BULLETIN OF MARINE SCIENCE, 36(2): 339-356, 1985 REEF PAPER

A REEF UNDER SILTATION STRESS: CAHUITA, COSTA RICA

Jorge Cortes N. and Michael J. Risk

ABSTRACT The at Parque Nacional Cahuita, Costa Rica, shows low live coral coverage and species diversity and large average colony diameters. Amounts of suspended and resuspended particulate matter are very high, and a large amount of terrigenous material is trapped inside massive . Coral growth rates are low, and are significantly inversely correlated with resuspension rates. We suggest that , which increased about 15 years ago, has increased the siltation stress on the reef. The siltation-specific characteristics of this reef may be used in managing modem reefs, in that the presence and degree of siltation stress may be quickly evaluated by determining the acid-insoluble residues of coral heads, and comparing these with growth rates through time, These characteristics may also be of use in interpreting possible mechanisms for decline and death of reefs in the fossil record,

The only well-developed coral reef on the Atlantic coast of Costa Rica is at Cahuita (9°45'N, 82°48'W: Fig. 1). On 24 September 1970, the Government of Costa Rica created Parque Nacional Cahuita, containing the coral reef and some 1,100 ha of adjoining land. Local inhabitants on the reef, and students and tourists come to visit and study the reef. There is evidence that this unique resource is being seriously damaged by siltation. Not a great deal has been published on the Cahuita reef. The first ecological description of the reef was by Wellington (1974). Valdez and Villalobos (1978) described the distribution of Diadema antillarum from Cahuita, and Risk et al. (1980) gave detailed descriptions of some of the reef habitats, with notes on the boring sponges. The Atlantic coastal lowlands of Costa Rica are hot and humid [type Br A'a, Humid-Megathermic: Instituto Panamericano de Geografia e Historia (1976)]. Annual precipitation is 2,000-4,000 mm/yr, falling mostly in the June-September rainy season. Watersheds above Costa Rica drain through Tertiary clastic and andesitic volcanic rocks (Direcci6n de Geologia, Minas y Petr61eo, 1968). The main crest of the Cahuita fringing reef arcs around the northern tip of Punta Cahuita, runs southeast about 4 km, then bends inshore towards Puerto Vargas (Fig. 1). A more detailed map of the reef, showing environments and locations of significant sample sites, is given in Figure 2. Two small crests occur near Puerto Vargas, and several small patch reefs occur in the . The reef to the north has well-developed buttresses, but the live coral coverage is very low. The highest live coral coverage occurs near Puerto Vargas, near a house now maintained by the Servicio de Parques Nacionales. Although previous workers (Wellington, 1974; Risk et aI., 1980) have com- mented on the poor coral development and the turbid water, lack of earlier work makes it difficult to conclude what changes, if any, have occurred with time. Considerable deforestation for lumber and agriculture has taken place in recent times. Clearing for banana plantations began in the late 1950's and early 1960's, and banana plantations now fringe part ofthe road south to Cahuita. The Servicio's house originally belonged to a lumber company, operating in the early 1960's in what is now the National Park. Most of the land around the Punta was logged off in the early 1960's: the forest was so dense that the valuable trees could only be

339 340 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985

0.5 f kllomelre&

g' PaCltic Ocean

6'

Figure I. (Left) The coral reef at Parque Nacional Cahuita. Figure 2. (Right) Detailed map of the Cahuita reef, showing environments, locations of sediment traps and sediment samples, and sample areas of heads analyzed for insoluble residues.

felled by cutting all the surrounding trees. Local inhabitants state that, years ago, the rivers ran clear in the dry season, whereas now they are muddy throughout the year. LANDSAT imagery produced at any time of the year shows sediment plumes along the coastline from Limon south to Cahuita (Fig. 3). Fishermen talk of declining catches and the increasing occurrence of patches of dying coral. Large (> I m) colonies of Mussa angulosa may be found, so recently dead that the septal teeth are perfectly preserved. The suggestion is one of a declining reef in a region of high and increasing siltation. Deforestation and subsequent increased siltation are problems throughout much of the Third World. Siltation has been identified as deleterious to coral reefs (Johannes, 1972; 1975). Ldio (1982) studied the Abrolhos reefs, off the coast of Brazil, the southernmost reefs in the Atlantic. She found that these reefs occurred in a muddy environment, with surrounding containing 40-70% siliciclastic material. Reef cavities and intraskeletal pores were frequently filled with muddy, quartz-bearing sedi- ment. Leao concluded that the depauperate nature of these reefs (fewer than half the number of coral species found on well-developed Caribbean reefs) was due to high rates of terrigenous sediment input. In this paper, we describe the depau- perate reef at Cahuita, and investigate some of the relationships between sedi- mentation and aspects of reef ecology. As a comparison with Cahuita, we have used the coral reefs off the southwest coast of Grand Cayman Island. At the time, Grand Cayman was a stopover on flights from Miami to San Jose (these flights have since been discontinued), and hence a very convenient study site. We felt that Grand Cayman, with its lush reefs, extreme water clarity, and isolation from CORTEs AND RISK: A SILTATION-STRESSED REEF 341

