BULLETIN OF MARINE SCIENCE, S1(3): 429-442, 1992 CORAL REEF PAPER

EFFECTS OF SUSPENDED SEDIMENT AND BURIAL ON

SCLERACTINIAN CORALS FROM WEST CENTRAL

FLORIDA PATCH REEFS

Stanley A, Rice and Cynthia L. Hunter

ABSTRACT

The distribution and abundance of eight species of endemic scleractinian corals were

determined for patch reefs off west central Florida in the depth range of7-18 m. Phyllangia

americana and Cladocora arbuscula were the most abundant species encountered. Five of

the eight species censussed were absent from the shallowest reef(7 m). Seven species of corals

were exposed to increasing levels of suspended sediment in the laboratory and seven species

were subjected to prolonged burial in sediment. Survival rates were not affected by IO-day

exposures to suspended sediment concentrations of49, 10 I, 165, or 199 mg' liter-'. Combined

mean growth rates for six species were significantly different between control and experimental

treatments at 165 mg' liter Isuspended sediment. Coral burial experiments produced survival

LT50 values of7 days for Scolymia lacera, 7.2 days for lsophyllia sinuosa, 10 days for Manicina

aereolata, 13.6 days for Siderastrea radians, 15 days for C. arbuscula, 16.2 days for Stepha nocoenia michelinii. and 15+ days for Solenastrea hyades. The results of these experiments

indicate that the species tested are among the most resistant corals in the Caribbean region

to the effects of suspended sediment and physical burial. These findings are consistent with

the fact that west central Florida patch reefs are exposed to more severe environmental

conditions, such as high turbidity and low light penetration (in addition to a broader range

of temperatures) than more tropical reefs to the south.

Since most vigorous coral reef development occurs in transparent tropical waters, it has become axiomatic that low levels of turbidity and suspended sediments

are essential for optimal hermatypic coral growth and survival. High turbidity

and sedimentation rates may depress coral growth and survival due to I) attenuation of light available to symbiotic zooxanthellae (Rogers, 1979), 2) redirection

of energy expenditures for clearance of settling sediments (Dallmeyer et aI., 1982;

Abdel-Salam and Porter, 1988), or 3) actual smothering of coral tissue (Rogers,

1983). However, tolerance of corals to high sediment loads may vary considerably

among species, with some corals being fairly resistant to low light levels and/or

sedimentation effects(Edmondson, 1929; Dodge and Vaisnys, 1977; Lasker, 1980;

Rogers, 1983; Hodgson, 1990). Sensitivity to these effects depends on sediment

clearing ability, calyx and colony morphology, orientation of growth, and the

types of sediment involved (Hubbard and Pocock, 1972; Bak and Elgershuizen,

1976; Dodge and Szmant-Froelich, 1985).

Rogers (1990), in a review of the effects of sedimentation on corals, suggested

that reefs with high sediment loads may have lower diversity, percent cover, and

growth rates of coral species, small colony sizes (or large colonies in areas in which

recruitment is limited by siltation), an upward shift in depth zonation, and a

predominance of resistant growth forms or species. In general, field observations

have confirmed a relationship between suspended sediment levels and coral distributions in a variety of geographic locations including: Palau (Motoda, 1939),

Australia (Mayer, 1918; Marshall and Orr, 1931); Fanning Island (Roy and Smith,

1971), Guam (Randall and Birkeland, 1978), Hawaii (Edmondson, 1928), Costa

Rica (Cortes, 1990), Florida (Hubbard and Pocock, 1972), Jamaica (Dodge et aI.,

1974), Puerto Rico (Loya, 1976; Acevedo and Morelock, 1988), and the Virgin

Islands (Rogers, 1983; Hubbard et aI., 1987). In many cases, the same species or

429
430 BULLETIN OF MARINE SCIENCE, VOL. 51, NO.3, 1992

genera are found in similar "suboptimal" turbid conditions in different geographic

locations (Colin, 1978).

There have been few studies that address the effects of suspended sediments

on growth and mortality of individual coral species. Field studies have demonstrated that growth rates of Montastrea annularis are inversely correlated with

sediment loads (Dodge et aI., 1974; Dodge and Vaisnys, 1977; Dodge and Lang,

1983; Tomascik and Sander, 1985; Hubbard et aI., 1987). However, other workers

have found that there was little or no evidence of decreased growth rate for

surviving colonies of Porites lutea, even in areas where high coral mortality has

been attributed to effects of sedimentation (Hudson et aI., 1982; Brown et aI.,

unpubl., in Rogers, 1990).

The present investigation was undertaken to test the effects of increasing sediment loads on the survival and growth rates of scleractinian corals from Gulf of

Mexico patch reefs offwest central Aorida. Although these patch reefs are atypical

of other, more luxuriant, coral reefs of the southern Aorida Keys and Caribbean,

they share common features with the latter in that they provide hard substratum

for concomitant diverse and unique biotic assemblages relative to the surrounding

soft bottom. Coral communities on the west central Florida patch reefs are characterized by low species diversity and abundance, and a predominance of species

with large, solitary polyps or small colony sizes. They are subject to seasonal

increases in suspended sediments due to winter storms and occasional hurricanes.

These reefs are also adjacent to designated dredge spoil disposal sites. Although

these patch reef corals may be surviving at or near their physiological limits

relative to a number of environmental variables (particularly temperature extremes), only the effects of increasing suspended sediment loads and burial were

addressed in this study.

