BLOOMS 1 2 Recurrence of bloom - forming scyphomedusae : 3 wavelet analysis of a 200 - year time series 4

8 Abstract Four meroplanktonic scyphomedusae, 9 Aurelia aurita, Chrysaora hysoscella, Cotylorhiza 10 tuberculata and Rhizostoma pulmo, and the holo11 planktonic, non-resident Pelagia noctiluca have 12 formed blooms in the northern Adriatic over the last 13 200 years. Published data about the historical occur14 rences of these five scyphomedusae, in combination 15 with our data, were used to analyse their long-term 16 fluctuations in this northernmost part of the Mediter17 ranean Sea. Analysis of the most recent blooms was 18 complemented with environmental descriptors (tem19 perature, salinity, pH, chlorophyll a, zooplankton dry 20 weight and major river discharges). Continuous 21 wavelet transformation analysis of the historical time 22 series of scyphomedusae occurrences and environ23 mental parameters revealed that the five species have 24 been present regularly in the northern Adriatic over 25 the last 200 years, with two major periods of jellyfish 26 proliferations. The first period in the years around 27 1910 was characterised by significant periodicity of 28 8–12 years for each species, while the second period 29 from the 1960s onwards was characterised by a 30 shortened significant periodicity of less than 8 years. 31 Pelagia noctiluca fluctuations were analysed in 32 greater detail for the last four decades, revealing 33 significant periodicities of *10 years, 2.5 years, 34 8–14 months, and 8 months. The significantly marked 35 periodicity of about 10 years in the P. noctiluca 36 spectrum indicates a pattern similar to that observed in 37 the western Mediterranean. Wavelet analysis showed 38 that the periodicity of occurrence of five jellyfish 39 species has shortened in recent decades and the 40 recurrence of blooms has increased, particularly for 41 A. aurita and R. pulmo.