Figure 3. LANDSAT imagery of the east coast of Costa Rica, showing southward deflection of sediment plumes (26 Aug 1979). Cahuita (arrow) is under the circular cloud. Scale bar 10 km. terrigenous sediment sources, would be a useful end-member comparison with Cahuita. Field work at Cahuita occupied the summers (June-August) of 1979 and 1980, with a 3-wk visit in December 1979. Grand Cayman was visited for a week in August 1979, and twice more for short periods (2-3 days) in January and August 1980. Of necessity, the number oflive coral heads collected from both areas was fairly low. At Cahuita, we were reluctant to worsen the situation, and our collecting permit for Grand Cayman restricted us to 5 kg coral.

MATERIALS AND METHODS

General Oceanographyof the Cahuita Reef-Surface water temperatures measured throughout the reef during August 1979, and December 1980, averaged 28"C (range 27-30"C). Surface salinities (hand refractometer) taken throughout the reef in July and August 1980 ranged from a low of 320/00after heavy rains to a high of 380/00,and averaged about 340/00.Current speed and direction were measured in July and August 1980 by taking bearings on drogues from two points on the shore. Current speed depended on prevailing winds, and ranged from 3-30 em/sec over the reef. Current direction was from northwest to southwest. Currents along the coast north of the reef were not measured, because of rough sea conditions. Shoreline geomorphology, however, indicates a southward trend of sediment movement, consistent with the southward bending of the sediment plumes seen on LANDSAT. The general setting of the reefs at Grand Cayman is summarized by Rigby and Roberts (1976). 342 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985

Salinity range is 35-380/00,and air temperature ranges from 24-30°C. The sample area chosen for comparison is south of Georgetown, immediately off Sunset House (19°17'N, 81°23'W). Community Description Methods. - Transects were laid out parallel to the main reef trend, and the line intersection distance of each coral colony measured (method of Risk, 1972). Sixteen transects (total length 201 m) were surveyed at Cahuita, and seven transects (52 m) at Grand Cayman. Com- parable zones were chosen at both reefs, although reef development is deeper at Grand Cayman. The transects at Cahuita were along both the inner and outer crests, near the southern end of the reef, where the coral coverage is relatively high. The Grand Cayman transects off Sunset House, just south of the Seaview transect of Rigby and Roberts (1976), were deeper (5-7 m, as opposed to 3-4 for Cahuita) but were along the main reef trend and dominated by the same coral species as the Cahuita transects, From the transect data live coral coverage, species abundance, average colony size and estimates of diversity were calculated. Determination of Suspended Particulate Matter (SPM).-One-liter samples of sea water were taken, poisoned with 0.001 M sodium azide, and filtered through preweighed prewashed Millipore 0.45-lLm HA filters. Filters were rinsed, dried, reweighed, and retained for X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis. About 250 SPM determinations were done for all environments at Cahuita, with some areas reoccupied several times in 1979 and 1980. Twenty-two values were obtained for Grand Cayman, during 1979 and 1980. Determination of Resuspended Sediments. - "Resuspended sediments" here refers to that material settling down on the reef surface, which will therefore be caught in vertically oriented sediment traps. The flux of this material is a measure of gross, but not net, rate. In most coastal areas, resuspension rate will be generally correlated with SPM values. It would be possible, however, to have a shallow reef where strong currents and wave action on a relatively cemented bottom resulted in suspended sediments passing over the reef, resulting in high SPM values and low resuspension rates. In terms of possible effects on the corals, high SPM values with low resuspension rates result in reduction oflight incident on the corals, while the reverse situation results in large amounts of sediment depositing on the coral colonies. To measure sediment resuspension, a series of sediment traps were constructed, similar to those described by Young and Rhoads (1971). Traps were plastic cylinders, area of opening 21.23 cm2, height to diameter ratio approximately 3: I (a good ratio for estimating vertical fluxes: Gardner I980a; 1980b). These traps were set out in triangular arrays, with openings 25, 50 and 75 em off the bottom. Sediment traps were emplaced at five stations (Fig. 2): stations I and 2 are northwest and southeast, respectively, ofa small patch reef 200 m off the coast between Punta Cahuita and Puerto Vargas (area 3 of Risk et aI., 1980); stations 3, 4 and 5 are (in order) north to south along the inner crest. Weather conditions on the outer crest and the northern area of the reef frequently make diving hazardous so no data are available from these areas. Traps were retrieved every 2 days, and the contents washed through preweighed filters, rinsed with distilled water, poisoned with sodium azide, and dried. Description of Bottom Samples. - Hand samples of bottom sediments were taken at both Cahuita and Grand Cayman. Samples were kept frozen until analysis. At Cahuita, sample locations were around each site, along both outer and inner reef crests, in the bay south of the reef, and at each measured transect (Fig. 2). At Grand Cayman, samples were taken at each transect site. Grain strews from both areas were described using petrographic and binocular microscopes. Pro- portion of organic matter was estimated as weight loss after digestion for one week in 30% hydrogen peroxide. Percentage of carbonate material was estimated as weight loss after digestion in 5% HC!. Determination of Coral Growth Rates. - Massive hermatypic corals are known to secrete annual cou- plets of skeletal density, which can be observed using radiography (Buddemeier, 1978). Coral colonies were cleaned (15% hydrogen peroxide, I wk), and slabs 0.5 em thick cut parallel to the corallites. X-ray radiographs of the slabs were produced with a Macrotank-L unit, 90 kV, 3 rnA, Kodak Tri-X Pan Professional Film, with exposure times of 4-6 min. Growth rates were calculated by direct measurement on contact prints of the negatives, and were checked by referring to calibrated stainless steel nails driven into the summits of massive colonies several years ago. Twenty or 30 of these nails were originally emplaced, but to date only two have been recovered (we suspect the others were all removed shortly after emplacement by grazing, wave action, etc., before the coral could grow around them, and seal them in). The two recovered were from heads of Diploria strigosa and Siderastrea radians, and in both cases showed growth rates identical (± I mm) to those determined radiographically. Determination of Acid-insoluble Residues in Coral Colonies. - For determination of insoluble residues, pieces of coral free from macroborings were cut from the colonies. For estimates of sediments trapped over the life of a colony, the X-ray negative was taped to the slab, and the reqUired year or years dissected with a diamond bandsaw (Gryphon Corp., Burbank, California). Coral samples were soaked CORTEs AND RISK: A SILTATION-STRESSED REEF 343

