No description available.

No description available.

The dinoflagellate Neoceratium is frequently observed in oligotrophic subtropical gyres (OSGs) where major inorganic nutrients such as nitrogen and phosphorus are depleted in the surface waters. In Chapter 2, continuous plankton recorder (CPR) data demonstrate that Neoceratium inhabit surface waters of the North Atlantic subtropical gyre (NAG) throughout the year. The presence of Neoceratium in surface waters contrasts with the fact that nutrients are severely depleted in the surface 100 to 130 m. In Chapter 3, it is shown that Neoceratium cannot grow when exposed to low nutrient concentrations comparable to those found in surface waters of OSGs. However, cells are able to survive and re-establish growth after a maximum of >3 weeks of “nutrient starvation”, with signs of stress becoming noticeable after 10 days. In Chapter 4, nutrient ratios in large areas of surface waters of OSGs in the Atlantic are shown to be favourable to mixotrophy (86 % of sites in the North and South Atlantic OSGs). It is estimated that 0.01 to 0.44 ciliates per day would need to be ingested for Neoceratium to survive in these waters. In Chapter 5, growth was only observed when cells were exposed to a range of irradiances (6 to 60 μmol quanta m-2 s-1), above 22 μmol quanta m-2 s-1; an irradiance that is typically only found 15 to 22 metres above the nutricline at midday, implying that VM would be required to access the nutricline. A mechanism for VM in OSGs is suggested whereby vertical movement is triggered by phosphate-limitation of cells. In Chapter 6, the thesis concludes with a conceptual model to explain how a number of the above findings likely interact to enable Neoceratium to successfully survive and grow in OSGs.

Taxonomy and temporal variability of deep-sea polychaete assemblages was assessed over a 9-year period. Macrofauna 300 μm fraction samples, taken with USNEL box core (0.25 m2), were studied from Porcupine Abyssal Plain, NE Atlantic Ocean from 8 cruises between August 1989 and September 1998. A taxonomic study at species level was carried out for the two most abundant families: Cirratulidae and Spionidae. 15 different morphotypes of Cirratulidae and 13 of Spionidae were recognized and described. For Cirratulidae eight morphotypes belonged to Chaetozone, there were six species of Aphelochaeta and one of Tharyx. For Spionidae three morphotypes belonged to Minuspio, two to Prionospio, one to Aquilaspio, two to Laonice, two to Spiophanes and two to indeterminate spionids. Aurospio dibranchiata also was recorded. The polychaete communities were characterized by high numbers of individuals (abundance) and high family richness. Highest abundance occurred in the upper 1 cm sediment layer (53.2% of total abundance). The most abundant families were the Cirratulidae, Spionidae, Opheliidae and Paraonidae. Surface deposit-feeders were the dominant trophic group (67.4% of total abundance). Significant temporal variability was evident with significant differences in polychaete abundance between sampling periods (cruises). There were stepwise increases in abundance in September 1996 and March 1997 coinciding with similar increases in abundance in large invertebrates (megafauna) in the same area (known as the ‘Amperima Event’ after a species of holothurian that increased in abundance by over three orders of magnitude). A similar trend was observed for abundance within different layers of the sediment, main families and trophic groups showing significant differences between cruises. A comparison made of samples taken 1) before the ‘Amperima Event’ (1989-1994) and 2) during the 'Amperima Event' (1996-1998) showed significant differences in the polychaete abundance in the upper 3 cm of the sediment. There were significant differences in some trophic groups (predators, deposit-feeders and burrowers) and the dominant families (Cirratulidae, Spionidae and Opheliidae). Changes in surface deposit feeders were particularly evident. The temporal variability is likely to be related to seasonal and interannual variability in organic matter input. Greater food supply in some years may allow the growth and development of deposit feeding polychaetes. However, not all elements of the polychaete community showed a response (e.g. the Paraonidae). At the species level, the most abundant cirratulid and spionid species not always appear to respond in the same way as the family. Only Aphelochaeta sp. 647D, Minuspio sp. 4 and Prionospio sp. 81 showed a clear response, with significant differences between cruises and between pre ‘Amperima Event’ and ‘Amperima Event’ periods. Chaetozone sp. 1, Chaetozone sp. 55A and Prionospio sp. 613 only showed significant differences between cruises, while Aphelochaeta sp. 13A and Aurospio dibranchiata did not show any significant change with time. In the Paraonidae, where no apparent response was detected, the species level response in the most abundant species was similar. Temporal changes in some polychaete species could be attributed to ‘Amperima Event’ conditions. However, for polychaete species that did not response in a clear way to the ‘Amperima Event’, their temporal variability observed appear to be related to interannual variations in organic matter input to the seabed throughout the deposition of phytodetritus. In general, seasonal and interannual fluxes in food supply appear to determine changes in polychaete assemblages at the Porcupine Abyssal Plain, affecting to a greater degree polychaete abundance, and to a lesser extent faunal composition.