Experiments were conducted under controlled laboratory conditions intended

to simulate the levels of suspended sediment that might be expected to occur due

to storms or dredge spoil disposal. Four different levels of suspended sediment

were tested in 10- and 20-day coral survival and growth experiments. Additional

laboratory experiments were employed to evaluate the resistance of seven coral

species to total burial under natural marine sediments. Results of these bioassays

were used to test the hypothesis that increasing suspended sediment load effects

survival and growth rate of these corals and contributes to the species distributional patterns observed in west central Florida patch reef environments. Field

observations included abundance estimates of eight species encountered on natural and artificial reefs in depths from 7-18 m.

METHODS

Field Surveys.-Species abundance and composition were censussed at three sites off west central

Rorida in between 1986-1989 (Stations A, B, C, Fig. I). SCUBA divers counted total numbers of

coral individuals and colonies within II, I m2 quadrats on a 15 m deep reef and 9-35 I m2 quadrats

on an 18 m deep reef (see Table I for sample size by coral species). On these two reef areas, quadrats

were placed over random sections of natural hard bottom. An artificial reef (7 m depth) was also

censussed by intentionally placing quadrats over sections of the reef where corals were present. Twelve

square meters of area were examined on the 7-m reef. This reef had been deployed for 3 years at the

time of sampling, and was construct::d of concrete rubble, culverts, pilings, and tires (some of which

had little or no growth on them) interspersed between areas of soft bottom.

Total Suspended Solids. - Nineteen seawater samples were collected from six stations located from I

to 18 nmi offshore in the GulfofMexico (Fig. I, Stations 1,4,7, 10, 12 and 18) during March-June,

1984 for determination of total suspended solid loads in near bottom water. Samples were collected

by divers or with a Niskin bottle within I m of the bottom and transferred to precleaned polypropylene

containers for transport to the laboratory.

Water samples were also collected periodically from the experimental and control exposure aquaria
c

•

280N+

83°W

o

~·I.

•B

10 20

III

mile.

RICE AND HUNTER: EFFECTS OF SEDIMENT ON CORALS 431

Figure I. Location of stations visited for coral collection, suspended sediment measurements, and

coral population estimates. Station I, 27: 19:ION, 82:36: II W; Station 4, 27: 17: 15N, 82:39: lOW; Station 7, 27: 16:II N, 82:42:09W; Station 10,27: 14:62N, 82:45:00W; Station 12,27: 14:34N, 82:45:55W;

Station 18, 27:31:45N, 83:04:09; Station A, 27:29:30N, 82:44:05W; Station B, 27:32:2IN, 82:54:

30W; Station C, 27:39: 15N, 83:0 I: 15W.

during the course of each suspended sediment bioassay. All suspended solid samples were stored at

4°C until analyzed, and all samples were analyzed in duplicate.

Polycarbonate filters with a pore size of0.45-J.Lm were vacuum rinsed with distilled-deionized water,

dried at 100°C for 2 h and weighed. A premeasured volume of well-mixed sample was then filtered,

followed by a second distilled-deionized water rinse to remove any salt. Filters were dried at Iaaoc

for 2 h and reweighed.

Sediments used in suspended sediment and burial studies were collected by SCUBA divers from

Station 12 (Fig. I). This reefis not in the vicinity of any known ocean disposal sites and the sediments

collected there are believed to be representative of naturally occurring deposits in the Gulf of Mexico.

Sediment samples were scooped into 4 liter, acid washed, polypropylene jars in the field and stored

at 4°C in the laboratory until used.

Collection and Handling of Experimental Corals. -Corals for suspended sediment and burial experiments were collected from Station 12 (Fig. I). Care was taken in collection and transport of test

organisms to minimize stress and damage to tissues. When a suitable specimen was encountered,

SCUBA divers gently maneuvered a knife blade or pry bar between the base of the coral and the

substratum and dislodged the colony using steady pressure. Any specimens damaged during collection

were not returned to the laboratory. Once dislodged from the bottom, specimens were placed into

nylon mesh bags and taken to the surface where they were rapidly transferred to tubs of clean seawater.

Corals were then sorted and placed into insulated containers for transport. Seawater in the containers

was replaced at least once per hour and lids were kept in place to avoid stress from direct sunlight.

In the laboratory, specimens were placed into aquaria with continuously flowing seawater and

allowed to acclimate for at least 7 days. Temperature (24-26°C), salinity (34.5-35.00/00), dissolved

oxygen (4.3-6.2 mg'liter-'), and pH (7.86-8.06) were monitored and maintained during the acclimation

and experimental periods. Photoperiod was set at 14:10 (light: dark) with a I-h transition period on

each end of approximately 75% full illumination. Incident light in the photosynthetically active range

(400-700 nm) was measured with a LiCor integrating photometer and found to be 2.0 and 2.5 J.LEs·

m 2'sec' for the control and experimental aquaria, respectively. This converts to approximately 0.3-

0.4% of full noon solar irradianee. Brine shrimp nauplii were added to each aquarium daily in the

late afternoon, since most corals feed during darkness.

