Carbon cycle in marine systems under different anthropogenic pressure

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.