ABSTRACT:

The skeletal growth of hermatypic (reef-building) corals is a sensitive indicator of environmental conditions and perturbations. Normalized annual growth (linear extension) rates of corals from Broward County, Florida were studied for a 26 year period from 1985-1960, with a common record for all corals of 16 years from 1985-1970. Star corals, Montastraea faveolata, and brain corals, Diploria labyrinthiformis, were collected from each of four reef sites at two depths, 9 and 18 m. Collection sites were located near previous beach renourishment projects to assess the possibly adverse effects from turbidity and sedimentation. Coral growth differences among sites at particular years and among years within sites were statistically evaluated. The general results indicated that beach renourishment projects had minor or no influence on nearby coral growth rates. Site averages were strongly correlated with one another and indicated that southern 9 m corals had higher growth rates than northern counterparts for M. faveolata. Comparison of growth-rate time series to recorded temperature and salinity revealed a positive relation with salinity (water density) variations. The salinity-growth effect may be impacted by freshwater outflow from the inland watershed.

BACKGROUND:

Scleractinian reef building corals can be long-lived (hundreds of years) and produce massive calcareous skeletons. These skeletons preserve a skeletal record of both time and growth. Alternating bands of high- and low-density skeletal material are present and are visible through X-radiography, where each pair of bands represents approximately one year's annual extension of the coral. The presence of time-dependent banding within coral skeletons can provide a useful tool for reconstructing environmental variables that affect coral growth.

Skeletal growth includes: physical lengthening of the coral skeleton over time (extension), material distribution within the skeleton (density), and mass accretion or thickening of the skeleton over time (calcification), all of which may be quantified. The chemical characteristics of the coral skeleton coupled with the inherent chronological record of the growth bands also can provide environmental information. Specifically, the carbon and oxygen stable isotopic composition of the skeleton is valuable for reconstruction of temperature, salinity, and light levels. The fluorescence of the skeleton can show the influence of terrestrial water input. Trace element composition of the skeleton can show water temperature (Sr/Ca), upwelling (Ba/Ca), radioactivity (e.g., U/Ca), and hydrocarbon pollution (e.g., V/Ca).

METHODS:

In this study we analyzed the skeletal growth (annual skeletal extension) of over 154 corals collected from reefs of Broward County, Florida. Specimens consisting of two coral species (star corals, Montastraea faveolata, and brain corals, Diploria labyrinthiformis) were collected at 4 sites at two depths (9m and 18m) along an approximately 30 mile north-south latitudinal gradient. Once collected, coral skeletons were sectioned using a large diamond bit masonry saw into slabs approximately 0.5-cm thick. Slabs were oriented normal to growth band surfaces. Multiple slabs were cut from each coral for replication and comparison of banding patterns. Each slab was X-radiographed onto film to reveal the density banding. X-ray negatives were printed into positives for the entire length of the coral growth record. Annual linear extension for each year (from top of one high density (HD) band to the top of the next HD band) was measured by caliper along two separate transects normal to growth band boundaries.

Normalized coral chronologies were constructed by dividing the mean extension of each transect into each annual extension value. Each normalized or index transect was then averaged into an indexed coral master chronology. Normalized annual linear skeletal extension rates of the corals were examined over 16 common years (1985-1970). Individual normalized coral chronologies were averaged into site, depth, and combined site-depth master chronologies for each of the two species.

Individual site master chronologies (by species and depth) were compared to environmental variables. Environmental data included water temperature and density (salinity) from Miami Beach, rainfall (Lower East Coast) as tabulated by SFWMD, and canal discharge (North New River canal USGS) data. Man-induced perturbations (i.e. beach renourishment projects) were also

compared.

RESULTS:

Site averages of absolute coral growth indicated that southern 9 m depth specimens had higher growth rates than northern counterparts for M. faveolata. In the southern collection sites, 9 m depth growth of both species tended to be greater than 18 m depth growth.

Correlation analysis indicated that the time series for coral extension was similar among sites, species, and depths. This was exemplified by the obvious agreement of site master chronologies, most dramatically exhibited by the common 1970 stress band.

Comparisons of chronologies to recorded environmental variables (water temperature, salinity, and canal discharge) revealed a positive correlation of coral extension with salinity, as represented by sea water density (sigma T).

DISCUSSION AND SUMMARY:

Coral growth at Hollywood, Fort Lauderdale, Pompano, and Deerfield sites exhibited significant correlation within and among species at both 9m and 18m depths. This correlation supports the premise that coral growth within Broward County responds in a generally similar manner to forcing environmental variations.

The positive relationship between coral extension and salinity (as represented by sea water density) may not be straightforward in cause, but is significant in correlation. Factors affecting salinity include temperature, rainfall, and freshwater discharge. Based on annual averages, there was no significant correlation between coral extension and temperature. Therefore, it is unlikely that the strongly significant correlation of coral extension with sea water density is attributable to temperature effects on salinity as this should also have been evident in the temperature-extension correlation. It is more likely that the salinity-growth effect is attributable to rain and resulting freshwater discharge.

This study proposes that freshwater sources, including those emanating from the Everglades Drainage canals may adversely affect offshore coral growth. A strong positive correlation is demonstrated between coral growth and sea water density as an indicator of salinity. This relationship suggests that increased freshwater input may likely reduce coral growth rates. Further investigations are needed to gauge the extent of past effects and to predict future impacts related to Everglades restoration.

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