitski, 1992). Highest productivity values are usually reported in mangroves associated with rivers (Twilley et al., 1986). River flow and tides transport a large fraction of mangrove pro- duction (on average 29.5%: Duarte & Cebrian, 1996) to nearby habitats in the form of leaf litter and propagules (e.g., Twilley et al., 1986; Hemminga et al., 1994; Panapitukkul et al., 1998). In addition a substantial fraction of mangrove production is buried in the sediments (10.4% on average: Duarte & Cebrian, 1996), causing a large fraction of the mangrove production (therefore, a large quantity of nutrients) to be lost from the ecosystem (Boto & Bunt, 1981; Twilley et al., 1986). Primary pro- duction of mangrove habitats therefore tends to depend on continuous nutrient sup- ply from land or sea (Duarte et al., 1998). This nutrient dependence led to the hypothesis that mangrove growth may be nutrient-limited, as has been shown by Boto and Wellington (1983) and Feller (1995). Growth of Rhizophora apiculata seedlings living at the edge of progressing man- grove forests at the study sites in the Philippines and Thailand is directly correlated to the nutrient and silt contents within the sediments (Duarte et al., 1998). Sites with low nutrients and coarse sediments yielded seedlings with very low growth rates. On the other hand, nutrient-rich, silty sediments yielded seedlings with much faster growth rates. The size of the watersheds drained by the rivers where mangroves grow has a strong linkage with, among others, sediment composition and mangrove growth (Duarte et al., 1998), while autochthonous substances are received by the mangrove itself (Boto & Bunt, 1981; Boto, 1984; Twilley et al., 1986). However, substantial 102 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC [...]... relationship among coral abundance and various inputs including sedimentation These are fuzzy logic procedures and watershed-based modeling The first is linked to a non-linear economic structure incorporating technical interventions (e.g., pollution treatment) and policy interventions (e.g., taxation) (Ruitenbeek et al., 1999) The result of the optimization process gives insights into the most cost-effective... Marine Ecology Progress Series 11, 63–69 Brouns, J.J & Heijs, F.M.L 1991 Seagrass ecosystems in the tropical west Pacific pp 371 –390 in Mathieson, A.C & Nienhuis, P.H (eds) Ecosystems of the World 24, Intertidal and Littoral Ecosystems Elsevier, Amsterdam, 572 pp Brown, B.E & Holley, M.C 1981 The influence of tin smelting and tin dredging on the intertidal reef flats of Phuket, Thailand pp 214 in Proceedings... recovery of the seagrass Halophila ovalis Journal Experimental Marine Biology Ecology 234, 1– 27 Lundin, C.G & Linden, 0 1993 Coastal ecosystems: attempts to manage a threatened resource Ambio 22, 46 73 Macnae, W 1968 A general account of the fauna and flora of mangrove swamps and forests in the Indo-Pacific region Advances Marine Biology 6, 73 – 270 Madsen, T.V & Sand-Jensen, K 1994 The interactive effects of. .. W.J 1992 Protecting Fish and Wildlife Habitat through a Better Understanding of the Minimum Light Requirements of Sub-Tropical Seagrasses in the South-Eastern United States and Caribbean Basin Ph.D thesis, North Carolina State University, Raleigh, 345 pp Kirk, J.T.O 1983 Light and Photosynthesis in Aquatic Ecosystems Cambridge University Press, Cambridge, 378 pp Klumpp, D.W., Salita-Espinosa, J.T., &... Vaisnys, 1 977 ) In Bolinao (NW Philippines), Wesseling et al (19 97) further found that Acropora completely buried with littoral sediment (16% silt, 38% fine sand, and 38% coarse sand) experienced high mortality This finding suggests a reduction in coral composition in reefs subjected to intense sedimentation Less sensitive taxa (e.g., Porites), however, were found to recover within a month of exposure The. .. from land or sea (Duarte et al., 1998) This nutrient dependence led to the hypothesis that mangrove growth may be nutrient-limited Growth of Rhizophora apiculata seedlings living at the edge of progressing mangrove forests at the study sites in the Philippines and Thailand is directly correlated to the nutrient and silt contents within the sediments (Duarte et al., 1998) Sites with low nutrients and. .. 3 07 315 den Hartog, C 1 970 The Seagrasses of the World North-Holland, Amsterdam, 275 pp Dennison, W.C 19 87 Effects of light on seagrass photosynthesis, growth and depth distribution Aquatic Botany 27, 15–26 Dennison, W.C & Alberte, R.S 1985 Role of daily light period in the depth distribution of Zostera marina (eelgrass) Marine Ecology Progress Series 25, 51–62 © 2001 by CRC Press LLC The Effects of. .. condition and turbidity in the Dutch Wadden Sea Aquatic Botany 37, 71 –85 Gordon, D.M., Grey, K.A., Chase, S.C., & Simpson, C.J 1994 Changes to the structure and productivity of a Posidonia sinuosa meadow during and after imposed shading Aquatic Botany 47, 265– 275 Grice, A.M., Loneragan, N.R., & Dennison, W.C 1996 Light intensity and the interactions between physiology, morphology and stable isotope ratios in. .. of sedimentation on growth and productivity of coral reefs may be manifested at the levels of the colony, population, and community Sedimentation affects coral metabolism by decreasing photosynthetic production, increasing relative respiration, and increasing carbon loss through greater mucus output (Riegl & Brance, 1995) Two recent procedures are used to generate a surface dose–response model of the. .. mangroves grow has a strong linkage with, among others, sediment composition and mangrove growth The nonlinear relationship between seedling growth performance and watershed size found in the study in Thailand identifies mangroves next to rivers draining watersheds larger than 10 km2 as the most profitable target areas in the efforts promoting natural and artificial colonization of Rhizophora apiculata . Ecosystems of the World 24, Intertidal and Littoral Ecosystems. Elsevier, Amsterdam, 572 pp. Brown, B.E. & Holley, M.C. 1981 The influence of tin smelting and tin dredging on the inter- tidal reef. apiculata seedlings living at the edge of progressing man- grove forests at the study sites in the Philippines and Thailand is directly correlated to the nutrient and silt contents within the sediments. 38% fine sand, and 38% coarse sand) experienced high mortality. This finding suggests a reduction in coral 100 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC composition in reefs