Experimental Exposure System. -All suspended sediment and burial experiments were carried out

in a climate-controlled laboratory. Two parallel recirculating seawater systems were used in suspended
432 BULLETIN OF MARINE SCIENCE, VOL. 51, NO.3, 1992

Table I. Coral densities on patch reefs in the vicinity of Tampa Bay, Florida. Values are mean density

in colonies or individuals per square meter ± one standard deviation. The 15 m and 18 m reefs are

natural patch reefs, the 7 m reef is artificial, composed of concrete pilings and culverts that had been

submerged for 3+ years at the time of sampling. All species are hermatypic except for Phyllangia

americana. N = number of I m2 quadrats sampled

Reef depth

Mean for all

Species 7m 15 m 18 m reefs·

Cladocora arbuscula 3.5 ± 2.0 22.0 ± 14.3 18.6 ± 13.9 15.5 ± 13.9

(N = 12) (N = II) (N = 23) (N = 46)

Stephanocoenia mitchelinii 0 1.5 ± 1.9 0.3 ± 0.5 0.7 ± 1.3

(N = 12) (N = II) (N = 23) (N = 34)

Isophy/lia sinuosa 0 1.6 ± 2.0 1.6±0.7 1.6 ± 1.4

N = 12) (N = II) (N = IS) (N = 26)

Siderastrea spp. 0 13.5 ± 11.3 0.9 ± 1.5 4.9 ± 8.7

(N = 12) (N = II) (N = 23) (N = 34)

Sofenastrea hyades 0.5 ± 1.0 1.1 ± 2.1 0.7 ± 0.8 0.7 ::!: 1.2

(N = 12) (N = II) (N = 23) (N = 46)

Scolymia lacera 0 0,6 ± 0.9 0.7 ± 1.2 0.7 ± 1.1

(N = 12) (N = II) (N = IS) (N = 26)

Phyllangia americana 44.8 ± 29.6 10.3 ± 6.5 18.8 ± 14.8 25.6 ± 24.9

(N = 12) (N = II) (N = 9) (N = 32)

Manicina aereofata 0 2.1 ± 1.9 3.2 ± 2.8 2.9 ± 2.6

(N = 12) (N = II) (N = 35) (N = 46)

Total coral density 48.8 52.7 44.8 52.6

• Includes only those reefs where specimens were observed Looccur.

sediment tests. These systems each consisted of a 200-liter polyethylene reservoir from which seawater

was pumped to a 55-liter head tank provided with two magnetic stirrers. Sediment from the field

collection site was screened through a 0.5-mm sieve and added to the experimental system reservoir.

The sediment was kept in suspension by vigorous aeration and the sediment-water slurry was pumped

to the head tank with a Flotec immersible centrifugal pump. Scdiment in the head tank was kept in

suspension with two magnetic stirrers, Water flowed by siphon from the head tanks to 60-literexposure

tanks where the experimental and control animals were housed. Constant aeration in the experimental

tank helped to keep the sediment in suspension. Some sediment accumulated on the bottom of the

experimental tank during experiments but the corals were not affected by this as they were placed on

a plastic grating 2 cm above the bottom. Flow rates from the head tanks into the exposure tanks

provided approximately 44 volume changes per 24 h. Overflow water from the exposure tanks flowed

back to the reservoirs. Total water volume in each system was I 15 liter. Seawater used in all experiments

was obtained from the 114,000-liter laboratory-wide system pumped from New Pass, Sarasota.

During initial acclimation periods, exposure tanks were connected to the laboratory-wide flowthrough seawater system. At least 2 days prior to beginning experiments, water flow through the

exposure tanks was switched to the I IS-liter recirculating system. Seawater in the two parallel recirculating systems was replaced with seawater from the laboratory-wide system after each lO-day experiment.

Coral burial experiments were carried out in 20-liter glass aquaria supplied with flowing seawater

from the laboratory system. A subgravel filter was placed in each aquarium and covered with 3.5 cm

of crushed coral gravel. Experimental animals were arranged on top of the gravel and their locations

within the aquaria were mapped. Sediment from the Gulf of Mexico was sieved through a 0.5-mm

screen and added to the aquarium until all specimens were covered. Subgravel filters provided sufficient

flow through the sediment to prevent anoxic conditions from occurring. At the end of each experiment,

specimens were removed from the sediment and placed into recovery aquaria provided with flowing

seawater. The health and general condition of each experimental animal was recorded immediately

following removal from burial. Some organisms survived burial, but appeared to be heavily stressed.

For this reason, each organism was checked again at 24 hand 7 days with final survival rates recorded

after 7 days. Seven species of corals were tested in burial experiments with sample sizes ranging from

9-28 specimens per species. A summary of the specific experiments performed, species tested, and

parameters measured is presented in Table 2.

