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Sublethal and lethal effects of sediments applied to common Caribbean Reef corals in the field
Marine Pollution Bulletin
Marine Pollution Bulletin, Vol. 14, No. 10, pp. 378-382, 1983 Printed in Great Britain
0025-326X/83 $3.00 + 0.00 Pergamon Press Ltd.
Sublethal and Lethal Effects of Sediments Applied to Common Caribbean Reef Corals in the Field CAROLINE S. ROGERS
c/o West Indies Laboratory, Teague Bay, Christiansted, St Croix, V100820, USA Experiments to measure natural rates of sedimentation and to assess the tolerance of coral species to increased sedimentation were conducted at San Cristobai Reef, Puerto Rico. Sedimentation rates were measured over an 18 month period. Calcareous sediments were applied to colonies of
Montastraea annularis, Diploria strigosa, D. clivosa, Acroporapalmata and A. cervicornis in different frequencies and in different doses. Mean sedimentation rates from sediment traps were 9 . 6 + 2 . 4 (S.E.) mg cm -2. day -~ at 10 cm above the bottom and 2.5_+_0.9 (S.E.) and 2 . 6 + 1 . 2 (S.E.) mg cm -2. day -~ for two sets of traps at 50 cm above the bottom. Sediment application experiments indicated A. palmata was the least tolerant of the species tested. Although A. cervicornis and D. strigosa colonies were not significantly affected, single applications of 800 mg cm -2 to M. annulariscolonies and of 200 mg cm -2 to A. palmata colonies caused death of underlying coral tissue. Algae colonized the smothered portions of these corals.
Coastal development and dredging in tropical areas increase sedimentation rates and constitute a serious threat to coral reefs (Brock et al., 1966; Roy, 1970; Maragos, 1972; Marsh & Gordon, 1974; Bak, 1978). It is essential to determine the impact of these activities on reefs and, specifically, the critical long- and short-term levels of sedimentation above which coral reefs are adversely affected. Adequate data on natural sedimentation rates and coral tolerance to increased sedimentation are required. Corals reject the sediments which fall on their living surfaces, primarily by using their tentacles and cilia, and by entrapping the particles in mucus which sloughs off. Field studies and laboratory experiments (Edmondson, 1928; Marshall & Orr, 1931; Hubbard & Pocock, 1972; Kolehmainen, 1974; Bak & Elgershuizen, 1976; Thompson, 1979; Lasker, 1980) on the response of corals to sediment application indicate large differences in their ability to reject sediments. Although corals in general do not survive burial for more than a few hours (Mayer, 1918; Marshall & Orr, 1931; Thompson, 1979), there is little information on lethal and sublethal effects of lesser amounts of sedimentation. The objective of this study at San Cristobal Reef, Puerto Rico, was to document natural sedimentation rates over an extended period of time, and to quantify the response of common Caribbean coral species to the application of sediments in different frequencies and in different doses. 378
Study Area
San Cristobal Reef is a small reef less than 2 km from shore off La Parguera, Puerto Rico. The study area was 2--4 m deep and was partially protected from strong wave action by the shallow Acroporapalmata zone at San Cristobal and by other seaward reefs (Fig. 1). Live coral cover by 20 hard coral species ranged from 20 to 70%. Acropora cervicornis and Montastraea annularis were the most abundant corals.
Methods
Sedimentation rates Sedimentation rates were measured from January 1975 to June 1976 with sediment traps at five sites spaced at 10 m intervals along an east-west transect. All traps were in about 4 m of water. At each station, two plastic, widemouth jars (diameter 8 cm, height 9.5 cm) were attached at 50 cm and 10 cm above the bottom, to a steel rod driven into the substrate. To determine the variability at a single location, 10 additional traps were placed within about a 30 m 2 area, 50 cm above the substrate and 15 m north of the other traps. These traps were monitored from February 1975 to June 1976. Traps were collected and replaced after periods of 15-53 days. In the laboratory, the water in the traps was siphoned off until about 100 ml remained. Encrusting organisms (mostly algae) were scraped off and the traps were placed in a drying oven. The difference between the final weight of the traps with the dry sediments and the initial weight of the traps represented the amount of sediment collected. After correction for the weight of salt (roughly 4 g in 100 ml), the results from each trap were converted into mg cm-2.day-~. (An alternative method involves filtration of trap contents on to preweighed filters, followed by rinsing with distilled water, and subsequent drying and weighing of the filters. Some fine materials may be lost in filtration.)
Application of sediments to corals Calcareous sediments were applied to in-situ colonies of A. cervicornis, A. palmata and M. annularis, species which were dominant at San Cristobal and are primary framework builders in the Caribbean, and to colonies ofD. strigosa and D. clivosa which were common in the lagoon (Rogers,
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Fig. 1 Locationof study site at San Cristobal Reef, Puerto Rico(adapted from Almy & Corri6n-Torres, 1963).
