Factors influencing the distribution of epibenthic megafauna across the Peruvian oxygen minimum zone Thomas Mosch n , Stefan Sommer, Marcus Dengler, Anna Noffke, Lisa Bohlen, Olaf Pfannkuche, Volker Liebetrau, Klaus Wallmann Helmholtz-Zentrum f¨ ur Ozeanforschung Kiel, GEOMAR, Wischhofstr. 1–3, 24148 Kiel, Germany article info Article history: Received 23 September 2011 Received in revised form 3 April 2012 Accepted 16 April 2012 Available online 29 June 2012 Keywords: Oxygen minimum zone Epibenthic megafauna Bacterial mats Internal tides Video imaging Peruvian margin abstract Current de-oxygenation of the oceans is associated with severe habitat loss and distinct changes in the species composition of bentho-pelagic communities. We investigated the distributions of epibenthic megafauna across the Peruvian OMZ (111S) at water depths ranging from  80 to 1000 m water depth using sea floor images. Likely controls of distributions were adressed by combining the abundances of major groups with geochemical parameters and sea-floor topography. In addition to bottom-water oxygen levels and organic-carbon availability, particular emphasis is laid on the effects of local hydrodynamics. Beside the occurrence of microbial mats at the shelf and upper slope, distinct zones of highly abundant megafauna, dominated by gastropods (900 ind. m 2 ), ophiuroids (140 ind. m 2 ), and pennatulaceans (20 ind. m 2 ), were observed at the lower boundary of the OMZ. Their distribution extended from 460 m water depth (O 2 levels o 2 mM), where gastropods were abundant, to 680 m (O 2 6 mM) where epifaunal abundances declined sharply. Bottom water O 2 represents a major factor that limits the ability of metazoans to invade deeply into the OMZ where they could have access to labile organic carbon. However, depending on their feeding mode, the distribution of organisms appeared to be related to local hydrodynamics caused by the energy dissipation of incipient internal M2 tides affecting the suspension, transport and deposition of food particles. This was particularly evident in certain sections of the investigated transect. At these potentially critical sites, energy dissipation of internal tides is associated with high bottom shear stress and high turbulences and coincides with elevated turbidity levels in the benthic boundary layer, increased Zr/Al-ratios, low sedimentation rates as well as a shift in the grain size towards coarser particles. In or near such areas, abundant suspension- feeding organisms, such as ophiuroids, pennatulaceans, and tunicates were present, whereas deposit- feeding gastropods were absent. The influence of local hydrodynamic conditions on the distribution of epibenthic organisms has been neglected in OMZ studies, although it has been considered in other settings. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Worldwide, oxygen minimum zones (OMZs) are expanding with integrated oxygen loss rates in the range of 37 mmol m 2 yr 1 (eastern tropical Indian Ocean) and 136 mmol m 2 yr 1 for (eastern tropical North Atlantic) (Stramma et al., 2008). These trends are associated with habitat loss and distinct changes in the species composition of bentho-pelagic communities (Chan et al., 2008; Diaz and Rosenberg, 2008). Whereas effects of hypoxia on the distribution of meio- and macrobenthic organisms have been studied at major OMZs worldwide (Levin, 2003; Gooday et al., 2009) less attention has been paid to epibenthic megafauna and environmental controls on their distribution. As the major OMZs differ in spatial extension as well as in the bottom water (BW) O 2 concentration (Helly & Levin, 2004), it is difficult to generalize distribution patterns of epibenthic mega- fauna. However, successional sequences of microbial and epifau- nal organisms appear to be common to all OMZs. At the upper boundary giant sulfur bacteria belonging to the genera Beggiatoa, Thioploca and Thiomargarita are prominent members of the epibenthos, for instance, off Pakistan (Schmaljohann et al., 2001), off Namibia (Schulz et al., 1999) and on the Peruvian/ Chilean continental margin (Gallardo, 1977; Gutie ´ rrez et al., 2008). Depending on the BW O 2 levels, the core regions of OMZs are mostly devoid of epibenthos (Levin, 2003; Murty et al., 2009). These core regions are characterized by persistently low O 2 levels, whereas at the upper and lower boundary of the OMZs bottom Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/dsri Deep-Sea Research I 0967-0637/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dsr.2012.04.014 n Corresponding author. Tel.: þ49 431 600 2267; fax: þ49 431 600 2928. E-mail address: tmosch@geomar.de (T. Mosch). Deep-Sea Research I 68 (2012) 123–135