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A 5km swath-width SeaMARC I sidescan sonar survey, conducted over the zone of overlap between the southern rift zone of Axial Volcano and the northern tip of the Vance spreading segment on the Juan de Fuca Ridge (between 45°24'N and 45°50'N latitude), was analyzed to locate the present position of the Juan de Fuca spreading axis, and to determine the tectonic and volcanic structure of the seafloor. Sidescan data were processed in concert with the ship's Loran-C navigation to construct navigated, orthorectified mosaics of the sidescan imagery. In order to navigate the sidescan swaths, a simple numerical model was developed to describe the tracking behavior of the towed sidescan vehicle. Successive positions and orientations of the sidescan towfish were estimated, and were used to assign latitude/longitude values to individual sidescan pixels. Navigated sidescan pixels were mapped by computer onto an absolute (latitude/longitude) reference grid, and the resulting sidescan mosaic was compared directly to existing high-resolution SeaBeam bathymetry in order to discriminate the effects of large- and small-scale roughness on the observed backscatter distribution. The Juan de Fuca spreading axis between 45°25'N and 45°39'N is located within the axial valley of the Vance segment. Relative age relationships, based on crosscutting and superposition principles, indicate that the most recent volcanism within the axial valley has occurred along the valley's central ridge, and that the most recent resolvable extension within the axial valley has been concentrated between the central ridge and west valley wall. The Vance segment terminates at 45°39'N, and is not associated with a transform fault. The south rift zone of Axial volcano is a constructional volcanic feature that is not faulted, and a discrete axis of spreading over the south flank of Axial volcano is not resolvable in the sidescan imagery; however, the spreading locus north of 45°39'N is constrained to a zone between 130°06'W and 129°54'W. The lack of a well-defined spreading axis north of 45°39'N indicates that the physical manifestation of the divergent plate boundary has been modified or masked by hotspot volcanic processes associated with Axial volcano such that a definitive locus of spreading is not expressed in the surface morphology. / Graduation date: 1989

Hydrothermal vents are deep-sea hot springs. Vents are home to luxuriant assemblages of animals that colonize the warm venting fluids. High biomass is fed by microbes that use hydrogen sulphide and other reduced chemicals in the vent fluid as an energy source to fix inorganic carbon. Individual vents may persist for a few years to several decades. The specialized animals must find new vents, cope with changing fluid conditions and foster their offspring. The composition and structure of vent communities vary in space and time. My research at Axial Volcano, a seamount on the Juan de Fuca Ridge (JdFR) in the northeast Pacific, aims to find pattern in this variation and to propose viable hypotheses of the mechanisms driving the patterns. Axial is an ideal location as it supports mature vent fields (venting for over 15 years) and young, developing vents initiated by a volcanic eruption in 1998. Thus, I was able to study both temporal and spatial variation in vent communities at the same site and relate patterns of developing assemblages to patterns observed at longer-lived vents. Pattern detection is the first critical step in any community ecology study as it justifies and focuses the search for process. I have refined existing statistical methods and developed novel techniques to test for pattern in vent species distributions and abundances. I modified an existing null model approach and showed that species distributions among sixteen vents differ from random in a long-lived (>15 years) vent field. I also developed a novel null model to confirm that initial patterns of community assembly seven months following the Axial eruption differ from random recruitment of species and individuals to new vents. My description of the community response to the Axial eruption is the first quantitative report of patterns of vent colonization and succession. My work documents that new vents are colonized quickly (within months) and that initial assemblages are variable. However, rapid community transitions and species replacements within the first few years cause new assemblages to resemble mature vents by 2.5 years post-eruption. Three habitat factors correlate with the development of nascent vent assemblages: the recruitment timing of the tubeworm Ridgeia piscesae post-eruption, vent age and vent fluid hydrogen sulphide content. I also describe a new polynoid polychaete discovered colonizing the new vents in high densities. My major contribution to vent community ecology is revealing species patterns through extensive sampling and rigorous statistical methods. These patterns are a necessary step towards understanding the processes that structure vent communities: they direct future research effort towards the key species and generate hypotheses to be experimentally tested. My work also elucidates how vent species respond to habitat destruction and creation, which is critical information for effectively managing Canada's only hydrothermal vent Marine Protected Area on the JdFR. / Graduate

