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During the past two centuries, human activities have greatly modified the exchange of carbon and nutrients between land, atmosphere, freshwater bodies, coastal zones and open ocean. This PhD thesis focuses on different aspects of carbon cycle, considering three case study for the evaluation of the effects of anthropogenic pressure and of the natural evolution of carbon cycle. The first case study focuses on anthropogenic impacts on contaminated coastal areas. These environments connect terrestrial and oceanic systems, and are highly subjected to anthropogenic pressure. An example of a contaminated coastal area is the Mar Piccolo of Taranto (Ionian Sea) and the results of two published papers are reported. In the first paper, the results of the analysis of nutrients and carbon fluxes at sediment-water interface showed that the multi-contamination of both inorganic and organic pollutants in the sediments is potentially transferable to the water column and to the aquatic trophic chain. On the other hand, in the second paper, the results of the analysis of a long time series of chemical-physical characteristics of the water column highlighted that the implementation of sewage treatment plants has positively affected the trophic status of the Mar Piccolo from being relatively eutrophic to moderately oligotrophic. The increase of atmospheric CO2 concentration has been recognised as one of the main causes of climate change. Carbon capture and storage technology (CCS) is expected to play a key role among mitigation strategies by reducing CO2 emissions into atmosphere from fossil fuel combustion. Although leakages from well-engineered storage sites are not expected, the environmental impacts related to potential CO2 seepages are a major issue for the acceptance of this approach and should be carefully monitored. Since different carbon sources have different δ13C values, the aim of the second case study is the evaluation of phytoplankton stable carbon isotopes as a tool for effective early warning of CO2 leakage from CCS. Two culture experiments were conducted under controlled conditions for monitoring δ13C changes in the diatom Thalassiosira rotula during growth in two different media. The isotopic composition of microalgae grown in natural seawater (NAT) was compared to that of diatoms grown in an artificial seawater (ASW), supplied with industrial CO2 and characterised by strongly 13C-depleted dissolved inorganic carbon values. The uptake of inorganic carbon in ASW resulted in a rapid and significant change in microalgae δ13C values, whereas in NAT phytoplankton δ13C did not show important deviations from the starting value, confirming the effectiveness of phytoplankton δ13C as a tool for detecting different CO2 sources. The third case study aims at a better understanding of carbon cycle, as the rate of changes not only depends on human activities, but also on natural biogeochemical processes. Marine dissolved organic matter (DOM) plays a crucial role in oceanic carbon storage, and the extent of its contribution on carbon sequestration depends on its bioavailability. In this study, a plug-flow bioreactor approach has been tested in order to evaluate DOM availability for microbial community in the Gulf of Trieste (Northern Adriatic Sea) by the analysis of dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) degradation and nutrient uptake. The bioreactor approach confirmed to be useful for bioavailable DOC assessment, but further research is needed for confirming its effectiveness for defining DON and DOP bioavailability and nutrient utilisation.

The Ross Sea is one of the most productive areas in the Southern Ocean, and is characterized by a pronounced export of particulate organic carbon to the mesopelagic (up to 50% of surface primary production). This organic carbon downward flux is deeply regulated by its interactions with prokaryotic communities within the entire water column. During austral summer 2017, we performed four microcosm experiments by providing freshly produced algal detritus to free-living prokaryotic communities (

The Northern Adriatic Sea is the northernmost basin of the Mediterranean Sea and one of its most productive areas, characterized by a shallow depth and by a dominant cyclonic circulation. The oceanographic and meteorological parameters show a marked seasonal and interannual variability. The major forcings of the system are represented by the significant river inputs along the Italian coast, by the Eastern Adriatic Current-EAC, which brings high salinity and oligotrophic waters from the southern basin, and by the notable sea-level range, relatively to the Mediterranean area. The NAS is subject to multiple anthropogenic impacts, e.g.: nutrient inputs, coastal urbanization, fishing activity, tourism, and maritime trade. The basin has undergone marked eutrophication followed by a phase of oligotrophication and then by a recent increase in nutrient concentrations. The NAS has also been subjected to frequent development of mucilage aggregates until the first decade of the 2000s. The LTER-Italy parent site NAS currently includes four research sites: the Gulf of Trieste, the Gulf of Venice, the Po Delta and Romagna Coast, and the Senigallia-Susak Transect. At each site meteo-oceanographic and biological data, mainly on plankton, are gathered both during oceanographic cruises and at fixed point observatories. Each site is supervised by a research institution that also manages the system of fixed sensors, which record data in near real-time

The Northern Adriatic Sea is the northernmost basin of the Mediterranean Sea and one of its most productive areas, characterized by a shallow depth and by a dominant cyclonic circulation. The oceanographic and meteorological parameters show a marked seasonal and interannual variability. The major forcings of the system are represented by the significant river inputs along the Italian coast, by the Eastern Adriatic Current-EAC, which brings high salinity and oligotrophic waters from the southern basin, and by the notable sea-level range, relatively to the Mediterranean area. The NAS is subject to multiple anthropogenic impacts, e.g.: nutrient inputs, coastal urbanization, fishing activity, tourism, and maritime trade. The basin has undergone marked eutrophication followed by a phase of oligotrophication and then by a recent increase in nutrient concentrations. The NAS has also been subjected to frequent development of mucilage aggregates until the first decade of the 2000s. The LTER-Italy parent site NAS currently includes four research sites: the Gulf of Trieste, the Gulf of Venice, the Po Delta and Romagna Coast, and the Senigallia-Susak Transect. At each site meteo-oceanographic and biological data, mainly on plankton, are gathered both during oceanographic cruises and at fixed point observatories. Each site is supervised by a research institution that also manages the system of fixed sensors, which record data in near real-time

In the shallow environment, the nutrient and carbon exchanges at the sediment-water interface contribute significantly to determine the trophic status of the whole water column. The intensity of the allochthonous input in a coastal environment subjected to strong anthropogenic pressures determines an increase in the benthic oxygen demand leading to depressed oxygen levels in the bottom waters. Anoxic conditions resulting from organic enrichment can enhance the exchange of nutrients between sediments and the overlying water. In the present study, carbon and nutrient fluxes at the sediment-water interface were measured at two experimental sites, one highly and one moderately contaminated, as reference point. In situ benthic flux measurements of dissolved species (O2, DIC, DOC, N-NO3 (-), N-NO2 (-), N-NH4 (+), P-PO4 (3-), Si-Si(OH)4, H2S) were conducted using benthic chambers. Furthermore, undisturbed sediment cores were collected for analyses of total and organic C, total N, and biopolymeric carbon (carbohydrates, proteins, and lipids) as well as of dissolved species in porewaters and supernatant in order to calculate the diffusive fluxes. The sediments were characterized by suboxic to anoxic conditions with redox values more negative in the highly contaminated site, which was also characterized by higher biopolymeric carbon content (most of all lipids), lower C/N ratios and generally higher diffusive fluxes, which could result in a higher release of contaminants. A great difference was observed between diffusive and in situ benthic fluxes suggesting the enhancing of fluxes by bioturbation and the occurrence of biogeochemically important processes at the sediment-water interface. The multi-contamination of both inorganic and organic pollutants, in the sediments of the Mar Piccolo of Taranto (declared SIN in 1998), potentially transferable to the water column and to the aquatic trophic chain, is of serious concern for its ecological relevance, also considering the widespread fishing and mussel farming activities in the area.