1979). The sediments, moderately sorted coarse sand with a mean grain size of 1.01 q0(0.5 mm), consisted mostly of Halimeda flakes, and all came from a barren area 2-3 m deep in the lagoon whereliving corals are scarce (see Fig. 2 for particle size distribution). All experimental corals grew at a depth of 2--4 m and were of moderate size. (Acropora cervicornis colonies were about 0.5-0.75 m across, A. palmata colonies 1-1.5 m across, and the head corals approximately 0.5 m across.) Sediments in three different doses were applied to nine colonies each of M. annularis, A. cervicornis and D. strigosa, with three replicate colonies receiving each dose.
Dose 1 was approximately 200 mg cm-2; dose 2, 400 mg cm-2; and dose 3, 800 mg cm 2. These doses were 1-3 orders of magnitude higher than mean sedimentation rates measured at the reef. Three colonies ofA. palmata received dose 1 only. Three other colonies of each species were designated as controls. In another experiment, nine colonies of each species (M. annularis, Diploria sp., A. cervicornis) received applications of about 200 mg of sediment cm-2 at varying intervals from 11 October to 26 November 1975. Three colonies received sediments daily, three weekly, and three monthly. 379
Marine Pollution Bulletin 40
substrate (ANOVA, p <0.01) (Fig. 3). Mean rates at 50 cm were 2.5 _+0.9 (S.E.) mg cm 2.day 1 (along the transect) and 2.6 _+ 1.2 (S.E.) mg cm 2. day-~ (north of transect), compared to 9.6 _+2.4 mg cm --~. day-1 for the 10 cm traps. Individual accumulation rates varied from nearly zero to about 30 mg cm-2. day-~. During Tropical Storm Eloise in September 1975, accumulation rates increased to 14.4 _+7.9 (S.D.) mg cm-2- day-~ for the bottom traps and to 5.6 _+2.2 (S.D.) for the 50 cm replicate traps north of the transect. Sedimentation rates at these northern traps (but not at the 50 cm transect traps) were significantly higher during the period including the tropical storm than rates from before or after this time (ANOVA, p < 0.01). The lower percentages of sand and gravel in the higher traps (Fig. 4) reflect the general inability of the waves to suspend significant quantities of coarser materials to an elevation 50 cm above the substrate.
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Fig. 2 Particle size distribution of the calcareous sediments applied to corals of San Cristobal Reef. Bars with horizontal lines represent gravel, bars with no lines represent sand. The symbol qois equal to - log 2 (diameter of the particle in mm).
Application of sediments to corals (a) Single application experiments (varying doses)
(A single D. clivosa colony received sediment once a day along with two D. strigosa colonies for the 45 days.) Treatments were assigned using a random numbers table. Sediments were collected the day of application or on the preceding day and applied wet. A 1 m 2 quadrat was placed around each colony, and sediments were applied as evenly as possible over the entire quadrat from a container which held 2 kg of sediment. For larger doses, more containers were used.
Acroporapalmata. Immediately after a single application of sediments (200 mg cm 2), the three A. palmata colonies released fine strands of mucus which floated off the corals, entrapping sediment particles and removing some of them from the colony surface. The following day, all colonies still had extensive amounts of sediments. After 6 days, algae were already growing on the smothered portions, both on the bleached sections of the corals and on the sediment accumulations. These colonies never recovered. Montastraea annularis. All of the Montastraea colonies which received 800 mg cm-2 retained heavy accumulations of sediments on relatively flat areas or in crevices. A few days after application, bleaching was evident around these accumulations. After two weeks, one colony had a dead patch about 100 cm 2 in area. Other colonies had smaller amounts of dead tissue. There was no evidence of recovery
Results Sedimentation rates Sediment traps near the bottom had significantly higher mean accumulation rates than traps 50 cm above the 20 18 16
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Fig. 3 Sedimentation rates at San Cristobal Reef.
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Volume 14/Number 10/October 1983 30
with all other experimental colonies, there was minor bleaching around the base of the colonies as surge transported the accumulating sediments back and forth, abrading the peripheral polyps. Bleaching of small patches (1-15 cm 2) also occurred on the surfaces of several other colonies. Diploria strigosa. No significant damage to D. strigosa colonies was observed.
