The Northern Adriatic Sea: selected results from the European program INTERREG III Italy–Slovenia (2000–2006)

The Northern Adriatic Sea is located between the Gulf of Trieste and the imaginary line from Pescara, in Italy, to Zadar, in Croatia, and has an average depth of 35 m in the northernmost area, comprising the most extensive region of shallow water in the Mediterranean Sea. The most important Italian river (Po) discharges its water in this sub-basin after having crossed the most developed region in Italy in terms of inhabitants and industrial and agricultural activities. Despite this, the Northern Adriatic Sea is still a site of intense biological activity and a good example of how the marine ecosystem copes with pollution, eutrophication, climate change and other human-mediated activities.

This special issue collates contributions from the line B of the European program INTERREG III A/Phare CBC Italy–Slovenia 2000–2006, dedicated to the study of the biological oceanography of the Northern Adriatic Sea (OBAS). The project is a collaborative effort of the Italian Regional Agency for Environmental Protection of the Veneto Region (ARPAV) and the Italian National Research Council (CNR), Institute of Marine Science (ISMAR) of Venice, funded by the European Union (AAVEN111034).

The project roots on previous programs in the area (mainly INTERREG II and MAT), mainly devoted to better defining the trophic state of the area in terms of dissolved nutrients and biological tracers, but also to investigating particular events such as hypoxic and anoxic phenomena, mucilaginous aggregate formation, phytoplankton blooms and the occurrence of toxic species (Rabitti et al. 2002; Giani et al. 2005; Bernardi Aubry et al. 2006a,b; Bragato et al. 2006; Pugnetti et al. 2006; Tedesco et al. 2007).

The INTERREG III-OBAS has developed these studies further, with the aim of achieving a better understanding of (1) the role of the microbial loop and its contribution to the pelagic carbon cycle, (2) the diatom blooms as related to the effect of toxic compounds on copepod reproduction, (3) the role of gelatinous zooplankton on the trophic food web, and (4) the factors triggering mucilage formation.

To this end, collaborations with other institutions were developed, namely, with (1) ARPAV Padova, for the study of the interactions between coastal and offshore waters, (2) the Italian Central Institute for Sea Research and Technology (ICRAM Chioggia), for the measurements of dissolved organic carbon as a precursor of mucilage, (3) the Marine Laboratory of Piran (Slovenia), to determine gelatinous zooplankton distribution, (4) the Rudjer Boskovic Institute of Rovinj (Croatia) to investigate the trophic conditions of the coastal Croatian waters, (5) the Stazione Zoologica Anton Dohrn of Naples, for assessment of picoplankton using flow cytometry, pigment analyses using HPLC and, most of all, the effects of toxic compounds produced by diatoms on copepod reproduction and recruitment, and (5) the Experimental Geophysical Observatory (OGS Trieste) for the study of the microbial loop.

To summarize, the general goal of the OBAS project was to study the Northern Adriatic in terms of standing stocks and processes within the plankton marine food web. The project was process-oriented, aimed at identifying the main actors during key moments of the year, and it therefore investigated bacterial activity, growth rates, bloom dynamics, primary and secondary production.

These process studies have been realized also with the aim of comparing the data with previous long-term studies, so as to interpret and highlight the dynamics of the basin as related to climatic conditions and their recent changes. The impact of these changes is evident from the increased occurrence of plankton blooms and the nuisance they represent, and also from the well-known phenomenon of mucilage aggregate formation.

The general philosophy of the papers selected for this special topic is process-oriented in its interpretation, also taking into account the interdisciplinary approach of the project and the spirit of collaboration among the participants.

The set of contributions present in the actual issue, includes a synthesis on the trophic status and on the relationships of autotrophic versus heterotrophic biomass in a highly variable pelagic system (Pugnetti et al.), while the paper of Del Negro et al. focus on the structure and functioning of the microbial food web. Camatti et al. consider the zooplankton distribution and advance hypotheses on its long-term trends, possibly related to climate change, and finally the contribution of lanora et al. deals with diatom blooms and their effects on copepod reproduction, with identification of new compounds that are responsible of the low reproductive success in these organisms.

A following set of papers, which will appear in the December issue, includes a general description of the hydrological, physico-chemical and biological features of the Northern Adriatic sub-basin, with an emphasis on the long term dynamics (Socal et al.), while De Lazzari et al. integrate these data with an analysis of the distribution and dynamics of dissolved and particulate organic matter, and their role in mucilage development. The contribution of Mozetic et al. describes the features and the effects of a sewage discharge, while that of Turk et al. regards the feeding activity of a jellyfish (Aurelia sp.), approached with an in situ experiment, and its implications for the microbial food web.

We strongly believe that the papers on this special topic contribute to the knowledge of the Northern Adriatic pelagic ecosystem in terms of comprehension of its functioning and dynamics. The papers also point to some relevant challenges that need to be investigated further and that appear to represent important factors in the evolution of this key area of the Mediterranean Sea.

We wish to thank all the authors of the published papers for presenting their best data and the reviewers for timely commentaries, both of which significantly contributed to the success of this initiative. This special topic strongly relies on the collaborative editorial effort between the Guest Editors and the co-Editor of the Journal, Dr M.C. Gambi, who gave constant, constructive and expert advice and feedback on the manuscripts and on the general structure of the topic.