Marine & Environmental Sciences Faculty Reports

Document Type

Report

Publication Date

9-27-1995

Abstract

A four-year study was undertaken to survey Broward County, Florida (southeast Florida) coral communities and infaunal marine biota in relation to possible effects from the Hollywood- Hallandale Beach renourishment project. Beach restoration involves dredging sand from offshore deposits and placing it on eroded beaches, activities which may cause sedimentation and turbidity. Coral reefs were assessed using transect and quadrat surveys at a total of 15 stations, unevenly distributed between dredging impact (n=9) and control (n=6) areas to characterize and quantify populations of sponges, gorgonians, scleractinian corals, as well as other less well represented groups. In addition, the infauna of sand areas were analyzed using 150 core samples collected from both control and dredging impact areas. The first study was conducted in 1990, one year prior to construction of the beach in 1991. Other surveys were conducted immediately after construction in 1991, and then in 1992 and in 1994.

The issue of the response of coral reefs and coral reef organisms to sedimentation and turbidity is complicated. These ecosystems have adapted over long time periods to be able to deal with certain low levels of natural sedimentation and turbidity. However, excessive or chronic sedimentation causes documented adverse effects. These can include mortality, as well as changes in growth, coverage, density, and community composition. The difficulty is that all of these parameters, while linked, change at different rates and in other ways which are largely unqualified for individual species, let alone the broad combinations of species and growth forms which ultimately create ecosystems. Consequently, predicting (and assessing) the effects of a particular event or events (e.g., a beach renourishment project) can be particularly difficult when effects are less than catastrophic (e.g., complete mortality).

The most consistent result obtained by this study is that a long term decline, indicated by many key taxonomic groups and indices has occurred in the study areas. Statistical analyses using repeated measures Analysis of Variance (ANOVA) often show a time effect for both control and dredging treatments. Declines in both control and dredging stations are especially obvious when 1990 Pre-construction parameters are compared with those of 1994 (although there may be unexplained fluctuations in between these times). Percent cover by scleractinian corals, as well as their mean density and coverage diversity are all lower (often significantly) in 1994 than they were in 1990. Coral coverage at dredging sites dropped continuously and lost 20% of its Pre-construction value. However, the largest percent decline among gorgonians occurred between the 1992 and 1994 surveys in which dredge stations populations decreased by 28.5% and control populations declined by 27.8%. An overall decrease in the mean number of sponges and scleractinian corals also occurred in the study areas, similarly not limited to dredge stations, but encompassing control stations as well.

Differences among treatment means were not statistically significant and consequently insufficient to indicate dredging effects. In some cases, however, effects of dredging were noted, especially for the gorgonian populations. The number of gorgonian corals declined 15.8% at the dredging sites between 1991 and 1992, while remaining constant at control sites. Most of these gorgonian losses occurred on nearshore stations just offshore of the restored beach where many colonies were found partially or completely dead and covered with a layer of silt. At the same time, however, the mean number of individual sponges and scleractinians increased at both control and dredging sites in the same period.

While the data do not demonstrate the absence of potential environmental impacts as a result of dredging and filling, the overall pattern is not consistent with a simple, single impact explanation. Storm events must also be factored into the pattern. During the study period, two major storms affected the area. Hurricane Andrew in August of 1992 occurred just a few weeks before the 1992 survey. The otherwise unnamed "Storm of the Century" took place in 1993, a year when no biological assessment was undertaken. In qualitative surveys following the storms, we specifically noted damage to the reef communities. Invertebrate populations were scoured from their points of attachment to the substrate and piled into crevices and depressions on the reef. Our data from the current study show that numbers of sponges, which had increased at both dredge and control sites in 1991, declined substantially after the storm, recovering slightly or leveling off in 1994. Gorgonian populations declined twice at dredging sites, in 1991 and again between 1992 and 1994. The first decline had no parallel on control sites, but the second decline was mirrored by a population decrease at control stations. Stony coral colonies increased or remained the same at dredge sites during the first three surveys, then similarly decreased between 1992 and 1994. Mean coral density and coverage diversity followed the same pattern.

Inshore and offshore core sites supported different macroinfaunal assemblages during this, project. Pre-construction faunal composition as reflected by most common organisms was generally similar at control and treatment sites both inshore and offshore, although one control (R90) and one treatment site (Till) differed considerably from the other inshore sites. With these two exceptions, macrofaunal abundances and species richness values increased at all inshore sites immediately post-dredging. By contrast, organism abundances, richness and diversity indices declined substantially at both offshore sites over the same period (1990-1991). In 1992, all inshore sites (except Till) recorded greater macrofaunal abundances than in the Preconstruction survey, although two control and three treatment stations declined from 1991 peaks. Similarly, species richness values continued to increase or at least remained higher than Preconstruction levels at six sites (again excepting R90 and Till). In 1994, organism abundances had declined to below Pre-construction levels at all sites with the exception of two inshore treatment stations (R106, R116) that had developed a different macrofaunal assemblage accompanied by peaks in nematode and harpacticoid numbers. Species richness declined at least slightly from 1991 or 1992 peaks at all inshore sites (except R106), but remained higher than before renourishment with two exceptions: richness at stations R90 and Till declined roughly continuously through all four surveys so that, in 1994, these two sites supported assemblages similar to those at most of the other inshore sites (T88, R92, R94, R120). Diversity indices showed no recognizable trend relative to control versus treatment over the course of the four surveys.

Of the dominant inshore organisms, the polychaetes, Dispio uncinata, Paraonis fulgens, Scolelepis texana, Spio pettiboneae and Armandia agilis, generally increased in numbers from 1990 through 1992 and almost uniformly declined in 1994, with much greater declines at the four treatment sites. S. texana disappeared from all treatment sites, while Prionospio multibranchiata appeared at all control sites. S. pettiboneae disappeared from all eight inshore sites. The inshore amphipods, Metharpinia floridana and Haustoriussp., remained abundant or increased in numbers at control sites. At treatment sites, both exhibited at least some immediately Post-

construction increases and then declined, with the former species disappearing in 1994. The bivalve, Tivela floridana, also exhibited 1991 peaks at several stations, but, in contrast with the amphipods, declined at all sites in 1992 and rebounded at three control and three treatment sites in 1994. At the offshore sites, Prionospio cristata generally remained the most abundant polychaete although it decreased in numbers at both stations in 1994. Both P. cristata and another polychaete, Chone cf. americana, occurred in greater abundance in the borrow area than at the control site in all three Post-construction surveys. However, of the three common nonpolychaete taxa, the bryozoan, Cupuladria sp., increased at the control site and decreased at the borrow area over the four surveys; the tanaidacean, Cirratodactylus floridensis, and the isopod, Xenanthura brevitelson, declined at the control site, though they remained in moderate numbers there, while both declined or disappeared at the borrow area after dredging.

The results of this assessment has indicated few major detrimental effects from the beach renourishment project. This would suggest that future renourishment projects could be expected to result in only minor impacts, if responsible construction practices were followed. However, it is also important to recognize the limitations of this study and possible confounding effects. These include small sample size (numbers of monitoring sites) within the dredging and control areas, confounding effects of reef community zonation with depth (e.g., First, Second, and Third Reefs), confounding effects of short-term disturbances (e.g., Hurricane Andrew) or long-term change (e.g., global warming, chronic pollution from other sources), and finally high natural, variability of reef communities, which decrease the ability of statistical tests to detect differences, regardless of the replication.

Report Number

Technical Report 95-03

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