PARTICLE SIZE DISTRIBUTION
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(b) Multiple application experiments (varyingfrequencies) The repeated application of 200 mg of sediment cm-2 to A. cervicornis, M. annularis and D. strigosa colonies resulted in no extensive damage. The singleD, clivosa colony was the only colony in the frequency experiments which showed extensive damage. After 38 daily applications of 200 mg cm -2, about 10% of the colony surface developed an abnormal colour overnight. The coral was subsequently less effectiveat removing the daily sediment doses, and a sediment layer gradually accumulated and covered about 50°7o of the coral until after the termination of the experiment, when water movement eventually removed the particles revealing the underlying dead area.
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in succeeding months as filamentous green algae grew over the sediment layers on all colonies. Less damage occurred when sediments accumulated on one M. annularis colony which received 400 mg cm- 2. This caused bleaching of the underlying polyps. Four weeks after sediment application, the sediments had been removed, presumably by surge, and this portion of the colony was regaining its normal colour. No other colonies receiving 200 or 400 mg cm-2 showed evidence of damage. Acropora cervicornis. Algae began growing on bleached areas which appeared on one of the A. cervicornis colonies which received 400 mg cm-2 and on one of the controls. As
Discussion The sediment application experiments indicated considerable coral resistance with death of local areas of one species, M. annular& occurring only after single applications of 800 mg cm-2. Acropora palmata, a species normally found in areas with strong wave action, was the least resistant to sediment application and succumbed to a single dose of 200 mg cm-2. Sediments accumulated on the flattened portions of A. palmata colonies and in crevices and depressions on colonies ofM. annularis. The cylindrical branches of A. cervicornis and the almost spherical morphology of D. strigosa better adapt these species to higher sediment loads. In the laboratory, Hubbard & Pocock (1972) found A. palmata to be less capable of rejecting sediment particles 0.5-2 mm in si7e (1.0 to - 1.0 phi) than A. cervicornis, M.
TABLE 1
Comparison of sedimentation rates for different reefs Location
Rates (mg cm -2. d a y - I
Comments
Study
Hawaii (Kaneohe Bay)
35-41 096, x-= 3498
Rates from lagoon slopes, stations Maragos (1972) 6 m deep or less; reefs subjected to several stresses, including heavier than normal runoff during study
Jamaica (Discovery Bay)
0.5-1.1 (means for traps 50 cm above substrate)
Reef lagoon, ca. 4 m deep
Dodge et al. (1974)
St. Thomas, U.S.V.I.
0.8+0.4-5.8+13.3 (means + S.E. for traps 10 cm above the substrate) 0. I +_0.1-1.6+__0.7 (means + S.E. for traps 50 cm above the substrate)
Five coral reef areas, 3-5 m deep
Rogers (1982)
Puerto Rico
1-15
Reef 9-33 m deep
Cintron et al. (1974)
Puerto Rico
2.5 ___0.9, 2.6 + 1.2 (means _+ S.E. for traps 50 cm above the substrate) 9.6+__2.4 (mean + S.E. for traps 10 cm above the substrate)
Backreef, 4 m deep
This study
381
Marine Pollution Bulletin annularis, D. c l i v o s a a n d D . strig, osa. K o l e h m a i n e n (1974) studied the effect of sediments applied to M o n t a s t r a e a cavernosa, S i d e r a s t r e a s i d e r e a a n d D. s t r i g o s a in the field. After 30 h, coral tissue which had been covered with a 6 m m thick layer of sediments disintegrated, while bleached coral tissue recovered its normal appearance if the sediments were removed in less than 24 h. Bak & Elgershuizen (1976) f o u n d that, in the absence of water movement, A . p a l m a t a and A. c e r v i c o r n i s were less efficient than M . a n n u l a r i s and D. s t r i g o s a in rejecting sediments similar to those applied to San Cristobal corals. Lasker (1980) applied ca. 75 mg of coarse sediment cm 2 and ca. 19 mg of fine sediment cm -2 to colonies of M . c a v e r n o s a in the laboratory a n d showed that over 70°7o of the sediment was removed after 8 h. It should be noted that the particle size distribution of sediments applied to corals and sediments accumulating in the traps differed (Figs 2, 4), with the traps collecting higher percentages of the finest particles. However, all size fractions of the applied sediments were represented in sediment trap samples. Also, the particle size distribution of the applied sediments probably resembles that of sediments which would settle on corals as a result of suspension by storm waves or possibly dredging. Sediment application experiments focus on the dosage required to kill portions of coral colonies. Smaller doses may be high enough to prevent establishment of coral planulae and to reduce coral growth. Dodge e t al. (1974) found that average growth of M . a n n u l a r i s in Discovery Bay, Jamaica, was significantly reduced by sedimentation rates of about 1.1 mg cm +2. d a y - t u n d e r n o r m a l conditions. Sedimentation rates at San Cristobal were similar to those at other unstressed reefs but lower than those reported by Maragos (1972) for disturbed reefs in Kaneohe Bay, Hawaii (Table 1). F r o m this a n d previous studies, it appears that normal sedimentation rates for coral reefs is of the order of 10 mg cm-2. day ~a n d less. In conclusion, baseline data from different coral reefs provide a basis for determining m a x i m u m sedimentation rates under natural conditions while sediment application experiments can help in establishing tolerance limits for reef organisms. The San Cristobal experiments involved transient stress from sediments. With the chronic introduction of sediments in lethal or sublethal doses, as in dredging, more damage would result.
