Erosion and Sedimentation

This portion of the web site is still under construction. A preview is provided below and a full discussion of the impact of sediment on coral reefs.

Views of erosion on the island:

Increased turbitiy resulting from upstream development



The Impact of Sediment on Coral Reefs

In many tropical regions, the development of the coastal zone to satisfy human needs and desires traditionally has received a higher priority than the protection of coral reef resources. Regions where such deleterious effects have been documented include Hawaii (Hunter and Evans, 1995), Puerto Rico (Loya, 1976; Edinger and Risk, 1994), Costa Rica (Cortes and Risk, 1985; Hands et al.., 1993), Nicaragua (Roberts and Murray, 1983), Barbados (Tomascik and Sander, 1985; Hunte and Wittenberg, 1992), St. Croix (Hubbard, 1985), Australia (Neil, 1990), the Red Sea (Hawkins and Roberts, 1994) and Kenya (McClanahan and Obura, 1995). However, as these ecological consequences have become apparent, officials have begun to rethink their priorities and are trying to implement coastal zone management plans. However, in order to develop effective and balanced management plans, coastal decision makers must be able to link particular activities on land with deleterious effects on the coral communities. Without this ability, management decisions will likely either unnecessarily curb development activities or inadequately protect the coral reefs.

Frequently, the cause-and-effect relationship between an action on land and the impact on the reef can be difficult to determine because of the complexity of interactions between land, sea, air, and man. One of the most implicated, yet understudied, causes of damage to corals is the introduction of sediment to coastal waters as a result of development activities. In order to understand the relationship between human activities and coral degradation, mechanisms must be developed to link the activities which cause sediment loading in near-shore waters to the impact it has on the coral communities. This review will look at the biological impacts of sediment on coral reef communities.

1. Sediment and Coral

This review synthesizes current knowledge on the relationship between coral and sediment. The mechanisms by which coral responds to sediment stress are discussed within the context of their effect on individual corals and on community and population dynamics.

1.1. Background

Coral reef communities are unique and important ecosystems. Coral covers about eighty million square miles of the earth’s surface, an area nearly twenty-five times the area of the United States (Cousteau, 1971). An extremely productive ecosystem, coral reefs directly support coastal fisheries and indirectly many pelagic species by providing nursing grounds for juveniles (Sorokin, 1993). Reefs offer protection from erosion by providing the sand that lines the beaches and the structures which buffer the effect of waves (Richmond, 1993). From a human’s perspective, reefs provide nourishment, recreational opportunities, medicinal products, and a draw for tourists (Richmond, 1993). Additionally, coral reefs have traditionally offered security and cultural value and utility (Salm and Kenchington, 1984).

Coral demands a specific combination of environmental conditions in order to survive. For long-term viability, coral needs: 1) waters that are warm (between 25-29 degrees C), low in nutrients, and relatively free of pollutants; 2) firm substrate for recruitment; 3) salinity above 28%; and 4) clear water to provide sufficient sunlight to allow the symbiotic algae to actively photosynthesize (Cousteau, 1971; Sumich, 1980; Edinger and Risk, 1994). As a result of these limitations, coral reefs are found only to depths of 135 feet and only in tropical and subtropical waters, lying no more than 32 degrees north or 27 degrees south of the equator (Cousteau, 1971). Within this narrow range of parameters, coral communities have established a complex ecosystem consisting of many symbiotic relationships and ecological specialists. Like other niche ecosystems, coral reefs are therefore highly susceptible to environmental perturbations.

In recent decades, the actions of humans have altered these conditions and threatened the health and viability of coral reefs. Approximately sixty percent of the world’s population live within the coastal zone (Green et al., 1996), and the by-products of their activities tend to find their way into coastal waters. Sewage, pesticides, heavy metals, runoff and other related compounds readily move from the land to the sea (Richmond, 1993). One of the most visible consequences of man’s activities is the polluting of coastal waters with high levels of sediments. Many activities contribute to increasing the sediment loads in waterways. Agriculture, grazing, development projects, dredging, beach replenishment and urbanization in general have led to increased soil erosion and sedimentation (Hunter and Evans, 1995; Richmond, 1993; Hawkins and Roberts, 1994). Tropical regions, in particular, are unduly affected because such activities are usually unregulated, and the warm climate, higher precipitation rates, and poorly developed soils make these areas more susceptible to erosion (Roberts and Murray, 1983). The impacts can be significant: one large highway construction project in Hawaii increased sediment loads in the nearby stream to six times the levels prior to construction (Hill and DeCarlo, 1991 as cited in Hunter and Evans, 1995) while another author noted that sediment transport was an order of magnitude higher in a coastal stream affected by development during a storm than during fair weather (Hubbard, 1986).

