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.

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.

The biological carbon pump (BCP) is a significant part of the global carbon cycle, exporting ∼10Gt of particulate organic carbon (POC) out of the euphotic zone each year. However, most of the exported POC is remineralized biologically within the upper few hundred metres of the mesopelagic, above the permanent thermocline. Gaining more understanding of the factors controlling the BCP is hence important for understanding and predicting the global carbon cycle better. This thesis investigates the BCP in the Iceland Basin, and during an artificial ocean iron fertilisation experiment in the South Atlantic. In the Iceland Basin, export during a spring diatom bloom was tracked using Lagrangian sediment traps and thorium-234 disequilibria. A large pulse of diatom detritus was exported suddenly at the end of the bloom, probably upon impending Si-limitation. The particles were rich in transparent exopolymer particles (TEP, sticky polysaccharides secreted by phytoplankton), and a comparatively large proportion (20–40%) of the exported POC sank past 750 m. This shows that diatom blooms can produce rapid pulses of particle sedimentation that are transferred efficiently through the mesopelagic, and suggests that aggregation and sinking are mediated by TEP. In contrast, alleviating iron limitation in low silicic acid waters of the South Atlantic with very high copepod grazing pressure only caused a modest phytoplankton response and no enhancement of downward particle flux. This was probably primarily due to grazing control and detritus-feeding by copepods, since diatom growth rates were apparently not strongly Si-limited. This suggests that future Fe-fertilisation experiments must investigate the role of zooplankton thoroughly to distinguish beween bottom-up control of export by nutrient concentrations and top-down control by zooplankton. Export measurements based on thorium-234 disequilibria compared well with net community production measured by O2:Ar ratios over the 39d experiment, suggesting that these two methods can be meaningfully compared over ∼month-long cruises. Further work was conducted with a mesoscale array of four time-series sediment traps deployed for eight moths in the Iceland Basin to study particle flux at 2000m. Large, fast-sinking acantharian cysts contributed up to 48% of POC flux during a specific flux event in early spring, demonstrating that the celestite shells of these protists do not necessarily dissolve in the upper mesopelagic as generally believed. The hypothesis is advanced that deep sinking of acantharian reproductive cysts during spring in this region enables juveniles to feed off seasonally sedimenting phytodetritus in the deep-sea. Finally, the full time-series of particle flux in the four deep traps was analysed. Fluxes peaked in late spring and again in mid-summer. Over the eight months, cumulative mass flux varied by 30% between traps without corresponding variation in the cumulative flux of thorium-230, implying genuine mesoscale variability in bathypelagic particle flux. Moreover, during any one of the two-week collection intervals total mass flux of particles varied 2–16-fold between traps, although it is unclear how much of this short-term variability was due to differences in collection efficiency between traps. Overall, the traps probably under-collected thorium-230 in absolute terms by at least 50%, but this estimate is very uncertain.

This study assessed the interaction of coastal defence structures (CDSs), namely shore-parallel ‘low crested breakwater structures’ (LCSs), design features and hydrodynamic regime on the quantity and timings of macroalgae deposition. The employed sampling strategy comprised a spatially and temporally stratitified approach of time-lapse photography of macroalgae deposition, verified with field observations. Field surveys determined the associated ecological assemblages of the sediment infauna and rocky shore epifauna associated with the breakwater scheme, as well as for two nearby beaches with groynes. Further analysis determined decay rates, decay processes, changes in C and N stable isotope values of dominant macroalgal species and the dependence of the faunal assemblages on the decaying macroalgae deposits. Temporal analysis highlights the main factors driving macroalgal deposition were differences in spring and neap tidal range, wave height and sea temperature. Greatest deposition occurred during months of lower wave height and fewer storms, when filamentous red algae and ephemeral green algal species dominated. Results indicated to greater amounts of macroalgae deposits around LCSs than around wooden or granite groynes. Beach elevation best explained the spatial variation, both vertically and horizontally, in the benthic assemblages within the breakwater scheme, with the abundance of detritivorous deposit feeders being significantly correlated with abundance of macroalgae deposits. Orientation of LCSs, relative to wave action, was an important driver of epifaunal assemblages on the CDSs, with the eastward ends of the breakwaters providing the optimum intermediate environment with regards to wave action, exhibiting the greatest abundances of epifauna. Stable isotope analysis showed that the carbon and nitrogen isotopic values of macroalgae changed during the decomposition and were both species and time dependent. Isotope analysis illustrated that decaying macroalgae deposits were of greater trophic importance to species within the LCS ecosystem where there was large macroalgal deposition, than to species within the groyne ecosystem where macroalgal deposition was lower. Key findings of the study illustrate the importance of decaying macroalgae deposits for the local ecosystem via modification of food chain energy flows. Though the ecosystem benefits from this allochthonous resource, deposits may be a nuisance requiring controlled human intervention.

