Abstract
In the southern part the Adriatic, gelatinous macrozooplankton fauna is poorly known compared to the other taxa. Therefore, our goal was to collect and systematize all available phenological data and abundances of gelatinous organisms based on scientific surveys from 1996 to 2020 and a ”citizen science“ sighting program in the southern Croatian waters. The inter-annual variability and seasonality of planktonic Cnidaria and Ctenophora were described. A total of 590 reports was received from citizens, which summed up to result in 870 data together with the scientists’ records. In total, 15 species were recorded. Of these, scyphomedusa Pelagia noctiluca accounted for 34 %, followed by ctenophora Cestum veneris (19 %) and the scyphomedusa Cothylorhiza tuberculata (17 %). Mass occurrence was most frequently found in P. noctiluca and then in C. tuberculata and C. veneris. Isolated mass occurrence was recorded for the freshwater hydromedusa Craspedacusta sowerbii, found in Lake Kuti near the mouth of the Neretva River, and for the scyphomedusa Aurelia cf. solida. The three species A. solida, Mnemiopsis leidyi and C. sowerbii are not native to European waters. Considered that alien species are invading the Adriatic Sea and human impact and global warming are increasing, it is crucial to carefully monitor the occurrence of gelatinous zooplankton taxa and to conduct studies focused on deciphering their ecological impact in marine ecosystems. Therefore, our empirical analysis of 25 years of observations provides essential information on the variation of gelatinous zooplankton in the Southern Adriatic region.
Highlights
Gelatinous, carnivorous macrozooplankton include several organisms commonly referred to as “jellyfish” belonging to Hydromedusae, Scyphomedusae, and Ctenophora
The species has been found in the Atlantic Ocean, Indo-Pacific Ocean and Mediterranean Sea [15, 38], including the Adriatic Sea [35, 69]
Since 2017, the species has been frequently present in the Northern Adriatic, and in October a mass occurrence was recorded near Pula (3-4 ind. m-3) [5]
Summary
Data on the occurrence and abundance of gelatinous macrozooplankton species, in the period from 1996 to 2020, were obtained based on research cruises and surveys conducted by scientists from several Adriatic oceanographic institutions. Observations for each species were pooled on a monthly basis to create a multi-year semi-quantitative dataset. Each month of the year is assigned a value between 0 and 3 according to the following criteria: 0—jellyfish not seen at all; 1—sporadic occurrence of individual organisms; 2—frequent occurrence of individual jellyfish and/or small aggregations; and 3—frequent occurrence of large aggregations. Annual semi-quantitative abundance was calculated from the monthly datasets using the following equation previously used by [62]: Ʃyear = [(t.o. x rel. 1, 2, 3 = relative abundance according to criteria described earlier
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