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Effects of large-scale topographic features on the benthic boundary layer (BBL) flow characteristics and on sediment transport are analysed in a natural shallow coastal site and in a laboratory flume. The main signature of flow/bedform interaction processes consists of largescale flow structures, generated downstream of the bedforms; these are characterised by alternating modules of lower and higher streamwise speeds than the mean flow. These structures modify the suspended sediment transport, generating high turbidity levels within macro-scale features of low flow speed, and the bedload transport, generating more frequent and larger bedload movements during macro-scale features of high flow speeds. These particularities result from variations in the turbulent bursting process within the macroturbulence structures. Ejection events generate increases in suspended sediment concentration; these are more frequent and more intense within the macro-scale structures of low flow speed. Sweeps are shown to generate most of the bedload transport; these events are dominant and are magnified during modules of increasing horizontal velocity. Enhanced (negative) contribution of outward and inward interaction events to the Reynolds stress, downstream of large-scale bedforms, compared to those reported for uniform BBLs, are observed in the field and laboratory investigations. This induces 'abnormally' low Reynolds stress values, which are believed to invalidate the use of the Reynolds stress parameter to predict sediment transport. Other aspects of the turbulence are little affected by the presence of bedforms upstream, such as the coupling between the turbulent velocities and the Reynolds stress, or, at the field site, the turbulent energy dissipation rate.

Patterns in polychaete abundance, body size and diversity were investigated at 12 sites between 4300 and 5000 m in the central Pacific and the north-east Atlantic. In the central Pacific, three of the sites (EqPac 0N, 2N and 5N) were observed to lie under high surface productivity regimes, and they were known to receive significant accumulations of food-rich phytodetrital material. The EqPac 9N, HOT 23N, DOMES A, ECHO 1 and PRA sites, which did not receive phytodetritus, were used as control sites with which to investigate the effect of this phytodetrital input. In the north Atlantic, one of the sites (PAP) was known to receive phytodetrital input, and one of the sites (MAP) had been subjected to a large-scale natural disturbance in the form of a turbidite emplacement. All specimens were identified to species level. Two families, the Pilargidae and Cirratulidae were selected for a more detailed alpha taxonomy assessment. New characters were developed for the identification of cirratulid thoracic fragments. Benthic polychaete abundance was correlated with surface productivity in both the Pacific and Atlantic Oceans. There was some evidence to suggest that there was a stronger benthic-pelagic link in the Pacific Ocean, where small changes in surface productivity generated larger changes in abundance than in the Atlantic Ocean. When data from previous studies are included, it is suggested that at levels of surface productivity above 200 gCm'2yr4, there is an upper limit to benthic polychaete abundance. Significant differences in body size between sites were found at species level, family level and for the entire polychaete taxon. At a species level, several abundant cosmopolitan deposit feeding species showed reduced body size in the food-rich phytodetrital sites. The only species to show increased body size in the food-rich sites were two predatory species. Polychaetes in the Atlantic Ocean responded more strongly in terms of body size reduction in phytodetrital sites than they did in the EqPac sites. Three hypotheses were put forward to explain these patterns: increased metabolic efficiency of large organisms in food-poor regions, seasonal recruitment pulses at phytodetrital sites and increased competition at phytodetrital sites. The former was favoured as the most likely explanation. Species diversity was shown to be highest in the phytodetrital sites in the central Pacific. It was hypothesised that this was the result of increased productivity, increased spatio-temporal heterogeneity and increased sediment heterogeneity at these sites. An increase in species diversity at phytodetrital Atlantic sites was not observed. The differences between north Atlantic and central Pacific sites were attributed to regional enrichment of local diversity in the Pacific. At the MAP turbidite site, alpha diversity was significantly lower than at other sites, and dominance was high, indicating the potential for large-scale natural disturbance in the abyss. A new spatio-temporal scale of disturbance was highlighted that may have evolutionary as well as ecological significance. Although levels of alpha diversity were generally shown to be high in the abyss compared to shallow water, the evidence did not suggest in favour of high beta diversity on scales of 1000 to 3000 km in either the central Pacific or north Atlantic. The low levels of beta diversity observed suggest that total species richness in the deep sea may not be as high as previously hypothesised, and that regional processes are likely to significantly impact local ecology in the deep-sea.

