We investigate upper crustal anisotropy of the Endeavour Segment of the Juan de Fuca Ridge using shear wave splitting measurements of ~3000 earthquakes recorded during three years using the Keck seafloor seismic network. We apply a new cluster analysis of shear-wave splitting measurements to our database. The methodology reduces the use of subjective criteria and improves the accuracy of measurements in the presence of noisy data. Fast polarization directions at a given seismic station are constant and stable during the deployment; however, fast-polarization directions between stations vary significantly. We presume that the lack of consistency of shear wave splitting among seismic stations reflects the spatial distribution of anisotropy in the vicinity of the ridge axis. We infer that the variation of fast polarization directions and delay times is caused by spatial variations in shallow hydrogeological structures and the stress field. Local faults and fissures are unlikely to be the primary cause of this anisotropy since most of the fast polarization directions are not consistent with the ridge parallel trend of faults. Stress perturbations induced by magmatic injection into the axial magma chamber or spatial variation in the rates of a hydrothermal heat transfer may contribute to the observed heterogeneity in seismic anisotropy.

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