Table 1. Comparison between the Cahuita and Grand Cayman reefs

Coral community Cahuita Grand Cayman H' (Shannon and Weaver, 1949)· 1.44 1.67 Evenness (Heip, 1974) 0.63 0.63 Richness (Margaleff, 1957) 1.00 1.61 No. species encountered in transects 10 14 • Diversity lower at Cahuita: P < 0.01. Zar (1974).

in I SOlohydrogen peroxide for 4-5 days, rinsed several times with distilled water, dried (lOO°C) and weighed. Corals were then decalcified in EDT A (Glover, 1961); the residue was caught on prerinsed, preweighed Millipore O.45-lLm HA filters, dried and weighed. X-ray Diffraction Analysis. -Samples of SPM from rivers and reef waters, plus samples of bottom sediments, resuspended sediments, and the material trapped inside coral heads, were analyzed by XRD analysis (Cu K-alpha radiation, 30 kV, 16 mA, 5-60° scan, Phillips unit).

RESULTS Coral Community Description.-Live coral coverage at Grand Cayman averaged 63% (range 45-76%). At Cahuita, coverage was consistently lower (average 40%, range 4-80%). The high values for Cahuita are from transects where mono specific stands of Agaricia agaricites (vertical growth habit) were encountered. Such mono- specific stands were not observed at Grand Cayman. Further data on species composition, abundances and depths are given in Cortes (1981) and Cortes and Risk (1984). Community comparison data are given in Table 1. The reef at Cahuita is significantly less diverse than at Grand Cayman. At Cahuita, diversity decreases as live coral coverage increases, reflecting the tendency. toward mono specific stands. This relationship does not occur at Grand Cayman. Another significant difference between these reefs is average colony size. Several of the common Carribean hermatypic species are absent or rare at Cahuita, and only three species were found near the transect locations at both areas in large enough numbers to allow comparisons: Agaricia agaricites (whole colonies), Porites astreoides and Sider- astrea radians. A summary of the comparison of average colony diameters is given in Table 2. All the differences are highly significant, and it is clear that, in these three species, the average colony at Cahuita is larger. The colonies measured for the diameter comparison in Table 2 were all either on or within 1 or 2 m of the measured transects. The variances are therefore less than would be expected from randomly sampling coral colonies over a whole reef(although reduced larval recruitment may also explain the low variances for Cahuita-see later discussion). Species lists are a partial indicator of reef richness. Risk et al. (1980) recorded 26 coral species from Cahuita, based on about 40 person-days' observations. After two additional field seasons' research (lCN and co-workers), this number had increased to 34 (Cortes, 1981). In contrast, 33 species were observed at Grand Cayman during the course of two short visits devoted to gathering other data. Study of species' presence and abundance suggest a pattern. As discussed in Risk et al. (1980), most of the dominant corals at Cahuita have good sediment- rejecting capabilities (after Hubbard and Pocock, 1972, and Bak and Elgershuizen, 1976), and many corals which are dominant or abundant on other Caribbean reefs, but which have poor sediment-resisting abilities, are rare or absent at Ca- huita. The depths at which corals occur are also shallower than optimum depths 344 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985

