ABSTRACT1. Hardbottom habitats of Biscayne Bay, a shallow lagoon adjacent to the city of Miami, Florida, USA, contain a limited number of coral species that represent a small subset of the species found at nearby offshore hardbottom and reef habitats of the Florida Reef Tract. Although the physical characteristics of this basin make it a marginal environment for coral growth, the presence of dense populations of Siderastrea radians and Porites furcata indicate that these, as well as other corals that are found at lower densities, are able to tolerate extreme and fluctuating conditions. Three factors, temperature, sedimentation, and salinity, appear to limit coral abundance, diversity, and distribution within Biscayne Bay.2. Temperatures exhibit high frequencies of extreme high and low values known to cause coral stress and mortality elsewhere. Similarly, sedimentation rates are very high and sediment resuspension caused by currents, storms and boating activities commonly bury corals under sediment layers. Sediment burial was shown experimentally to influence growth and mortality of S. radians.3. The salinity of Biscayne Bay is influenced by freshwater inputs from canal, sheetflow and groundwater sources that create a near-shore environment with low mean salinity and high salinity fluctuation. Coral communities along this western margin have the lowest coral density and species richness. Chronic exposure to low salinity was shown experimentally to cause a decrease in the growth of S. radians.4. The location of Biscayne Bay, downstream of a large restoration effort planned for the Everglades watershed, highlights the need to understand the relationship between the physical environment and the health of benthic communities. The data presented here provide the type of scientific information needed so that management decisions can take into account the potential impacts of human activities on the health of coral populations that are already near their tolerance limits for temperature, salinity, and sedimentation.

The 1983 mass mortality of the sea urchin Diadema antillarum greatly decreased grazing intensity on Caribbean reefs, contributing to widespread increases in algal abundance and exacerbating decreases in coral cover. Urchin populations have been recovering in some areas, most notably the reefs of Jamaica's north coast. We manipulated the density of D. antillarum in the buttress zone of a previously unstudied Jamaican reef where the recovering urchins have a clumped distribution. Some buttresses have a large number of urchins while others nearby have none. We transplanted half of the urchins from high urchin density donor buttresses to low urchin density recipient buttresses. Transplantation significantly decreased the percent cover of macroalgae and increased the amount of bare space. These changes occurred despite a generally low retention of transferred urchins on recipient buttresses. Those urchins remaining on the recipient buttresses aggregated at rugose locations around which algae-free barrens appeared. Transplantation of urchins decreased their local density while maintaining overall density on the reef. The increase in algal consumption after transplantation implies that aggregated urchins compete for algae. Whereas aggregated D. antillarum tend to graze within the same area and have only a localized effect on algae, dispersed urchins compete less and eat more. Increased bare space could enhance recruitment of corals, further improving reef health. Our methods could potentially be used as an inexpensive reef restoration tool. Such restoration projects would be most effective if recipient sites with natural or artificially increased rugosity are used.

Traditional analyses of feeding experiments that test consumer preference for an array of foods suffer from several defects. We have modified the experimental design to incorporate into a multivariate analysis the variance due to autogenic change in control replicates. Our design allows the multiple foods to be physically paired with their control counterparts. This physical proximity of the multiple food choices in control/experimental pairs ensures that the variance attributable to external environmental factors jointly affects all combinations within each replicate. Our variance term, therefore, is not a contrived estimate as is the case for the random pairing strategy proposed by previous studies. The statistical analysis then proceeds using standard multivariate statistical tests. We conducted a multiple choice feeding experiment using our experimental design and utilized a Monte Carlo analysis to compare our results with those obtained from an experimental design that employed the random pairing strategy. Our experimental design allowed detection of moderate differences among feeding means when the random design did not.

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