1 & 2. Euphausiids comprise a major component of ecosystems in the pelagic realm, the world's largest habitat, but basin scale drivers of euphausiids diversity and abundance are poorly understood. Mid-Ocean Ridges are the largest topographical feature in the pelagic realm and their benthic and pelagic fauna have only just recently become the focus of research. This thesis present new analyses on the drivers of euphausiids species richness in the Atlantic and the Pacific, giving specific attention to the influence of Mid- Ocean Ridges. New information is given on the biogeography of euphausiids and pelagic food-web trophology of the Mid-Atlantic Ridge, and on the biogeography of pelagic decapods on the South-West Indian Ocean ridge. 3. A Generalized Additive Model framework was used to explore spatial patterns of variability in euphausiid species richness (from recognized areas of occurrence) and in numerical abundance (from the Continuous Plankton Recorder survey) in conjunction with variability in a suite of biological, physical and environmental parameters on, and at either side of, the Mid-Atlantic Ridge. Euphausiid species richness peaked in midlatitudes and was significantly higher on the ridge than in adjacent waters, but the ridge did not influence numerical abundance in the top 10 m significantly. Sea surface temperature (SST) was the most important single factor influencing both euphausiid numerical abundance (-76.7%) and species richness (34.44%). Dissolved silicate concentration, a proxy for diatom abundance, significantly increased species richness (29.46%). Increases in sea surface height variance, a proxy for mixing, increased the numerical abundance of euphausiids. GAM predictions of variability in species richness as a function of SST and depth of the mixed layer were consistent with present theories, which suggest that pelagic niche-availability is related to the thermal structure of the near surface water. 4. Using a Generalized Additive Model in the Pacific, the main drivers of species richness, in order of decreasing importance, were found to be sea surface temperature (explaining 29.53% in species variability), salinity (20.29%), longitude (-15.01%, species richness decreased from West to East), distance to coast (10.99%), and dissolved silicate concentration (9.03%). An additional linear model poorly predicted numerical abundance. The practical differences in drivers of species richness in the Atlantic and Pacific Ocean were compared. Predictions of future species richness changes in the Pacific and Atlantic were made using projected environmental change from the IPCC A1B climate scenario, suggesting an increase in species richness in temperature latitudes (30° to 60° N and S) and little to no change in low latitudes (20° N to 20° S). 5. New baseline information is presented on biogeography, abundance and vertical distribution of euphausiids along the Mid-Atlantic Ridge (40° to 62° N). 18 species were recorded, with Euphausia krohni and Thysanoessa longicaudata being most abundant. Eight species had not been recorded in the area previously. The Subpolar Front is a northern boundary to some southern species, but not a southern boundary to northern ubiquitous species that show submergence. Four major species assemblages were identified and characterised in terms of spatial distribution and species composition. Numerical abundance was highly variable but decreased by orders of magnitude with depth. The Mid-Atlantic Ridge showed only a marginal effect on euphausiid distribution and abundance patterns. 6. Zooplankton and micronektic invertebrate epi- and mesopelagic (0-200 and 200- 800m) vertical distribution (e.g. Euphausiacea, Decapoda, Amphipoda, Thecosomata, Lophogastrida) on either side of the Subpolar Front of the Mid-Atlantic Ridge is described. Dietary relationships are explored, using stable isotope ratios and fatty acid trophic marker (FATM) composition. An increase in trophic level with size was observed. Individuals from southern stations were higher in dinoflagellate Fatty Acid Trophic Markers (FATM) (22:6(n-3)) and individuals from northern stations were higher in Calanus spp and storage FATMs (20:1(n-9) and 22:1(n-9)) reflecting primary production patterns in the two survey sectors. Observations on the geographical and vertical variability in trophodynamics are discussed. 7. New baseline information is presented on the biogeography, abundance, and vertical distribution of mesopelagic (200-1000 m), crustacean micronekton on- and offseamounts of the South-West Indian Ocean Ridge (26° to 42° S). Species richness and numerical abundance were typically higher near seamounts and lower over the abyssal plains, with several species being caught uniquely on seamounts. Observations suggest that the ‘oasis effect' of seamounts conventionally associated with higher trophic levels is also applicable to pelagic micronektic crustaceans at lower trophic levels. Biophysical coupling of micronekton to seamounts may be an important factor controlling benthopelagic coupling in seamount food-webs. 8. Euphausiid and pelagic diversity is driven primarily by geographical variability in temperature, by longitudinal patterns in upwellings, and by variability in nutrient concentration. Mid-Ocean Ridges modify pelagic ecology, by raising the seafloor and by bringing in proximity true pelagic and bathypelagic predators associated with the seabed. The increase in specialized fauna and biomass associated with ridges and seamounts serves to deplete zooplankton in the near bottom layer (0-200 m) and affect systems in and above the benthic boundary layer (<200 m from the seafloor), and the benthopelagic faunal layer. Mid-Ocean Ridges may serve to structure pelagic faunal distribution, and increase the overall diversity of the world ocean. The influence of ridges in the ocean basin may be comparable to that of hedges in a farmland; whilst delimiting the extent of crops (or zooplankton assemblages), hedges serve as local hotspots of mammal and avian diversity.