382
Most of the work described here was part of a dissertation presented to the Graduate Council of the University of Florida in partial fulfilment of the requirements for the degree of Doctor of Philosophy. M. Canals, B. Hildner and F. Wadsworth helped in the field and in the laboratory. The Puerto Rico Nuclear Center (now the Center for Energy and Environment Research) generously provided laboratory facilities. Funding was from Contract 14-26-2767 to the Department of Natural Resources in San Juan, Puerto Rico, under the Pittman-Robertson and Dingell-Johnson Acts, Bureau of Fish and Wildlife Service, U.S. Department of the Interior. D. Hubbard constructively criticized the manuscript. This article is contribution I01 from the West Indies Laboratory. Almy, C. C., Jr. & Carri6n-Torres,C. (1963). Shallow-waterstony corals of Puerto Rico. Carib. J. Sci., 3, 133-162. Bak, R. P. M. (1978). Lethal and sublethal effects of dredging on reef corals. Mar. Pollut. Bull., 9, 14-16. Bak, R. P. M. & Elgershuizen, J. H. B. W. (1976). Patterns of oilsediment rejection in corals. Mar. Biol., 37, 105-113. Brock, V. E., van Heakelem, W. & Helfrich, P. (1966). An ecological reconnaissanceof Johnston Island and the effects of dredging. Hawaii Int. Mar. Biol. Tech, Rep. 11.
Cintron, G., McKenzie, F. & Olazagasti, R. (1974). Studies at the PRINUL site, Final Report: Missions 3 and 5. Dept. of Natural Resources, San Juan, Puerto Rico. Dodge, R. E., Aller, R. C. & Thomson, J. (1974). Coral growth related to resuspensionof bottom sediments.Nature, Lond., 247, 574-577. Edmondson, C. H. (1928). The ecology of an Hawaiian coral reef. B. P. Bishop Mus. Bull., 45, 1-64. Hubbard, J. A. E. B. & Pocock, Y. P. (1972). Sediment rejection by Recent scleractiniancorals: a key to paleo-environmentalreconstruction. Geol. Rundsch,, 61,598-626. Kolehmainen, S. E. (1974). Siltation experiments on corals in situ. In Puerto Rico Nuclear Center A nnual Report 1973, pp. 77-80. Lasker, H. R. (1980). Sediment rejection by reef corals: the roles of behavior and morphology in Montastrea cavernosa (Linnaeus). J. exp. mar. Biol. Ecol., 47, 77-87. Maragos, J. E. (1972). A study of the ecology of Hawaiian reef corals. Ph.D. thesis. Univ. Hawaii. Marsh, J. A., Jr. & Gordon, G. D. (1974). Marine environmentaleffects of dredging and power plant construction. Univ. Guam Mar. Lab. Rep. 8.
Marshall, S. M. & Orr, A. P. (1931). Sedimentation on Low Isles Reef and its relation to coral growth. ScL Rep. Great Barrier Reef Exped. 1, 94-133. Mayer, A. G. (1918). Ecology o f the Murray Island Coral Reef Carnegie Inst. Wash. Publ. 213. Rogers, C. S. (1979). The effect of shading on coral reef structure and function. J. exp. mar. Biol. Ecol., 41,269-288. Rogers, C. S. (1982). The marine environments of Brewers Bay, Perseverance Bay, Flat Cay and Saba Island, St. Thomas, U.S.V.I., with emphasis on coral reefs and seagrass beds: November 1978-July 1981. Division of Natural Resources Management, Dept. of Conservation and Cultural Affairs, Government of the Virgin Islands. Roy, K. J. (1970). Changes in bathymetric configuration, Kaneohe Bay, Oahu, 1882-1969. Hawaii Inst. Geophys. Res. Thompson, J. H., Jr. (1979). Effects of drilling mud on seven species of reef-buildingcorals as measured in field and laboratory. Final Report to the US GeologicalSurvey. TexasA & M University.29 pp.
Marine Pollution Bulletin, Vol. 14, No. 10, pp. 378-382, 1983 Printed in Great Britain
0025-326X/83 $3.00 + 0.00 Pergamon Press Ltd.
