Few studies have been reported of the effects of power-plant discharges on tropical and semi-tropical marine environments where plants and animals in non-stressed conditions live close to their upper lethal thermal limits. This paper describes a multidisciplinary three-years' study made in Card Sound, Florida, before, during, and after, thermal effluents were released from a power-plant. Biomass, growth, and production, of Thalassia and macroalgae, were estimated. Animals were collected by trawl, drop-net, suction dredge, in artifical habitats, and on mesh-panels. Except in an area of 2–3 ha adjacent to the canal mouth, little damage to the benthic community was observed. This was very different from the effect of the first canal emanating from the same power-plant at Turkey Point, where a large area had been affected by heated effluents.