Limnol. Oceanogr., 44(6), 1999, 1415–1422

A tank system is described for long-term exposure of sessile organisms to well-defined ranges of particle loads on a background of natural flowing seawater. Using low technology and a simple mathematical model, the concentration of suspended particulate matter (SPM) and the rate of sedimentation could be predicted and sustained with high precision. The system and operational procedures were tested in an 8-week experiment investigating the effect of SPM concentrations on the growth rates of two species of symbiotic scleractinian coral (Goniastrea retiformis and Porites cylindrica). To also evaluate the effect of shading by SPM on coral growth, two light levels corresponding to 3–4-m depth at the low and high particle concentrations were included in the design. The growth rates of corals in control tanks were not significantly different from those of conspecifics in situ. However, the patterns of growth rates vs. SPM and shading treatments differed between species. The growth rates of G. retiformis generally increased as a function of SPM concentration (range 5 ;1–16 mg L21), whereas the growth rates of P. cylindrica were unaffected by particle load. The shading effects corresponding to 16 mg SPM L21 at 3–4-m depth resulted in significantly reduced growth rates in both species. I hypothesize that the different growth patterns displayed by the two species are the results of different abilities to utilize SPM as a food source or different susceptibilities to SPM as a mechanical stress factor. The high level of environmental control and the constancy of SPM treatment levels were reflected in the absence of tank effects on growth rates and provided sufficient statistical power to detect relatively small differences in growth rates between corals from different treatments. SPM is a universal component of aquatic systems, and the ecological role of SPM for both pelagic and benthic communities has received increasing attention over the past three decades (for review, see Wotton [1994] and references therein). The perceived role of SPM, however, varies between ecosystems. In temperate benthic habitats, detrital SPM may constitute an important source of food for suspension feeders (Seiderer and Newell 1985; Barille et al. 1997), especially below the euphotic zone (Genin et al. 1992; Rosenberg 1995). On coral reefs, however, there is a general consensus that high loads of suspended and sedimenting particles represent a stress factor by smothering tissues and attenuating light for photosynthesis (reviewed by Rogers 1990). While a number of field studies have addressed the problems of turbidity and sedimentation on coral assemblages, their findings have been inconsistent. The reported effects of high turbidity levels and rates of sedimentation on corals range from high mortality (Dodge and Vaisnys 1977; StaffordSmith 1992) to no discernible effect (Dollar and Grigg 1981; McClanahan and Obura 1997). Most studies have been conducted in environments with high spatial and temporal variability, potentially precluding the identification of tur-