Four suspended sediment experiments were completed using scleractinian corals exposed to natural
RICE AND HUNTER: EFFECTS OF SEDIMENT ON CORALS 433

Table 2. Summary of coral bioassay experiments and species tested in each trial. Results of these

experiments and statistical comparisons can be found in Tables 4-9

Number

lesled

per

Suspended Parameters trealExperiment sediment load Duration measured Species tested menl

Suspended Sediment I 49 mg' liter-I 10 days survival and 1,2,3,4 20

(SSE I) growth

Suspended Sediment 11 101 mg'liter

'

10 days survival and 1,2,3,4 20

(SSE 11) growth

Suspended Sediment III-I 165 mg' liter-I 10 days survival and 1, 2, 3, 6, 7, 8 30

(SSE III-I) growth

Suspended Sediment 111-2 199 mg'liter I 10 days· survival and I, 2, 3, 6, 7, 8 30

(SSE II1-2) growth

Burial I I day survival 1,2,3,5 22

Burial II 4 days survival 1,2,3,5 22

Burial III 6 days survival 2,3,6,7 22

Burial IV 8 days survival 6, 7 13

Burial V 10 days survival 1,2,5,8 24

Burial VI 15 days survival 1,3,5,8 27

Species codes: I = Cladocora arbuscula; 2 = Manidna aereolala; 3 = Solenaslrea hyades; 4 = Phyllangia americana; 5 = Siderastrea

radians; 6 = IsophyJ/ia sinuosa; 7 = Scolymia lacera; 8 = Stephanocoen;a mitchelinii .

• This experiment was a continuation on SSE 111·1(see Methods).

sediments from the Gulf of Mexico. Each experiment lasted 10 days with survival and growth rates

measured for each coral specimen. A different suite of experimental animals was used in each experiment except for the final two suspended sediment tests in which the same animals were exposed for

20 days and checked for survival and growth at 10-day intervals [Suspended Sediment Experiment

III (SSE III-I and 2)].

Coral Growth Rates. -During each of the four suspended sediment bioassays, the beginning and final

weight of each coral colony or individual was measured using a modification of the buoyant weight

technique described by Dodge et al. (1984). A basket was suspended beneath an American Scientific

Products DTL 2500 toploading electronic balance and was positioned over a 38-liter aquarium so

that the basket and coral were completely submerged during weighing. Each coral was weighed to the

nearest 0.01 g and the temperature and salinity of the water in the weighing aquarium measured after

every fifth weighing. Buoyant weights were converted to equivalent weights in air using the formula:

W, = [WJ(I-(DJDm)))

where W, = weight in air, Ww= buoyant weight, Dw = density of water, and Dm = density of coral.

Dmwas assumed to be 2.94 (the density of pure aragonite) and Dwwas calculated from the temperature

and salinity of the seawater in the weighing aquarium using Table 2.1 in Riley and Chester (1971).

The resulting weights in air (W,) were then used to compute growth rates. The change in weight of a

specimen over the course of the experiment was divided by the initial weight of the same coral to

obtain a normalized value expressed as mg growth·g I body weight· 10 days-I. Normalizing the growth

data reduced the coefficient of variation (Sokal and Rohlf, 1969) in each analysis.

Determination oj Survival. -Survival rates were determined for each species in both suspended sediment and burial experiments. Some corals, such as Scolymia and Phyllangia. represented single

individuals and could be easily classified as alive or dead. Most species tested were colonies consisting

of numerous corallites and for these species individual corallites were considered alive if soft tissues

appeared normal, or if tentacles were extended in feeding posture during the recovery period.

Sublethal responses were recorded for corals exposed to suspended sediment or used in burial

experiments. Loss of color associated with expulsion of zooxanthellae was a common response in

Manicina, Scolymia. Isophyl/ia. and Solenastrea. Shrinkage of soft tissue and exposure of underlying

skeletal features was observed in Manicina. Scolymia, and Isophyllia. Most corals produced mucus

from their soft tissues, and this feature was used to assess relative health. In cases where it was difficult

to determine if a coral was alive, mucus production was used to infer the presence of living tissues.

Surface mucus was absent on all corals considered to be dead.

Data Analysis. -All treatment means, standard deviations, and probabilities were calculated using

Stat View SE + Graphics software on a Macintosh computer. This software package performs a standard

ANOVA using the Schelre F-test (Schelre, 1959) to test for significant differences. Treatment means
434 BULLETIN OF MARINE SCIENCE, VOL. 51, NO.3, 1992

Table 3. Total suspended solids in seawater samples collected from the GulfofMexico. These stations

represent a transect WSW from New Pass, Sarasota, Florida, to the coral collection site. Samples were

collected between March and June 1984

Number of times Distance from Mean lotal suspended

Station number sampled shore (nmi) Waler deplh (m) solids ± SD (mg ·Iiler ')

Station I 4 I 7 25.9 ± 12.1

Station 4 4 4 11 18.1±11.3

Station 7 4 7 12 19.9± 14.8

Station 10 4 10 13 16.3 ± 12.0

Station 12* 1 12 15 26.2

Station 18t 2 18 26 15.1 ± 10.9

• This station was the site of coral collection for bioassays.

t This site represents a dredged material disposal site before the beginning of disposal operations.

were considered to be significantly different when the F-test probability was less that 0.05 (95%

confidence).

RESULTS

Coral Distributions and Abundance. -Distributions and densities of eight coral

species were determined on reefs at three different depths (Table 1). Data for

Siderastrea includes both S. radians and S. sideria since it was difficult to distinguish between these similar species in the field. The most abundant species encountered, particularly on the shallowest reef (7 m), was the ahermatype, Phyllangia americana. Five of the eight species surveyed were absent from the 7 m

reef and two species, Cladocora arbuscula and Siderastrea spp., displayed highest

densities on the intermediate depth reef (15 m). All eight species were present on

the two deeper reefs (15 m and 18 m). Overall, Scolymia lacera, Solenastrea

hyades, and Stephanocoenia mitchelinii were the least abundant corals in the depth

ranges sampled. The distributions of all species were patchy as indicated by the

large standard deviations that equal or exceed the mean densities in many cases

(Table 1).