No description available.

The aim of this project was to determine whether otolith trace dement chemistry can be used [Q track migrations in fully marine fish. This question was addressed through a semi-controlled experiment where Irish Sea and North Sea plaice (Pleurononectes platessa) were maintained in a monitored environment. The relationships between water, blood and otolith chemistry were assessed and with reference to environmental and physiological variables and through comparisons of otolith trace clement chemistry in wild plaice tagged by data storage tags (DST).

No description available.

Marine dinoflagellates are an ecologically important group of protists within the plankton, performing key process such as photosynthesis, heterotrophy and toxin production. Some dinoflagellates are also capable of producing bioluminescence and they are the most abundant protists that produce light in the surface waters of the oceans. This study employed molecular tools to investigate the identity of bioluminescent species, the genetic basis, diversity and functional regulation of bioluminescence, and the distribution of marine bioluminescent dinoflagellate populations. Using “universal” and taxon specific PCR primers designed to amplify the luciferase gene (lcf), the distribution of this gene within dinoflagellates was found to be taxon specific and the first lcf sequences obtained from four genetically distant genera revealed a previously unknown high diversity of this gene. The luciferin binding protein gene (lbp) was detected for the first time in three genera of ecologically important gonyaulacoid dinoflagellates, showing that this understudied gene is common in dinoflagellate bioluminescence systems. Phylogenetic analyses of both lcf and lbp provided new insight on species divergence within the toxic genus Alexandrium and pointed out important pitfalls in using protein coding genes for phylogenetic studies. The lcf PCR primers were employed in the first field study using a molecular approach to detect natural populations of bioluminescent dinoflagellates, showing that this approach outperforms optical bioluminescence measurements and that their distribution and composition is intimately tied to hydrographic patterns that create distinct environmental zones. The lcf PCR primers were also employed to study the regional variation in bioluminescence of Noctiluca scintillans, showing for the first time that the environment maintains or eliminates bioluminescence, by finding that a hydrographically isolated non bioluminescent variety in the west coast of the USA has “switched off” its bioluminescence system while diverging to possibly become a different species. Also, a novel lbp was discovered in N. scintillans which improved the current model for the evolution of bioluminescence genes in dinoflagellates. This thesis is the first study to employ molecular tools in a comprehensive and multifaceted investigation of dinoflagellate bioluminescence and has contributed significantly to building a fundamental understanding of this remarkable phenomenon.

The eco-physiological thresholds controlling the distribution of marine invertebrates are of significance in understanding the evolution of marine diversity. This includes the direction of species radiation throughout the oceans. Range expansions occur as a result of evolutionary adaptations, or through environmentally or anthropogenically driven shifts in distribution. The success of such events is centred around a species ability to adapt; in order for a migration to be successful, all life history stages must tolerate the conditions of the new habitat. This thesis examines the thermal and hyperbaric thresholds affecting range extension in the marine environment. It focuses on the larval development of the shallow-water North Atlantic gastropod Buccinum undatum (Linnaeus 1758). The Buccinidae family consists of a wide range of shallow and deepwater species distributed globally. Improved knowledge on this topic will contribute to our understanding of the adaptations influencing both historical and modern shifts in the distribution of species. The thermal and hyperbaric ranges observed during development indicate B. undatum to have the capacity to develop at both temperatures and pressures outside its current distribution. Thermal acclimation to low temperature was also found to increase pressure tolerance during development. A shift in number of embryos developing and nurse egg partitioning per embryo indicate a decrease in developmental success at temperatures above those it is naturally exposed to. An increase in energetic expenditure, with both increasing temperature and pressure, relates to a rise in the metabolic cost associated with development under either condition. These results, combined with the known life history of B. undatum, suggest range expansion into deep water may be a plausible scenario, but tolerance of warmer conditions remains questionable due to the cold-induced spawning observed in this species. The results of this thesis support theories of high-latitude migrations into the deep sea via cold, isothermal waters, and indeed, suggest polar temperatures may promote the rate at which such range expansions occur. Additionally, the observed metabolic cost associated with development suggests hydrostatic pressure may induce bathymetric limits, which explain patterns observed in reproductive trends, eco-physiological adaptations, and faunal distribution in marine invertebrates throughout the oceans.