Many organisms live in aggregations. For marine organisms attached to the substratum, the presence of neighbors may reduce both the water flow and light encountered by an individual within a group. We used the marine macroalga Turbinaria ornata to investigate how water velocity and photon flux density were affected by aggregations under natural flow and light conditions in the field, and to explore how the ability to stand upright due to buoyancy impacted light interception by individuals in aggregations. We found that the flow experienced by thalli in the middle of a group in a backreef habitat was only about half the speed of the water flow they encountered after their neighbors were removed. This suggests that aggregated algae are subjected to lower hydrodynamic forces than are solitary thalli, but may also experience reduced rates of transport of dissolved nutrients and gases. Light sensors placed along the length of thalli positioned within and outside of clumps of T ornata showed that the tops of buoyant upright thalli experienced similar light levels to solitary thalli, but that neighbors shaded the lower portions of thalli in aggregations. Individuals that were rendered non-buoyant (by filling their airbladders with water) could not support their own weight and those in aggregations experienced lower light at all points along their lengths than did upright buoyant individuals. Using in situ determinations of the rate of photosynthesis of T ornata as a function of irradiance, we converted our field measurements of light interception to mg carbon fixed over the course of a day for whole fronds. Such estimates indicated that carbon fixation was higher for solitary buoyant and non-buoyant T ornata than for buoyant individuals in aggregations, all of which were greater than for non-buoyant thalli in those clumps. Our results indicate that living in aggregations reduces the productivity of T ornata, but this effect is mitigated by the buoyancy of thalli. (c) 2006 Elsevier B.V. All rights reserved.