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 mm) 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. Microscopic life, both eukaryotic and prokaryotic, is responsible for about half of global primary productivity and most of the nutrient cycling on the planet. Microbial community structure is important in determining the ultimate fate of carbon and energy transfer in the ocean, which has repercussions for all higher trophic levels (Arrigo 2005). Picoplankton (0.2–2 mm) dominate both photosynthetic and heterotrophic processes over much of the world’s oceans (Whitman et al. 1998; Worden et al. 2004) but factors governing their community structure and spatial distribution, or biogeography, are not well understood (Martiny et al. 2006; Šlapeta et al. 2006). Picoplankton community structure can be influenced by shifts in circulation patterns and changing hydrographic conditions (Greene and Pershing 2007). The limits on picoplankton mobility and sinking rates imposed by their very small size mean the mesoscale distribution of these free-living cells is primarily determined by passive lateral advection and vertical mixing in the water column. Picoplankton are, therefore, ideal subjects for investigating species patterns and their distribution in relation to the physical properties of the water column. It is imperative to understand the extent that physical oceanographic factors influence microbial community and food web structure in the sea because of the likelihood of rapid changes in global circulation in response to climate forcing (Sun and Bleck 2006). This is especially important in the Arctic, where climate models consistently predict that changes in circulation, stratification, temperature, salinity, and ice1 Corresponding author ([email protected]).