The predicted cycling of the whole ocean through hydrothermal plumes is comparable to the mixing time of the oceans (few thousand years). Hence, understanding hydrothermal plume processes is crucial if their impact on the global geochemical cycles of elements is to be assessed. One of the most important processes that has been demonstrated to modify the gross chemical flux from venting to the oceans is the oxidative precipitation of dissolved Fe (II). It has been hypothesised that this might vary significantly from one ocean basin to another along the path of thermohaline circulation. To test that hypothesis, hydrothermal plume samples were collected from the first confirmed hydrothermal vent fields in the Indian Ocean, at Kairei and Edmond, close to the Rodriguez Triple Junction, during the RRS Charles Darwin cruise CD128 in 2001. The samples were analysed to determine the concentrations of dissolved iron and manganese and particulate Fe, Mn, Al, Ca, Mg, Cu, Zn, P, V, As, Y and the rare earth elements. For a subset of the samples, the concentrations of Fe, Mn, Cu and P in different size fractions of the particulate phase were also measured. Dissolved Fe and Mn concentrations are high in the Kairei and Edmond hydrothermal plumes compared to Atlantic and Pacific hydrothermal plumes previously studied. Particulate Fe concentrations are also high while particulate Mn concentrations remain low throughout the plume. Of the total (i.e. particulate plus dissolved) Fe which emerges from the vents, approximately 20-30% is lost from the plume via the removal of Fesulfide phases formed early in the buoyant plume. Further loss of Fe due to the oxidation and formation of particulate Fe-oxide phases results in a total Fe loss of 50-70%. For the very young non-buoyant plume samples, there is very little in situ particulate Fe present. The behaviour of the chalcophile elements (Cu, Zn, Cd and Pb), elements which exist as oxyanions in seawater (P, V and As), as well as the rare earth elements and Y are consistent with previous studies of elemental behaviours in hydrothermal plumes in the Atlantic and Pacific Oceans. The observed behaviours of these elements with respect to particulate Fe suggests that the differing Fe (II) oxidation rates between ocean basins do not impact the processes taking place within hydrothermal plumes. In addition, fractionation of Fe, Mn, Cu and P in the Edmond hydrothermal plume between the dissolved, colloidal and fine and coarse particulate phases shows consistency with previous conclusions based on only one of these phases. The P/Fe and V/Fe ratios of the hydrothermal particulate samples are intermediate to those of particulate samples from the Atlantic and Pacific Oceans suggesting that as previously hypothesised, these ratios are dependent on dissolved ambient phosphate concentrations. Hence, there remains the potential to use these ratios from sediment cores as paleo-proxies for dissolved phosphate concentrations.

In temperate shelf seas, a deep chlorophyll maximum (DCM) persists within the thermocline during summer stratification. This study explores the significance of primary production (PP) within the DCM and provides a detailed investigation into the mechanisms of phytoplankton adaptation to this tidally dynamic and dimly-lit environment. Comparison is drawn to the DCM in a range of hydrographic regimes in the open ocean. Data are presented from two cruises in the Celtic Sea during summer (2003 and 2005) and one from the Atlantic Ocean (AMT15, 2004). Phytoplankton physiological measurements were obtained from 14C Photosynthesis vs. Irradiance (P vs. E) experiments and a bench-top Fast Repetition Rate Fluorometer (FRRF). Water-column profiles of PP were empirically modelled using P vs. E parameters and measurements of spectral in situ irradiance. Across the Celtic Sea shelf the DCM was located towards the base of the thermocline and was tightly coupled to the nitracline. The thermocline is presented as a 3-layer system, the top and bottom layers dominated by physical mixing and a mid-layer which is relatively stable and occurs at the top of the nitracline. Nitrate flux into the thermocline from the BML was maximal at spring tides and could support almost all the PP in the thermocline. It appears that nitrate supply and utilisation is roughly balanced over spring - neap timescales. Within the shelf sea thermocline, layering of phytoplankton taxonomy was observed, with the maxima of cell concentrations decoupled from those of cellular pigment concentrations. The FRRF-derived effective absorption cross section of photosystem II (σPSII) and photosynthetic efficiency (Fv/Fm) co-varied with pigment composition, and did not appear to contribute to photo-acclimation. Data indicated that the mechanism of photo-acclimation was by the number, rather than size, of PSII reaction centres. In contrast to the surface mixed layer, phytoplankton at the DCM remained light limited and PP in the thermocline was more sensitive to incident irradiance than in the surface. In the Celtic Sea, the potential range in water-column integrated PP between a cloudy and sunny day was greater than the spatial range in PP across the region. PP within the thermocline contributed 28 – 84 % of vertically-integrated daily PP. The mechanism of photoacclimation appeared to be consistent between the shelf sea and open ocean environments. Throughout the Atlantic Ocean PP in the DCM contributed between 15 – 80 % of total water column production. It is shown that physical – biological coupling is a major determinant on phytoplankton taxonomy, physiology and productivity in the DCM.

