Upwelling generates a unique refugium from climate change in the fast warming Eastern Mediterranean Sea

The identification of areas resistant to climate change is a strategy to support biodiversity conservation on a warming planet. Such ‘refugia’ are often detected by modeling future climate and species distributions. More rarely, refugia hypotheses are tested in the field. The easternmost sector of the Eastern Mediterranean is affected by the largest climate-driven collapse of native biodiversity hitherto documented. However, a 150-km-long upwelling system occurs along the south-western Cyprus coastline, keeping summer seawater temperatures 2–3 °C lower than at any other location in the whole sector. We here test the hypothesis that this area is an in-situ climate refugium. We sampled mollusks from 5 to 30 m depth along two transects with Posidonia oceanica meadows and on rocky substrates inside and outside the upwelling area. We reconstructed historical baselines from shelly death assemblages, that is, the accumulations of molluscan shells on the sea bottom. As expected, native species richness in the upwelling area was several fold higher than in the warmer site, irrespective of habitat type. Biodiversity loss was considerably lower in the upwelling area in Posidonia meadows. Rocky substrates showed significant biodiversity loss at both sites in shallow water (5–15 m) reflecting the loss of macroalgal cover. Deeper (20–30 m) rocky substrates were covered by macroalgae and showed considerably lower biodiversity loss, even at the warmer site. These results suggest to target also the lower photic zone when identifying refugia. The upwelling system of Cyprus is the only refugium for the collapsing eastern Mediterranean Sea biota and may prove the final stronghold of its endemic species.