Zooplankton in Warming and more Oligotrophic Coastal Sea: the Northern Adriatic Case

The northern Adriatic is a shallow (<60m) northernmost area of the Mediterranean Sea that is characterised by high river inputs (50km³yˉ¹) and a broad range of environmental conditions. Due to shallowness and geographic location, its biota is exposed to variable/extreme hydrographic conditions. SST (surface temperature) has increased at a rate of 0.08±0.01°C y ^-1 in the last two decades, and the warming bottom T trend was even more pronounced. Analysis of extreme temperature events since 1980s has shown that the occurrence of marine heatwaves (MHW) increased and marine cold-spells (MCS) have declined. In parallel to a warming trend, the last period was also characterised by lower phytoplankton biomass. We used zooplankton data from 5 stations located along the east-west and south-north trophic gradient to explore community response to environmental changes during 2006–2016, and compared these results to the literature and own data from an earlier period. The total zooplankton dry mass decreased and showed less variability during 2006-2016 compared to 1991-2001. Using the literature designations, we categorized zooplankton into four trophic levels (carnivorous-C, herbivorous-H, omnivorous-O and omnivorous/herbivorous-OH), and this analysis of feeding traits revealed a robust seasonal pattern. C were dominated by gelatinous taxa (Cnidarian, Ctenophora), abundant during late summer-early autumn. Cladocera and Thaliacea feeding on small-sized pico/nanoplankton (H) showed a peak in the warmest months. OH group consisted mainly of small calanoid copepods and larvae of benthic organisms, which prevailed in the winter-spring period. In general, gelatinous taxa showed an increasing trend in the last period, while Penilia avirostris, cold water H and OH copepod taxa declined. We focused specifically on years with the highest number of MHWs (2015), the highest number of MCSs (2006), a year with the most MHW and MCS extremes (2012), and the year with fewest extremes (2014).