This work seeks to explore current ecological theory through application to communities inhabiting hydrothermal vents. This thesis aims to: (1) add to and synthesise knowledge of species and their distributions at the intra- and intersegment scale; and (2) evaluate vent community patterns and speculate on processes. Samples used are submersible grabs of low temperature (<60°C) tubeworm assemblages on basalt and sulphide surfaces. Species abundances and distributions on three segments of the Juan de Fuca Ridge (Axial, Cleft, and CoAxial) are described. Community descriptors such as species density, Simpson's and the Shannon-Wiener diversity indices, evenness, species richness, species abundance-distribution models, species percent-average relative abundance and density are used. Vent community structure is compared among segments using these descriptors, visual descriptions, pairwise correlations, Friedman tests of distributions, cluster and correspondence analysis, rarefaction, complementarity, a test for saturation, and Whittaker's beta diversity. Vent community composition on Axial, north Cleft, and CoAxial is similar at the segment and inter-segment scale. The limpet Lepetodrilus fucensis is the most abundant species at all sites. Differences among communities are best seen temporally, not spatially. Senescent communities can be distinguished from active vent assemblages. Pioneer communities, however, are statistically indistinguishable from intermediate communities when sampled two or more years post-eruption. Axial and Cleft species dispersion fits the core-satellite hypothesis. The exceptions are the polynoids Branchinotogluma sp., Lepidonotopodium piscesae, and Levensteiniella kincaidi, which are widespread and present in low local abundances. Both local and mesoscale regional mechanisms explain observed local diversity. Spatial isolation, not habitat differences, influences between-habitat diversity (beta diversity) on Axial, Cleft, and all three segments combined. Meiofauna are important for species richness estimates, identifying differences among structurally similar communities, and understanding input/output between vents and the deep-sea. Measurements such as species richness and diversity indices may be poor at distinguishing among vent communities because vents are species poor and uneven. The Michaelis-Menten, Jackknife 2, and Chao 2 nonparametric vent species richness estimators perform well with small samples. Vent communities should be compared to habitats of similar diversity and evenness as well as disturbance and productivity regimes. Candidate comparison communities include communities in early successional states, selected taxocenes such as carabid beetles on fungi, or high disturbance and/or low diversity systems like the rocky intertidal, organically polluted sediments and oxygen minimum zones below upwelling regions in the deep-sea. / Graduate

Sediments from the northern end of Juan de Fuca Ridge are Late Quaternary in age and contain widely correlatable cycles of turbidity current and hemipelagic sedimentation. Sediments from the Ridge were examined for their mineralogy, structure, components of the sand fraction, rates of sedimentation and grain size distribution to establish processes of sedimentation, stratigraphy, correlation and local hydrothermal relationships. Ten gravity and Phleger core sites along two profiles of the Ridge were examined in detail, one section was perpendicular to West Valley, the main spreading centre, and one section was within and parallel to West Valley. Sediment from Cascadia Basin was compared to the results of the Ridge study. Changes in sedimentation defined by core X-radiograph structure, components of the sand fraction and grain size distribution, indicated cycles of relatively coarse sediment overlain by finer bioturbated sediment with a repeated stratigraphic relationship in all but one Juan de Fuca Ridge core. Changes in sediment composition are attributed to brief, episodic, continent derived turbidity current deposition followed by lengthy periods of hemipelagic sedimentation for each cycle. Differences in composition exist between sediment of ridges and valleys, with a greater winnowed foraminiferal-hemipelagic and a lesser turbidity current influence in the former area. Radiocarbon dated foraminiferal-rich intervals from ridge sediments were exclusively Late Pleistocene with Middle Ridge sediment having an inferred 9000-9500 B.P. Late Pleistocene-Holocene boundary. Similar sedimentation cycles between Middle Ridge and valley localities enabled correlation of ridge and valley stratigraphy and the Late Pleistocene-Holocene boundary. A stratigraphic relationship based on the episodic deposition of continent derived turbidites exists between the northern end of Juan de Fuca Ridge and the continental Pacific Northwest. Pulses of turbidity current sedimentation coincide with initial interglacial warming trends during the Late Pleistocene. Holocene sedimentation for Juan de Fuca Ridge is of hemipelagic origin with rare local turbidity current deposition. Hydrothermal minerals were not detected. