Dissolved organic carbon accumulation during a bloom of invasive gelatinous zooplankton Mnemiopsis leidyi in the northern Adriatic Sea; case of the anomalous summer in 2017

Abstract In the so-called “anomalous” summer in 2017, an increased concentration of DOC was measured in the entire water column at the station SJ107 (eastern part of the Po River delta-Rovinj transect, Graphical abstract) in the northern Adriatic (NAd). Surface DOC ranged between 1.56 and 3.10 mg l−1, being 2.5 times higher than the long-term (1989–2017) average for the same area. Such enlarged concentration of DOC, typical for eutrophic lagoons or lakes and not for open waters, was most probably a consequence of an intensive bloom of Mnemiopsis leidyi, which was recorded with an exceptionally high population density (up to 76 ind. m−3) in the same waters. The number of jelly combs showed a decrease from the east (SJ107) to the west (SJ101) of the transect (Graphical abstract). Abundance in the central part of the transect was also quite low. Maximum of DOC corresponded with the abundance of the M. leidyi smallest specimens ( A large anticyclonic gyre which developed from April to November in 2017 in the area of SJ107, supported calm water column conditions and abundant resources of zooplankton, which could support the residence and accumulation of the invasive ctenophore M. leidyi. For the first time, the presence of a long-lasting gyre (duration of at least 4 months) is documented both with hydrographic measurements and by numerical modelling. Such a situation characterised by the production of highly reactive DOC leads to an extreme eutrophic episode. A dispersion model indicated a high probability of M. leidyi spreading by the end of winter from the Venice Lagoon, its hypothetical winter residing area, to the Istrian coast (Croatia).