No abstract

No abstract

Little is known concerning environmental factors that may control the distribution of virioplankton on large spatial scales. In previous studies workers reported high viral levels in eutrophic systems and suggested that the trophic state is a possible driving force controlling the spatial distribution of viruses. In order to test this hypothesis, we determined the distribution of viral abundance and bacterial abundance and the virus-tobacterium ratio in a wide area covering the entire Adriatic basin (Mediterranean Sea). To gather additional information on factors controlling viral distribution on a large scale, functional microbial parameters (exoenzymatic activities, bacterial production and turnover) were related to trophic gradients. At large spatial scales, viral distribution was independent of autotrophic biomass and all other environmental parameters. We concluded that in contrast to what was previously hypothesized, changing trophic conditions do not directly affect virioplankton distribution. Since virus distribution was coupled with bacterial turnover times, our results suggest that viral abundance depends on bacterial activity and on host cell abundance.Viruses are the most abundant dynamic component among the microorganisms in the surface waters of the world's oceans. The distribution of virus abundance has been examined in many locations and habitats worldwide. Viral counts from all sorts of environments (coastal, offshore, deep sea, and tropical to polar latitudes) have been found to range from 10 4 particles ml Ϫ1 in oligotrophic systems to over 10 8 particles ml Ϫ1 in eutrophic systems (30).Over the past decade, much effort has been devoted to improving virus quantification and to obtaining a better understanding of the ecological role of this component in the biogeochemical cycling of carbon (10, 18) and nutrients in marine systems (28). However, most studies have been focused on phage-host cell interactions and have been confined to experimental studies or small-scale field investigations. Information on viral and bacterial distribution on a large (i.e., basin or regional) scale based on synoptical samples is practically nonexistent.The problem of the spatial scale is crucial for understanding which environmental factors influence viral distribution (6). The available data indicate that viral abundance decreases with water depth (2, 6, 13), and there is increasing evidence that physical and chemical changes (water temperature or salinity gradients) can also play a role in viral abundance and distribution (27). As a consequence, information on the relationship between hydrological features and viral distribution is needed for a predictive understanding of viral development in response to environmental changes. A reexamination of bacterial abundance data and viral abundance data collected synoptically over small spatial scales indicated that these two components are significantly correlated (30). However, studies examining larger data sets (e.g., studies based on regression analysis of reported va...

[1] Wind forcing plays a key role in controlling the water column structure and circulation in the northern Adriatic Sea. Through shipboard observations and numerical modeling, we have documented the changing of oceanographic features before, during, and after a sequence of cold northeasterly bora wind pulses that occurred during stratified conditions in late September 2002. High-resolution meteorological, hydrodynamic, and wave model outputs were related to in situ observations of hydrologic parameters, dissolved nutrients and oxygen, suspended matter biogeochemical properties, and phytoplankton. The bora intensified the southward flowing coastal current along the Italian coast, establishing a frontal system that typically exists in winter. The bora also caused complete vertical mixing to 20-25 m in the water column, an influx of warm salty water from the south along the Croatian coast, and increased resuspension and southward transport of bottom sediments for the combined effects of currents and waves. The effects on the bottom were limited to the western coastal belt, as in the deeper central part of the basin hypoxic conditions were present during the whole observing period. During the bora, the concentration of inorganic dissolved nutrients in the bottom water increased consistently with the release of nutrients from the sediments and with the mineralization processes. Resuspension of bottom layer sediment represents an important source of nutrients for the water column in this period. The higher level of nutrients was observed together with an increase in phytoplankton biomass, suggesting a potential trigger for the autumnal phytoplankton bloom in the northern Adriatic. Finally, bora events seem to be able to generate a relevant increase of nutrient export from the northern Adriatic through the intensified Adriatic western coastal current, so they could play a relevant role in the nutrient balance of the basin.

[1] The influence of the Po plume on the northern Adriatic Sea was observed during two seasons in 2003 under distinct physical forcing regimes. During the winter, the plume was cool, low in both salinity and chlorophyll, but with higher chlorophyll concentrations occurring along the plume boundary. The plume mixed deeply in the water column in response to the strong wind forcing. The northern Adriatic and the Po plume cooled significantly during the observational period, and therefore salinity alone was the best discriminator of water mass variability. In contrast to the strong forcing of the winter period, the late spring was characterized by weak wind forcing, and below-average Po River discharge ($600 m 3 /s) which was about one third of the typical discharge for this period. As in winter, salinity was again the best discriminator of water mass variability. The Po plume advected southward along the Italian coast and in some locations portions of the coastal plume were transferred offshore in filament-like features. However, the one observed filament was quite low in chlorophyll and was quite thin vertically, extending downward less than 5 m from the surface. The spring observations provide a distinct contrast in the effects of the physical forcings of river flow and wind stress from two different seasons. The strong winter forcing resulted in deep mixing of the plume despite its low salinity and buoyancy, whereas the weak summer flow under weak winds resulted in a very shallow plume (<5 m) that was high in chlorophyll.

No abstract

No abstract

Long-term data series (1971–2015) of physical and biogeochemical parameters were analyzed in order to assess trends and variability of oceanographic conditions in the northern Adriatic Sea (NAS), a mid-latitude shallow continental shelf strongly impacted by river discharges, human activities and climate changes. Interpolation maps and statistical models were applied to investigate seasonal and spatial variability, as well as decadal trends of temperature, salinity, chlorophyll-a and nutrients. This analysis shows that sea surface temperature increased by +0.36% year−1 over four decades. Annual mean flow of the Po River markedly changed due to the occurrence of periods of persistent drought, whereas the frequency of flow rates higher than 3000 m3 s−1 decreased between 2006 and 2015. Moreover, we observed a long-term decrease in surface phosphate concentrations in Po River water (−1.34% year−1) and in seawater (in summer −2.56% year−1) coupled, however, to a significant increase in nitrate concentration in seawater (+3.80% year−1) in almost all seasons. These changes indicate that the nutrient concentrations in the NAS have been largely modulated, in the last forty years, by the evolution of environmental management practices and of the runoff. This implies that further alteration of the marine environment must be expected as a consequence of the climate changes.