1.2. Impacts of Sediment on Coral

Both the popular press and scientific journals contains numerous qualitative and quantitative accounts of how increased sediment concentrations negatively effects on coral reefs. Based on inference from personal observations, SCUBA divers and naturalists have long associated heavy sedimentation with fewer coral species, less live coral, lower coral growth rates, and greater abundance of branching forms (Rogers, 1990). Accounts of such relationships abound from coral reefs the world over (see introduction). Cousteau (1971) attributes the complete absence of coral off the otherwise suitable coasts of Brazil, India, and West Africa to the vast quantities of mud that is discharged by the great rivers of those regions. While most accounts are qualitative, many simple quantitative studies have correlated increased sedimentation with a decrease in growth rates and coral mortality (Dodge and Vaisnys, 1977). One of the best documented cases of such a relationship is the increase in coral abundance and decrease in algal cover associated with the dramatic and rapid decrease in nutrient levels, turbidity, and phytoplankton abundance following the diversion of sewage outfalls in Kaneohe Bay, Hawaii in 1986 (Hunter and Evans, 1995). While recent research has shown that this trend has reversed itself due to other sources of pollution, the study clearly exhibits the direct link between water quality and coral reef abundance and health (Hunter and Evans, 1995).

1.3. Mechanisms of Impacts of Sediment on Coral

Controlled laboratory and field experiments have provided insight into the physical and biological mechanisms by which coral reefs respond to, and are affected by, sediment. Increased concentrations of suspended sediment alter the aquatic environment in three primary ways: reducing in the amount of available light, increasing the rate of sedimentation, and increasing nutrient concentrations in the coastal waters. The response of coral communities to these alterations will be discussed, as will the effects that these responses have on changing the coral-algal symbioses, shifting competitive interactions, increasing reproductive failure and causing insufficient recruitment (Richmond, 1993).

1.3.1. Turbidity and decreased light availability

As is well known, reef building colonial corals contain within their soft bodies symbiotic, unicellular algae called zooxanthellae (Preobrazhensky, 1993). The corals obtain the majority of their nutritional requirements via translocation of metabolites from their photosynthesizing symbiotes (Richmond, 1993). Studies have shown that suspended sediments decrease the quality and quantity of incident light levels, resulting in a decline in the photosynthetic productivity of zooxanthellae (Dallmeyer et al.., 1982). Because of the dependency of choral on zooxanthellae, such a decrease in algal productivity causes a requisite drop in the nutrition, growth, reproduction and depth distribution of corals (Richmond, 1993). Such stress has also been shown to decrease growth rates due to the disruption of the coral-algal relationship which is essential in the calcification process (Pastorok and Bilyard, 1985). In certain coral species, this drop in productivity actually can result in the starvation of the coral since the corals can neither feed nor adjust their respiration sufficiently to compensate for the loss in energy (Richmond, 1993). Additionally, long-term turbidity stress can shift the species composition of reefs through the death of more light demanding corals and the subsequent replacement by deeper, more shade tolerant ones (Pastorok and Bilyard, 1985).

1.3.2. Increased sedimentation rates

In addition to blocking light penetration, sediments can physically interfere with coral metabolism. Coral rely on their cilia and tentacles to catch prey in order to supplement the energy provided by the zooxanthellae. As sedimentation rates increase, these feeding surfaces become coated by silt and sand. Although corals possess an active, epidermal mucociliary system to remove sediment (Hubbard and Pocock, 1972), the removal of sediment necessitates the expenditure of energy. Telesnicki and Goldberg (1995) illustrated this increase in expenditure by comparing differences in oxygen consumption between stressed and unstressed corals. Thus, sediment not only disrupts feeding, but also extracts a high cost in energy through the process of cleaning their cilia, energy which otherwise could be used for food capture, growth, skeletal repair or reproduction (Richmond, 1993; Peters and Pilson, 1985; Dodge and Vaisnys, 1977). Examples of effects from varying rates of sedimentation are given in Table 1.

