Although the deep sea is considered a relatively stable environment, it is affected by climatic and environmental variations with implications for functioning of its ecosystems and food webs. We used stable isotopes to compare trophic levels of 21 species of deep-sea megafauna (fish, decapods and other invertebrates) between 2 periods separated by ca. 20 yr (1985-1989 vs. 2007-2011) and characterized by different climatic conditions (based on the NAO index), by increases in temperature and salinity and by a decrease in O2 concentration in intermediate (Levantine Intermediate Waters, LIW) and deep (at the Benthic Boundary Layer, BBL) water masses in the Balearic Basin. We found a decrease in the δ15N of benthopelagic fishes (1.5‰ on average), benthic invertebrates (1.1‰) and pelagic organisms (0.8‰) from 1985-1989 to 2007-2011, a temporal decline confirmed with additional analyses performed after compiling published data collected in 1996, 1998, 2003/2004 and 2007/2008 from the same area. Changes in trophic levels (based on δ15N) were correlated (Spearman's R) to temperature increase, i.e. species seemed to feed on lower trophic levels during warmer periods. According to generalized linear models, the main explanatory variables for changes in δ15N were O2 in the BBL and at the core of the LIW, chlorophyll a concentration at the surface and salinity near the bottom. We hypothesize that the changes in oceanographic conditions in the Mediterranean, driven by both global change and river damming, have influenced benthic organisms by reducing their biomass and/or size, shifting megafauna (mainly fish) to feed on more pelagic or smaller benthic prey, with lower δ15N values, in 2007-2011 compared to 1985-1989. © Inter-Research 2016.

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