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How anthropogenic changes to species composition and diversity are likely to affect the properties of the ecosystems of which they are an integral part, and by extension the goods and services humans derive from them, is a key question in ecology. Despite over a decade of vigorous empirical research and theoretical developments, there remain many unknowns. Using intertidal rockpools and laboratory marine mesocosrns, I used a variety of approaches to address several of these relatively poorly studied issues. In particular, the work presented here focused on the relative roles of species composition and richness, as well as the extent to which such effects are context-dependent. The first study (Chapter II) takes advantage of a successional gradient of macroalgal species composition and diversity resulting from the periodic addition of artificial rockpools to a coastal defense structure. The results show that the focal ecosystem properties (macroalgal biomass and productivity) were largely determined by species composition (and functional traits). Macroalgal species evenness, but not diversity, peaked at intermediate stages during the chronosequence, but no measure of diversity had a detectable influence on primary productivity. The results confirm the prediction that effects of species diversity will be outweighed by compositional changes during succession. I used an experimental approach in Chapters III to V, manipulating the composition and richness of intertidal molluscan grazers (Chapters III and V) and intertidal predatory crabs (Chapter IV) and measuring their effects on prey assemblages as focal ecosystem processes. In a 13-month field experiment (Chapter III) I found that effects on the composition and functioning of developing rockpool communities were determined by grazer composition, not the number of species. Laboratory mesocosm experiments show that the influence of species richness on ecosystem processes can be context-dependent. The effect of resource partitioning (of the multi-species prey assemblage) among predators was only detectable at high predator densities where competitive interactions between individual predators were magnified. A factorial experiment using the rate of algal consumption by molluscan grazers as a response variable, provides the first empirical test of the prediction that the balance between species richness and identity effects can be determined by the degree of spatial heterogeneity (Chapter V). Species identity had strong effects on homogeneous substrates, with the identity of the best-performing species dependent on the substrate. The strengths and limitations of the predominantly small-scale experimental approach employed here are discussed (Chapter VI).

Coral reefs are the iconic ecosystem of tropical seas and yet they are under increasing pressure as a result of multiple climatic stressors. This thesis uses observations and models to further understanding of environmental impacts on coral reefs. In particular it examines the impact of rising Sea Surface Temperature (SST) and ocean acidification on coral growth and the frequency of coral bleaching events. UK ocean biogeochemical models are assessed for implementation in the next UK Earth System Model. This analysis finds little evidence that more complex ocean biogeochemical models provide better simulations of large scale biogeochemical features. An established wavelet-based spatial comparison technique is used to analyse the spatial scales that Earth System Models can skillfully simulate patterns of SSTs. It is shown that in coral regions, current models cannot skilfully simulate patterns of historical SST anomalies at sub-regional (<32◦) scales. These findings are used in combination with SST and aragonite saturation state outputs from Earth System Models to show that historical Caribbean coral growth has been influenced by anthropogenic aerosol emissions over the 20th Century. Earth System Model outputs are also used to make projections of global coral bleaching throughout the 21st Century. It is shown that under even the most extreme conventional mitigation scenarios the majority of the world’s coral reefs are projected to experience levels of thermal stress induced bleaching that cause reef degradation throughout the 21st Century. Geoengeering scenarios involving the injection of SO2 into the stratosphere can reduce the projected thermal stress on coral reefs relative to conventional mitigation scenarios but such benefits are shown to be highly dependent on the sensitivity of coral bleaching thresholds to ocean acidification.

