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This paper presents a winter carbon budget for the northern Adriatic Sea, obtained through direct measurements during two multidisciplinary cruises and literature data. A box model approach was adopted to integrate estimates of stocks and fluxes of carbon species over the total area. The oligotrophy at the basin scale and the start of primary productivity well before the onset of spring stratification were observed. In winter, the system underwent a complete reset, as the mixing of water masses erased any signal of previous hypoxia or anoxia episodes. The northern Adriatic Sea was phosphorus depleted with respect to C and N availability. This fact confirms the importance of mixing with deep-sea water for P supply to biological processes on the whole. Despite the abundant prokaryotic biomass, the microbial food web was less efficient in organic C production than phytoplankton. In the upper layer, the carbon produced by primary production exceeded the fraction respired by planktonic community smaller than 200?m. On the contrary, respiration processes prevailed in the water column below the pycnocline. The carbon budget also proved that the northern Adriatic Sea can be an effective sink for atmospheric CO2 throughout the entire winter season.

We have investigated here the structure of the pelagic microbial food web and quantified the carbon fluxes from viruses to microplankton along trophic gradients in the Mediterranean Sea. To explore the complex trophic pathways of the pelagic food web, we conducted independent and replicated experiments to measure (i) predation on prokaryotes by microzooplankton, (ii) predation on prokaryotes by heterotrophic nanoflagellates, (iii) virus-induced prokaryotic mortality, and (iv) microzooplankton grazing on nanoplankton and microphytoplankton. Our study covered more than 5000 km, from the Atlantic Ocean to the Levantine basin, and from conditions of high primary production and nutrient availability to ultraoligotrophic and phosphate-limited waters. Microphytoplankton abundance and biomass were typically scarce across the entire Mediterranean basin, with almost negligible levels in the eastern part. Also, nanoplankton biomass was typically low. Conversely, prokaryotes, and particularly the heterotrophic components, were abundant and represented the only significant food source for both nanoplankton and microplankton grazers. Viral infections were not the primary agents of prokaryotic mortality, but in some areas, such as the Ligurian Sea, they had a key role in prokaryotic dynamics. The scenario depicted in this study in summer reveals the pivotal role of microzooplankton in the pelagic food web of the Mediterranean Sea, with a key role in the potential transfer of biomass to higher trophic levels. We also show that converse to theoretical expectations, the microbial food web was relatively complex under the mesotrophic conditions (Atlantic and western Mediterranean) and was much more simplified in the ultraoligotrophic conditions of the eastern Mediterranean.

Background: Regime shifts are abrupt changes encompassing a multitude of physical properties and ecosystem variables, which lead to new regime conditions. Recent investigations focus on the changes in ecosystem diversity and functioning associated to such shifts. Of particular interest, because of the implication on climate drivers, are shifts that occur synchronously in separated basins. Principal Findings: In this work we analyze and review long-term records of Mediterranean ecological and hydro-climate variables and find that all point to a synchronous change in the late 1980s. A quantitative synthesis of the literature (including observed oceanic data, models and satellite analyses) shows that these years mark a major change in Mediterranean hydrographic properties, surface circulation, and deep water convection (the Eastern Mediterranean Transient). We provide novel analyses that link local, regional and basin scale hydrological properties with two major indicators of large scale climate, the North Atlantic Oscillation index and the Northern Hemisphere Temperature index, suggesting that the Mediterranean shift is part of a large scale change in the Northern Hemisphere. We provide a simplified scheme of the different effects of climate vs. temperature on pelagic ecosystems. Conclusions: Our results show that the Mediterranean Sea underwent a major change at the end of the 1980s that encompassed atmospheric, hydrological, and ecological systems, for which it can be considered a regime shift. We further provide evidence that the local hydrography is linked to the larger scale, northern hemisphere climate. These results suggest that the shifts that affected the North, Baltic, Black and Mediterranean (this work) Seas at the end of the 1980s, that have been so far only partly associated, are likely linked as part a northern hemisphere change. These findings bear wide implications for the development of climate change scenarios, as synchronous shifts may provide the key for distinguishing local (i.e., basin) anthropogenic drivers, such as eutrophication or fishing, from larger scale (hemispheric) climate drivers.