Table 2. Comparison of coral colony diameters

Coral colony diameters Cahuita Grand Cayman Agaricia agaricites Avg. diam. 21.0 em 6.6 em s 3.0 2.1 N 105 76 Porites astreoides Avg. diam. 9.3 em 7.0 em s 1.9 1.8 N 48 8 Siderastrea radians Avg. diam. 26.6 em 4.6 em s 2.4 2.1 N 32 3

recorded from Jamaica (Goreau and Wells, 1967). For example, Agaricia agari- cites in Jamaica has an optimum depth occurrence of 7-20 m (minimum 3 m), while at Cahuita it is common to dominant in 2-4 m. The morphologies of some of the Cahuita corals are also consistent with sediment rejection capabilities or growth in low light intensities. Agaricia agaricites and Acropora palmata both occur in vertical plates or branches, a morphology better able to shed sediments (Bak and Elgershuizen, 1976). Among the massive species, Porites astreoides and Diploria strigosa exhibit, in shallow water, the platey or curtain-like morphology usually characteristic of depths of 20 m or more on other Caribbean reefs. The few colonies of Montastrea annularis found at Cahuita occur in 2-3 m, and display the plate-like morphology found at depths of25 m or more in other areas (Dustan, 1975; Graus and Macintyre, 1982). Possibly, this reef may never have developed to the status of some of the flourishing and well-known Caribbean areas. The coastline in the region is pre- dominantly sandy, with little available initial hardground. Recruitment oflarvae from the nearest other reef area, Bocas del Toro, Panama (about 50 km to the southeast) would be against the prevailing currents. Suspended Particulate Matter (SPM). -A summary of Cahuita SPM values, to- gether with those from Grand Cayman and the Rios Estrella and Banano (both of which drain into the ocean north of Cahuita) is given in Table 3. SPM values for Grand Cayman were low, and much of the mass consisted of diatom frustules. Values for Cahuita were much higher-in fact, approaching values cited for the upper reaches of the Bay of Fundy (Amos and AIfoldi, 1979), an where surface waters are sufficiently turbid as to inhibit (Yeo and Risk, 1979). River loadings were also very high. One set of samples taken near the mouth of the Rio Estrella during heavy rains recorded values of 6,200 mg per liter. Resuspended Sediments. - Seventy resuspended sediment samples were retrieved. Data are given in Table 4 (1981), and a summary presented in Figure 4. Resus- pension rates were highest near the bay, and lowest at the patch reef. At anyone sample area, resuspension rates were highest near the bottom, and decreased upwards. For the time intervals corresponding to resuspended sediment samples, sea state was estimated (independently, by at least two observers) as belonging to one CORTEs AND RISK: A SILTATION-STRESSED REEF 345

Table 3. Suspended particulate matter (SPM) data summary for Cahuita, rivers north of Cahuita, and the study reef at Grand Cayman (all samples taken at the surface)

Samples Average SPM (No.) Range Cahuita Outer crest 2.6 mglliter 7 0.3-4.6 Inner crest 5.1 50 1.4-18.8 Lagoon 7.1 41 0.2-36.6 Bay (south of reef) 8.6 38 1.8-54.0 Ri vers (Estrella and Banano) 450 26 1.4-6,200 Grand Cayman All stations 1.6 14 <0.1-2.4

of 5 categories, as follows: 1, very rough, large (2-m) waves, strong (> 20 cm· sec-I) currents; 2, rough, large (1-m) waves, moderate (15-20 cm·sec-I) currents; 3, relatively calm, small (0.5-m) waves, moderate (5-15 cm·sec-I) currents; 5, very calm, no waves, undetectable currents. At all elevations off the bottom, for all sediment trap locations, increased suspended sediment values correlated di- rectly with rough water (rank correlation coefficients for all areas and elevations: P always <0.05). During one very rough period, resuspension rates < 1 g cm-I. day-I were recorded. SPM values were also recorded at the trap sites during the monitoring periods. SPM values did not correlate significantly with resuspension rates. Bottom Sediments. -Organic matter in bottom sediments was higher at Cahuita (x = 1.0%, SD = 0.1 %, N = 11) than at Grand Cayman (x = 0.57%, SD = 0.17%, N = 3). Percentage of acid-insoluble material was much higher at Cahuita (x = 33.4%, SD = 30%, N = 11) than at Grand Cayman (x = 0.16%, SD = 0.08%, N = 3). Both of these differences are significant (t-test, P < 0.05). The insoluble residue at Grand Cayman is largely composed of diatom frustules and sponge spicules. The amount of insoluble material in Cahuita samples is a function oflocation: sediments from patch reefs in the lagoon are 3-5% insoluble, whereas values from the southern part of the reef are 30-50%, and samples from the floor of the bay south of the reef are 70% insoluble. The insoluble residues from Cahuita sediments contained rounded to subrounded quartz grains, mag- netite, zoned plagioclases, rounded to angular grains of olivine and prisms of hornblende, and abundant clay-size material. Coral Growth Rates. - Growth rates of corals at Cahuita seemed low, compared with determinations for the same species elsewhere in the Caribbean (although comparison of growth rates is very difficult, given differences in regional settings and environments). The mean growth rate of Siderastrea radians at Cahuita was 5.2 mm 'yr-I (SD = 1.0, N = 10 heads; about 90 slabs were X-rayed, and the pooled variance calculated). The same coral from a comparable environment at Grand Cayman showed a mean growth rate of 12.4 mm·yrl (SD = 5.0, N = 4 heads; 11 slabs were X-rayed). These growth rates are significantly different (t- test; P < 0.01). The Siderastrea colonies from Cahuita exhibiting the highest growth rates were taken from the patch reef and the outer crest, areas of relatively low SPM and resuspended sediment values. Diplaria strigosa from Cahuita grew 4.0 mm· yr-I (SD = 1.1, N = 7 heads), 346 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985