Suspended Solids Measurements. - The highest suspended solids loads measured

among the six sites sampled occurred at Stations 1 and 12 (Fig. 1) at a distance

of 1 and 12 nmi offshore respectively (Table 3). Two samples taken on different

days in the vicinity of a proposed ocean disposal site for dredged material contained 22.8 mg' liter-I and 7.4 mg'liter-I

total suspended solids. All of the values

reported in Table 3 represent sediment loads during relatively calm conditions

and are considered to be low relative to higher loads following storms and ocean

disposal activities.

Suspended Sediment Experiments-Survival. -Specimens of Siderastrea spp. exhibited stress reactions and bleaching under laboratory conditions. Four out of

five control colonies and two out of five experimental colonies died after 17-27

days. Therefore, growth and survival data for these species were eliminated from

the analyses. No deaths of control or experimental colonies of other species were

recorded in the IO-day exposures to 49,101,165, or 199 mg' liter-Iof suspended

sediments. Overall survival of these species was not affected by exposure to the

levels of suspended solids tested during the time course of these experiments.

A few colonies became partially buried at their base as a result of sediment

accumulation during the course of the experiments. At the end of SSE III-I, the

following species in the experimental treatment displayed partial polyp mortality

or bleaching in some individuals: Scolymia lacera (1 specimen), Stephanocoenia

mitchelinii (1 specimen), and Solenastrea hyades (2 specimens). One control spec-
RICE AND HUNTER: EFFECfS OF SEDIMENT ON CORALS 435

Table 4. Mean growth rates and initial mean specimen size (weight in air) for corals exposed to a

suspended solid load of49 mg'liter-' for 10 days (SSE I). Values are mg increase (or decrease) per

gram of initial weight ± standard deviation

Species Control

Treatment

Experimental

Probability

(F-test)

A. Mcan growth rates in mg·g-I initial weight ± one SD. N = 5 per treatment per species.

Cladocora arbuscula 9.25 ± 2.35 10.99 ± 8.40 0.667

Manicina aereolata 7.17 ± 6.25 8.85 ± 7.72 0.716

Solenastrea hyades 5.30 ± 2.09 6.12 ± 7.78 0.825

Phyllangia americana 7.09 ± 5.66 3.14 ± 12.53 0.538

All species combined (N = 20) 7.20 ± 4.37 7.27 ± 9.07 0.975

B. Mean initial weight (g) in air by species and treatment. N = 5 per treatment per species.

Cladocora arbuscula 15.42 ± 6.90 14.09 ± 4.70 0.731

Manicina aereolata 29.83 ± 2\.40 28.48 ± 22.00 0.924

Solenastrea hyades 22.44 ± 4.93 1\.98 ± 5.79 0.022

Phyllangia americana 5.07 ± 3.28 4.65 ± 3.36 0.848

imen of S. hyades also exhibited partial bleaching. At the end of SSE111-2(second

10 days), color loss was noted in the following specimens of the experimental

treatment: S. lacera (1 specimen), Manicina areolata (2 specimens), S. hyades (4

specimens), Isophyllia sinuosa (2 specimens), Cladocora arbuscula (1 specimen).

Similar color loss was observed in control individuals ofM. areolata (2 specimens),

and S. hyades (4 specimens). No attempt was made to quantify the amount of

bleaching on coral specimens since bleaching patterns and coral shapes were

irregular and difficult to measure. On the basis of number of individual specimens

affected, regardless of the extent of bleaching, 14% of experimental and 10% of

control colonies experienced some color loss over the 10-20 days of exposure.

Suspended Sediment Experiments - Growth Rates. - Growth rates for control and

experimental treatments for each of the four suspended sediment experiments are

presented in Tables 4-6. Data are summarized as the average growth rates for

each treatment by species and as the combined average growth rates for all species

tested in that experiment. The mean initial weight of specimens was not significantly different between control and experimental treatments for all species tested

except for Solenastrea in SSE-l where the control treatment mean initial weight

was significantly larger than the experimental mean initial weight.

Table 5. Mean growth rates for corals exposed to a suspended solid load of 101 mg· liter-I for 10

days (SSE II). Values are mg increase (or decrease) per gram of initial weight ± standard deviation.

N = 5 per treatment per species, except where noted

Species

Treatment

Control Experimental

Probability

(F-test)

Cladocora arbuscula

Manicina aereolata

Solenastrea hyades

Phyllangia americana (N = 5, 4)

All species combined (N = 20, 19)

All species combined excluding Phyllangia

(N = 15)

-0.99 ± 2.52

2.39 ± 2.35

4.41 ± 17.47

-9.75 ± 33.90

-1.39 ± 16.65

1.40 ± 3.82

1.31 ± 5.52

0.38 ± 2.05

2.51 ± 3.69

12.37 ± 48.98

4.14 ± 22.22

1.94 ± 9.79

0.422

0.187

0.817

0.448

0.384

0.844
436 BULLETIN OF MARINE SCIENCE, VOL. 5L, NO, 3,1992

Table 6, Mean growth rates for corals exposed to a suspended solid load of 165 mg,liter I for 10

days (SSE III-I), B. Mean growth rates for corals exposed to a suspended solid load of 199 mg' liter I

for 10 days (SSE III-2). Values are mg increase (or decrease) per gram of initial weight ± standard

deviation. N = 5 per treatment per species

Treatment

Probability

Species Control Experimental (F·test)