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Three gastropods occupy a range of habitats along gradients in hydrothermal flux at Juan de Fuca Ridge vents. I examined how these species co-exist and identified mechanisms driving their abundances. First, I measured temperatures and spatial patterns in adult densities of the three species at three distances from vents to test if thermal regime relates to their habitat selection. Lepetodrilus fucensis and Depressigyra globulus were most dense in-vent (0-25 cm) at variable temperatures (10+5°C): 2100 and 240 incl. dm -2 (respectively). Provanna variabilis was most abundant far-vent (51-75 cm: 60 ind. dm 2) at stable temperatures (3±0.5°C). Thermal conditions are key in their habitat selection: behavioural experiments showed that these gastropods select fluid temperatures<18°C. L.fucensis and D. globulus preferred 5-15°C, while P. variabilis preferred 4-12°C. The next studies sought to explain how Lepetodrilus fucensis reaches order of magnitude higher densities in comparison to other gastropods. First. I quantified L. fucensis recruitment and sex ratio patterns to identify innovative life history traits. I meaured size structure and density at in- and far-vent locations. Early postlarval juveniles occupied far-vent at remarkable densities (2419 ind. dm-2). To test for sex ratio biases, I sexed animals from different habitats and sizes. Populations nearest vents hosted the largest females (>6.0 mm), while peripheral habitats were male-biased. A transplant experiment showed that female survivorship and gonad fullness were significantly lower than males in far-vent locations. Sex ratio biases are driven by two mechanisms: females maximize their reproductive output by selecting optimal habitats and suffer relatively higher mortality in low flux. Next, I hypothesized that the Lepetodrilus fucensis gill symbiosis is a key adaptation. I used multiple approaches to determine if the prevalence of the association and relationship to the limpets condition support this hypothesis. FISH probes specific to the 16S rRNA molecule of a gamma-Proteobacteria hybridized where bacteria were present. Direct sequencing using symbiont-specific primers gave a single unambiguous sequence. indicating high specificity. Light and TEM micrographs of gill tissue from a range of species also showed that the symbiosis is ubiquitous. In addition, the gills of in-vent animals had high surface area. dense symbiont populations and healthy tissues, while far-vent animals showed the reverse trend, suggesting that the symbiosis benefits L. fucensis. Carbon fixation by gill tissues was stimulated by inorganic sulfide and related to the abundance of bacteria on the gill. These data indicate a persistent and specific symbiosis that is dependent on access to sulphide. I further examined feeding by Lepetodrilus fucensis to determine if the bacteria contribute to their host's nutrition. The morphology of feeding structures were compared among Lepetodrilus species. L. fucensis exhibited specialized features: the gill is enlarged. the lamellae are free of the mantle. do not narrow and are stabilized by ciliary junctions. The radula and stomach of L. fucensis are also reduced. Shipboard observations confirmed suspension feeding by L. fucensis. In addition, the symbiont may be ingested because its phylotype was well-represented in food material on the gill. The limpet's morphological specializations are consistent with dependence on suspension feeding and/or symbiont farming; however, L. fucensis also grazes. a mechanism likely important in peripheral locations. Lepetodrilus fucensis populations are partitioned by size and sex along environmental gradients near vents. Peripheral populations are dominated by recruits and adults tend to be male: grazing is likely their primary feeding mode. Larger animals form stacks in venting fluids and are female-biased. These populations access suspended particles for food and sulphide, which generates dense symbiont populations for ingestion. Multiple feeding modes sustain high L. fucensis densities in a space-limited environment and may be an innovative strategy that drives its remarkable abundances.

