Nutrient manipulation methods for coral reef studies: A critical review and experimental field data M.M. Littler a, ⁎ , D.S. Littler a,b , B.L. Brooks a , B.E. Lapointe b a Department of Botany, MRC 166, P.O. Box 37012, National Museum of Natural History, Smithsonian Institution, Washington, DC 20013, USA b Division of Marine Science, Harbor Branch Oceanographic Institution, 5600 US 1 North, Ft. Pierce, FL 34946, USA Received 15 December 2005; received in revised form 26 April 2006; accepted 19 May 2006 Abstract The results reported in this paper demonstrate suboptimal experimental designs in some of the previously published manipulative methods and provide insights for the improvement of in-situ nutrient studies on coral reefs. Overgrown 0.5-liter porous clay-pot diffusers (“mini-reefs”—following a decade of recruitment, colonization and competition) were utilized to evaluate protocols for studies of controlled nutrient enrichment on coral reefs. A commonly used fertilizer, Tree Food Stakes resulted in detrimental 11-fold and 20-fold decreases of fleshy algae and calcareous coralline algae, respectively, relative to the Control treatments; while blue-green algae (Cyanobacteria) became significantly (6 times) more abundant. Osmocote-filled mini-reefs showed no such negative differences in mortality from the Controls for any functional group. By avoiding the pitfalls of inappropriate sources of enrichment, insufficient durations of colonization/competition studies, suboptimal study areas and inadequate nutrient detection limits in future research, the potential to provide new insights into the nutrient status of coral reefs is greatly increased. Published by Elsevier B.V. Keywords: Algae; Corals; Coral reefs; Methods; Nutrients 1. Introduction A fundamental goal in ecology is to understand the mechanisms by which natural and anthropogenic factors may alter or maintain interactions and structure in biotic communities. The concepts of “top–down” and “bot- tom–up” controls have been used (e.g., Atkinson and Grigg, 1984) to describe mechanisms where either the actions of predators or resource availability regulate the structure of coral-reef communities. These factors provide a useful perspective to assess human activities that affect the interactive mechanisms controlling stable states and phase shifts (e.g., eutrophication, destructive overfishing) among the dominant functional groups on healthy tropical reefs (see Littler et al., in press, Relative Dominance Model, RDM). Top–down control by abundant populations of large mobile herbivores has been documented repeatedly for nearly half a century (i.e., since the time of the classic exclusion study by Stephenson and Searles, 1960), in strong support of the prominent role of herbivory on coral reefs. Many workers (reviewed in Steneck, 1989; McCook, 1999; Bellwood et al., 2004) have verified that decreasing Journal of Experimental Marine Biology and Ecology 336 (2006) 242 – 253 www.elsevier.com/locate/jembe ⁎ Corresponding author. Tel.: +1 202 633 0956; fax: +1 202 786 2563. E-mail address: littlerm@si.edu (M.M. Littler). 0022-0981/$ - see front matter. Published by Elsevier B.V. doi:10.1016/j.jembe.2006.05.014