A. Cladocora arbuscula 1.88 ± 6.01 -6.46 ± 10.89 0.172

Manicina aereolata 0.55 ± 1.38 0.64 ± 1.34 0.915

Solenastrea hyades 4.06 ± 3.13 -2.84 ± 3.67 0.013

Scolymia lacera 2.37 ± 7.55 0.40 ± 4.64 0.631

Isophyllia sinuosa 0.59 ± 3.89 -0.12 ± 1.41 0.712

Stephanocoenia michellini 3.80 ± 7.76 -0.69 ± 2.32 0.251

All species combined (N = 30) 2.21 ± 5.18 -1.51±5.37 0.008

B. C1adocora arbuscula 16.27 ± 7.52 20.27 ± 7.97 0.172

Manicina aereolata 0.55 ± 1.07 3.28 ± 3.10 0.100

Solenastrea hyades 7.85±6.51 13.61 ± 4.20 0.135

Scolymia lacera 0.71 ± 2.53 0.48 ± 2.46 0.888

Isophyllia sinuosa 0.05 ± 1.90 -0.95 ± 1.37 0.369

Stephanocoenia michellini 2.84 ± 3.43 3.61 ± 2.23 0.685

All species combined (N = 30) 4.71 ± 7.19 6.72 ± 8.64 0.334

In SSE I (Table 4), where the experimental treatment was exposed to 49 mg'

litecl of suspended sediment, the experimental group displayed a slightly higher,

although not significantly different, growth rate than controls. Cladocora arbuscula

had the highest growth rate in both the control and experimental treatments in

this test. In SSE II (Table 5), with twice the suspended sediment load as the

previous experiment, the difference between control and experimental treatments

was not significant. Solenastrea hyades had the highest mean growth rate in the

control while Phyllangia americana had the highest mean growth rate in the

experimental treatment. Data for one specimen of P. americana from the experimental treatment was excluded from analysis due to an apparent weighing error,

hence the smaller sample size compared to the control. The variation between

individuals in both treatments for P. americana was high (coefficients of variation:

control = 80%; experiment = 399%) and the size of the specimens tested was

Table 7. Combined growth rates of corals from all experiments comparing control and experimental

treatments. Values are mg increase (or decrease) per gram of initial weight over the lO-dayexposure

period

Species

Treatment

Control Experimental

Proba·

biLity

(F.tesl)

Cladocora arbuscula (N = 20)

Manicina aereolata (N = 20)

Solenastrea hyades (N = 20)

Scolymia lacera (N = 10)

Isophyllia sinuosa (N = 10)

Stephanocoenia michellini (N = 10)

Phyllangia americana (N = 10, 9)

All species combined (N = 100,99)

All species combined excluding Phyllangia

(N = 90)

6.61 ± 8.34

2.67±4.21

5.40 ± 8.86

1.54 ± 5.38

0.32 ± 2.90

3.32 ± 5.68

- 1.33 ± 24.57

3.24 ± 9.40

3.75 ± 5.88

6.53 ± 12.87

3.29 ± 5.30

4.85 ± 7.73

0.44 ± 3.50

-0.53 ± 1.38

1.46 ± 3.12

7.24 ± 31.65

3.84 ± 12.29

3.50 ± 8.65

0.982

0.683

0.834

0.593

0.413

0.377

0.516

0.699

0,823
RICE AND HUNTER: EFFECTS OF SEDIMENT ON CORALS 437

Table 8. Results of coral burial experiments expressed as LT,o (burial time in days necessary to kill

half of the experimental animals)

Species

Sco/ymia /acera (N = 15)

Isophyllia sinuosa (N = 9)

Manicina aereo/ata (N = 23)

Siderastrea radians (N = 21)

Cladocora arbuscu/a (N = 24)

Stephanocoenia michelinii (N = 12)

So/enastrea hyades (N = 28)

LTm in days

7.0

7.2

10.0

13.6

15.0

16.2

>15

small (all were less than 5 g weight in air). For these reasons, the data are presented

with and without inclusion of this species in Tables 4, 5 and 7.

In SSE III-I, mean growth rates were significantly higher in control colonies

(F•.58 = 7.45; P = 0.008) than in the experimental group. The same group of

specimens exposed to 199 mg·litecl(SSE 111-2) of suspended sediments for a

second 10-day period displayed no significant difference in growth rates compared

with controls (Table 6B). Growth rates for the entire 20-day period (SSE III-I +

2) were not significantly different between control and experimental treatments

when analyzed collectively. Average growth rates of corals were higher in the

second phase of SSE III than the first phase for both treatments (Table 6). This

may indicate that the corals were still adapting to laboratory conditions during

the first phase. The specimens used in this experiment were acclimated to laboratory conditions for 17 days prior to the beginning of Phase I, and both phases

of SSE III were conducted under virtually identical physical and chemical conditions. Solenastrea hyades displayed the highest mean control growth rate in SSE

111-1while Cladocora arbuscula had the highest mean control and experimental

growth rate in SSE III-2. The combined mean growth rates for seven species of

corals exposed to suspended sediment loads (SSE I, II, and III) are presented in

Table 7.