Lipids are a group of biologically important compounds that may be used as indicators of relationships between an organism and its environment. Lipid class composition identifies types of storage lipid which can suggest a pelagic or benthic existence and may be used to measure the condition of an organism. Fatty acid composition may reflect trophic ecology. Lipid profiling has been used to elucidate elements of the ecology of organisms from Mid-Atlantic and Juan de Fuca Ridge hydrothermal vents and shrimp from below a permanent oxygen minimum zone. Lipid profiles of these organisms from extreme deep-sea environments are compared. Lipid profiles of adult Mid-Atlantic Ridge hydrothermal vent shrimp, Rimicaris exoculata. support a benthic existence and primarily chemoautotrophic bacterial nutrition. Postlarval alvinocarid shrimp lipid profiles suggest a pelagic life cycle stage with a reliance on phototrophically derived organic matter. This result supports previous molecular evidence that Rimicaris exoculata travels between hydrothermal vents on the Mid-Atlantic Ridge. The branchial area of Rimicaris exoculata has been reported to be colonised with bacteria and elevated levels of bacterial fatty acids are present in these tissues. Lipid analyses of the reproductive organs of Rimicaris exoculata reveal increases in triglycerides, sterols, phosphatidyl choline and the proportion of n-3 fatty acids with advancing reproductive maturity. Preliminary studies of the lipid profiles of the Mid-Atlantic vent shrimp Alvinocaris markensis support the hypothesis that it is a scavenger and contains a lower proportion of bacterially derived fatty acids. The lipid profiles of the deep-sea shrimp, Nematocarcinus gracilis, from the Indian Ocean, were consistent with a benthic lifestyle, opportunistic feeding and a reliance on phototrophically derived organic matter. Despite the presence of a permanent oxygen minimum zone above the habitat of the shrimp, Nematocarcinus gracilis contain low levels of highly unsaturated fatty acids, reflecting a reduction in the availability of labile organic matter with depth. The hydrothermal vent tube worm Ridgeia piscesae contains only low levels of storage lipid, reflecting an extreme reliance on endosymbiotic bacteria. Wax esters detected are thought to be stored in oocytes. The fatty acid profile of Ridgeia piscesae reveal mainly bacterial biomarker fatty acids, but some phototrophically derived fatty acids are also present. Adult Ridgeia piscesae have no gut, so the mechanism by which these fatty acids are assimilated is uncertain. Lipid profiles of the hydrothermal vent palm worm, Paralvinella palmiformis vary with the conditions of the different microenvironments in which specimens were found. Fatty acids reflect a mixed diet based mainly on chemoautotrophic bacteria, but with significant inputs of phototrophically derived organic matter, such as diatom debris. Phototrophically derived fatty acids were also detected in the hydrothermal vent clam Calyptogena pacifica. suggesting that the reduced filter-feeding ability of this species is used to supplement nutrition from endosymbiotic bacteria. Endosymbiont-bearing gill tissues contain the highest proportion of bacterial fatty acids and also high concentrations of triglycerides. The presence of triglycerides in gill tissues suggests that energy may be transferred from symbionts by hydrolysis of bacterial symbiont membrane lipids and their conversion to triglyceride. The lipid profiles presented in this work increase understanding of the life history strategies and ecology of the species studied. This thesis shows that even the most highly adapted hydrothermal vent organisms do not rely solely on a chemoautotrophic source of nutrition, but also appear to require phototrophically derived elements in their diet.