Community patterns on a submerged barrier reef at Barbados, West Indies

101

Censuses were taken at 8 coral reefs in southwest Puerto Rico to identlfy assemblages of flshes and to determine whether water motion, depth, and substrate were correlated with fish distributlon and abundance. Each reef was sampled by recording species and abundance of fishes and substrate sizes within quadrats placed at different depths (from reef crest to an average of 6 m deep) along transects placed on forereefs and backreefs. Water motion was examined by measuring the amount of dissolution of plaster-of-Paris blocks w h~c h gave a relative index of the degree of water movement wlthin and among reefs. Data collected on 14 fish specles and measurements of water motion, degree of reef slope, and 4 sizes of substrate over 32 transects were analyzed to identify species groups and examine ecological correlates. The analyses indicated 3 species assemblages which d~ffered in distnbution and abundance within reefs (backreef, shallow forereef, and deep forereef) and among reefs (nearshore, intermediate, and offshore reefs). Similarly, water movement, depth, reef slope and substrate size occurred as gradients that differed significantly within and among reefs. Fishes in Assemblage 1 included primarily Ophioblennius atlanticus. Thalassoma bifasciatum, Stegastes dorsopunicans, and Microspathodon chrysurus. These species were most common on the shallow forereef, particularly at offshore reefs (those farthest from land); these sites were characterized by a high degree of water motion, gentle forereef slope, and boulder-sized substrate. Assemblage 2 species included Stegastesplanifrons, Hypoplectrus chlorurus, and Holocentrus rufus. These fishes were most abundant on the deep forereef, especially at nearshore reefs (sites closest to land), which were typified by relatively low water movement, steep slope, and smaller substrate sizes. Fishes in Assemblage 3 (Stegastes leucostictus and Malacoctenus macropus) were most numerous in the backreef where water motion, slope inclination, and substrate sizes were minimal compared with the forereef. While biological variables are undoubtedly important in determining species distributions, the different assemblages were found to be correlated with variations in water motion, depth, and substrate type. These 3 physical variables occurred as interrelated gradients, but the analyses suggested that water movement energy was the environmental factor most strongly associated with species distributions.

Section: Discussionmentioning

confidence: 82%

Correspondence between assemblages of coral reef fishes and gradients of water motion, depth, and substrate size off Puerto Rico

McGehee¹

1994

101

“…For example, traps with high height to aperture ratios will be less subject to resuspension of their contents than shallower traps. Traps placed on or just above the substrate collect more sediments than those higher in the water column (Ott 1975). Mean sedimentation rates for reefs which are not subject to stresses from human activities range from less than 1 to about 10 mg cmp2 dC1 (Table 1).…”

Section: Historical Records Of Sedimentation On Coral Reefsmentioning

confidence: 99%

Responses of coral reefs and reef organisms to sedimentation

Rogers¹

1990

1,146

833

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.

“…Coral reef systems are characterized by limited spatial and temporal variation in temperature, oxygen and salinity (Ott, 1975;Dana, 1976;Ohlhorst, 1980), but there are significant gradients in exposure to water movement, irradiance and disturbance (Roberts et al, 1977;Van den Hoek et al, 1978;Bak and Luckhurst, 1980). The possible response to such gradients in reproductive patterns of the dominant reef builders, the stony corals (Scleractinia), is essentially unknown.…”

Section: Introductionmentioning

confidence: 99%

Reproductive strategies in two closely related stony corals (Agaricia, Scleractinia)

Moorsel¹

1983

103

Samples of the scleractinian coral Agaricia agaricites (Limaeus) were studied for time of planula release, planula production and related characteristics. The corals were collected over a 2 yr period on the fringing reef of Curaqao at depths from 5 to 30 m. Two distinct reproductive pattems emerged. Based on these patterns and on consistent differences in skeletal morphology, such as size of calices and n b b e r of septa, the separation of the form A. agaricites humilis Vemll as a distinct species, A. hurnilis, from the other forms of A. agaricites is proposed. A. humilisplanulated throughout the year. A. agaricites shed planulae in spring and summer, a period of increasing sea water temperatures. In A, humilis 78 % of the colonies (N = 133) released planulae within 1 wk after collection, compared with only 26 % ( N = 91) in A. agaricites. Maxlmum diameter of the smallest planulating colonies of A. humilis was about 4 times less (28 mm) than in A. agaricites (108 mm). Planula production per unit living tissue area was 4 times larger (0.86 planulae cm-2 wk-') in A. humilis than in A. agaricites (0.23 planulae cm-2 wk-l). However, planula volume was 5 times lower (0.166 mm3) than in A. agaricites (0.860 mm3). It appears that A. humilis employs an opportunistic reproductive strategy compared with A. agaricites. A. humilis is mainly confined to the shallow reef flat and drop off zone (5 to 12 m) and colony diameters rarely exceed 100 mm. A. agaricitesreaches its greatest abundance on the deeper reef slope (10 to 30 m), growing to a maximum size of 600 mm. The differences in reproductive strategies can be explained in relation to the predictability of their habitats.