Experimental Error for Growth Rate Determinations. - The experimental error

associated with the buoyant weighing method was estimated by weighing the same

specimens 3 days apart, and subtracting the expected amount of growth (determined from means for each species in control treatments) from the observed

3-day mean change in weight. The remainder represents an estimate of experimental error. Thirty-five specimens from SSE 1II-2 were weighed and then reweighed 3 days later with an average weight change of 0.337 mg·g-I.day-I. The

expected growth was determined to be 0.374 mg·g-I ·day-I. The difference between

these two, ±0.037 mg·g-'·day-', represents the estimated experimental error.

Burial Experiments. -Six burial experiments were undertaken in which coral

specimens were completely covered with natural marine sediment (from the same

batch of sediment used in SSEs) for periods of 1-15 days (Table 2). Not all species

of corals were used in each experiment. Survival rates for individual species of

corals (expressed as LT5o) are presented in Table 8. The LT50 represents the burial

time necessary to kill half of the experimental organisms. Scolymia lacera and

Isophyllia sinuosa had the lowest tolerances to burial, with 50% mortality at about

7 days. Solenastrea hyades was most resistant to burial with 100% survival after

15 days burial (the longest burial period).

Many coral specimens tested in burial experiments suffered sublethal damage

that was expressed as color loss, soft tissue damage, invasion by algae, or reduced
438 BULLETIN OF MARINE SCIENCE, VOL. 51, NO.3, 1992

feeding activity in recovery aquaria, Each specimen was examined 7 days after

being disinterred and if still alive, considered to have survived the experiment.

Holding these specimens in recovery aquaria for longer periods of time would

likely have resulted in additional mortality; therefore, the LT50 values reported

in Table 8 should be considered minimum estimates,

DISCUSSION

Seven out of eight scleractinian corals from west central Florida patch reefs

showed no significant decrease in growth rate or survival when exposed to a range

of suspended sediment loads in the laboratory that equal or greatly exceed those

to which they are exposed in the field. LT50 values for complete burial of corals

ranged from 7 to more than 15 days for the six species tested, suggesting a very

high tolerance of these patch reef corals to the most severe effects of sedimentation,

While the remote location of west central Florida patch reefs makes them

difficult to access for controlled experiments in the field, corals also can be notoriously challenging animals with which to work in the laboratory, Growth rates

may often vary among individuals of the same species (Rogers, 1979; Brown and

Howard, 1985) or among clones of the same individuals (Hunter, 1988). Coral

growth rates have been shown to vary between day and night and in some cases,

growth may be greatly reduced during periods of "mucus tunic" production as in

Porites porites (Davies, 1989). In the present study, variability within treatments

may have obscured our ability to observe statistically significant differences in

growth rates between control and experimental groups. Davies (I989) has refined

the buoyant weight technique and incorporated corrections for coral tissue density

and for the density of skeletal material which may vary between coral species.

A number of corals in our study experienced a weight loss in both control and

experimental treatments, although more commonly in the latter. Special care was

taken to avoid damage to coral tissues or skeleton during weighing and handling,

so this was probably not a factor in the observed weight losses. Skeletal loss may

have occurred in some specimens through the activity of boring organisms such

as polychaetes, bivalves, and sponges. Dodge et al. (I 984) recommended collecting

the residue produced by borers and weighing it along with the corals, although

this was not possible in the present study due to the accumulation of sediment

around corals in the experimental treatment. Cryptic non-boring invertebrates

(such as shrimp, echinoderms, and amphipods) were discovered on some corals

(especially Cladocora arbuscula), and may have left the host or switched hosts

during experiments. Attempts were made to limit these associated animals as

much as possible and the mass ofthe animals that escaped detection was unlikely

to be large enough to account for observed weight losses. There was no reason to

suspect that boring organisms or associated fauna were more abundant on experimental corals than on control corals, since test animals were selected at random

and assigned to treatments.

Another possible mechanism of weight loss may have been related to excessive

mucus production in the corals exposed to heavy suspended sediment loads.

Several authors have reported copious mucus production by sediment stressed

corals (Marshall and Orr, 1931; Rogers, 1983). In light of Davies (I989) findings

that coral tissue is not necessarily of the same density as seawater, heavy mucus

secretion may result in general shrinkage of tissue mass and result in apparent

weight loss of the colony.

Anomalous estimates of growth rates, such as those recorded for Phyllangia

americana, may have been due to the small size of specimens used. This species
RICE AND HUNTER: EFFECTS OF SEDIMENT ON CORALS 439

was difficult to collect in large monospecific colonies and as a result small individuals (less than 2 g) were used in the first two experiments, exaggerating the

experimental error incurred in weighing.

A possible solution to the problem of variable growth rates within species would

be to chart individual growth of corals for longer periods of time (several weeks

or more) in the laboratory prior to beginning experiments. This would provide a

baseline for comparison with subsequent experimental treatments and would help

to establish adequate acclimation times for growth studies.

