Unusual winter zooplankton bloom in the open southern Adriatic sea

Abstract

The paper reports an unusual response of the microzooplankton community to oceanographic conditions observed during the winter of 2015 in the open southern Adriatic. Record-breaking nauplii abundance of 13,734 ind. m–3 was sampled for the open southern Adriatic by 50-μm net sampling. This could be explained by (i) warmer-than-usual surface and intermediate ocean temperatures, (ii) higher precipitation that freshened and widened the surface layer, pushing saline Levantine Intermediate Water below a 400-m depth, and (iii) strong wind episodes that transported nutrients from the coastal zone to the open ocean and induced limited vertical mixing. Neritic tintinnids and viable photoautotrophs well below the photic zone support the last possibility. Average seasonal maximum of zooplankton abundance was shifted from spring and early summer to the late winter months. Our results document large and fast variations in production conditions rarely found to occur in oligotrophic waters such as the southern Adriatic Sea.