Sublethal and Lethal Effects of Sediments Applied to Common Caribbean Reef Corals in the Field CAROLINE S. ROGERS
c/o West Indies Laboratory, Teague Bay, Christiansted, St Croix, V100820, USA Experiments to measure natural rates of sedimentation and to assess the tolerance of coral species to increased sedimentation were conducted at San Cristobai Reef, Puerto Rico. Sedimentation rates were measured over an 18 month period. Calcareous sediments were applied to colonies of
Montastraea annularis, Diploria strigosa, D. clivosa, Acroporapalmata and A. cervicornis in different frequencies and in different doses. Mean sedimentation rates from sediment traps were 9 . 6 + 2 . 4 (S.E.) mg cm -2. day -~ at 10 cm above the bottom and 2.5_+_0.9 (S.E.) and 2 . 6 + 1 . 2 (S.E.) mg cm -2. day -~ for two sets of traps at 50 cm above the bottom. Sediment application experiments indicated A. palmata was the least tolerant of the species tested. Although A. cervicornis and D. strigosa colonies were not significantly affected, single applications of 800 mg cm -2 to M. annulariscolonies and of 200 mg cm -2 to A. palmata colonies caused death of underlying coral tissue. Algae colonized the smothered portions of these corals.
Coastal development and dredging in tropical areas increase sedimentation rates and constitute a serious threat to coral reefs (Brock et al., 1966; Roy, 1970; Maragos, 1972; Marsh & Gordon, 1974; Bak, 1978). It is essential to determine the impact of these activities on reefs and, specifically, the critical long- and short-term levels of sedimentation above which coral reefs are adversely affected. Adequate data on natural sedimentation rates and coral tolerance to increased sedimentation are required. Corals reject the sediments which fall on their living surfaces, primarily by using their tentacles and cilia, and by entrapping the particles in mucus which sloughs off. Field studies and laboratory experiments (Edmondson, 1928; Marshall & Orr, 1931; Hubbard & Pocock, 1972; Kolehmainen, 1974; Bak & Elgershuizen, 1976; Thompson, 1979; Lasker, 1980) on the response of corals to sediment application indicate large differences in their ability to reject sediments. Although corals in general do not survive burial for more than a few hours (Mayer, 1918; Marshall & Orr, 1931; Thompson, 1979), there is little information on lethal and sublethal effects of lesser amounts of sedimentation. The objective of this study at San Cristobal Reef, Puerto Rico, was to document natural sedimentation rates over an extended period of time, and to quantify the response of common Caribbean coral species to the application of sediments in different frequencies and in different doses. 378
Study Area
San Cristobal Reef is a small reef less than 2 km from shore off La Parguera, Puerto Rico. The study area was 2--4 m deep and was partially protected from strong wave action by the shallow Acroporapalmata zone at San Cristobal and by other seaward reefs (Fig. 1). Live coral cover by 20 hard coral species ranged from 20 to 70%. Acropora cervicornis and Montastraea annularis were the most abundant corals.
Methods
Sedimentation rates Sedimentation rates were measured from January 1975 to June 1976 with sediment traps at five sites spaced at 10 m intervals along an east-west transect. All traps were in about 4 m of water. At each station, two plastic, widemouth jars (diameter 8 cm, height 9.5 cm) were attached at 50 cm and 10 cm above the bottom, to a steel rod driven into the substrate. To determine the variability at a single location, 10 additional traps were placed within about a 30 m 2 area, 50 cm above the substrate and 15 m north of the other traps. These traps were monitored from February 1975 to June 1976. Traps were collected and replaced after periods of 15-53 days. In the laboratory, the water in the traps was siphoned off until about 100 ml remained. Encrusting organisms (mostly algae) were scraped off and the traps were placed in a drying oven. The difference between the final weight of the traps with the dry sediments and the initial weight of the traps represented the amount of sediment collected. After correction for the weight of salt (roughly 4 g in 100 ml), the results from each trap were converted into mg cm-2.day-~. (An alternative method involves filtration of trap contents on to preweighed filters, followed by rinsing with distilled water, and subsequent drying and weighing of the filters. Some fine materials may be lost in filtration.)
Application of sediments to corals Calcareous sediments were applied to in-situ colonies of A. cervicornis, A. palmata and M. annularis, species which were dominant at San Cristobal and are primary framework builders in the Caribbean, and to colonies ofD. strigosa and D. clivosa which were common in the lagoon (Rogers,
Volume 14/Number 10/October 1983
PUERTO
RI CO
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6 7"02'30"
~
~
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Laurel
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t
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Kilometers
1
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ra.oo
Media
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~
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LEGEND
OUTLINE OF REEF AT OR NEAR SEA LEVEL OUTLINE OF REEF ABOVE SEA LEVEL SHORELINE ,~__~IO~-~CONTOUR LINES BELOW SEA LEVEL ( Contour Interval: 30 FEET )
Fig. 1 Locationof study site at San Cristobal Reef, Puerto Rico(adapted from Almy & Corri6n-Torres, 1963).