With the exception of the ahermatype, Phyllangia americana, Cladocora arbuscula had the highest mean growth rate among control corals tested and Isophyllia sinuosa had the lowest. P. americana appeared to be a weedy, fast-growing

species, occurring in high numbers on the young artificial reef. It was also dominant, although in lower densities, on the 18 m reef where growth of hermatypic

species may be limited by decreasing light penetration. C. arbuscula densities were

highest on the deeper, natural reefs suggesting that, with its relatively fast growth

rates, it is a competitively dominant species.

Results of other field studies on coral distributions have indicated a negative

correlation between suspended sediment loads and hard coral abundance. Mayer

(1918) concluded that only corals that were tolerant to higher temperatures and

turbid water were found near shore. He found Siderastrea radians to be among

the most resistant corals that he tested, able to survive total burial for more than

73 h. Edmondson (1928) tested the resistance of Hawaiian corals to siltation and

burial in the laboratory, and found that tolerance ranged from 12 h to 10 days.

Marshall and Orr (1931) determined that corals with branching colonies or those

with large polyps were best able to clean themselves of sediment. Studies by

Hubbard and Pocock (1972) on coral morphology and behavior revealed clues to

environmental distribution patterns relative to sediment removal abilities in 26

coral species. Their observations indicated that coral species vary considerably

in their efficiency of sediment removal, and that many species are size-specific

sediment rejectors. Several species tested in the present studies were also tested

by Hubbard and Pocock (1972, p. 604). These authors found Solenastrea hyades

to be a poor sediment rejector, while the following species are listed in order of

increasing sediment rejection efficiency: Cladocora arbuscula, Siderastrea siderea.

Manicina areolata, and Isophyllia sinuosa. The efficiency of sediment rejection

(rate of sediment removal) measured by Hubbard and Pocock (1972) should not

be equated with tolerance of a species to sediment loads, since the present studies

indicate that Cladocora arbuscula was among the most tolerant species, based

upon burial and suspended sediment studies, while Isophyllia sinuosa was the

least tolerant to these conditions.

Trends from these and other field and laboratory studies indicate that many

coral species can tolerate natural sediment suspended loads and burial for relatively short periods of time (i.e., the duration of most laboratory tests) but that

long-term exposure to suspended sediment can cause reduced coral growth and

reduced reef development.

Sedimentation may be the major cause of reef degradation on a global scale

(Johannes, 1975; Kuhlman, 1988; Rogers, 1990). The effects of sediment resuspension on coral reefs as a result of anthropogenic activities have been documented

in reviews of dredging, filling, dredged material disposal and mining effects(Levin,

1970; Courtenay et ai., 1974; Endean, 1976; Bak, 1978; Rogers, 1990). The addition of complicating factors such as freshwater flooding or toxic pollutants

adhered to sediments may significantly aggravate effects on corals.

In the present study, only natural uncontaminated sediments were employed
440 BULLETIN OF MARINE SCIENCE, VOL. 51, NO.3. 1992

in laboratory tests. The species of hard corals inhabiting reefs off the west central

portion of Florida appear to be among the most tolerant species tested with regard

to suspended sediment and burial effects. These species have been found to withstand continuous exposure to 150-200 mg' liter-I of suspended sediment for 20

days with no mortality and no significant reduction in growth rate. No literature

reports were found that suggested that any coral specimen could survive total

burial for more than 10 days, and most species previously tested could not tolerate

more than 2 days of burial. For the corals in the present study, the shortest burial

LT 50 was 7 days, with that of three species greater than or equal to 15 days.

The natural distribution of corals may provide an indication of general tolerance

of a species to environmental factors. Many of the corals inhabiting patch reef

communities offwest central Florida have been found to occupy the least favorable

habitats in more tropical settings such as lagoons and near-shore reefs (Griffin,

1974; Colin, 1978). Some species (e.g., Manicina areo/ala and Isophyllia sinuosa)

have been recorded to occur as unattached colonies in lagoon habitats and have

the ability to right themselves if turned upside down in the sediment (Hubbard

and Pocock, 1972).

Compared to more tropical habitats, the patch reefs off west central Florida are

exposed to more severe environmental conditions, such as low winter water temperatures, high turbidity, and decreased light penetration. It is not surprising that

species that inhabit these areas demonstrate high tolerances to environmental

stress, but it also may be that these species are living near their physiological

tolerance limits of environmental conditions and may be more sensitive to additional prolonged adverse conditions (i.e., chronic siltation stress) than their

tropical congeners. Laboratory tests conducted during the present study were

intended to simulate environmentally advantageous conditions for coral survival

and growth with regard to temperature, salinity, food availability, and light levels.

Further experiments under a range of different physical regimes, or with contaminated sediments may be useful in elucidating synergistic effects of a number of

stressful parameters on reef corals.

ACKNOWLEDGMENTS

This research was supported in part by funding from the U.S. Environmental Protection Agency

and by a Faculty Grant from the University of Tampa to S.A.R. The following students at the University

of Tampa contributed to portions of the field data collection and their participation is gratefully

acknowledged: P. Dolan, M. Hirschman, C. Nowicki, and B. Moffat. The assistance of J. Kucklick

with the laboratory portion of this work is greatly appreciated.

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DATE ACCEPTED: April 13, 1992.

ADDRESSES: (S.A.R.) Department of Biology, University of Tampa, Tampa. Florida 33606; (C.L.H.)

Hawaii Institute of Marine Biology. University of Hawaii at Manoa. P.O. Box 1346. Kaneohe. Hawaii

96744.