Contents lists available at ScienceDirect Marine Environmental Research journal homepage: www.elsevier.com/locate/marenvrev Benthic mucilage blooms threaten coralligenous reefs L. Piazzi a,* , F. Atzori b , N. Cadoni b , M.F. Cinti b , F. Frau b , G. Ceccherelli a a Dipartimento di Chimica e Farmacia, Università di Sassari, Via Piandanna 4, 07100, Sassari, Italy b Area Marina Protetta Capo Carbonara-Villasimius, Via Roma 60, Villasimius, (CA), Italy ARTICLE INFO Keywords: BACI design Benthic mucilaginous aggregates Coralligenous assemblages Diversity Mediterranean sea Rocky habitat ABSTRACT Mucilaginous aggregates produced by planktonic or benthic algae are considered ecological threats to marine systems. The study evaluated the effects of the spread of benthic mucilaginous aggregates on the structure of coralligenous assemblages. The assemblage and the quality of a site subjected to a benthic mucilage bloom were compared to those of two reference sites using a Before/After-Control/Impact (BACI) design. Results showed the α and β-diversity, ESCA and COARSE quality ecological indices and the cover of encrusting algae and bryozoans were lower at the impact site after the mucilage event than at the control sites and at the impact site before the mucilage event. An opposite pattern was observed for the necrosis of gorgonians and the cover of algal turf. This study describes for the first time the impacts of ephemeral mucilage blooms on the whole coralligenous as- semblage, identifying a further threat of this habitat and the need of adequate monitoring programs. 1. Introduction Mucilaginous aggregates consist of gelatinous organic material with colloidal properties, mainly composed of high molecular weight poly- saccharides released by marine organisms (Wimpenny et al., 2000). They may be pelagic (aggregates suspended in seawater) or benthic (cloud-like aggregates adhering to solid substrates), primarily produced by planktonic or benthic algae, respectively (Innamorati, 1995; Myklestad, 1995; MacKenzie et al., 2002; Schaffelke et al., 2004; Caronni et al., 2016). Mucilaginous aggregates are considered an eco- logical threat to marine systems (Devescovi and Iveša, 2007; Yentur et al., 2013) and also damage tourism and fisheries (Mingazzini and Thake, 1995). In the Mediterranean Sea, intensification of algal blooms with mu- cilaginous aggregates has been associated with global warming (Innamorati et al., 2001; Russo et al., 2005), because mucilage events have increased in frequency and duration (Rinaldi et al., 1995; Giani et al., 2005; Precali et al., 2005) since the second half of the nineteenth century, when sea surface temperature began to increase (Moron, 2003). In fact, the main algae producing mucilaginous aggregates are favored by warming (Schiaparelli et al., 2007). Though less studied than pelagic events, benthic mucilaginous ag- gregates have become an increasing concern in many areas of the Mediterranean Sea (Sartoni and Sonni, 1992; Welker and Bressan, 1994; Calvo et al., 1995; Innamorati, 1995; Olianas et al., 1996; Hoffmann et al., 2000; Lorenti et al., 2005; Schiaparelli et al., 2007; Aktan and Topaloğlu, 2011; Caronni et al., 2014). Benthic mucilaginous aggregates may develop over different types of substrate and a wide depth range, usually from early spring to the end of summer. They are caused by five filamentous macroalgae: free-living forms of two brown algae, Acinetospora crinita (Carmichael ex Harvey) Kornmann and Tri- bonema marinum J. Feldmann (Sartoni and Sonni, 1992; Sartoni et al., 1993; Giani et al., 2016), and three fast growing benthic chrysophytes Nematochrysopsis marina (Feldmann) Billard, Chrysonephos lewisii (Taylor) and Chrysophaeum taylorii Lewis and Bryan (Sartoni et al., 1995, 2008; Caronni et al., 2016, 2017). Species composition is quite variable, although chrysophytes usually occur in spring down to a depth of 20 m, whereas A. crinita spreads deeper. Live bacteria, mi- croalgae, especially diatoms, and particles of detritus are commonly embedded in the mucilage (Thornton, 2002; Faraloni et al., 2003; De Philippis et al., 2005). Mucilaginous aggregates can seriously damage benthic organisms by overgrowth (Rinaldi et al., 1995; Giuliani et al., 2005; Schiaparelli et al., 2007), reducing light penetration (Lorenti et al., 2005) and causing benthic hypoxia due to degradation of large amounts of organic material (Cornello et al., 2005). Accumulated mucilage may suffocate sessile or less mobile organisms by clogging their siphons and burrow openings, or may determine anoxic conditions near the bottom, with the same negative effects (Rinaldi et al., 1995; Pellegrini et al., 2003). However, the effects may differ in relation to habitat: no significant alterations have been observed in seagrass meadows (Lorenti et al., 2005) while deep subtidal rocky assemblages seem the most threatened, probably due to longer lasting episodes, higher sensitivity of the or- ganisms and different mucilage composition, mostly due to A. crinita https://doi.org/10.1016/j.marenvres.2018.06.011 Received 10 March 2018; Received in revised form 11 June 2018; Accepted 13 June 2018 * Corresponding author. E-mail address: lpiazzi@uniss.it (L. Piazzi). Marine Environmental Research xxx (xxxx) xxx–xxx 0141-1136/ © 2018 Elsevier Ltd. All rights reserved. Please cite this article as: Piazzi, L., Marine Environmental Research (2018), https://doi.org/10.1016/j.marenvres.2018.06.011