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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.

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Determination of the percentage cover and residence time of drift algae over permanently marked 0.25 m 2 plots at 8 study sites indicated that drift algal cover was relatively low (< 20% at most sites) and did not remain in the same location longer than 2 to 3 mo. Experimental maintenance of high drift algal cover for 2 to 3 mo produced a ca. 25% reduction in above-ground biomass compared to plots free of drift algae, but the magnitude of the decline in biomass did not increase with an additional 3 mo of algal cover. Below-ground biomass and shoot density, however, were not affected by the presence of drift algae, suggesting that a change in plant morphometrics contributed to reduced above-ground biomass and limited long-term negative impacts as root reserves were not depleted. We investigated the effects of drift algal cover on short-term growth of Thalassia testudinum at 2 different sites in 2 different seasons. In neither instance did we observe any decrease in growth rates of T. testudinum in the presence of drift algae. The lack of a negative effect of drift algae on seagrass growth is likely due to the clonal nature of the seagrass plants, their ability to share resources among shaded and unshaded short shoots, and the availability of stored reserves in robust roots and rhizomes. We also conducted growth experiments assessing the effects of epiphytes on growth of T. testudinum. Unlike drift algal cover, the presence of encrusting calcareous epiphytes was shown to have a negative impact on growth of T. testudinum. This is likely due to a combination of decreased light levels as well as a reduction in exchange of materials across the boundary layer at the leaf surface. Collections of seagrass leaves from under drift algae and from areas free of drift algae indicated that filamentous epiphyte loads are decreased in the presence of macroalgae. Our results suggest that temporary, moderate cover of macroalgae may benefit seagrass by reducing epiphyte loads if the epiphyte cover negatively impacts the seagrass. KEY WORDS: Plant/plant interactions · Macroalgae · Seagrass · Epiphytes · Thalassia testudinum Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 279: [81][82][83][84][85][86][87][88][89][90][91] 2004 community (Fry 1984, Virnstein & Carbonara 1985, Heijs 1987, and provide additional structure to seagrass meadows and important habitat for numerous estuarine species (Kulczycki et al. 1981, Herrnkind & Butler 1986, Virnstein & Howard 1987, Schneider & Mann 1991, Holmquist 1994.Seagrass, epiphytes, and macroalgae occur together with their abundances influenced both directly and indirectly by top-down (grazing) and bottom-up (nutrient) controls (e.g. Neckles et al. 1993, Williams & Ruckelshaus 1993, McGlathery 1995, Hauxwell et al. 1998, Gacia et al. 1999, Heck et al. 2000. Under enhanced nutrient inputs typical of many coastal estuaries, blooms of epiphytes (Tomasko & Lapointe 1991, Neckles et al. 1993, Williams & Ruckelshaus 1993, Coleman & Burkholder...

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