Major contribution of prokaryotes to carbon fluxes in the pelagic microbial food webs of the Mediterranean Sea

In this study, we carried out dilution experiments at the surface and in the mesopelagic and bathypelagic layers at 15 sites in the Mediterranean Sea that covered a wide range of trophic conditions. The main aim was to test the hypothesis that prokaryotes, and particularly heterotrophic prokaryotes, are pivotal in sustaining both nanoplankton and microzooplankton energy requirements at all of the considered trophic states. These data highlight that bacterivory is the major pathway of organic carbon transfer in the oligotrophic and meso-eutrophic environments. The microzooplankton mostly feed on prokaryotes, directly or indirectly (through nanoplankton exploitation), rather than on microalgae. Under eutrophic conditions, herbivory is the main trophic pathway; however, the heterotrophic prokaryotes always represent an important source of carbon. The lowest food-web efficiency (i.e., ratio between productivity of the highest trophic level and productivity of the lower trophic levels) was determined for the eutrophic status due to possible grazer satiation, which translates into an excess of autotrophic biomass available for export or transfer to higher trophic levels. The food-web efficiency is higher under mesoeutrophic and oligotrophic conditions, where the main pathway is bacterivory. In the mesopelagic and bathypelagic layers, only nanoplankton predation on heterotrophic prokaryotes was investigated. The food-web efficiency in these layers was relatively high and nanoplankton appear to efficiently exploit the available biomass of heterotrophic prokaryote

Microzooplankton composition in the winter sea ice of the Weddell Sea

Sympagic microzooplankton were studied during late winter in the northern Weddell Sea for diversity, abundance and carbon biomass. Ice cores were collected on an ice floe along three dive transects and seawater was taken from under the ice through the central dive hole from which all transects were connected. The areal and vertical microzooplankton distributions in the ice and water were compared. Abundance (max. 1300 ind. l-1) and biomass (max. 28.2 μg C l-1) were high in the ice cores and low in the water below the sea ice (max. 19 ind. l-1, 0.15 μg C l-1, respectively). The highest abundances were observed in the bottom 10 cm of the ice cores. The microzooplankton community within the sea ice comprised mainly aloricate ciliates, foraminifers and micrometazoans. In winter, microzooplankton represent an important fraction of the sympagic community in the Antarctic sea ice. They can potentially control microalgal production and contribute to particulate organic carbon concentrations when released into the water column during the ice melt in spring. Continued reduction of the sea ice may undermine the roles of microzooplankton, leading to a reduction or complete loss of diversity, abundance and biomass of these sympagic protists

Climate Change and the Potential Spreading of Marine Mucilage and Microbial Pathogens in the Mediterranean Sea

Background: Marine snow (small amorphous aggregates with colloidal properties) is present in all oceans of the world. Surface water warming and the consequent increase of water column stability can favour the coalescence of marine snow into marine mucilage, large marine aggregates representing an ephemeral and extreme habitat. Marine mucilage characterize aquatic systems with altered environmental conditions. Methodology/Principal Findings: We investigated, by means of molecular techniques, viruses and prokaryotes within the mucilage and in surrounding seawater to examine the potential of mucilage to host new microbial diversity and/or spread marine diseases. We found that marine mucilage contained a large and unexpectedly exclusive microbial biodiversity and hosted pathogenic species that were absent in surrounding seawater. We also investigated the relationship between climate change and the frequency of mucilage in the Mediterranean Sea over the last 200 years and found that the number of mucilage outbreaks increased almost exponentially in the last 20 years. The increasing frequency of mucilage outbreaks is closely associated with the temperature anomalies. Conclusions/Significance: We conclude that the spreading of mucilage in the Mediterranean Sea is linked to climate-driven sea surface warming. The mucilage can act as a controlling factor of microbial diversity across wide oceanic regions and could have the potential to act as a carrier of specific microorganisms, thereby increasing the spread of pathogenic bacteria

The Mediterranean Sea Regime Shift at the End of the 1980s, and Intriguing Parallelisms with Other European Basins

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

A MSFD complementary approach for the assessment of pressures, knowledge and data gaps in Southern European Seas : the PERSEUS experience

PERSEUS project aims to identify the most relevant pressures exerted on the ecosystems of the Southern European Seas (SES), highlighting knowledge and data gaps that endanger the achievement of SES Good Environmental Status (GES) as mandated by the Marine Strategy Framework Directive (MSFD). A complementary approach has been adopted, by a meta-analysis of existing literature on pressure/impact/knowledge gaps summarized in tables related to the MSFD descriptors, discriminating open waters from coastal areas. A comparative assessment of the Initial Assessments (IAs) for five SES countries has been also independently performed. The comparison between meta-analysis results and IAs shows similarities for coastal areas only. Major knowledge gaps have been detected for the biodiversity, marine food web, marine litter and underwater noise descriptors. The meta-analysis also allowed the identification of additional research themes targeting research topics that are requested to the achievement of GES. 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license.peer-reviewe