Increased sedimentation rates also have been shown to affect reproductive success and the probability of recruitment by larvae. Using water collected from a sediment stressed reef on the night of a mass spawning event, Richmond (1993) found a 77% reduction in the numbers of embryos surviving to the planula larva stage as compared to controls. Although the impacts of the lowered salinity level and the sediment could not be separated, Richmond (1993) warns that the reliance of coral on annual spawning events makes them susceptible to decreased water quality. These results support earlier findings on the effects of sedimentation on the settlement of Acropora millepora larva (Babcock and Davies, 1991). This sensitivity to sediment has been shown to persists after settlement has occurred. Newly settled larvae have been known to "bail out" of their exoskeleton in response to sedimentation (Te, 1992; Samarco, 1982).

1.3.3. Increased nutrients

Increased nutrient competition has the most direct impact on community dynamics. High nutrient concentration in sediments usually originate from either sewage outfalls or areas such as golf courses where fertilizers and pesticides are used extensively (Richmond, 1993). Nutrients exacerbate the problems caused by sedimentation and reduced light levels by shifting the competitive balance of the community to favor the growth of algae and benthic organisms over the growth of coral (Edinger and Risk, 1994). Coral evolved to fill a niche in nutrient poor waters and primarily maintain their competitive advantage through the symbiotic relationship with zooxanthellae. With the addition of nutrients, the coral lose their advantage, and dense algal mats begin to form. In Kanehoe Bay, Hunter and Evans (1995) found this relationship when nutrients were removed from the system, and the dominant species subsequently changed from the green bubble alga, Dictyosphaeria cavernosa, to more coraline species. Additionally, these furrowed algal mats tend to trap sediment, and inhibit the successful colonization of coral larvae by essentially burying all hard substrate (Hunte and Wittenberg, 1992; Richmond, 1993). Therefore, the alteration of the nutrient balance not only shifts the population dynamics, but also can have dramatic effects on the rate of recruitment.

1.4. Gradients of Sediment Stress and Thresholds

Despite these responses to sediment stress, some coral species exhibit negligible, and even beneficial, effects from increases in sediment concentrations. Studies have shown that damage from sediments may be minimal as long as adequate nutrients, from either internal (zooxanthellae) or external (zooplankton, particulates) sources, are available to the coral animal to meet the additional energy expenditures required to rid themselves of sediment (Peters and Pilson, 1985). In fact, some corals grow most rapidly under moderate to high nutrient concentrations (Tomascik and Sander, 1985); but this growth is quickly offset by competition from non-calcified organisms such as benthic and planktonic algae as well as bioerosion from boring sponges and worms (Edinger and Risk, 1994).

Such responses to sediment are highly species dependent. The relative tolerance of some coral species to sedimentation are given in Table 2. In general, those species that tend to grow in near-shore waters are more tolerant to high concentrations of suspended sediment than species found on the deeper, seaward fringing reefs (Pastorok and Bilyard, 1985; Roberts and Murray, 1983). Figure 1 empirically illustrates this trend by comparing coral coverage, water turbidity and depth. Field experiments on three coral species found that species differ on the way they express stress from sediment; Porites lutea had lower mortality rates than Acropora or Pocillopora, but reduced its growth rates at lower levels of stress (McClanahan and Obura, 1995). Such differences between species in their ability to tolerate sediment stress appears in the geologic record as well. While 50% of all coral genera in the Caribbean went extinct between 16 and 24 million years ago, nearly all coral genera tolerant of both turbidity and cool water survived (Edinger and Risk, 1994).

These data indicate that all coral species lie along a gradient of relative tolerance to stress from sediment. Each coral species, therefore, has its own set of "threshold values" representing the concentrations of sediment which produce sublethal or lethal effects. Tomascik and Sander (1985) in-situ experiments support the concept of thresholds when they determined that after a certain maximum concentration, reduction of growth occurs due to smothering, reduced light levels and reduced zooxanthellae photosynthesis. These thresholds limits are an important concept for managers, because they can provide insight into the levels of sediment pollution that the local coral community can tolerate. Additionally, they provide an understanding of how sensitive measurements of suspended sediment concentrations must be in order to be useful in managing coral reefs.

2. Satellite Imagery Analysis and Sediments

Knowing the effects of suspended solids on coral is only the first step towards developing an effective coral reef management plan. A critical second step is to understand the a priori mechanisms by which sediments come in contact with corals. From a management perspective, the critical question is: how will a given action affect the sediment loads in coastal waters? One approach to answering the question is to wait until the activity has started and then directly measure the increase in sedimentation as compared to measurements taken prior to the onset of the activity. Another approach which draws more directly on the spatial nature of the question is to develop a geographic information system model which mimics the erosion and sediment transport processes. Using either approach, satellite imagery analysis can assist in determining the concentration and extent of suspended sediment.