The Endeavour Ridge is a segment of the Juan de Fuca Ridge, an active spreading centre which lies off western North America between the Pacific and Juan de Fuca plates. This segment is a bathymetric high and a site of hydrothermal activity—both characteristics suggest an underlying heat source such as an axial magma chamber which is associated with crustal generation. To investigate the creation and evolution of oceanic crust, a detailed refraction survey was carried out over the Endeavour Ridge in the fall of 1985. As a component of this survey, a diamond-shaped array consisting of eight OBS along a 20-km line across the ridge and two OBS placed along it at distances of 10 km on either side of the cross-ridge line was deployed to define the shallow crustal structure near and beneath the ridge, especially the possible existence of an axial magma chamber. Airgun shots at 0.2 km intervals along ~300 km of profiles provide conventional reversed and unreversed refraction lines as well as multiple full azimuthal coverage of the region. Travel-time and amplitude data from fifteen in-line airgun profiles recorded on the inner array were forward modelled using an algorithm based on asymptotic ray theory with a starting model obtained from a concurrent study. Two-dimensional models were constructed and then combined to obtain the three-dimensional structure of the region. These models consist of four layers, with the average model correlating well to the classic model of oceanic crust. Layer 2A averages 0.40 km in thickness and has velocities of 2.6 km/s and 2.8 km/s at the top and bottom of the layer, respectively. To achieve such a low velocity, Layer 2A must consist of highly fractured vesicular basalts. A sharp velocity increase to 4.8 km/s marks the transition to Layer 2B. This velocity discontinuity is also visible as a reflector on a. multichannel reflection line obtained through the centre of the study region and is caused by an abrupt decrease in porosity. Layer 2B averages 0.67 km in thickness, has a velocity of 5.4 km/s at its base and consists of less fractured pillow basalts and sheet flows. The Layer 2B-Layer 2C interface is a velocity increase to 5.8 km/s and is the pillow basalt-sheeted dike contact. A small velocity increase from 6.3 to 6.5 km/s delineates the base of the 0.95 km-thick Layer 2C which is the boundary between the sheeted dikes and cumulate gabbros in Layer 3. Layer 3 has the lowest velocity gradient (0.30 s⁻¹) and a velocity of 7.3 km/s at 4.65 km below the seafloor, the maximum depth constrained by the modelling. Lateral heterogeneities on the scale of 2-3 km are superimposed on this basic velocity structure. These heterogeneities are effects of porosity changes, differential pressure changes, and alteration caused by hydrothermal circulation. Layer 2A thins and increases in velocity away from the ridge; ridge-parallel cracks create a velocity anisotropy of ~10-25%, the faster direction parallel to the ridge. Velocities within Layers 2B and 2C also increase by 0.1 km/s away from the axis of the ridge. Layer 3 velocities decrease by 0.1 km/s for arrivals travelling under the ridge. Increased Layer 2 velocities at the ridge crest reveal high lateral velocity constrasts in very young crust, but within 0.03 Ma the oceanic crust at the ridge has matured to the off-ridge structure. No firm evidence exists for a large magma chamber under Endeavour Ridge. Although the bathymetric high and high-temperature hydrothermal discharges are evidence for a magma chamber, the lack of recent sheet flows at the ridge crest and the presence of a rift along the crest indicate the magma chamber is waning and must be of a size (<1 km in width) not resolvable by seismic refraction data. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

