How to use biological time series in Mediterranean ecosystem studies: can hydromedusae be indicator species?

When looking into rational explanations for disturbances in an ecosystem we regularly try to get as many data as possible from historical databases, and, more often than not, we face the problem of incomplete data sets characterized by gaps in the information base. One approach to identify patterns, regularities and irregularities is to focus on regions possessing extensive long-term research, documented in large number of published scientific papers. This is the case of the Adriatic Sea - especially its northern part - the focus of this paper. The research of various aspects of the North Adriatic Ecosystem goes back to early modern oceanographic and marine biological investigations. Plankton research was very fashionable in the 19th and 20th centuries. Contributions from marine research expeditions such as R/V R. Virchov in 1909, 1911, R/V Najade in 1912-1914, R/V Hvar in 1948-1949 were notable. In addition, a number of other research cruises took place in the Adriatic sea, although a large number of samples and biological material was were not examined and therefore potentially valuable information was lost. Benovic et al. (1987) analyzed the hydromedusan fauna and environmental factors in the North Adriatic Sea. Based on comparisons of species composition from almost 100 years of research with recent data, they suggested that changes in environmental factors resulting from the discharge of terrigenous material by the northern Adriatic rivers probably caused changes in hydromedusan fauna and depletion of many species. In addition, they predicted that in the future environmental changes would take place on a large scale in the north Adriatic. Further papers dealing with blooms ofplankton, mucilages and other disturbances in the North Adriatic that were published after 1987, confirmed those predictions. Can a hydromedusa be an indicator species? Analyzing the list of species and trying to understand populations in different regions of the sea, Benovic and Lucic (1996) speculated about possible repopulations of the North Adriatic by species shifted from southern regions. Though somespecies appeared, they were in very small numbers, thus not having the potential to repopulate altered environment of the North Adriatic. These species can be considered as indicator species of some regions, but they cannot serve as indicators of the entire environment. In conclusion, studies of hydromedusae may be useful tools in Mediterranean ecosystem studies. However, only kn