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The global mid-ocean ridge (MOR) system is ~60 000 km long and accounts for 9% of the seafloor. Deep-sea organisms living on MOR have two potential energy sources; chemosynthesis and the downward flux of photosynthetic organic matter. This study examines the trophodynamics of benthic fauna collected from non-vent sites north and south of the Charlie-Gibb Fracture Zone (CGFZ) on the Mid-Atlantic Ridge (MAR) and hydrothermal vents fields (E2 and E9) on the East Scotia Ridge (ESR) using stable isotopes of carbon (δ13C), nitrogen (δ15N) and sulphur (δ34S). δ13C and δ34S values revealed the MAR benthos was sustained by photosynthetic primary production and no chemosynthetic food source was detected. δ15N values of benthic invertebrates were lower than the surficial sediments at the southern site but this did not occur at the northern site. Benthic invertebrates appeared to comprise a separate food chain to bentho-pelagic fishes and crustaceans but size-based trends in δ13C and δ15N revealed at certain life history stages bentho-pelagic fishes may consume benthic fauna. Size-based trends in δ13C and δ15N trends varied spatially and temporally in some bentho-pelagic fishes, which suggested differences in feeding plasticity among the species. Spatial differences among sites were observed in δ13C, δ15N and δ34S of the ESR vent fauna. These were thought to reflect differences in the vent fluid chemistry, vent derived carbon fixation pathways and incorporation of photosynthetic organic matter into the vent system depending on the species and the magnitude of the difference among sites. Size and sex were important determinants of intra-population variability in stable isotope values of three species of vent fauna but this was not consistent among sites. Abstract ii The present study revealed the importance of undertaking a tri-isotope approach to deep-sea trophic studies in order to elucidate production sources and at different sizes deep-sea organisms can link different trophic pathways.

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Rockall Bank is a large undersea bank situated in the north-east Atlantic. The region features steep bathymetry with the depth of Rockall Trough reaching 3000 m, rising to the surface at Rockall rock itself. Winter convective mixing in this area is strong and can reach 600 m or more. As this is deeper than a significant proportion of the bank, the water column above the bank becomes cooler than in the surrounding area, and a "cold water patch" forms. This water has been observed moving off the slope as a dense water cascade, a process that is important for shelf-ocean exchange and ventilation and which is also biologically important. This research contributes to knowledge of the dense water formation over Rockall Bank by analysing remotely-sensed data as well as numerical model results. Within this study: i) satellite sea surface temperature data were obtained for a ten year time series and the presence and properties of the surface cold water patch were investigated, and ii) the POLCOMS numerical model was adapted to simulate the Rockall Bank under different climatological conditions. It was demonstrated that a surface cold patch could be seen regularly in the satellite SST data, though more often there is a cold front associated with the Bank with no distinct cold patch. The duration of the cold patch and the duration of the front are inversely linked, and the cold patch is seen for longer in winters with a greater winter temperature difference across the Bank. The model successfully simulated the formation of the dense water in three test years, and the results show that the Bank greatly amplifies the effect of changing meteorological conditions. Dense water formation is very sensitive to changes in the winter air temperature, with a 2°C temperature decrease leading to a three-fold increase in the density difference between the water over the Bank and the surrounding area. The model results show a limited amount of cascading during the cooler years, but no cascading at all in the warmer year. Such large changes in the dense water formation caused by a relatively small change in air temperature suggests that climate change will have a significant impact on this system.

Eutrophication of marine and coastal waters is a growing concern throughout Europe's regional seas and an historical problem in the Black Sea and regions of the North-East Atlantic, particularly the North Sea. As the base of the marine pelagic food web, phytoplankton are sensitive indicators of environmental change and therefore may be used as indicators of eutrophication for the monitoring, management and mitigation of the effects of nutrient loading on coastal and marine ecosystems. However, due to the interactive effects of climate and eutrophication, it can be difficult to separate the climatedriven response of phytoplankton from changes induced by excess nutrients. Therefore, the aim of this work is to separate these two signals in order to explore eutrophication effects. Without historical knowledge of 'pristine' or unimpacted ecosystem states it is difficult to identify and assess the severity and magnitude of change. Even w~re spatially and temporally comprehensive ecological datasets are available, equivalent nutrient timeseries are rare and a method of linking phytoplankton dynamics to eutrophication is required. Because open sea ecosystems are less impacted by anthropogenic nutrients than those near shore, offshore regions may be used as reference areas in comparison with coastal systems to investigate the effects of nutrient loading. Changes observed solely in coastal systems are mo$1likely a result of local processes (such as eutrophication) while those observed in both open sea and coastal areas are probably a response to large-scale drivers (such as climate). Therefore the comparison of coastal and open sea data may reveal different (or similar) patterns ofchange in phytoplankton indicators. Throughout most of the North-East Atlantic climate appears to override the effects of nutrients on phytoplankton dynamics, although the two drivers have been found to have synergistic effects resulting in increasing chlorophyll levels in the coastal North Sea. Additionally, the 1980s North-East Atlantic regime shift is clearly visible in coastal and open sea chlorophyll concentrations and diatom and' dinoflagellate abundances, demonstrating the sensitivity of phytoplankton as indicators at both the biomass and functional group scales. In the Black Sea, an observed decrease in chlorophyll appears to be at least partially a result of changes in climate and is not solely attributable to the 'recovery' of the Black Sea ecosystem. Black Sea chlorophyll has also undergone a possible recent (2002) regime shift, although its significance is difficult to determine due to the short time-series of chlorophyll data available. . The successful use of phytoplankton as indicators of eutrophication in these two disparate sea regions at two different ecological scales suggests that the method of comparing coastal and open sea phytoplankton data could be applied to other European seas as a means of distinguishing betWeen the effects of climate and eutrophication.