Table 4, Data from the five sediment traps: months in Roman numerals, sea state estimated as in text. The 25 cm trap height was added after mid-J une 1980. SPM values for No.2 were not measured, as they are virtually identical to those from No. I, less than 50 m away. No.5 was not sampled by boat, but by swimming out from shore; hence, data gaps occurred due to loss or contamination of samples. For location of traps, see Figure 2

Height above Trap substrate Resuspended sediments No. Date Sea state SPM (em) (mg·em-l·day-I) I/VII/80 rough 3.5 75 109.1 50 136.2 16/VII/80 calm 3.7 75 28.4 50 32.7 23/VII/80 very calm 4.] 75 27.5 50 29.7 25 34.8 30/VII/80 very rough 1.6 75 77.6 50 1]2.7 25 148.8 7/VIII/80 relatively calm 6.1 75 72.2 50 90.9 25 120.8 2 16/VII/80 calm N.M. 75 18.2 50 19.2 23/VII/80 very calm N.M. 75 24.9 50 25.2 25 32.6 30/VII/80 very rough N.M. 75 69.2 50 92.2 25 111.9 7/VIII/80 relatively calm N.M. 75 38.8 50 65.9 25 112.8 3 2/VII/80 rough 2.2 75 41.1 50 46.9 8/VII/80 calm 3.7 75 24.4 50 26.8 16/VII/80 calm 3.0 75 14.2 50 15.7 23/VII/80 very calm 3.4 75 17.0 50 19.5 25 20.9 30/VII/80 very rough 2.0 75 21.8 50 35.2 25 64.9 7/VIII/80 relatively calm 5.5 75 12.8 50 15.3 25 20.6 4 4/VII/80 relatively calm 4.0 75 289.2 50 358.7 16/VII/80 calm 3.7 75 28.4 50 36.5 23/VII/80 very calm 3.7 75 34.9 50 47.3 25 69.3 30/VII/80 very rough 4.8 75 528.1 50 701.6 25 1,060.3 CORTEs AND RISK: A SILTATION-STRESSED REEF 347

Table 4. Continued

Height above Trap substrate Resuspended sediments No. Date Sea state SPM (em) (mg·em-2·day·')

7/VIIV80 relatively calm 8.6 75 116.9 50 140.4 25 253.1 5 26/VI/80 very rough 10.8 75 651.5 50 747.4 2/VII/80 rough 13.2 75 233.6 50 278.9 l6/VII/80 calm 4.4 75 (not recovered) 50 43.7 23/VII/80 very calm (not recovered) 75 53.1 50 60.9 25 76.4 30/VIV80 very rough (not recovered) 75 666.9 50 889.4 25 1,179.9 7/VIII/80 relatively calm (not recovered) 75 114.2 50 135.6 25 158.1

500 o'"' 't:I...• N E ...•u 400 0> E

en W l- et 300 a: INNER CREST z o III Z W a.. 200 III ::l III W a:

l- 100 Z W ~ OUTER CREST o W III PATCH REEF

20 40 60 80 DISTANCE FROM BOTTOM (em) Figure 4. Sediment resuspension rates versus elevation off the bottom. Sites I and 2 are just north and south, respectively, of a small patch reef in the lagoon; sites 3, 4 and 5 are in order, north to south, on the inner reef crest. 348 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985

