Interannual variations in Adriatic Sea zooplankton mirror shifts in circulation regimes in the Ionian Sea

We investigated potential connections over the past 2 decades between mesoscale circulation regimes in the Ionian Sea and newly observed species and the concurrent rise in sea temperature in the Adriatic Sea. Analyses of plankton samples from 1993 to 2011 in the southern Adriatic revealed marked changes in the non-crustacean zooplankton community. Eleven species were recorded for the first time in the Adriatic, while 3 species reappeared after years of absence. We found that pluriannual changes in the zooplankton community tracked the continuum of circulation regimes in the Northern Ionian Gyre (NIG). The occurrence of Atlantic/Western Mediterranean species coincided with anti-cyclonic circulation in the NIG, probably due to the advection of Modified Atlantic Water into the Adriatic, while the presence of Lessepsian species coincided with the cyclonic pattern, which governs the entry of Eastern Mediterranean waters. The impact has been that newcomers now make a significant contribution to the zooplankton community in the southern Adriatic and, in certain cases, have replaced native species. Our results provide new evidence of the influence of teleconnection processes between the North Atlantic and Eastern Mediterranean on the dynamics of water masses in the southern Adriatic. The synergistic effects of these processes, together with warmer Mediterranean waters, raise concerns over dramatic changes in the marine biodiversity of the Adriatic.