1979). The sediments, moderately sorted coarse sand with a mean grain size of 1.01 q0(0.5 mm), consisted mostly of Halimeda flakes, and all came from a barren area 2-3 m deep in the lagoon whereliving corals are scarce (see Fig. 2 for particle size distribution). All experimental corals grew at a depth of 2--4 m and were of moderate size. (Acropora cervicornis colonies were about 0.5-0.75 m across, A. palmata colonies 1-1.5 m across, and the head corals approximately 0.5 m across.) Sediments in three different doses were applied to nine colonies each of M. annularis, A. cervicornis and D. strigosa, with three replicate colonies receiving each dose.
Dose 1 was approximately 200 mg cm-2; dose 2, 400 mg cm-2; and dose 3, 800 mg cm 2. These doses were 1-3 orders of magnitude higher than mean sedimentation rates measured at the reef. Three colonies ofA. palmata received dose 1 only. Three other colonies of each species were designated as controls. In another experiment, nine colonies of each species (M. annularis, Diploria sp., A. cervicornis) received applications of about 200 mg of sediment cm-2 at varying intervals from 11 October to 26 November 1975. Three colonies received sediments daily, three weekly, and three monthly. 379
Marine Pollution Bulletin 40
substrate (ANOVA, p <0.01) (Fig. 3). Mean rates at 50 cm were 2.5 _+0.9 (S.E.) mg cm 2.day 1 (along the transect) and 2.6 _+ 1.2 (S.E.) mg cm 2. day-~ (north of transect), compared to 9.6 _+2.4 mg cm --~. day-1 for the 10 cm traps. Individual accumulation rates varied from nearly zero to about 30 mg cm-2. day-~. During Tropical Storm Eloise in September 1975, accumulation rates increased to 14.4 _+7.9 (S.D.) mg cm-2- day-~ for the bottom traps and to 5.6 _+2.2 (S.D.) for the 50 cm replicate traps north of the transect. Sedimentation rates at these northern traps (but not at the 50 cm transect traps) were significantly higher during the period including the tropical storm than rates from before or after this time (ANOVA, p < 0.01). The lower percentages of sand and gravel in the higher traps (Fig. 4) reflect the general inability of the waves to suspend significant quantities of coarser materials to an elevation 50 cm above the substrate.
PARTICLE SIZE DISTRIBUTION
-r
30
i2o kZ t~
O w
10
0 2
-1
0
1
2
3
4
5
6
7
8
9
012¢5 u n i t s
4.00
0.002
mm
Fig. 2 Particle size distribution of the calcareous sediments applied to corals of San Cristobal Reef. Bars with horizontal lines represent gravel, bars with no lines represent sand. The symbol qois equal to - log 2 (diameter of the particle in mm).
Application of sediments to corals (a) Single application experiments (varying doses)
(A single D. clivosa colony received sediment once a day along with two D. strigosa colonies for the 45 days.) Treatments were assigned using a random numbers table. Sediments were collected the day of application or on the preceding day and applied wet. A 1 m 2 quadrat was placed around each colony, and sediments were applied as evenly as possible over the entire quadrat from a container which held 2 kg of sediment. For larger doses, more containers were used.
Acroporapalmata. Immediately after a single application of sediments (200 mg cm 2), the three A. palmata colonies released fine strands of mucus which floated off the corals, entrapping sediment particles and removing some of them from the colony surface. The following day, all colonies still had extensive amounts of sediments. After 6 days, algae were already growing on the smothered portions, both on the bleached sections of the corals and on the sediment accumulations. These colonies never recovered. Montastraea annularis. All of the Montastraea colonies which received 800 mg cm-2 retained heavy accumulations of sediments on relatively flat areas or in crevices. A few days after application, bleaching was evident around these accumulations. After two weeks, one colony had a dead patch about 100 cm 2 in area. Other colonies had smaller amounts of dead tissue. There was no evidence of recovery
Results Sedimentation rates Sediment traps near the bottom had significantly higher mean accumulation rates than traps 50 cm above the 20 18 16
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Fig. 3 Sedimentation rates at San Cristobal Reef.
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Volume 14/Number 10/October 1983 30
with all other experimental colonies, there was minor bleaching around the base of the colonies as surge transported the accumulating sediments back and forth, abrading the peripheral polyps. Bleaching of small patches (1-15 cm 2) also occurred on the surfaces of several other colonies. Diploria strigosa. No significant damage to D. strigosa colonies was observed.
