Effects of Terrigenous Sediments Influx on The Coral Cover and Linear Extension Rates of Three Caribbean Massive Coral Species

Juan L. Torres and Jack Morelock
University of Puerto Rico at Mayagüez
Department of Marine Sciences
PO Box 908
Lajas, PR 00667

Correspondence address:
PO Box 3210
Lajas, PR 00667 USA
E-mail: jltores@caribe.net

Key words: Montastraea annularis, Siderastrea siderea, Porites astreoides, sedimentation, linear extension rates, Puerto Rico

Abstract

During the past three decades, high sediment input from urbanization and river discharges have changed the structure of many Puerto Rican coral reefs, from primary reef-builders to scattered colonies of secondary species and hardground-type areas. We measured the coral cover and the linear extension rates of the Caribbean corals Montastraea annularis, Siderastrea siderea, and Porites astreoides at five sites in Puerto Rico. Sediment deposition rates varied from very low to high. Although the cover of M. annularis decreased significantly in reefs with a high content of terrigenous sediments, the cover of S. siderea and P. astreoides was not affected. The linear extension rates of the three species were not significantly affected. Differences in linear extension of both density bands appear to be related to sediment regime, but it is not such a controlling factor that we can disregard other metabolic processes. The input of a large amount of terrigenous sediments into a reef environment resulted in the loss of sediment-sensitive species (e.g., M. annularis), but did not markedly affect sediment-tolerant species (e.g., S. siderea and P. astreoides).

INTRODUCTION

Decrease in the clarity of the water column caused by high sedimentation is one of the most important factors limiting reef development (Loya, 1976; Acevedo et al., 1989; Rogers, 1990) and can even cause the "turn-off" of a reef system (Buddemeier and Hopley, 1988). Increased sedimentation can decrease living coral cover (Morelock et al., 1983; Rogers, 1990) and depth in coral species zonation (Acevedo, 1986; Acevedo et al., 1989).

Most work on the growth response of Caribbean massive corals has focused on the massive coral Montastraea annularis (Dustan, 1975; Weber and White, 1976; Torres, 1998, 2001). Only a few studies exist on the linear extension rates of other Caribbean massive coral species, such as Montastraea cavernosa (Weber and White, 1976), Diploria clivosa, and Stephanocoenia michellinii (Hubbard and Scaturo, 1985). This study examines changes in coral cover and linear extension rates of the mountainous coral Montastraea annularis, the massive starlet coral Siderastrea siderea, and the mustard hill coral Porites astreoides under different natural sedimentary regimes. These species are commonly found in a variety of conditions varying from clear to turbid waters; both P. asteroides and S. siderea have been characterized as efficient sediment removers under laboratory conditions (Bak and Elgershuizen, 1976), and as resistant to sediment stress (Acevedo et al., 1989). We report on the upward growth response, in terms of linear extension rates, for a five year period, as well as changes in coral cover as a result of increased sediment influx.

MATERIALS AND METHODS

Study sites

Five study sites (Table 1) were chosen, ranging from no exposure to high exposure to terrigenous sediments. Coring was done from April to May 1996 at all sites. Cores (three/species/site) were taken from similarly sized-colonies located at depths between 3 and 8 m (Table 1). Cores were cut with an hydraulic drill (operated by an 8 HP compressor) equipped with a 0.5 m long and 6.3 cm internal diameter diamond-tip core bit. Cement plugs were hammered into the bores to prevent algae colonization and entrance of boring organisms. Cores were sliced parallel to their longitudinal axis and X-rayed to expose skeletal density bands. Linear extension rates were measured to the nearest 0.1 mm (vernier caliper) along the major axis of growth. Three replicates were measured in neighbor corallites spaced approximately 0.5 cm apart and the results were averaged. Three colonies of each species were sampled at each site. A Repeated Measures ANOVA was used to find differences in linear extension rates of the three species, separately among individuals of the same site, and between sites.



















Coral cover analysis

During the same day, three (30 m long) photo-transects were performed following the coring depth contours. With the Sigma Scan/Image Qandel Scientific) program, estimates of coral cover were performed on 10 arbitrarily chosen quadrants within each transect. This allowed measures of cover by species and total coral cover at the sampling depth. Results were analyzed and linear regressions were performed to examine possible relationships between species cover and percent of terrigenous sediments recovered in sediment traps.

Sedimentation analysis

Sedimentation rates were obtained from two sediment traps placed at each site and collected monthly from all sites. The traps were plastic wide-mouth Nalgene 1-liter bottles tied to a steel bar (these traps are particularly effective- see Gardner, 1980). Collection was performed from May 1994 to May 1996. The content of each trap was washed with distilled water to remove salts and allowed to dry at ambient temperature to avoid lithification of the mud fraction. Contents were then mixed by shaking the bottle and two samples were obtained by decantation into a small beaker; these samples were used for grain size and composition analysis. Grain size was determined following the procedures of Folk (1968). For composition analysis, the sample was rinsed twice with distilled water to remove salts; the water was decanted and the samples were oven-dried at 80 °C, weighed, and treated with sodium hypochlorite (50 % solution) to dissolve the organics. The sample was dried, weighed again, treated with a 10 % HCL solution to dissolve all the carbonates, dried, and reweighed. The remaining sediments were assumed to be terrigenous.

RESULTS

Surface temperature at the study sites varied from 26 to 28 oC and was not considered to influence the results. Salinity was 34 or 35 PSU at all sites at sampling depth. Tide regimes were not considered because they are semi-diurnal in Puerto Rico and the difference between high and low tide is less than 0.5 m through the year. Rainfall data from stations closest to the study reefs were provided by the United States Geological Survey.

