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Biohydrography of eukaryotic microorganisms in a cold-ocean ecosystem Hamilton, Andrew Kent

Abstract

Picoeukaryotes (0.2 - 3 urn) dominate the planktonic biomass of the Arctic Ocean for most of the year, strongly influencing primary production and carbon and nutrient cycles. Despite their importance in this cold-ocean ecosystem, little is known about factors controlling picoeukaryote diversity and distribution. Picoeukaryote community composition and distribution in relation to the physical characteristics of the water column was investigated, and we introduce the term biohydrography to describe studies of this type. Samples were collected at 6 depths across 9 stations in the North Water Polynya (NOW), a large recurring polynya in northern Baffin Bay. The hydrography of the NOW was determined from 52 CTD casts, and several different water masses were identified by their distinct temperature and salinity characteristics. The circulation of water masses in the region was reported and evidence of interleaving and mixing was found along the frontal zone where water masses converged. Picoeukaryote community composition was determined from denaturing gradient gel electrophoresis (DGGE) profiles; samples showed 42 distinct band types or operational taxonomic units (OTUs) overall, with 8 to 22 OTUs per sample, and considerable variation in OTU composition among samples. Similarity analysis of DGGE profiles showed assemblages from different depths at the same station shared as little as 6% similarity, whereas assemblages from locations hundreds of kilometers apart shared as much as 90% similarity. Similarity among picoeukaryote communities was most closely related to the origin of the water mass sampled; for example, Arctic derived waters showed a unique and very different community than those of Atlantic origin. Separate community assemblages were also identified along the frontal zone, suggesting water masses maintain their signature community until further physical mixing disperses the organisms. Matching of excised DGGE band sequences identified organisms from taxonomic groups Acantharea, Cercozoa, Chrysophyceae, Bacillariophyceae, Dinophyceae, Prasinophyceae, and Prymnesiophyceae; however, many sequences matched uncultured organisms, whose function in the environment is unknown, highlighting the need for both culture and ecosystem-based studies. Canonical correspondence analysis (CCA) revealed that latitude, depth, chlorophyll levels, and community size structure were important factors that partially explained 42.6% of the variability in assemblages, indicating contemporary environmental conditions influence picoeukaryote community structure. A detailed understanding of water mass distribution, circulation patterns, and physical mixing processes was required to further explain assemblage relatedness among sites, revealing the importance of investigating hydrographic processes in studies of picoeukaryote community dynamics.

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