which compares with a rate of 5.6 mm .yr-I recorded for that species at the same depth in Horida (Shinn, 1976; within-head variances not given). Growth rates of Montastrea annularis at Cahuita have a mean value of 5.3 mm 'yr-I (SD = 2.3, N = 5). Montastrea annularis is a well-studied coral, and a large number of growth estimates are available in the literature (Lewis et al., 1968; Aller and Dodge, 1974; Hein and Risk, 1975; Dustan, 1975; Shinn, 1976; Dodge, 1982; Hudson, 1981; 1982). These values typically range from 20 mm· yr-1 in shallow water (Lewis et a1., 1968) to less than 2 mm'yel in depths greater th an 30 m (Dustan, 1975). The Cahuita values are lower than any reported Montastrea annularis growth rates from shallow water. The highest individual value from Cahuita, 6.3 mm· yr-I, compares with a value of6.2 mm'yr-I for slow-growing colonies from areas of high sediment resuspension in Discovery Bay, Jamaica (Aller and Dodge, 1974). Sediments Trapped Inside Coral Heads.-Sixty values of internal acid-insoluble sediment amounts were determined for Cahuita, and 10 for Grand Cayman corals (Table 5). Replicate samples from the same depth within coral heads gave identical values (t-test, N = 20, P < 0.05). Values of trapped sediment in Cahuita corals were significantly higher than from Grand Cayman (Mann-Whitney V-test, P < 0.05). SEM observations of the insides of coralla (Fig. 5) show the Grand Cayman corals to be pristine, while the insides of Cahuita corals were often clogged with clay-sized material. Siderastrea radians from Cahuita typically had from 0.03-0.1 % by weight acid-insoluble material, with higher values recorded from areas with high rates of sediment resuspension. Amounts of trapped sediment in corals from the same general sample areas were comparable. Of the corals studied, Diploria strigosa exhibited the highest values, commonly 0.2% by weight. Such high values of trapped silici- clastic sediment should be considered in studies of the trace element geochemistry of corals. Grand Cayman corals showed no trend in trapped sediment values for samples taken from various places within colonies. Amounts of trapped sediment in Ca- huita coral samples laid down relatively recently (1974-1977) were significantly higher than in older (1963-1965) skeletal material (Wilcoxon Signed-Rank Test, N = 9 pairs, P < 0.05). We were reluctant to sacrifice large colonies at Cahuita, so the oldest data come from skeletal material laid down in the late 1940's. Trapped sediment values pre- 1950 never exceeded 0.05% by weight in the corals examined (Siderastrea radians, Montastrea annularis, Diploria strigosa, Stephanocoenia michelinil). Care must be exercised, however, in interpreting trapped sediment results. The bases of most corals are heavily bored and, at Cahuita, surrounded by high amounts of resus-

T.able 5. Amount of acid-insoluble residue in coral heads from Cahuita and Grand Cayman, at different levels in the heads. For Grand Cayman samples, the insoluble residue consists largely of diatom frustules and sponge spicules. The residues from Cahuita are overwhelmingly composed of detrital siliciclastic material, including quartz, feldspar, kaolinite, illite, and montmorillonite. For location of Cahuita corals, see Figure 2

Subsample (cm from Approximate year of Insoluble top of colooy) skeletal deposition (%) Grand Cayman G.c. I 2 1979 0.0111 Diploria strigosa 3.5 1974 0.0170 CORTEs AND RISK: A SILTATION-STRESSED REEF 349

Table 5. Continued

Subsample (cm from Approximate year of Insoluble top of colony) skeletal deposition (%)

5 1970 0.0159 G.C.2 2 1979 0.0114 Diploria strigosa 6 1969 0.0218 10 1960 0.0063 G.c. 3 2 1977 0.0288 Siderastrea siderea 7 1970 0.0172 G.c. 5 5 1974 0.0217 Siderastrea radians 10 1969 0.0288 Cahuita 5 3 1973 0.2019 Diploria strigosa 8 (bottom) 1963 0.2414 8 2 1976 0.0582 Siderastrea radians 5 1970 0.0443 8 1964 0.0377 12 (bottom) 1956 0.0440 15 2 1975 0.0259 Stephanocoenia michelinii 8 1967 0.0405 14 1963 0.0315 20 1957 0.0200 26 1951 0.0320 32 1945 0.0213 16 2 1975 0.2468 Siderastrea radians 4 1971 0.1214 8 1963 0.0611 10 1959 0.0539 12 1955 0.0377 16 1947 0.0379 17 1 1977 0.0140 Siderastrea radians 2 1975 0.0268 4 1971 0.0133 18 1.5 1977 0.0168 Siderastrea radians 3 1973 0.0407 4 1971 0.0298 5.5 1969 0.0339 20 2 1976 0.0339 Siderastrea radians 3 1974 0.0366 5 1970 0.0284 6 1964 0.0151 8 1961 0.0210 9.5 0.0156 21 1 1977 0.0920 Siderastrea radians 2.5 1974 0.0550 3.5 1972 0.0586 6 1967 0.0551 7.5 1964 0.0369 11 (bottom) 1955 0.0877 1 2 1977 0.0634 Montastrea annularis 5 1974 0.0516 6 1972 0.0360 7 1971 0.0393 8.5 1970 0.0462 IO 1968 0.0195 13 1965 0.0157 15.5 (bottom) 1963 0.0521 350 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985

Figure 5, Scanning electron microscope photographs of the insides of corallines from Cahuita (upper) and Grand Cayman (lower). Scale bars 10 )Lm. Note the clastic material in the Cahuita corallite. Both micrographs show the inside margin of a corallite. Top of the colonies is toward the viewer. pended sediments. In some of the corals analyzed (about one-third), the bottom sample in the head, no matter the date of deposition, recorded elevated values of trapped sediments. We have interpreted these values as anomalous, and as caused by horizontal percolation of sediment through the borings, into the col- onies. The trend with time of amount of trapped sediment for a suite of five corals collected on the inshore reef crest (Fig. 2: area 8 of Risk et al., 1980) is shown in Figure 6. These corals all show a trend of increasing values of trapped sediment from the late 1960's to the present. The colony showing the highest trapped amounts (head No. 16), and the most abrupt post-I970 increase, is close to the location of sediment trap No.5, which recorded the highest rates of sediment