The Juan de Fuca Ridge, located 400 km off the coast of Washington State, is home to unstable and unpredictable hydrothermal vent sites where chemosynthetic communities flourish. In 2007 the manned submersible ALVIN retrieved a Ridgeia piscesae tubeworm community in its entirety from the side of the Tower sulfide edifice from the Endeavor segment of the Juan de Fuca Ridge (47 55.416720 N, 129 6.487020 W, at a depth of 2269 m) using the Bushmaster Jr. collection device. The collection was analyzed for community structure and the data collected were compared to that from several other hydrothermal vent communities. It was determined that substrate composition is a factor that heavily influences community structure. The data were then compared to the community succession model developed by Sarrazin et. al. in 1997 and 1999 (Sarrazin et. al. 1997, Sarrazin and Juniper 1999). The Tower community was found to expand the model as a new community succession classification; that of community iii low flow. The Tower community was then analyzed for diversity, structure and tubeworm morphology in conjunction with two other communities from differing substrata. The Ridgeia piscesae tubeworms were found to be of the "long skinny" morphotype, one that was previously thought to only reside on basaltic substrate. The Tower community has similar species richness and higher species evenness than those from basaltic substrate, but similar richness and lower evenness that those from sulfide. This community type combines the characteristics of those from both substrata, resulting in a community with diversity and structure that is an intermediary between sulfide and basaltic substrates. / Biology

The hydrothermal vent environment, in its extreme spatial and temporal variability, offers the opportunity to study habitats that are naturally fragmented and unstable. The vestimentiferan tubeworm Ridgeia piscesae is a foundation species inhabiting hydrothermal vent habitat in the Northeast Pacific Ocean. R. piscesae is a phenotypically plastic species and is arranged in a metapopulation spatial structure, with each local population displaying one of a range of morphotypes. Ridgeia piscesae participates in an obligate symbiosis that is dependent on hydrogen sulphide in the hydrothermal vent fluid that supplies each local population. Hydrothermal fluid flow is highly variable in the hydrothermal vent environment and hydrogen sulphide flux is a limiting nutrient for R. piscesae; this variability may create differences in habitat quality. The objective of this study is to determine whether local populations of R. piscesae centered on high and low flux hydrothermal fluid outputs are similar in body condition, reproductive condition, and juvenile recruitment. Using the submersibles ROPOS and Alvin, I collected high flux and low flux sample pairs from within meters of each other at multiple sample sites on Axial Seamount and the Endeavour segment of the Juan de Fuca Ridge. I used morphological measurements, histology and lipid analysis to assess physiological and reproductive condition. I also determined the relative abundances of new and older recruits in high and low flux local populations. I found that low flux habitat was inferior in its ability to support Ridgeia piscesae at all stages in the tubeworm’s life cycle. In terms of body condition, local populations in low flux habitat had lower body weight, greater body length, smaller anterior tube diameter, lower trophosome volume, lower total lipid volume, and lower branchial plume condition. With respect to reproductive condition, local populations in low flux habitat had lower proportions of reproductive individuals, less sperm transfer, lower gonad volume, and fewer mature oocytes; there was no difference in sperm development stages between high and low flux habitat. From the perspective of the individual, low flux tubeworms live longer, and lifetime reproductive output may be comparable to high flux tubeworms. However, turnover is higher in the high flux habitat, so reproductive output of high flux populations is greater than that of low flux populations. Juvenile recruitment was biased toward high flux habitat, although this trend was not significant and recruitment to low flux habitat was still notable. The differences between reproductive output and juvenile recruitment between these habitats support a source-sink model of population dynamics. From the perspective of the metapopulation, low flux habitat is inferior in its ability to support Ridgeia piscesae at all stages in the tubeworm’s life cycle. This distribution of relative contributions to the overall population of a key species in a Marine Protected Area (MPA) should factor into management decisions affecting MPA boundaries and use. / Graduate