Sediment characteristics in reef areas influenced by eutrophication-related alterations of benthic communities and bioerosion processes V. Chazottes a, ⁎ , J.J.G. Reijmer a,1 , E. Cordier b a Laboratoire de Géologie des Systèmes Carbonatés, FRE 2761, EA 4229 Université de Provence, Place Victor Hugo, 13331 Marseille cedex 03, France b Laboratoire des Sciences de la Terre de l'Université de La Réunion, UMR 7154, 15 Avenue René Cassin, 97715 Saint-Denis, La Réunion, France Received 15 June 2007; received in revised form 3 December 2007; accepted 20 January 2008 Abstract This study investigated sediment deposits and suspended particulate matter in three reef areas of Reunion Island characterized by varying benthic communities and bioerosion processes, as a result of different eutrophication levels. The sediments were sampled using two different methods, hand-collecting directly on the reef bottom and sampling by traps. This approach allowed a comparison between the composition of reefal bottom sediments and suspended particles. In locations where algal communities prevailed over corals as a result of nutrification, a shift from coral to coralline algae dominated sediments could be observed. In addition, a decrease in sediment production and a prevalence of very fine sand to mud sized grains over medium to fine sands existed in the nutrient-enriched areas. This grain-size difference probably is caused by a decrease in grazing activity in the enriched areas. High proportions of coralline algal debris and sponge spicules were specifically found in the sediments within areas receiving high nutrient input. These two diagnostic components were abundant in both the suspended particulate matter and in the fine fractions (b 0.40 mm) of the bottom sediments. The implications for the interpretation of the fossil record of these bioerosion and sedimentation processes in recent reefal environments are discussed. © 2008 Elsevier B.V. All rights reserved. Keywords: coral reefs; sediment deposits; suspended particles; eutrophication; Réunion Island 1. Introduction Deposition rates in tropical reefs are strongly influenced by framework and particle breakdown by eroding organisms. Even though the contribution of skeletons from molluscs, foramini- fers, crustaceans or echinoderms is also important for the overall sediment production, they are generally of minor importance when compared with the volumes of sediments introduced to the sedimentary system by bioerosion (Hubbard et al., 1990). Thus, the composition of the bioeroder communities may in part control sediment production, and may have implications on the grain-size distribution within the reefs and associated deposi- tional environments. Sammarco et al. (1987) pointed out that grazing activities of organisms like echinoids or scarid fish resulted in coarser sediments than for environments with minor grazing activity in which the sediment particles produced result mainly from the activity of micro and macro-borers. Chazottes et al. (2002) investigated the variation of bioero- sional processes in relation to disturbances of reefal communities due to eutrophication. The La Saline fringing reef at La Reunion Island (Southern Indian Ocean) is subjected to increased nutrient input from the adjacent land by submarine groundwater discharge which favours the development of macro-algae. Chazottes (1996) and Chazottes et al. (2002) showed that an increase in nutrient supply (particularly NO 3 - and NH 4 + ) in reefal waters favours the development of algal covers, especially crustose coralline algae and the macrophyta Lobophora variegata. This shift from a coral to algal dominated community is the response of a reef community related to a change in the nutrification processes Marine Geology 250 (2008) 114 – 127 www.elsevier.com/locate/margeo ⁎ Corresponding author. Tel.: +33 491 106 754; fax: +33 491 108 523. E-mail address: veronique.chazottes@univ-provence.fr (V. Chazottes). 1 Present address: Vrije Universiteit, Faculty of Earth and Life Sciences (FALW), Department of Sedimentology and Stratigraphy, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands. 0025-3227/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2008.01.002