The understanding of the role of the pteropods Limacina helicina in the ecosystem has become of greater interest as the debate on ocean acidification and its consequences for calcifying organisms has increased. Four incubation experiments were carried out in January and February 2006 in Terra Nova Bay Polynya (Ross Sea) to identify the faecal pellets (FPs) produced by L. helicina. Mean FP production rates were 6.1+ 1.3 and 10.9+2.1 pellets day21 individual21 in January and February, respectively. FPs produced by L. helicina had an oval shape with a more lengthened side. The identification of L. helicina faeces allowed us to quantify the amounts of L. helicina FPs in the material collected by sediment traps deployed in the same area from 1998 to 2001. We found that L. helicina FPs flux ranged from 71 103 FP m22 year21 to 362 103 FP m22 year21 and reach maximum values in March–April every year. The FPs flux of this organism contributed 19% of the particle organic carbon flux. The carbon pump may be modified if the L. helicina population declines as a consequence of the predicted acidification in polar and subpolar waters.

This paper reports the rate of change of mesozooplankton biomass on seasonal, inter-annual and decadal time scale in the Gulf of Trieste (NE Mediterranean, Northern Adriatic). We measured variability in mesozooplankton dry weight (mg DW m−3), organic carbon and nitrogen contents of the DW in relation to mesozooplankton taxonomic structure and some environmental parameters. The study is based on data obtained from mesozooplankton samples collected monthly by a vertical WP2 net (200 μm) from January 1986 to September 2005 at one monitoring station, a coastal site in the Gulf of Trieste. We considered mesozooplankton DW in relation to copepods, phytoplankton taxonomic structure, water temperature and North Atlantic Oscillation. For further analyses we counted also on data for DW for the period 1972–1980, monthly data for organic carbon (mg C m−3) and nitrogen contents (mg N m−3) of the DW for the period 1991–2005, determined by a CHN Elemental Analyzer.We explored statistically our high temporal resolution time series data picking out the main features: seasonal components and trends. Mesozooplankton DW ranged from only 1 mg m−3 (January 1977) to 95 mg m−3 (March 1990) in the coastal ecosystem of the Northern Adriatic during the period 1972–2005. The annual cycle of the DW was found to be bimodal with maximum in late winter–spring and a weaker one in late summer. Maximum DW were regularly recorded when Copepods prevailed the mesozooplankton community structure. Similarly, high organic carbon and nitrogen contents were detected when copepods dominated, although wide-ranging on a seasonal scale. Irregular intra- and inter-annual fluctuations were typical mostly during the 1990s. Mesozooplankton DW sharply shifted since 2001–2002 to the level exceeding the observed one during the regime of the 1980s. Our results indicate substantial changes in the seasonal timing of mesozooplankton DW, which together with decadal inter-annual fluctuations match significantly with shifts in copepods, phytoplankton taxonomic structure, temperature and NAO index. The discussed zooplankton descriptors (DW, C and N contents and C:N ratio) being to a certain degree predictive of phyto- and mesozooplankton composition, in essence might foretell the phenological response of plankton communities to environmental changes in the coastal ecosystem, including climate change.

This synthesis was drafted by all workshop participants, under the coordination of Ferdinando Boero and with the support of Paula Moschella. Frédéric Briand reviewed and edited the entire volume whose physical production was carried out by Valérie Gollino.

Cluster and time series analyses were applied to a 1970–1999 data set of zooplankton collected from the Gulf of Trieste (Northern Adriatic Sea). Two time periods were compared: 1970–1980 and 1986–1999. Associations of copepod species were identified and an attempt was made to reveal their long-term dynamics and significant patterns over the study period. Two main groups with 2 and 3 subgroups each were identified. We observed a seasonal alternation of the 2 main groups, albeit with remarkably different patterns during the 2 periods. Time-series analyses suggested recent changes in the periodicity of the associations. The long-term annual mean copepod abundance showed an increase in the late 1990s in comparison to the 1970s; however, this increase was not constant. Small-sized copepods increased in the second period, indicative of changes in the phytoplankton size spectrum. Shifts in the 1990s appeared to be related to climate change in the Northern Hemisphere that appeared abruptly after 1987. Climate change is thus recognized as the most important factor (more so than anthropogenic change) affecting inter-annual and inter-decadal variability of plankton dynamics. The strong signal recorded in the zooplankton community seems to reflect this, identifying the Gulf of Trieste as one of the most sensitive areas in the Mediterranean.

In the summers of 1988, 1989, and 1991 large quantities of sticky mucilaginous masses occurred in the Adriatic, particularly in its northern part. The mucilage phenomenon has been studied by scientists during past events, but the previous scientific reports back to the thirties of this century. Great efforts have been made since 1988 to understand the nature of the phenomenon. Although remarkable scientific results have been achieved, many questions related to such a complex phenomenon have remained open. In this paper results and hypotheses related to the chemical and biological composition, causes, triggering mechanisms, and responsible organisms for the mucilage phenomenon are briefly reviewed. Finally, some suggestions for future researches are proposed.