Environmental boundaries of marine cladoceran distributions in the NW Mediterranean: Implications for their expansion under global warming

• Environmental preferences of marine cladocerans in the NW Mediterranean Sea have been identified.
• Cladoceran distributions are associated with productive, stratified, riverine waters.
• Extreme heat waves during July 2003 in southern Europe impaired Penilia avirostris survival.
• The lack of strong diel migrations seems associated with cladoceran life history traits.
• Warming-driven poleward expansion of P. avirostris may impact the pelagic food-web structure.

We studied the horizontal and vertical distributions of marine cladocerans across the Catalan Sea shelf (NW Mediterranean) in July and September 2003, and in June and July 2004. At the seasonal scale, Penilia avirostris appears first in June in the southern region, where temperatures are warmer, and its populations develop northward during the summer. Evadne-Pseudevadne did not show a clear pattern, likely because several species were pooled. In 2003 successive heat waves affecting southwestern Europe resulted in surface seawater temperatures about 2 °C higher than usual across the whole study region. These high temperatures were associated with much lower abundance of P. avirostris. Overall, the mesoscale distributions of cladocerans were associated with the presence of low salinity, productive and stratified waters of continental origin, and negatively linked to the intrusion of offshore waters. On the vertical scale P. avirostris was located within or above the thermocline, whereas Evadne-Pseudevadne was much shallower; no evidence of diel migration was detected in either group. Our study provides new insights regarding the environmental limits for marine cladocerans in the NW Mediterranean; in the particular case of P. avirostris that knowledge can define the likely boundaries of its new distributions as it expands poleward under climate change.