Responses of coral reefs and reef organisms to sedimentation

Abstract

Unprecedented development along tropical shorelines is causing severe degradation of coral reefs primarily from increases in sedimentation. Sediment particles smother reef organisms and reduce light available for photosynthesis. Excessive sedmentation can adversely affect the structure and function of the coral reef ecosystem by altering both physical and biological processes. Mean sediment rates and suspended sediment concentrations for reefs not subject to stresses from human activities are < 1 to ca 10 mg cm-* d-' and < 10 mg I-', respectively. Chronic rates and concentrations above these values are 'hlgh'. Heavy sedmentation is associated with fewer coral species, less live coral, lower coral growth rates, greater abundance of branching forms, reduced coral recruitment, decreased calcification, decreased net productivity of corals, and slower rates of reef accretion. Coral species have different capabilities of clearing themselves of sediment particles or surviving lower light levels. Sedlment rejection is a function of morphology, orientation, growth habit, and behavior; and of the amount and type of s e l m e n t . Coral growth rates are not simple indicators of sediment levels. Decline of tropical fisheries is partially attributable to deterioration of coral reefs, seagrass beds, and mangroves from sedimentation. Sedimentation can alter the complex interactions between fish and their reef habitat. For example, sedimentation can lull major reef-building corals, leading to eventual collapse of the reef framework. A decline in the amount of shelter the reef provides leads to reductions in both number of individuals and number of species of fish. Currently, we are unable to rigorously predict the responses of coral reefs and reef organisms to excessive sedimentation from coastal development and other sources. Given information on the amount of sediment which will be introduced into the reef environment, the coral community composition, the depth of the reef, the percent coral cover, and the current patterns, we should be able to predict the consequences of a particular activity. Models of physical processes (e.g. sediment transport) must be complemented with better understanding of organism and ecosystem responses to sediment stress. Specifically, we need data on the threshold levels for reef orgarusms and for the reef ecosystem as a whole -the levels above which sedimentation has lethal effects for particular species and above which normal functioning of the reef ceases. Additional field studies on the responses of reef organisms to both temgenous and calcium carbonate sediments are necessary. To effectively assess trends on coral reefs, e.g. changes in abundance and spatial arrangement of dominant benthic organisms, scientists must start using standardized monitoring methods. Long-term data sets are critical for tracking these complex ecosystems.

Cryptic regime shift in benthic community structure on shallow reefs in St. John, US Virgin Islands

“…The environmental factors having the greatest effects on community structure, rainfall and hurricanes, have strong effects on scleractinians and coral reefs. Rainfall modulates terrestrial runoff, salinity, sedimentation, and nutrient concentrations, all of which can negatively affect scleractinians (Rogers 1990, Fabricius 2005. The association between multivariate community structure and rainfall is likely to reflect the role of this variable in integrating the effects of other factors including seawater temperature, storms, underwater light intensity, and the AMO.…”

Effects of two sediment types on the fluorescence yield of two Hawaiian scleractinian corals

“…Reefs not subject to human stress receive 1-10 mg cm -2 d -1 of sediment deposition (Rogers, 1990), but in heavily populated high-island systems like Hawaii, sediment levels may be considerably higher. The average sediment loads applied in this study were 1.5- (Bothner et al, 2006).…”

“…In her seminal review paper on coral reefs and sedimentation, Rogers (1990) noted that scientists are "unable to rigorously predict the responses of coral reefs and reef organisms to excessive sedimentation." Rogers specifically identified a need for threshold levels for lethal effects.…”

“…Sedimentation stress has the additional danger of tissue loss or necrosis due to smothering. Rogers (1990) also recommended that studies compare the effects of terrigenous and carbonate sediment. The two sediment types in the present study were extensively characterized in terms of sediment size and trace metal content (Tables 1-2); in addition, nutrient-related properties (total organic carbon, nitrogen, phosphorus, chlorophyll) can be the primary driver of sedimentation stress (Weber et al 2006).…”

“…The effect of sedimentation on corals depends on colony characteristics, sediment type and environmental conditions (see reviews by Rogers, 1990;Fabricius, 2005).…”