PARTICLE SIZE DISTRIBUTION
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(b) Multiple application experiments (varyingfrequencies) The repeated application of 200 mg of sediment cm-2 to A. cervicornis, M. annularis and D. strigosa colonies resulted in no extensive damage. The singleD, clivosa colony was the only colony in the frequency experiments which showed extensive damage. After 38 daily applications of 200 mg cm -2, about 10% of the colony surface developed an abnormal colour overnight. The coral was subsequently less effectiveat removing the daily sediment doses, and a sediment layer gradually accumulated and covered about 50°7o of the coral until after the termination of the experiment, when water movement eventually removed the particles revealing the underlying dead area.
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in succeeding months as filamentous green algae grew over the sediment layers on all colonies. Less damage occurred when sediments accumulated on one M. annularis colony which received 400 mg cm- 2. This caused bleaching of the underlying polyps. Four weeks after sediment application, the sediments had been removed, presumably by surge, and this portion of the colony was regaining its normal colour. No other colonies receiving 200 or 400 mg cm-2 showed evidence of damage. Acropora cervicornis. Algae began growing on bleached areas which appeared on one of the A. cervicornis colonies which received 400 mg cm-2 and on one of the controls. As
Discussion The sediment application experiments indicated considerable coral resistance with death of local areas of one species, M. annular& occurring only after single applications of 800 mg cm-2. Acropora palmata, a species normally found in areas with strong wave action, was the least resistant to sediment application and succumbed to a single dose of 200 mg cm-2. Sediments accumulated on the flattened portions of A. palmata colonies and in crevices and depressions on colonies ofM. annularis. The cylindrical branches of A. cervicornis and the almost spherical morphology of D. strigosa better adapt these species to higher sediment loads. In the laboratory, Hubbard & Pocock (1972) found A. palmata to be less capable of rejecting sediment particles 0.5-2 mm in si7e (1.0 to - 1.0 phi) than A. cervicornis, M.
TABLE 1
Comparison of sedimentation rates for different reefs Location
Rates (mg cm -2. d a y - I
Comments
Study
Hawaii (Kaneohe Bay)
35-41 096, x-= 3498
Rates from lagoon slopes, stations Maragos (1972) 6 m deep or less; reefs subjected to several stresses, including heavier than normal runoff during study
Jamaica (Discovery Bay)
0.5-1.1 (means for traps 50 cm above substrate)
Reef lagoon, ca. 4 m deep
Dodge et al. (1974)
St. Thomas, U.S.V.I.
0.8+0.4-5.8+13.3 (means + S.E. for traps 10 cm above the substrate) 0. I +_0.1-1.6+__0.7 (means + S.E. for traps 50 cm above the substrate)
Five coral reef areas, 3-5 m deep
Rogers (1982)
Puerto Rico
1-15
Reef 9-33 m deep
Cintron et al. (1974)
Puerto Rico
2.5 ___0.9, 2.6 + 1.2 (means _+ S.E. for traps 50 cm above the substrate) 9.6+__2.4 (mean + S.E. for traps 10 cm above the substrate)
Backreef, 4 m deep
This study
381
Marine Pollution Bulletin annularis, D. c l i v o s a a n d D . strig, osa. K o l e h m a i n e n (1974) studied the effect of sediments applied to M o n t a s t r a e a cavernosa, S i d e r a s t r e a s i d e r e a a n d D. s t r i g o s a in the field. After 30 h, coral tissue which had been covered with a 6 m m thick layer of sediments disintegrated, while bleached coral tissue recovered its normal appearance if the sediments were removed in less than 24 h. Bak & Elgershuizen (1976) f o u n d that, in the absence of water movement, A . p a l m a t a and A. c e r v i c o r n i s were less efficient than M . a n n u l a r i s and D. s t r i g o s a in rejecting sediments similar to those applied to San Cristobal corals. Lasker (1980) applied ca. 75 mg of coarse sediment cm 2 and ca. 19 mg of fine sediment cm -2 to colonies of M . c a v e r n o s a in the laboratory a n d showed that over 70°7o of the sediment was removed after 8 h. It should be noted that the particle size distribution of sediments applied to corals and sediments accumulating in the traps differed (Figs 2, 4), with the traps collecting higher percentages of the finest particles. However, all size fractions of the applied sediments were represented in sediment trap samples. Also, the particle size distribution of the applied sediments probably resembles that of sediments which would settle on corals as a result of suspension by storm waves or possibly dredging. Sediment application experiments focus on the dosage required to kill portions of coral colonies. Smaller doses may be high enough to prevent establishment of coral planulae and to reduce coral growth. Dodge e t al. (1974) found that average growth of M . a n n u l a r i s in Discovery Bay, Jamaica, was significantly reduced by sedimentation rates of about 1.1 mg cm +2. d a y - t u n d e r n o r m a l conditions. Sedimentation rates at San Cristobal were similar to those at other unstressed reefs but lower than those reported by Maragos (1972) for disturbed reefs in Kaneohe Bay, Hawaii (Table 1). F r o m this a n d previous studies, it appears that normal sedimentation rates for coral reefs is of the order of 10 mg cm-2. day ~a n d less. In conclusion, baseline data from different coral reefs provide a basis for determining m a x i m u m sedimentation rates under natural conditions while sediment application experiments can help in establishing tolerance limits for reef organisms. The San Cristobal experiments involved transient stress from sediments. With the chronic introduction of sediments in lethal or sublethal doses, as in dredging, more damage would result.