>- I {!) W 015 \ (0.25%) :i: \ 1977 >- m \

Vl -' 'Ji 0.10 o u

>- Z llJ ~ GJ 0.05 ,, Vl a "U-'""""" llJ a.. a.. ' ...o------...{) « a: >- ° 1975 1970 1965 1960 1955 Figure 6. Amount of insoluble material trapped inside coral colonies from the inner reef crest at Cahuita, at different times during growth. Dashed line Montastrea annularis, solid lines Siderastrea radians. Lowest S. radians is Head No. 20; highest is Head No. 16. CORTEs AND RISK: A SILTATION-STRESSED REEF 351

Q

M Q

RS

Figure 7. X-ray diffractograms ofCahuita Reefand Rio Estrella sediments: RS, river sediments; SS, SPM over the reef, north end; STr, sediment trap #3, 50 em off the bottom; BS, bottom sediment; TS, sediments trapped inside coral heads. Letters indicate peaks due to identified minerals: Q, quartz; F, feldspar; M, montmorillonite; I, illite; A, aragonite.

resuspension, adjacent to the clastic sediments of the bay bottom. The colony demonstrating the lowest values oftrapped sediments in Figure 5 (Head No. 20), and a relatively moderate post-l 970 increase, is from the north-south trending part of the crest, where resuspension rates are lower. Geochemical Analyses. -Grand Cayman sediment samples consisted of aragonite, calcite, and quartz. Analysis of the small amount of material trapped inside Grand Cayman heads showed no mineral material whatsoever. SPM in the Rios Estrella and Banano contained magnetite, feldspar, quartz, illite, montmorillonite and kaolinite, as would be expected with rivers draining first through andesitic terrain, then clastic sediments, and finally crossing a humid coastal plain. On the reef, samples were analyzed ofSPM, resuspended sediments from the traps, bottom sediments, and material trapped inside coral colonies. All of these traces are essentially identical (Fig. 7), with the exception that the incoming river sediment contains no aragonite, nor does the material trapped inside the colonies (aragonite is dissolved during preparation of these samples). 352 BULLETIN OF MARINE SCIENCE, VOL. 36. NO.2, 1985

J;. JAMAICA 10.0 • CAHUITA GROWTH J;. J;. RATE J;. • (mm/yr.l 5.0 • ••

0.1 1.0 10 100 1000 2 SEDIMENT RESUSPENSION RATES (mg /cm / day) Figure 8. Relationship between growth rate of Montastrea annularis and rate of sediment resuspen- sion. Triangles, Discovery Bay, Jamaica (Aller and Dodge, 1974); circles, Cahuita.

Suspended sediments carried to the sea by, for example, the Rios Estrella and Banano, are transported south by longshore drift to the reef at Cahuita (as seen by LANDSAT). Some of this material deposits on the reef, and is intermittently resuspended; some is trapped inside the coral skeletons, and some accumulates in the bay south of the reef.

