Water masses and biogeography of picoeukaryote assemblages in a cold hydrographically complex system

We investigated the influence of geographic distance, environmental variables, and water mass origin on picoeukaryote ( phytoplankton and other protists < 3 mu m) assemblages to assess the presence of biogeographic patterns. The study region was an area of converging Arctic and Atlantic currents where several distinct water masses were overlain and intersecting. Denaturing gradient gel electrophoresis ( DGGE) profiles of assemblages revealed 42 distinct band types overall, with minimum richness ( 8 band types) in Arctic surface water and Atlantic deep water, and maximum richness ( 22 band types) in regions of water mass mixing. Sequencing of DGGE bands revealed that most sequences ( 78 of 98) matched uncultured clones from major taxonomic marine groups, including the Acantharea, Bacillariophyceae, Cercozoa, Chrysophyceae, Dinophyceae, Prasinophyceae, Prymnesiophyceae, and stramenopiles, as well as the novel marine stramenopiles ( MAST), alveolate groups I and II, and picobiliphytes. Multivariate statistical analysis of DGGE profiles revealed that picoeukaryote assemblage composition was positively correlated with geographic proximity, abiotic environmental conditions ( salinity, photosynthetically active radiation, and transmissivity), and biotic community structure ( total phototrophic biomass and size class). Picoeukaryote assemblage similarity was also strongly associated with water mass origin; assemblages in close spatial proximity ( horizontally or vertically) showed less similarity if located in different water masses, while spatially distant assemblages showed higher similarity if located within the same water mass. This study highlights that ocean hydrodynamics must be considered to fully explain the distribution and diversity of microbes in this fluid realm.