engleski

Repopulation of hydromedusans in the northern Adriatic Sea

A dramatic drop of the hydromedusan biodiversity in the northern Adriatic Sea after 1960s was caused by increased eutrophication and frequent hypoxic/anoxic events in the near bottom layers. It has been suggested that composition and abundance of meroplanktonic hydromedusae might be indicators of such environmental conditions (Benović et al., 1987, 2000). During last two decades an increasing oxygenation of northern Adriatic bottom waters and recovering of macrozoobenthic communities were documented (Giani et al., 2012). Following concern about substantial changes in the northern Adriatic, in this study we present the comparative results on hydromedusan fauna of northern Adriatic, pointing to the possibility of species repopulating recent years. Zooplankton samples were taken by vertical tows from bottom to the surface used Nansen plankton net (WP 2) at four fixed stations (Figure 1). We tested the hydromedusan population’s variations by comparing their species composition and abundance observed in 3 periods: 1999-2002, 2003-2008, and 2009-2015. The higher number of species and the higher abundance might be related to the recent hydrographic and productive changes of northern Adriatic ecosystem. Hypoxic events were especially reduced after 2005 (Ivančić, unpublished). Besides, some species may have been introduced with the prevailing currents. Particularly high values for Clytia spp (Figure 3) and Solmaris spp., as well as occurrence of the typically open-sea medusa Persa incolorata, coincided with strong intrusion of Levantine Intermediate Water in 2011 (Supić, unpublished). The high mean and maxima of metagenetic hydromedusans indicate the possibility that they are establishing stable populations in the northern Adriatic, although the abundance of hydrozoan species varies during their life cycles (Boero at al., 2008). These changes are consistent with changes in plankton composition and abundance, possibly caused by a climatic forcing and recognized oligotrophication of the entire northern Adriatic (Giani et al., 2012). Our results confirm previous suggestions of Benović et al., (1987, 2000) that hydromedusae may be used as indicators of change in shallow marine ecosystems.

5th International Jellyfish Bloom Symposium

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