Grain size was not considered in the analysis because it did not vary between sites (Table 2). Overall composition of suspended sediments is shown in Figure 1. Total carbonates were significantly higher at Turrumote, showing a dominance of in-situ produced sediments. The terrigenous sediment fraction was dominant at all sites, except Turrumote. Turrumote may be considered an "unstressed" site because the amount of terrigenous sediments was negligible. The organic matter present in all samples was less than 8 %. An earlier unpublished study found no significant differences in the concentration of Dissolved Inorganic Nitrogen (DIN), N02-, N03, phosphates, and chlorophyll a between Turrumote and Cayo Caribe; therefore, effects of eutrophication at these two sites were not considered.




The average linear extension rates for the three species are shown in Table 3. The data are limited to 1991-1995 (period for which extension rates for all three species were available). Data of density bands from 1996 were not included because these were still forming during the sampling dates. Only partial Low Density (LD) bands were observed, and High Density (HD) bands were not formed by May, indicating that their deposition occurs later during the year.

Slightly higher linear extension rates were found in M. annularis from Turrumote, but these were not significantly different from the stressed sites (p = 0.29). However, a trend of decreasing linear extension rates can be seen in this species with an increase in suspended terrigenous sediments (r2 = 0.36, Fig. 2). The percentage of terrigenous sediment at each site and the amount of surface covered by M. annularis were inversely related (Fig. 3). At sites with high terrigenous sediment influx (Rodriguez, Cardona, Caribe) there was less than 2 % cover by M. annularis, compared with high percent cover on Turrumote and modest cover at Corona La Laja (Table 4). Only scattered colonies of several secondary corals appeared within the analyzed quadrants, predominantly the brain corals Colpophyllia natans and Diploria spp., and the very sediment-resistant species Montastraea cavernosa (Bak and Elgershuizen, 1976). Acevedo et al. (1898) reported a similar relationship for reefs between Ponce and Guayanilla.



























The linear extension rates of Siderastrea siderea were not significantly different among the sites (p = 0.15, Fig. 2, Table 3), confirming the suggestion of Acevedo et al. (1989) that this species is not affected by high influx of terrigenous sediments. The species' cover showed no difference among sites (p = 0.18; Fig. 3).














Measurable growth bands for P. astreoides were obtained only for the cores from Turrumote, Corona La Laja, Caribe, and Cardona. No bands could be measured from the cores of Rodriguez. Results were similar to those for S. siderea; no differences among sites were found for linear extension rates (p = 0.53, Fig. 2, Table 3) or species cover (p = 0.90, Fig. 3, Table 4).





















DISCUSSION

This study confirms the high resistance of Siderastrea siderea and Porites astreoides to sedimentation, as well as their slow extension rates even where entrance of terrigenous sediments is negligible. Also, although colonies of the massive reef-builder Montastraea annularis can continue their slow growing rate in sediment-stressed reefs, their cover is greatly reduced. Cortes and Risk (1985) found a relationship between sedimentation rates (Fig. 4) and linear extension rates of M. annularis but their data is inconclusive.

The relationship between the amount of terrigenous sediments collected in the sediment traps, the coral cover by species, and the extension rate of the three species indicates that M. annularis cover is drastically reduced in areas with high terrigenous sediment influx while cover by P. asteroides and S. siderea remains unchanged. The M. annularis colonies that remain alive at stressed sites, and the P. asteroides and S. siderea colonies, continue growing at an extension rate relatively unaffected by the sediment stress. Sediment stress can resuce the coral cover by certain species (including M. annularis) but does not have an appreciable effect on growth rate (as indicated by lineal extension rates). This indicates that colonies in sediment-stressed reefs can survive short periods of high sediment influx, as suggested by long-term time-series analyses (Torres, 1998, 2001). Coral recruitment rates are probably more affected because available substratum may be covered by fine terrigenous muds and turf algae, which impede settlement and development of some coral species. Working at Escollo Negro, near Mayagiiez, Loya (1976) found a similar trend with low coral cover and diversity in waters with high turbidity and high sedimentation; some detrimental effects found by this author were colony death, smothering, abrasion, and inhibition of planula larvae settlement and development.

Siderastrea siderea, M. cavernosa, and P. astreoides are the most dominant species at the sediment-stressed sites (Torres and Morelock, 1999). The small variability found between years may be due to intrinsic factors, such as reproductive cycles, in St. Croix, with the highest growth occurring at 12 m (as in S. siderea). Gladfelter et al. (1978) reported avera e linear extension rates of 3.3-3.5 mm/yr in shallow waters and 3.0 mm yr 1 at 10 m. No relationship has been reported between growth and temperature. Huston (1985) found significant differences in linear extension rates of P. astreoides from 0 to 30 m at Discovery Bay (Jamaica), with the highest rates occurring at 5 m. Considerable variation occurred within and among individual colonies at the same depth (2-6.3 mm yr 1 between 0-5 m, and 1.9-2.6 mm yr-1 below 25 m). We found no significant difference in the linear extension rates of this species between sites. We have no conclusive explanation for the lower linear extension rates of P. astreoides at Turrumote, where clear water predominates year-round.

ACKNOWLEDGEMENTS

We greatly appreciate the assistance of Milton Carlo and Godoberto L6pez with coral coring, while Dr. Antonio Frontera and Dr. Eduardo Figueroa kindly provided the X-ray facilities. Part of this work was supported by a fellowship to J.L.T. from the John C. Stennis Space Center (NASA) (NGT13-72700).

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