From Seasonal to Decadal Inter-Annual Variability of Mesozooplankton Biomass in the Northern Adriatic Sea (Gulf of Trieste)

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.JRC.H.3-Global environement monitorin

Long-Term Copepod Dynamics in the Gulf of Trieste (Northern Adriatic Sea) - Recent Changes and Trends

Cluster, time series analyses were applied to 1970-1999 data set of zooplankton collected in the Gulf of Trieste (North Adriatic Sea). Two time periods (1970-1980, 1986-1999) were compared. Copepod associations were identified. Two main groups with two and three subgroups each were identified. Time-series analyses suggest changes in the periodicity of the associations. The long-term annual dynamics of copepod abundance showed an increase in the late 90s compared to the 70s. Small-sized copepods increased in the second period. Changes in the 90s appeared to be related to the climate change in the Northern hemisphere that abruptly arose after 1987.JRC.H.5-Rural, water and ecosystem resource

Microzooplankton composition under the sea ice and in the open waters in Terra Nova Bay (Antarctica)

The aim of this study was to describe the microzooplankton composition under the sea ice in the Ross Sea (Antarctica) and its possible role as seed for the open-water community. We compared the under-sea-ice data to the communities present in the ice-free waters and at the retreating ice edge in order to test the hypothesis that the microzooplankton composition and abundance depend on the temporal development of the ecosystem. In December 2011, microzooplankton were sampled through two boreholes in sea ice, and 1 month later, in nearby stations that had been free from ice for a short time. The data from these samples were compared with a previous campaign (January 2011) when all of the area had been free of ice for a longer period. Our data indicate that under the sea ice the communities were very different from those of the ice-free waters, both in terms of abundance and diversity, with the prevalence of heterotrophic dinoflagellates and aloricate ciliates. Tintinnids were almost absent below the sea ice, and their abundance increased during the sampling at the intermediate depths. The present study provides evidence for the effect of the environment on the microzooplankton distributions, and indicates that the microzooplankton presence mainly depends on the sampling period. The community under the sea ice appears not to seed the open water community once the ice has retreated. The presence of the ice appears to influence the whole water-column production, which might thus not feed the deeper communities. The most important constraint appears to be the time that has lapsed from the presence of ice cover to the ice-free period

Dynamics of biogeochemical properties in temperate coastal areas of freshwater influence: Lessons from the Northern Adriatic Sea (Gulf of Trieste)

High spatial and temporal variabilities of biogeochemical properties are prominent features of regions under freshwater influence as a result of multiple factors. Understanding the ecological functioning of these ecosystems, which provide important services for humans, is challenging since it requires adequate observational strategies and efforts. Multi-years (1999e2006) continuous observations in the northernmost part of the Adriatic Sea (Gulf of Trieste) allowed us to compute a climatological description of seasonal dynamics of biogeochemical properties for three relevant sites: a coastal area directly influenced by a river, an off-shore area located in the centre of the Gulf and a coastal area located far from potential source of external nutrients. The analysis of the climatologies provides a quantitative corroboration of the conceptual scheme for biogeochemical and ecological seasonal dynamics of temperate coastal areas under freshwater influence already proposed in literature, highlighting the role of river input, lateral transport, stratification regime and interaction with bottom environment as driving factors. While all areas follow a common pattern of succession of ecological processes, spatial variability accounts for a significant decrease of the absolute trophic state, and for a phase delay in biogeochemical dynamics. Results show that spatial heterogeneity is an inherent structural feature of coastal ecosystems, suggesting that the evaluation of the quality status of coastal ecosystems should be made by using different reference terms for different sub-areas

The (un)coupling between viruses and prokaryotes in the Gulf of Trieste

Viruses and prokaryotes represent the smallest and the most abundant biological entities in marine environments. The interest for viruses and their interactions with marine organisms is continuously rising but the studies are generally limited to short-time investigations. This study conducted in the Gulf of Trieste on monthly resolution investigates for the very first time relationships between viruses and prokaryotes (both heterotrophs-HP and autotrophs-AP) over ten years (2000e2010). From our results emerged that no clear relationship between the abundances of viruses and prokaryotes is observed unless for rather restricted time intervals. Some of the sporadic peaks of viral abundances can be attributable to infections occurred during the autumn phytoplankton blooms, thus probably contributing to the end of the bloom. We infer that the general uncoupling between viruses and prokaryotes in the Gulf of Trieste is due to the variety of factors that regulate viral infection, proliferation and persistence such as the diversity of viral life cycles that are determined by environmental factors, the abundance and the physiological status of their hosts