Macrofauna are known to have a significant effect on intertidal sediment stability and biogeochemical properties. A series of manipulative in situ mudflat studies at Breydon Water, Great Yarmouth, UK investigated the effect of biodiversity on selected biogeochemical sedimentary properties related to mudflat sediment stability including the sediment erosion threshold and relative erosion rate, microphytobenthos biomass and health, sediment particle size and size distribution, sediment water content, chlorophyll a and b concentration, and colloidal carbohydrate concentration. Mudflat sediment macrofaunal biomass was removed using cryo-defaunation and the abundances of three common mudflat species Hediste diversicolor, Hydrobia ulvae and Corophium volutator manipulated to examine different aspects of macrofaunal biodiversity including species identity, density, biomass distribution, and richness. An additional laboratory study enabled two and three dimensional high resolution visualisation of fluid and particle mixing as a result of organism sediment bioturbation. Species identity was found to have a significant effect on sediment properties. The three species have distinct bioturbatory actions with consequences for sediment stability. In some circumstances a single organism was found to have as great an effect on selected ecosystem processes as a whole community. Variations in species density significantly changed the effect of the species on the measured sedimentary processes. Species richness effects were negatively interactive, with species mixtures underyielding in comparison to their monoculture counterparts. Changes in species biomass distribution and richness resulted in significant context dependent changes to sediment properties, moderated by inter- and intraspecific interactions. Species were also observed to exhibit a functional abundance threshold, below which they did not contribute significantly to ecosystem processes. Temporal and spatial variability observed in the experiments emphasised the potential of environmental and abiotic factors to also influence ecosystem processes. Investigating these subtle aspects of biodiversity will be key in the determination of the relationship between biodiversity and ecosystem processes.

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The purpose of the present study, of which some of the results are here presented, was to investigate and describe the Upper Estuarine Series and its sediments. It has been shown that the Series is best described as a bipartite formation rather than tripartite, as has been the case until now. The two divisions have been named, the Lower Freshwater Sequence, and the Rhythmic Sequence. Both names describe the general character of the divisions involved. The former name retains its original connotation. it can be demonstrated that the upper two thirds of the Series which have been celled until now, the Upper Estuarine Limestone, and the beds above the Upper Estuarine Limestone, are essentially rhythmic in character. Both limestones and clays have a repetitive sedimentary form, which is generally an upward change from a laminated texture to a homogeneous texture. Limestones may be developed in any of the rhythms, but are more common in the higher ones. The position within a rhythm determines the limestones characterestics. Those limestones which are developed at the base tend, like the crays, to be laminated, when they occur at the top of a rhythm they tend to be bioturbated and homogeneous in texture. The lowest rhythm which has been traced wherever the Upper Estuarine Series overlies the Lincolnshire Limestone is invariably marked by the occurrence or Lingula kestevenensis within the lower two feet. Lingula has never been found above this horizon. Upwards there is evidence of a passage sequence of sediments into the Great Oolite Limestone. The occurrence of a lacustrine-deltaic complex in the Lower Freshwater Sequence can be demonstrated, and shows "estuarine" to be a misnomer in the strictest sense. The ostracod fauna is described and there is evidence that, although marine conditions were dominant, it is likely that at times freshwater and brackish conditions influenced the Rhythmic Sequence. A study of the clay miner logy has shown that changes occur upwards through the two rhythms investigated, and it is considered that leaching during upper Estuarine time was the cause. An occurrence of echinoderms in the limestones at Blisworth has been studied and their preservation has been found to reflect the mode of death and burial, and has also shed some light on the conditions of deposition in the rimes tones. The thick, well developed, limestones of the Upper Estuarine Series have been studied and it has been found that lateral sedimentation played an important role in their deposition. Directional indicators are scarce in the upper Estuarine Series, but when they are present they give every indication of a southerly derivation for the sediment, both for the Lower Freshwater Sequence and the Rhythmic Sequence.

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