382
Most of the work described here was part of a dissertation presented to the Graduate Council of the University of Florida in partial fulfilment of the requirements for the degree of Doctor of Philosophy. M. Canals, B. Hildner and F. Wadsworth helped in the field and in the laboratory. The Puerto Rico Nuclear Center (now the Center for Energy and Environment Research) generously provided laboratory facilities. Funding was from Contract 14-26-2767 to the Department of Natural Resources in San Juan, Puerto Rico, under the Pittman-Robertson and Dingell-Johnson Acts, Bureau of Fish and Wildlife Service, U.S. Department of the Interior. D. Hubbard constructively criticized the manuscript. This article is contribution I01 from the West Indies Laboratory. Almy, C. C., Jr. & Carri6n-Torres,C. (1963). Shallow-waterstony corals of Puerto Rico. Carib. J. Sci., 3, 133-162. Bak, R. P. M. (1978). Lethal and sublethal effects of dredging on reef corals. Mar. Pollut. Bull., 9, 14-16. Bak, R. P. M. & Elgershuizen, J. H. B. W. (1976). Patterns of oilsediment rejection in corals. Mar. Biol., 37, 105-113. Brock, V. E., van Heakelem, W. & Helfrich, P. (1966). An ecological reconnaissanceof Johnston Island and the effects of dredging. Hawaii Int. Mar. Biol. Tech, Rep. 11.
Cintron, G., McKenzie, F. & Olazagasti, R. (1974). Studies at the PRINUL site, Final Report: Missions 3 and 5. Dept. of Natural Resources, San Juan, Puerto Rico. Dodge, R. E., Aller, R. C. & Thomson, J. (1974). Coral growth related to resuspensionof bottom sediments.Nature, Lond., 247, 574-577. Edmondson, C. H. (1928). The ecology of an Hawaiian coral reef. B. P. Bishop Mus. Bull., 45, 1-64. Hubbard, J. A. E. B. & Pocock, Y. P. (1972). Sediment rejection by Recent scleractiniancorals: a key to paleo-environmentalreconstruction. Geol. Rundsch,, 61,598-626. Kolehmainen, S. E. (1974). Siltation experiments on corals in situ. In Puerto Rico Nuclear Center A nnual Report 1973, pp. 77-80. Lasker, H. R. (1980). Sediment rejection by reef corals: the roles of behavior and morphology in Montastrea cavernosa (Linnaeus). J. exp. mar. Biol. Ecol., 47, 77-87. Maragos, J. E. (1972). A study of the ecology of Hawaiian reef corals. Ph.D. thesis. Univ. Hawaii. Marsh, J. A., Jr. & Gordon, G. D. (1974). Marine environmentaleffects of dredging and power plant construction. Univ. Guam Mar. Lab. Rep. 8.
Marshall, S. M. & Orr, A. P. (1931). Sedimentation on Low Isles Reef and its relation to coral growth. ScL Rep. Great Barrier Reef Exped. 1, 94-133. Mayer, A. G. (1918). Ecology o f the Murray Island Coral Reef Carnegie Inst. Wash. Publ. 213. Rogers, C. S. (1979). The effect of shading on coral reef structure and function. J. exp. mar. Biol. Ecol., 41,269-288. Rogers, C. S. (1982). The marine environments of Brewers Bay, Perseverance Bay, Flat Cay and Saba Island, St. Thomas, U.S.V.I., with emphasis on coral reefs and seagrass beds: November 1978-July 1981. Division of Natural Resources Management, Dept. of Conservation and Cultural Affairs, Government of the Virgin Islands. Roy, K. J. (1970). Changes in bathymetric configuration, Kaneohe Bay, Oahu, 1882-1969. Hawaii Inst. Geophys. Res. Thompson, J. H., Jr. (1979). Effects of drilling mud on seven species of reef-buildingcorals as measured in field and laboratory. Final Report to the US GeologicalSurvey. TexasA & M University.29 pp.