The identification and analysis of suspended sediment using satellite imagery has traditionally progressed along two lines of research: 1) directly calculating the concentration of suspended sediments within a known plume using coincident field sampling ; and, 2) using hydrologic and sediment transport models to predict sediment concentrations. This review will present the methodologies behind each approach, and will look at the role that satellite remote sensing plays in both types of analyses.

2.1. Background

Satellite remote sensing has been used to study the coastal zone since the first LANDSAT satellite was launched in 1972. With the complexity and rapidly changing nature of the coasts, remote sensing readily lent itself to monitoring and measuring coastal resources. Now, over twenty years later, routine applications of satellite imagery analysis include the mapping of littoral and shallow marine habitats, the detection of changes in resources, mapping coastal bathymetry, and the study of sediment plumes and coastal currents to assess impacts of coastal development (Green et al., 1996). Four satellite sensors have been primarily used for these purposes: LANDSAT MSS, LANDSAT TM, SPOT XS, and SPOT XP. As indicated in table 3, difference between these sensors include variations in their spatial resolution, periodicity of measurement, spectral resolution and image size. It is beyond the scope of this review to compare and contrast these sensors in regards to their applicability for specific coastal analyses; instead, the potential for the whole class of satellite sensors will be explored. Readers interested in the capabilities of specific sensors are referred to Lillesand and Kiefer (1994).

2.2. Suspended Sediments: Direct Measures

The direct calculation of suspended sediments has been pursued by researchers from a variety of disciplines (Pattiaratchi et al., 1992; Holyer, 1978; Whitlock et al., 1982; Ritchie and Cooper, 1991). Despite this diversity, the underlying goal of the body of work has remained the same: to derive algorithms which relate the spectral reflectance of suspended solids to in-situ field measurements of suspended sediment concentrations, sechi disk measurements, and/or chlorophyll a counts (Muller et al.., 1993). Once an algorithm is derived, the suspended sediment concentration for each pixel of water in the image can be calculated using only spectral reflectance values. One of the long-sought goals of this research is to develop a universal multispectral suspended sediment algorithm (Holyer, 1978). This algorithm would describe a constant relationship between the reflectance values measured by a particular satellite sensor and the actual concentration of suspended sediment in water. While eliminating the need for ground truthing would greatly expand the application of this method, deriving a standardized relationship has proven illusory. Recently, however, increased consistency between water quality calibration algorithms suggest that it still may be possible to develop an algorithm for widespread use (Ritchie and Cooper, 1991).

Researchers have had much success in deriving calibration algorithms between reflectance values from specific images and the coincidentally measured water quality parameters (Munday and Alfoldi, 1979; Muller et al., 1993). Numerous studies have illustrated that accurate estimates of suspended sediment concentrations can be made using satellite images. Holyer (1978) derived an algorithm which had a less than 0.05 variance down to a detection threshold of 25 mg/l, an accuracy which satisfied the standards set by the US EPA for sediment monitoring purposes. Many other studies have made estimates that were statistically significant at the 0.05 level, an accuracy which has been suggested to translate to a plus or minus 10% accuracy in sediment concentration estimates (Verdin, 1985; Lathrop, 1992; Munday and Alfoldi, 1979; Holyer, 1978; Lathrop and Lillesand, 1989; Ritchie and Cooper, 1991). While most previous work has been done on in-land lakes and rivers, these techniques are successfully being applied to coastal systems as well (Green et al.., 1996).

The value of directly calculated suspended sediment concentrations depends on the type of question being asked. Since direct measurement estimates can never be done before an event occurs, this methodology has no predictive abilities for a specified event. However, through periodic analysis, direct measurement can be used as a monitoring tool to determine changed in sediment concentration in an area over time. A coastal manager could use this data to determine areas which are exceeding specified water quality standards. This information could be used to curtail activities in these regions or simply to help determine where to allocate often scarce research funds. Multitemporal analysis can also be used to determine times of the year when sediment loading is a problem, data which could provide the basis for setting "seasons " for certain types of activities such as dredging or beach nourishment.

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Last modified on May 14, 1997.