DISCUSSION Environmental conditions at Cahuita, with the exception of the amount of sediment in the water, seem conducive to coral growth. SPM values over the reef at Cahuita generally were 5-10 mg per liter (weighted average 7.4 mg per liter), higher than any other Caribbean reef for which values have been obtained (Glynn, 1973; Rogers, 1977), although values approaching these (3-5 mg per liter) have been reported over inner reefs of the Florida Reef Tract (Griffin, 1974). Similarly, sediment resuspension rates at Cahuita, roughly 20-1,000 mg·cm-2·day-l, are much higher than in Discovery Bay, Jamaica (0.45-1.1 mg·cm-2·day-1; Aller and Dodge, 1974) or Puerto Rico (1-21 mg·cm-2·day-'; Rogers, 1977). Corals remove sediments by tentacular and ciliary action, body extension, and mucus sheet entanglement (Hubbard, 1973; Bak and Elgershuizen, 1976; Lewis and Price, 1976). All of these represent an energy drain on the coral. Corals also depend on light for growth (among many papers, see Goreau, 1959, and Muscatine et aI., 1979), and turbid water reduces light penetration. Rogers (1979) showed experimentally that reduction of incident light by suspended sediments has a greater effect on some coral species than the sediment itself. Finally, normal feeding activities of corals may be interfered with, especially those corals which suspension feed (Lewis, 1976). Whatever the means of action, the results of high sediment loading are decreased coral growth rates (Aller and Dodge, 1974), depressed spat recruitment (Sammarco, 1980) and, sometimes, coral death (Thompson, 1979; Rogers, 1979). In this study, coral growth rates are reduced in areas of high sediment resus- pension rates. Our data for Mantastrea annularis were combined with those of Aller and Dodge (1974), because the design of the sediment traps used in both studies was very similar. Significant inverse correlation exists (r = - O. 77, P < 0.05) between resuspension rates and growth rates (Fig. 8). Although Figure 8 CORTEs AND RISK: A SILTATION-STRESSED REEF 353 combines data from two different areas, it appears that the growth rate decrease of M. annularis is a linear inverse function of sediment resuspension rate, in the ranges studied. Although high resuspension rates probably exclude M. annularis from many areas at Cahuita, the relationship is not simple. From Figure 8, the resuspension rate corresponding to zero growth rate (coral death?) is approximately lOs mg ·cm-2·day-l, or a gross sedimentation rate of 150 m 'yr-I! It may be difficult to kill an adult Montastrea annularis by sediment resuspension alone. The effect of high resuspension rates on reef communities is, however, not restricted to suppression of growth rates of adult corals. Larval recruitment is also affected. High recruitment has been reported to result in large numbers of small colonies (Loya and Slobodkin, 1971; Sammarco, 1980). On the other hand, it has been argued (Dodge, 1978) that coral colonies will be smaller in turbid water, as they could more efficiently reject sediments. At Cahuita, average colony size is clearly larger than at Grand Cayman, and few small colonies occur at Cahuita. This is consistent with low larval recruitment. Larvae are, however, present in the : little scleractinian colonies developed within two weeks on the metal legs of the frame for the sediment trap arrays, but only at 50 cm or more off the bottom. Risk (1981) has reported similar success in larval recruitment on artificial surfaces emplaced in turbid areas, but elevated above the bottom, where water is somewhat less. The reef at Cahuita shows reduced live coral coverage and species diversity. Coral colonies may frequently be seen buried in sediments and bleaching at colony fringes. Reduction in growth rates would hamper a coral's ability to overcome sediment accumulation. Under high sediment loading, recovery from any wound or injury is likely to be hindered. To the extent that Cahuita may be used as a model, we suggest that reefs under severe terrigenous siltation stress will show the following characteristics (the sug- gested values in points 1-3 are estimated from the results of this study, and may or may not have wider application): (1) high concentrations of SPM (probably > 5 mg per liter); (2) high concentrations of resuspended sediments (> 30 mg' cm-2·day-l, 50 cm off the bottom); (3) high amounts of terrigenous material trapped inside the skeletons (>0.05% by weight); (4) reduced coral growth rates and diversities; (5) low live coral coverage, except for monospecific stands of siltation-tolerant species; (6) morphology changes in surviving coral species, and (7) size distribution of corals shifted towards larger colonies, indicating low re- cruitment rates. Previously, identification of reefs under siltation stress has usually involved description of areas that have already undergone considerable damage, and com- parison of these areas with either nearby healthier reefs or with earlier descriptions of the same reef (often poorly documented). The corals themselves may preserve a record of changing environmental conditions (Barnard et al., 1974). The trends described in this paper may become useful tools in reef management, allowing early identification of siltation stress. The information required is not difficult to collect, nor are the analytical procedures beyond the capabilities of most tropical countries. These results may be of use in ancient as well as modem coral reefs. Organic reefs, some of them made by corals, are found throughout the Phanerozoic, and are important exploration targets for minerals and hydrocarbons, largely because of porosity and permeability contrasts with country rocks (Chapman, 1977). The demise of individual fossil reefs has usually been attributed to siltation, because most are overlain by shales. The way in which a reef died, however, may have a profound effect on porosity and permeability of the uppermost reef unit. To that 354 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985 end, we propose, for reefs in the geologic record that were killed by other than subaerial exposure: (1) Reefs killed by rapid (anastrophic) sediment influxes will show no significant biofacies changes toward the top, other than a thin zone of intercalated terrigenous sediments and calcarenites. Detailed study of facies re- lationships should make such reefs easy to identify. (2) Reefs killed by chronic siltation will show, towards the top, a reduction in total species number, and size increases and morphology changes in surviving species. Due to reduced live cov- erage, an increased ratio of matrix to autochthonous components may occur which, combined with an increase in average colony size, may lead to a transition from matrix-poor bafflestone to matrix-rich framestone. Colonies of the herma- typic organisms themselves will show increased amounts of trapped terrigenous material (already verified for Devonian patch reefs: Risk, unpubl. data). The result of these trends will be to produce a zone of lowered primary porosity and per- meability at the top of the reef.

ACKNOWLEDGMENTS

We thank M. M. Kandler for help in the field, plus many students at the Departmento de Biologia at the Universidad de Costa Rica. M. M. Murillo and J. A. Vargas helped in transportation and equipment needs. The cooperation ofthe Servicio de Parques Nacionales de Costa Rica was essential, and deeply appreciated. Figures and plates were prepared with the photographic guidance of J. Whor- wood. An earlier draft of this paper was read by M. Murillo. M. Belec typed the manuscript. Supported by CONICIT (Consejo Nacional de Investigaciones Cientificas y TecnoI6gicas), CESO (Canadian Executive Service Overseas) and NSERC (Natural Science and Engineering Research Coun- cil, Canada). The thesis by the senior author, upon which this report is based, won the 1981 Costa Rican National Science and Technology Award (el premio "Cladomiro Picado Twight").

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DATEACCEPTED: March 12, 1984.

ADDRESSES: (J.C.N.) Centro de Investigaciones en Ciencias del Mar y Limnologfa (CIMAR), Univer- sidad de Costa Rica, San Jos~, Costa Rica; (M.J.R.) Department of Geology, McMaster University, Hamilton, Ontario Canada L8S 4MI.